Fuel pressure relief valve adjustment

[Cervan]

Im not sure what its called, but on my 616 4cylinder there is a small banjo bolt with a check ball valve with a small spring that limits the ammount of pressure that the lift pump allows into the injection pump. So i was doing some stuff and i had it off, i took the spring out and stretched it a small ammount and put everything back together, i have to say the results are astounding, gained alot more torque in the low end but its also a little harder to start.

It regulates the fuel pressure the lift pump provides. Stretching the spring probably restored the pressure lost to fatigue wear over time. I might have to give that a try myself.

[barry123400]

I like the ideal of time /use/fatigue. I think the injection pump should retain 14 lbs pressure before the relief return valve opens.

My understanding is that if you tee in to the line from the lift pump you should see 14 lbs. What you are really seeing is that return valve when it is open. This may vary depending on the model of pump on 123s but I doubt it. Too simular in general design.

So a lift pump pressure test in itself is suspect if you get a low reading. I guess you should pinch off your return from the pump to see if you exceed the 14lbs to be safe. Some pumps are not designed to have a positive output restriction in place though. So the mentioned volume flow approach with time probably makes more initial sense. It is not absolutly conclusive though it just proves you are getting volume really.

Probably the best way to retension the spring is to insert an adapter into the input or output of the injection pump and read the operating pressure. A new spring from the pump manufacturer may be pre calibrated and pretty cheap as an alternative solution.

If so better than trying to adjust the calibration ourselves. It would not surprise me if that spring is changed out at every major pump service or the pressure retention calibration of the pump at least checked. I hope they do not miss it. A pump guy could quote on that or anyone with specific knowledge.

This kind of sounds like it should be a tune up item on a 123 diesel. If your spring has gotten weak with time and age it will not be the only one. I have to wonder what your pressure is now though. Plus it might be subjecting the lift pump to new dynamics that may not keep it happy long. Thats if you are overpressured. Not sure either why the car is harder starting. Theoretically it should be easier unless you are way over pressure perhaps. You in effect might be well advancing the timing if the elements are taking on more fuel than designed

I also feel that if the pressure is too low inside the pump some elements act like they are starving a little. Or to be correct just not taking on their design volume of fuel. I have noticed lots of examples of this on poor injector pump supply problems posted over time. Since you noticed an improvememnt perhaps that was the reason.

I had wondered for a long time why nobody ever really expressed or checked this item for proper function. It may be because the car still runs but not as good as it should and some other possible unknown is just blamed like lower compression. Or the injectors are old.

I have been kind of watching and hoping someday the reason for inferior milage on some 240ds in comparison to others was discovered. The same effect is seen on 300ds but not as bad a percentage spread. . Low retention pressure in the injection pump could in my opinion result in uneven injector element loading and feeding on some cylinders. This would cause an imbalanced firing at speed but likely not noticed by the driver. Its probably like trying to hear your watch tick while riding in a rotating cement mixers drum. Probably could reduce the general milage as well.

Remember some people have claimed it appeared marginal fuel filters hurt milage in their experience. This may have been the injection pump was not getting enough fuel to balance off the injector elements equally. That could also cause enough less fuel flowing that the injection pump would not reach the 14 lb operational pressure. The pump was consuming fuel faster than a designed operational pressure could build up or accumulate.

Originally I did not discount people that claimed a partially obstructed fuel filter hurt milage. Yet I could not figure out why this was occuring. In my opinion lesser performance levels because of lack of enough fuel should be equal to running lean. Or since it is a diesel and I have no flame suit handy underfuelled on elements. If instead it causes an unequal element loading in the pump though that would do it. This return pressure calibration might even be critical to economical operation.

Now it looks as though what you have dived into may be a factor even with a good fuel filter. I hope others will jump on this and express opinions and thoughts. Maybe we can get all our 240ds on an equal miles per gallon basis yet someday. I still feel it is a generally unknown common denominator that causes the milage spread that is greater than just trying to brush it off by stating it is driver related.

Generally when highway cruising the operator does not have enough influence to cause a 22 to say 30 mpg spread on different yet the same cars. For a long time I felt it may be a tired centrifical advance mechanisim on some cars but could find nothing concrete to indicate it.


Top speed on the 240s would have been non existant for example. Pump problems like element timing drifting off sequence drifted across my mind. . Though that did not really gell either as indirect old fashioned engines have great tollerance compared to sophisticated systems. This type pump if properly supplying fuel at idle is going to generally supply at higher rpms by it's general design. If you undersupply the elements at idle the problem should be worse at higher revs. That is kind of a new consideration of mine since you posted.

Cervan could you keep an open mind and notice if the milage on your car seems better or worse. I do not think it will be the same.

That would indicate this area should really be investigated. Even though I am longwinded I have been watching for anything that even had the potential of being the common denominator. This area might fill the bill. Really glad you posted on this. If there is anything positive that results what a cheap and easy fix it would be for many. If the springs are pre calibrated and cheap I will change mine out if milage is inferior when doing other jobs around the cars.

There has been constant indicators that the amount of pressure applied to the general fuel waiting to be loaded into the elements is important. I suspect it prevents forming of air bubbles at the time of element lloading. The 14 lbs pressure may be very critical. It may be one of the considerations involved in how much fuel is loaded in the element as well. I have not decided if the elements piston at the bottom of its stroke is still filled with fuel left from the last compression stroke. Since fluid in neither compressable or mallable by vacuum. Does a vaccum induced bubble present itself when the piston decends to the bootom where a slot allows fuel to enter as in a two stroke engine?

Cervan if you have more residual pressure in the general fuel supply now you might be intaking a little more fuel into the element. Thats especially if you are now higher than the 14lbs design. The notacable increased torque is a slight change in the effective timing if the idle sounds about the same as it was in my opinion. If the residual pressure was say just 8 lbs and yoiur stretching the spring has raised it to about fourteen pounds then perhaps the increase torque is because all elements are now able to get a balanced loading. The new milage you get will tell one way or another I think.

[Cervan]

Yeah i thought about that, since im going to be subjecting the lift pump to more pressure than it was used to (or more than it was desiened for) it could cause it to fail sooner, but the lift pump itself doesnt cost very much, so im willing to take the chance. Im not quite sure why its harder to start, but what im wondering is if by raising the pressure in the pump its also advancing the timing somehow because it also knocks a bit more than normal.

[barry123400]

Yes it has to be you must have changed the element loading amount. This causes the opening pressure of the injector to be hit much earlier. Thats the new torque with the advanced timing. Or perhaps it's the equal loading lessening the imbalanced firing in the engine creating the additional torque. All cylinders are generally contributing more equally. Also possible is a combined effect.

I am pretty sure a new spring is minimal cost and precalibrated if available. Unfortunatly because of manufacturing tollerences you may have to get a complete replacement pre calibrated relief valve. In my opinion otherwise you have to have a look at your holding pressure with an accurate pressure gauge.

The question that is lurking in my mind right now. How many pumps have been replaced or rebuilt for a rough idle and or poor fuel milage that just had a seriously tired relief valve? An external pump part that is easy to check and change if required.

Still good for general knowedge you went there. From now on I will check my pressure on any car I feel the milage is substandard or idle is rough. It cannot hurt and I could even learn something. On cars at the top milage for type. I kind of know that pressure will already be about fourteen pounds I think.

I guess the item you really brought to life for me was how detrimental unequal element fueling would obviously impact milage. The engine in my opinion would be dumping much more fuel in some cylinders than others especially because it was unbalanced. Or in the unbalance state the problem was constantly changing to some degree. The loss of engine efficiency could be twenty five percent or so I imagine fairly easily. Or the rolled back timing effect might cause the milage drop. Even a combination perhaps.

All of a sudden the guys reporting better fuel milage after changing fuel filters may be making some sense. Again I never doubted their statements before I just was not cognizant of how that senario could be occuring.


Another one of my by now notorious thoughs. If the residual pressure were really low say the relief valve was stucjk open. I suspect you might have a terrible rough idle. To find out how it would be I think I will remove the relief valve or take out the spring and see just how the engine runs when I get a little time. I am aware the timing will also be a little retarded as well. Still the increased disparity of fuel amounts in the elements should easily overcome that. I never remember anyone suggesting that this be considered on a rough idle and/or running complaint. It has always been grouped in with air perhaps if anything.

Once again if this results in the reasons being found on the milage discrepancies between otherwise identical cars. Cervan you will have made one of the major site accomplishments so far in my opinion. This milage discrepancy problem has occupied my mind for a long time with the 240ds especially. Yet I was and have been unable to develop a sound theory of why.

Cervan what you might have proved in my opinion already. Amount of element loading is directly dependant of fuel pressure in the pump. This by your timing advance alone. There are many solid indications that quality therfore consistancy of all loading of the elements is also highly dependant on this operating residual pressure as well. If it is off so will a few other things be as well.

It will still be awhile until a percentage of the relief valves that are way off calibration with age emerges. There may be virtially none or quite a few. Time will tell and our knowledge about injection and lift pumps increases a little more.

Paragraphs added on today with some modification of the above.

As I was driving last night it occured to me that a possible reason the milage was worse with a poor flowing fuel filter. This has been reported too many times on site not to be true. Also it would have to be quite a drop for site members to have noticed it after changing out the fuel filter. I know I mentioned it earlier a little but it's importance is worth thinking seriously about . As my thoughts evolve I will modify present statements and insert information that may help others rationalise or recall issues from the past that tie in well.

I hate a wild goose chase in general yet suspicious things need examined. My own drive or compulsion is still this milage discrepancy thing. I know it is generally not a combined problem defect. The difference seems clear cut enough it is one item causitive in my opinion. There has been no basic pre chamber or head design rehash with these engines. In a gas engine if what I am observing was occuring the combustion chamber for design differences is the first place I would look. There just is no difference there on these engines.


It might effectivly roll the timing back a long ways. Or that and the unequal loading component of the elements if present. If this is so there is justification to change that filter out on a lessning miles per gallon basis rather than just waiting until it obstructs so bad you have to change it.

I personally ignore frequency of spark plug changing for example and change the spark plugs out if milage sags or car is starting harder. There are too many variables to give specific milage intervals. Some spark plugs will need changed out for example before the recommended milage interval but the majority seem to easily go much further in my experience. This type of possibly abstract reasoning just might apply to the fuel filters.

The manufacturer cannot control actual field conditions so he generalises with a safety margin with his recommendations. Again just my opinions. I think load of dirty fuel for example might do it. The car would still run decently yet your milage per gallon is comprimised. Now if you combne this with a weak regulator spring if they indeed change their calibration with time and use you are never going to see good fuel milage. This has all yet to be proven. Parts of it already seem to be.

Cervan. It is possioble to find how far your timing is advanced in my opinion by the milli volt method. Or a magnetic pulse type device if known to be accurate. Not worth the effort and you do not want to be there anyways.

Part of my intensity with the milli volt attempt to time these engines was it averages all the lurking variences in an old engine that an owner is not addressing. Or he cannot locate and finds the best available timing for an individual engine. The pulse device cannot do this and the manufactures timing recommendation is based on everything in good/excellent condition. Unfortunatly the stiffness in the injector lines make this approach not practical on these engines in my opinion. It cannot even be used to indicate you have underlying problems by indicating the peak efficiency point is well removed from the factory recommended timing.

Those stubborn hard injector lines negate any such effort. Perhaps yet in the future some practical way will be found to verify the factory recommended timing position is right for your individual engine with this method. The present methods do not even allow for the fuel we use today with lower celane numbers. This applies for instance to changing timing requirements mind any wear or defects.

From my past experience on volkswagon timing too far advanced can possibly loosen up the harmonic balancer. Your new hardness to start is possibly an effect of considerable timing advance. This whole senario may not be the best. I would just try to make sure the relief valve is calibrated to whatever the manufacturer states and observe my milage.

You have no baseline on where it was prior to the spring stretch in my opinion. Still after everything is restored see if you are getting better milage in general. That might kind of prove the residual operating pressure was indeed low before. Or that you had about 14 lbs pressure prior to the adjustment. Thats if the current milage is the same after restoration if required. Also I would move my timing up two degrees that a lot of people have reported over the factory recommended setting. I have never heard any difficulty reported by doing this. It also sounds reasonable just by the ongoing changes such as celane ratings in our present fuels over the last twenty or so years since these cars were made alone.

The lowered celane values compared to europe are real by the way and it is starting to appear amight be the true cause of the 3.5 diesels going bad as the same block in europe with their higher celane rated fuel is holding up I am led to believe. How the lower celane value fuel is doing this is another issue. I see no issue then of boosting the celane with aditives on a 3.5. If this area sorts itself out perhaps they might be even be a safe buy at some time in the future.

Right now I would only take one home if close to free and in good condition. Thats just me though. I still feel it is almost criminal they left so little material between cylinders. Thin material is not only weaker but will distort or warp easier as well.

I visualise investigating over time observations of items or effects observed by members until the culprit for the milage discrepancy is found. I am out of ideals myself. So I just try to keep an open mind until an item like Cervans is posted. It almost has to be somewhere in the complete fuel system.

On the 240ds the discrepancy allowing for different drivers habits is still 15-25 percent overall. Why this percentage spread is not showing up on 300ds is beyond me.

Or is it a requirement to see thirty miles per gallon to have almost static compression greater than 400 lbs? One day a member getting 30 or low thirty plus miles per gallon with a 240d will report his static compression.

If you get through this you are older but not wiser. I still would like to see disscussion in this area increase.

[barry123400]

Bumping this insanity up. Gotta be some opinions and thoughts out there. It might even be an important item. If not the resultant entertainment value might be worth it.

[Cervan]

yeah i belive that atleast 70% of diesels that are abandoned or sold because they are "not running" is because of a simple fuel problem and not a hard part going bad.

[kuhns34]

For kicks i pulled mine apart today and streched it a little bit and wow gave me alot more power!

[Cervan]

Originally Posted by kuhns34

For kicks i pulled mine apart today and streched it a little bit and wow gave me alot more power!

Thats exactly what i noticed.

[barry123400]

Try to notice if milage is better or worse and post on this thread. You might help a lot of people. Do you remember your average milage prior to stretching the spring?

What I still think is occuring is the injection pump elements are fuelling better or more equally. If the loading quantity is greater as well than design the timing relationship of the pump to the engine is changed as the injectors pop off earlier. Timing is in effect advanced.

The important item is before the spring was stretched . What was the release pressure point of that return valve? What is in now I wonder? Is it possible to put a gauge somewhere between the lift pump and the injection pump to read it? I an kind of hoping some old 240d and 300d engines have a substandard release pressure on their relief valves becuse af aging and usage.

If either of you where to presently read abouit 14-15 pounds pressure now it would mean before your spring stretching the pressure release was low. I am pretty certain that if you drip timed an engine with a substandrd release pressure. You are wasting your time. When that pump is returned to running you would have retarded timing in reality.

A lesser quantity of fuel is being loaded into the elements than the pump designers intended. So the injectors pop off later. If the presure is really low erratic loading may also occur from what I have observed on earlier posts. This at present is the best current possibility of where the missing miles per gallon might be on some of those 240ds.

It did occur to me since the pump and relief valve was simular on the 300ds the problem should be almost identical. . Milage loss for example would be simular. Instead it appears a somewhat lesser problem on them. The reason I suspect is more power strokes on a five cylinder than the four I think. The inefficiency is less pronounced with them perhaps.
The relief valve release pressure may have quite an effect on the quality of operation as well it is starting to look like. I originally thought the residual fuel pressure was just there to make sure fuel was available if needed.

Too bad we do not have a member with extensive pump knowledge. With a little time the answers will emerge anyways. You did not mention if your car is a 240d or 300d.

Any more black smoke than normal at startup and running? It is also quite possible for there to be less than before if efficiency has increased.

The curse of this may turn out to be be that before we all set our engine timing you should probably make sure the residual pressure in the injection pump is correct on all 123 diesels.

[kuhns34]

I just filled up today so i'll have to come back with MPG but i was getting around 25 in my 300sd. so far there seems to be less smoke and the engine has a crisper sound at highway speeds.

[Cervan]

Well after i stretched the spring i also retarded the timing as much as i could to try and alleiviate some of the knocking. But i couldnt retard it enough to stop all of it. My milage was about 25-30 around town. Im going to de-stretch the spring a little bit and see how that changes everything.

[barry123400]

You in my opinion can do some homework if you wish and find out the intended pressure release point of that valve when these cars where new. In my humble opinion if you gain any more than 2-4 miles per gallon you are too far over pressure. It is very hard to get a handle on fuel milage with any accuracy.

Yet two to four miles per gallon should be kind of felt intuitivly if there. By the same token if you are getting less mpg. Not a great probability . Remember to try to drive the car in a simular fashion to what you were before the stretch. Power tends to corrupt remember. You would sure notice it.

The best approach would be to tee a fairly accurate pressure gauge into the line somewhere past the lift pump and see what you have. Do not read the pressure before the filters if you feel they may be comprimised.

For the present people should not all rush out and stretch their relief springs. Yet if your milage is substandard to the good examples of your type. Reading the injection pump residual pressure might help establish a problem exists. It's pretty early with this yet promising.

[barry123400]

Originally Posted by Cervan

Well after i stretched the spring i also retarded the timing as much as i could to try and alleiviate some of the knocking. But i couldnt retard it enough to stop all of it. My milage was about 25-30 around town. Im going to de-stretch the spring a little bit and see how that changes everything.

My guess and it is only a guess is that your 240d was close to a top milage example before the spring stretch. I have never heard of one exceeding 32 mpg on long highway cruises. Exception being ebay of course. So your old calibration of that spring was about right or fairly close in my opinion. I Think this area needs investigation on any 240d standard getting less than about 27 mpg on the highway being driven in a reasonable fashion keeping up with traffic.

Now the five cylinders are a little trickier. They too can be deficient in milage but not so noticeable. So it is more subjective. At present. Suspicion should occur if you feel yours is substandard to the top 20 percent of your type examples.

The best approach is to read the residual pump pressure. If low I suspect in every case that they are returned to what they should be the engine will have more accurate timeing when the engine is running if the drip method was used. In fact if you find you have to restore the original pressure your timing with the engine running will match the drip method that was used to set it better again. Restoration of proper pressure also gets rid of the retarded timing effect inherent with low residual pressure I suspect.

Another important consideration is a ragged idle. This might indicate low pressure in the pump and unequal element loading . These cars are now so old deteriouration of something like relief springs is not unreasonable on some of them. You have to make sure the filters are not bad enough to limit pressure in the pump when reading. We know from others low pressure in the injection pump from poor filter flow hurts milage. Too many cases reported to dispute. I am at last almost certain the cause is now established.

If your milage is high average for type you can almost assume the pressure is about right. Rather than running out and stretching spring people with low mpg examples might read the pressure in their pumps. If lower than what it should be then adjust or replace the relief valve. First make sure the lift pump can reach past the required pressure perhaps by a quick pinch off test once the meter is installed. Volume flow to maintain pressure is another consideration. The filters have to be eliminated as a causitive as well. Restricted flow can result in low final pressures.

One last important item. It is apparent and was suspected an effective timing change does take place with different relief pressures or pump residual pressures if you preffer. I think the swing is substantual. This can be graphed with the milli volt method if bosh does not already have that. The tremendous increase in power these gentleman are feeling is the heavily advanced timing over their pre spring stretch timing.

The best and safest approach is to have the residual pressure at the high end of the original tollerance by bosh. If yours was substandard to that you will certainly feel an improvement. Especially in milage I hope. At this point we should establish the manufactures original intention and if he modified this value over the production time run. I suspect they did not.

If absolutly nothing else the method of insuring reasonable accurate running timing by pre checking the residual pressure of the pump is no longer much in question. If thaty pressure is off much you can drip time until the cows come home and the running timing will still be wrong. The relief valve in almost if not all cases of change will weaken with time. I have this gut feeling bosh wanted it really close to design requirements. A pound off might be 1-2 degrees of engine timing.

[Cervan]

Well i went to de-stretch the spring and it popped out of my hands and that was the last i saw of it lol. So i removed it completely and put the top screw back on, And i have to say that im now feeling even more power from the entire powerband. I have a theory on this and ill post it later on when i get back.