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  #1  
Old 04-01-2009, 03:28 PM
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fuel pressure

There has been a lot of discussion about the fuel pressure made by the lift pump and its effect on starting, fuel economy and even bearing failure. So I decided to install a fuel pressure gauge that I can monitor while I drive.

Initial reports show that at idle I have ~0.75bar right in the middle of the manuals 0.6-0.8bar min. The interesting thing is that I stretched my spring out last summer ~ 17k miles ago. At that time the pressure at idle was closer to 0.9bar.

If I rev. the engine up to ~3000rpm I get 1 bar above the 0.8bar min.

Now here is the interesting thing. When I go for a spirited drive the fuel pressure drops to 0.75bar at higher rpm and full throttle. I wonder if the lift pump can keep up with demand on the highly modified engines.

I'll update as times goes on.
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  #2  
Old 04-01-2009, 03:51 PM
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FWIW I think mine is starting to fail. Not able to keep up with demand. Do the cavitate?
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  #3  
Old 04-01-2009, 04:15 PM
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>>0.75bar at higher rpm and full throttle

Sounds like your pump's doing just fine.

Are you measuring downstream of the secondary filter?
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  #4  
Old 04-01-2009, 04:44 PM
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It is measured down stream of the filter.
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  #5  
Old 04-01-2009, 05:12 PM
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Can you give a little more info, like what kind of gauge you installed?
I want to do the same thing to monitor the fuel filter (mainly the VO filter). I want to put the pressure sensor just after the lift pump. Did you use an electronic or mechanical gauge?
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  #6  
Old 04-01-2009, 05:46 PM
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I have successfully used Turbo Boost gauges as fuel pressure gauges, although if the fittings are not tight, the fuel can travel up the hose and cause needle jitter.
To allow it to monitor the pressure of both fuels (I use separate fuel pumps and filters for my start fuel and veggie), I installed it at the IP inlet.
Dedicated fuel pressure gauges are available on eBay and should be suitable for this application.

Tony
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  #7  
Old 04-01-2009, 06:59 PM
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Your .75 bar at maximum high fuel feed rates when accelerating hard at speed is probably about as good as it gets. If not it is certainly more than adaquate.

You also mentioned that normal pressure is good for cold starting. It was suspected earlier and since proven I believe. I credit Mr.Hunter by the way for this.

I think thats principally why our Mr. Hunter has rebuilt quite a few lift pumps this past winter. I still wish he would post any viewpoints he has developed in this area. It is pretty obvious he has probably been checking pressures.

I still feel this cheap mechanical dampened gauge (about ten dollars) should be a permanent installation. It could be hidden in the glove compartment or somewhere. You really only need to check it periodically.

Although it must be visable at speed when you want to examine the pressure. Perhaps the present originator of this thread can describe where he placed his?

There is real established evidence of fuel milage decline with low pressure. To what extent I am not aware but it is claimed to be easily noticeable when the pressure is really low.

This often reported effect originally got me thinking and starting to post in this area some time ago. The fuel milage to a simple guy like myself should have been better.

Anyways it makes me feel good that serious consideration of this area is now occuring. Hopefully many more posts of gauge users will occur with time until we have a lot more knowledge. I look forward to learn a thing or two from you fellows that are taking the effort.

If the cam lobe for the lift pump reactivates the lift pump cycle when number one element is loading.Or just prior to it loading. Remember the stored energy stroke to the lift pump is only applied once per injection pumps complete cycle.

Then after exhaustive thought I think it might be reasonable to saddle the failures or at least the constant heavier loading of the number one rod bearing. On very low to almost non existant base fuel pressures.

More of the engines running work load is being serviced by the number one cylinder then. Other effects like individual cylinder timing being more retarded in comparison to the number one cylinder are contributing I believe as well.

It may always be running under more stress or loading than it should when pressure is substandard. On this basis it is just going to wear that number one rod bearing out quicker over time than the other bearings. .

As time goes on this earlier speculation of mine is becoming both more logical and other than absolute poof being presented it should probably be taken very seriously . At this point if you own a 616 engine especially.

If you run with virtually no fuel pressure in the base of the injection pump at speed. There is more than a fifty-fifty chance you are loading the number one rod bearing somewhat more compared to any other individual rod bearing in the engine. Just too many items now point directly there to ignore.

Your car can still run fairly well with very low supply pressure. So low fuel supply pressure might be quantified at this point as a potential silent engine killer. Why take the risk? Check your pressure. It really might just save your engine over time. There are also many other important benifits to having reasonable pressure as well..

Last edited by barry123400; 04-01-2009 at 08:27 PM.
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  #8  
Old 04-01-2009, 07:30 PM
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I'm not usually a fan of idiot lights, but this might be a good application for one. Inexpensive pressure switches are available with a range of opening pressures. It would be very simple to wire up a light that would warn any time the fuel pressure dropped below the opening pressure. Easier installation than a gauge, and safer than running a fuel line to the dashboard or glove box.
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  #9  
Old 04-01-2009, 07:44 PM
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I removed the ashtray & mounted a boost gauge and fuel pressure gauge as well as a couple of 12v sockets on a woodden panel in place of the ashtray. All visible while driving and lights up at night with the dash lights.
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  #10  
Old 04-01-2009, 07:45 PM
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Answer

Quote:
Originally Posted by barry123400 View Post
As soon as you notice the pressure starting to really drop off. It's probably time for a filter change. Besides all the side issues. When the fuel pressure drops down below .6 bar during normal cruising I think the fuel mileage is starting to decline.

Partially obstructed filters should have enhanced mileage as fuel to the engine was being restricted.. Until we get a lot of feedback we will not know how much fuel mileage is lost by very low pressure to the injection pump. There is no doubt it does hurt at this point.
Correct:
When the fuel pressure drops, the first diagnostic step is checking/changing fuel filters.

Misleading/incomplete question:
Partially or fully obstructed filters do NOT enhance MPG, because the injection pump is starving = random weak uneven fuel delivery + suffering internal heating and lubrication loss (this does damage the injection pump).
Most drivers exacerbate the issue by heavier throttle application, driving until shutdown...

Note:
The injection pump fuel delivery piston and cylinder are cooled and lubricated by diesel fuel = drop the fuel pressure low enough/long enough (bad lift pump) or drive with plugged filters long enough = the injection pump will loose calibration and/or fail.

(as of April 2009)

Lift pump repair kits cost $10.00
Fuel pre filter cost $2.15
Fuel secondary filter cost $10.00


Average OM617 Injection Pump Rebuild cost $850.00





Have a great day.
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  #11  
Old 04-01-2009, 08:26 PM
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A very intriguing thread !
Browsed some docs and found a difference in where the place the measurement takes place:
-between fuel pump and fuel main filter(w116 om617.95x)
-between fuel main filter and IP.(w115,w123)
Incorrect translation maybe?
The values given are the same.However,other manuals suggest that a clean fuel filter restriction may amount to 0,2-0,3 bar difference.
No document decribes testing the fuel delivery pressure under load-only at idle and high idle(3k rpm)..

I`ve applied pressurized air (>2 bar)to the outlet port of the fuel pump,expecting at least some air loss at the roller follower side-nothing(but no fuel dilution in the oil sump either)...just slight air leak from the inlet valve(I don`t expect them to seal completely)...and one can feel how easier is to push the rear plunger when the main spring is compressed. Think it`s good to change the main spring after decades of operation. ..Has anyone checked the delivery end pressure?
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  #12  
Old 04-01-2009, 08:49 PM
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An associated thread of interest...Robert

My Results timing advanced to 28 degrees 617.952
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  #13  
Old 04-01-2009, 08:51 PM
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Interesting....I am going to change my filters tomorrow!
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  #14  
Old 04-01-2009, 09:15 PM
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Quote:
Originally Posted by vox_incognita View Post
A very intriguing thread !
Browsed some docs and found a difference in where the place the measurement takes place:
-between fuel pump and fuel main filter(w116 om617.95x)
-between fuel main filter and IP.(w115,w123)
Incorrect translation maybe?
The values given are the same.However,other manuals suggest that a clean fuel filter restriction may amount to 0,2-0,3 bar difference.
No document decribes testing the fuel delivery pressure under load-only at idle and high idle(3k rpm)..

I`ve applied pressurized air (>2 bar)to the outlet port of the fuel pump,expecting at least some air loss at the roller follower side-nothing(but no fuel dilution in the oil sump either)...just slight air leak from the inlet valve(I don`t expect them to seal completely)...and one can feel how easier is to push the rear plunger when the main spring is compressed. Think it`s good to change the main spring after decades of operation. ..Has anyone checked the delivery end pressure?
The recommended tests with an unloaded engine do not absolutly indicate if proper pressure will still be present under heavy load or just cruising load. The strategic position of the gauge to read the actual base pressure present in the injection pump when cruising seemed far more important to me originally.

I disgarded the concept of the static tests for our needs. This was after Yellit a fellow member tried a dynamic test that proved it seemed a much sounder approach.

A solid side benifit was it alerted you when a filter change is really required. Or something less likely had occured to start dropping the pressure under load. It is really bad to just drive these cars until the effect of a partially blocked filter becomes obvious.

With a gauge permanatly installed this is not going to happen. The car will still be runing fine but you notice on the highway especially the base pressure is starting to sag. When it drops to a number you settle on change the filter.

This will and should occur far before any deterioration of performance is obvious. Also keeping your fuel milage at the individual cars higher end.

A dynamic load test will always be preffered to a static type test. Under very light feed conditions even with obstructions the pressure might seem adaquate. As soon as you up the flow rate it might impact things. This of course you never know without a gauge in the passenger compartent. .

I believe one of our members (yellit) had a real time getting reasonable pressure for highway usage. Ultimatly it was the lift pump I believe. If he had not read the gauge on the highway the true condition would have not been known. Unfortunatly he had the 616 engine. They just seem more sensitive to the rod issue. So it is wise in my opinion to get it right on them especially.

At the present state of knowledge the delivery end pressure of the lift pump seems to vary somewhat. This is understandable as the pressure relief valve on the injection pump releases any pressure higher than its output in operation..

Basically as long as the lift pumps output is higher than the minumin requirement under load there is no issue. Again the static test may fool a person on occasion. Far better than no test at all though.
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  #15  
Old 04-01-2009, 10:06 PM
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I think the oiling diagram may yield some cluse to the rod bearing issues. This version is not that easy to read, but it would appear that the oil is pumped to the rear of the engine first, through the filter and to the #1 rod bearing/main journal last.

IIRC, Pascal's law dictaes that all points in a pressurized system will see the same pressure, but that doesn't take oil starvation, out of round main saddles, etc., into account.

The theory that the #1 cylinder works harder than the others is inaccurate. If all cylinders have equal compression and similar fuel delivery, all will impart the same amount of force to the crankshaft. The position of #1 at the front of the engine doesn't make it any more of less susceptible to damage by reason of it 'seeing more load' than any other cylinder.

However, if it is the last to receive oil, then a reduction in volume could create an issue. My 617.950 never drops below 30 psi...ever...Robert
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