So I read this article and would like some input before I spend $40 for a genuine Bosch replacement:
https://mopar1973man.com/cummins/articles.html/24-valve-2nd-generation_50/51_engine/59_fuel/bosch-vp44-injection-pump-overflow-valve-r19/

 

 

Symptoms

1. At idle in park I maintain a steady ~18 PSI.
2. Driving it can drop to ~11 PSI, but sometimes rises to nearly 20, doesn't seem to be directly load related - sometimes it is low going up a hill like the filters are plugged, but then sometimes the pressure is HIGHER going up hill (gentle rollers).  This can be within a couple miles, steady interstate travel.
3. Some times on LONG, flat interstate stretches the pressure will rise and fall about every six to ten seconds (complete cycle, peak to peak).
4. I parked my truck uphill for a couple of days while I traveled, when I came back the truck needed to crank a bit to start (assume fuel drained back to tank).
5. No fuel leaks between VP44 and injector crossover tubes.
6. No fuel leaks that I can find anywhere else.
7. The couple of oil analyses I've had done came back clean, even after new injectors.
8. This has been going on for quite some time, but MAY have become more noticeable (or started) when the draw straw was installed - however, there was so much crap caked around the factory sock that pressures were never very high to begin with and probably never even reached 14 PSI.
9. Was neither better nor worse winter versus summer.
10. Like an electrical gremlin, this isn't an all-the-time kind of problem, making testing the valve a bit of a crap shoot.
11. Did this some with the stock steel lines and banjos, does it now with the 3/8" lines and no banjos.
12. My fuel temps go UP when the pressure is HIGH!  Fuel temps go DOWN when my pressure is LOW!  To me, this is the biggest clue.

 

Other Pertinent Info

1. ISSPRO gauge and sender - yes, sender is on a 1/8" grease hose and not air brake line.  There is also the snubber device (fixed orifice).
2. Fuel filters have been changed numerous times in the time I've noticed this phenomenon (owned the truck a little over 3 years)
3. Original (to me) lift pump was relocated to frame, then another Carter pump was installed, then a relay was installed to help out the ECM, then the DDRP was installed.
4. I even 'hotwired' the pump relay direct (bypassed ECM) and went for a spin, same deal.

 

My hypothesis

I think the overflow valve is "sticky" - it doesn't open until pressure gets a little high, then blasts open and drops the pressure to around 10 PSI when it finally closes again.

Would love some additional opinions on the matter.

Do you have 1/2" lines all through out or just bigger lines in and factory out. If so that your problem in psi swings. 

Just 3/8" to filter, and 1/2" filter to VP44 - 1/2" weren't popular when I bought the 3/8" lines.

12 hours ago, LorenS said:

Would love some additional opinions on the matter.

 

It seems to me that there are three possible things to try....

 

1.   Replace the overflow valve - usually I don't recommend parts changing, but sometimes when diagnostic tools are not available, changing an inexpensive is a good solution.

2.  Test the overflow following Mike's procedure.

3.  Install a temporary mechanical gauge and strap it to the windshield wiper arm and test drive for a day or two.  This should confirm whether or not you regular gauge is giving you reliable data.

 

Typically, the overflow valve is fairly bullet proof.

 

- John

I could test it, but it doesn't act up all the time.

Since the fuel temp reacts as the gauge moves, I think I the pressure must be representative - it may not be accurate or precise, but in the ballpark.

$40 won't break the bank.

Let us know what you find.... don't leave us hangin'!

 

- John

Not a lot to stick in the overflow valve as it is ball and spring. I suppose it could have been replaced with one without the small hole which does actually flow a lot of fuel back to tank below this 14psi figure.

For once that valve is cheaper in the UK. Paid £6 for genuine which made no difference whatsoever to my PSI

47 minutes ago, wil440 said:

I suppose it could have been replaced with one without the small hole which does actually flow a lot of fuel back to tank below this 14psi figure.

 

I believe the small hole is for venting air such as after a fuel filter change when using the bump start feature.   I also believe that when the engine is running at any rpm and if lift pump pressure is below 14 psi that the overflow valve is still in full operation because the lift pump feeds directly into the inlet of the positive displacement internal vane pump.  The flow out of the vane pump is regulated at a much higher pressure and is in common with the 14 psi overflow valve as well as other components inside the VP44.  It is my belief that as soon as the engine starts, the 14 psi overflow valve is in operation. 

 

- John

9 minutes ago, Tractorman said:

 

I believe the small hole is for venting air such as after a fuel filter change when using the bump start feature.   I also believe that when the engine is running at any rpm and if lift pump pressure is below 14 psi that the overflow valve is still in full operation because the lift pump feeds directly into the inlet of the positive displacement internal vane pump.  The flow out of the vane pump is regulated at a much higher pressure and is in common with the 14 psi overflow valve as well as other components inside the VP44.  It is my belief that as soon as the engine starts, the 14 psi overflow valve is in operation. 

 

- John

That may well be the case. To test your theory on my truck I could check return with a overflow with the little hole and then check with a overflow without the hole.

As you know I've tested mine and I have a lot of fuel returning to tank at 8 to 10

1 hour ago, wil440 said:

To test your theory on my truck I could check return with a overflow with the little hole and then check with a overflow without the hole.

 

I didn't know the overflow valve was made both ways.  It would be an interesting test to see if the results were different.

 

- John

Just now, Tractorman said:

 

I didn't know the overflow valve was made both ways.  It would be an interesting test to see if the results were different.

 

- John

There was someone on the here a few weeks ago that had got one without the small hole. I commented that my truck has a small hole and it does return fuel all the time regardless of the pressure or at least pressure below 14psi and I recommended that he got the correct overflow valve 

9 hours ago, wil440 said:

Not a lot to stick in the overflow valve as it is ball and spring

I agree, but even hammers can wear out.  Over 330k miles, and I have no idea if it's ever been replaced.  I also question the previous owners' maintenance regimen so it may have ingested some trash in its day.  Hopefully I'll receive the new one in time for installation next weekend.

Overflow valves are replaced every injection pump replacement. REQUIRED for warranty of the VP44. 

2 hours ago, Mopar1973Man said:

Overflow valves are replaced every injection pump replacement. REQUIRED for warranty of the VP44. 

Guess that means that Bosch knows they go bad. By next weekend I should have the new one.

They really don't go bad like the old P-pump overflow valves. The 24V used harden metals that typically they last the entire life of a injection pump. Like I said before its done part of the warranty. I see the tag wire tied to the bag notifying the installer to replace the overflow valve for warranty purpose and you must return the old valve with the pump. 

 

Again they really don't go bad...