Fuel pressure relief valve adjustment - Page 30 - PeachParts Mercedes-Benz Forum

fireman1073 · #**436** 11-24-2012, 09:00 AM

Tork Teknology this company makes adjustable ones for cummins engines with good sucsess at alleviating hard start and low power issues so there is definately an effect on performance with a bad or weak overflow valve.

I wonder if they would make them for a mercedes

Doktor Bert · #**437** 11-24-2012, 09:40 AM

I think one could easily estimate this condition by pinching off the return line to raise the pressures. In fact, it would be easier to put an adjustable regulator between the relief valve and the filter.

Stevo · #**438** 11-24-2012, 11:21 AM

Not sure whats going on with these pressure relief valves but heres my experience. I rebuilt an 80 engine for my 79 240D, new pistons, liners, the works, it ran great. A while back when pressure relief valves became a hot topic around here I bought the set up to check my cars just for fun. The 79 had ZERO pressure and was running great and had been since the rebuild seven or eight years past. I had installed just a banjo fuel fitting and no pressure relief valve at all, oops. I got the proper pressure relief valve from one of my parts cars and rechecked the pressure, 10 or 12 lbs, dont remember exactly, but it was up where it should be. It made NO difference in performance or fuel millage but like I said the car ran great in all respects since the rebuild with NO pressure relief valve. ???

Doktor Bert · #**439** 11-24-2012, 11:26 AM

Quote:

Originally Posted by Stevo

Not sure whats going on with these pressure relief valves but heres my experience. I rebuilt an 80 engine for my 79 240D, new pistons, liners, the works, it ran great. A while back when pressure relief valves became a hot topic around here I bought the set up to check my cars just for fun. The 79 had ZERO pressure and was running great and had been since the rebuild seven or eight years past. I had installed just a banjo fuel fitting and no pressure relief valve at all, oops. I got the proper pressure relief valve from one of my parts cars and rechecked the pressure, 10 or 12 lbs, dont remember exactly, but it was up where it should be. It made NO difference in performance or fuel millage but like I said the car ran great in all respects since the rebuild with NO pressure relief valve. ???

Interesting point you make. I have seen some engines 'wake up' from an increase in base pressure.

On my 617.950 I can pinch the return line shut and it doesn't change anything....

Stevo · #**440** 11-24-2012, 11:47 AM

Quote:

Originally Posted by Doktor Bert

Interesting point you make. I have seen some engines 'wake up' from an increase in base pressure.

On my 617.950 I can pinch the return line shut and it doesn't change anything....

Yah, I posted about it last year some time and nobody had much of a theory or explanation.

Doktor Bert · #**441** 11-24-2012, 11:54 AM

But on some cars, it makes a HUGE difference.....wonder why????

barry12345 · #**442** 03-22-2013, 11:02 AM

Without using a fuel pressure gauge there is only guesswork.

barry12345 · #**443** 03-22-2013, 11:07 AM

Quote:

Originally Posted by Stevo

Not sure whats going on with these pressure relief valves but heres my experience. I rebuilt an 80 engine for my 79 240D, new pistons, liners, the works, it ran great. A while back when pressure relief valves became a hot topic around here I bought the set up to check my cars just for fun. The 79 had ZERO pressure and was running great and had been since the rebuild seven or eight years past. I had installed just a banjo fuel fitting and no pressure relief valve at all, oops. I got the proper pressure relief valve from one of my parts cars and rechecked the pressure, 10 or 12 lbs, dont remember exactly, but it was up where it should be. It made NO difference in performance or fuel millage but like I said the car ran great in all respects since the rebuild with NO pressure relief valve. ???

Hard to figure out why exactly this was experienced. I know that some 616s will run with basically no fuel pressure. Usually getting the presure back up made a noticable difference on most of them.

Stevo · #**444** 03-22-2013, 12:02 PM

Quote:

Originally Posted by barry12345

Hard to figure out why exactly this was experienced. I know that some 616s will run with basically no fuel pressure. Usually getting the presure back up made a noticable difference on most of them.

It is odd that there was no difference in performance, MPG or anything after going from zero pressure to 12 lbs after adding the valve. I will say that since I dont keep track of my fuel millage on a regular bases there COULD be a small variation there. My 79 240D (80 engine) diffidently ran just the same with and without a pressure the relief valve. I remember after the rebuild reading Der Diesel doctor's instructions on timing by the bubble method, and not knowing what he was talking about when he said to "remove the pressure relief valve"

barry12345 · #**445** 03-22-2013, 07:06 PM

Quote:

Originally Posted by Stevo

It is odd that there was no difference in performance, MPG or anything after going from zero pressure to 12 lbs after adding the valve. I will say that since I dont keep track of my fuel millage on a regular bases there COULD be a small variation there. My 79 240D (80 engine) diffidently ran just the same with and without a pressure the relief valve. I remember after the rebuild reading Der Diesel doctor's instructions on timing by the bubble method, and not knowing what he was talking about when he said to "remove the pressure relief valve"

I assume you read your pressure without the normal relief valve in place. If not there is a small chance some partial obstruction past the injection pump may have enabled at least some fuel pressure to exist.

There was another member that made me aware a few years back is that some of these cars will still operate at zero pressure. Or at least with the relief valve out of the circuit.

Dr. diesel might have intended to mean remove the number one pump elements delivery valve.

Eighteen to nineteen pounds fuel pressure is about the optimum today. Going from say twelve pounds to that may or may not make a differance in your car.

Stevo · #**446** 03-22-2013, 08:18 PM

Quote:

Originally Posted by barry12345

I assume you read your pressure without the normal relief valve in place. If not there is a small chance some partial obstruction past the injection pump may have enabled at least some fuel pressure to exist.

There was another member that made me aware a few years back is that some of these cars will still operate at zero pressure. Or at least with the relief valve out of the circuit.

Dr. diesel might have intended to mean remove the number one pump elements delivery valve.

Eighteen to nineteen pounds fuel pressure is about the optimum today. Going from say twelve pounds to that may or may not make a differance in your car.

Hm, I didn't read the pressure at all until I installed the relief valve so I guess its possible that something was restricting the fuel, but when I installed the valve it read normal, forgot just what it was
No, he meant the pressure valve as you cant blow air through the pump with it installed.

I should have taken the pressure reading before I installed the valve, DOH

buch32 · #**447** 03-23-2013, 09:05 AM

Quote:

Originally Posted by barry12345

I assume you read your pressure without the normal relief valve in place. If not there is a small chance some partial obstruction past the injection pump may have enabled at least some fuel pressure to exist.

There was another member that made me aware a few years back is that some of these cars will still operate at zero pressure. Or at least with the relief valve out of the circuit.

Dr. diesel might have intended to mean remove the number one pump elements delivery valve.

Eighteen to nineteen pounds fuel pressure is about the optimum today. Going from say twelve pounds to that may or may not make a differance in your car.

SEE... FUEL PRESSURE PROBLEM SOLVED '80 300SD..2/21/13

DrLou · #**448** 05-30-2013, 06:05 PM

OK, I'm gonna weigh in here, if only to round out the dataset.

I'm beginning to work through a general loss of power under load problem, and have been reading all the pertinent threads for ideas.

Symptom is that the engine feels fuel starved under load, eg when climbing our local hills at speed. Really gets sluggish, not very responsive. Driven around town, however, under light load, the ol' girl is very smooth. Idle is very smooth.

Points:
Car is due for a valve adjustment, so that's first.
Fuel filters are relatively new, but will replace soon, too.

For kicks, I checked the relief valve spring you guys are discussing here.
Mine was about 22mm; I extended it to 27 and have no difference perceptible.

Car starts easily, and idles smoothly, as it did before.

The injection pump's rack damper bolt appears to be tightened to its fullest extent; I have never adjusted this bolt since purchase. (I've put over 200k miles on 'er)

Am also evaluating claims here - and elsewhere - that I simply must replace injector nozzles on a 400k+ car, almost as a matter of course. Do you all agree?

I've done cleaning of the nozzles a few times over the years; even attempted to re-set pop pressures on them at some point, but have never replaced. Thoughts?

Am being directed to the Bosio replacements, which are available in two sizes for my injectors, apparently. Injection Pump is a 617.951.

Finally, given that nothing has been done to this injection system in 20 yrs, is the rebuild of the lift pump a suggested maintenance item?

Thanks!

barry12345 · #**449** 05-31-2013, 10:29 AM

A systematic approach might be to first eliminate the tank filter as a culprit. Just disconnect the hose and see if a good gravity flow of fuel from it is there. If so rebuild the lift pump as they do wear with time or valves in them can crud up.

Since you do not have a pressure gauge. Establish if there is any flow from the return line on the injection pump. If absolutely none you know you have an inadequate pressure and flow problem. The system is designed to have the flow out the relief valve when running. Keeping it simple speeds up the location process.

The problem with the injectors is usually we do not know the cars history well. They may be the original injectors never even having been checked. Or they could have been done not long before our acquisition of the cars. They can be checked for spray pattern and pop pressure before deciding if they need rebuilt.

We will always be working with wear and aging symptoms. So before anything and especially with your high milage. Have someone press the fuel pedal to the metal in the car. You observe if the throttle linkage is moving the lever on the injection pump to it's end point.

You may have already done this but if not it is one of the highest rates of failures issue. Just a good possible candidate for your issue as well.

One last item is I am not a believer in if it is not broken do not fix it. It is straightforward and cheap to rekit a lift pump. At somepoint with milage it will give up. This could cause the need for a tow truck and all the associated inconvienience that goes with a car quitting. So I see it as just sensible pre emptive care.

Once again as we usually have so little actual history on these older cars we have no way of knowing what has or has not been done in the past. Fortunatly they are a simple straightforward design and keeping them reliable is usually not all that expensive.

DrLou · #**450** 05-31-2013, 01:22 PM

Barry1234, Thanks much for your feedback, and notes...

Quote:

Originally Posted by barry12345

A systematic approach might be to first eliminate the tank filter as a culprit. Just disconnect the hose and see if a good gravity flow of fuel from it is there. If so rebuild the lift pump as they do wear with time or valves in them can crud up.

Right. Am gonna do both of these first. Expecially as I've suspected - and today confirmed - that things seem to get worse as the fuel tank empties. A few of you here have indicated this as a symptom of a clogged fuel tank filter/strainer. Hey, it's been 30 years in there...(!)

Quote:

Originally Posted by barry12345

...Establish if there is any flow from the return line on the injection pump...

... They can be checked for spray pattern and pop pressure before deciding if they need rebuilt.

I've had them out a few times; lots of cleaning, and some attempts at re-setting pop pressures. Surely never replaced though, of this I'm sure!

Quote:

Originally Posted by barry12345

Have someone press the fuel pedal to the metal in the car. You observe if the throttle linkage is moving the lever on the injection pump to it's end point.

You may have already done this but if not it is one of the highest rates of failures issue. Just a good possible candidate for your issue as well.

One last item is I am not a believer in if it is not broken do not fix it. It is straightforward and cheap to rekit a lift pump. At somepoint with milage it will give up.

This rebuild is now firmly on The List, as are your other suggested items.

Quote:

Originally Posted by barry12345

Once again as we usually have so little actual history on these older cars we have no way of knowing what has or has not been done in the past...

Fortunately, I've got most records from the car's early life, and have kept meticulous records myself - so I'm sure nothing has been done on the fuel supply/injection side of the house in about 200k miles!