Engine/Transmission (1998.5 - 2002) 24 valve fuel pressure what is acceptable

[blkdragon]

what fuel pressure should a 24 valve be at

 

[KCanepari]

i just put a gauge in my 2000 after putting a new lift pump. puts out between 13-16 psi at idle. I would say below 10 psi at idle you should think about replacing pump.

 

[Cummins724]

The vp44 shouldn't see less than 5psi at anygivin time. Starts to fry at 3-4psi... . from what I've read.

 

[16919]

Like others have said... less than 5 your in trouble, 10 or so I would worry 15 is all you need. I have read that any more than that and you can damage the seals in the pump.

 

[rweis]

I have tried to get a really accurate reading from anyone that really works with these things.



From what I can gleen is the VP44 is calibrated to 13. 5 psi. The there is some sort of tolerance from that. Generally thought to be +-3psi due to the intermediate stage diaphram seals.



From a post by II (Brady) about a month ago he implies that there is hydraulic loading (from the internal transfer pressure) especially at higher rpms if the input psi is much off the calibration psi. I get the impression that at idle rpms (low stress) you can be a little higher or lower than 13. 5 psi, but as the rpms increase toward cruise (2k or abouts) and the stress on the pump increases it is desirable to be closer to 13. 5 than farther away from it.



Again, just my take from what I read and interpret.



Bob Weis

 

[AFarmer]

I've also heard that if you go below 10 psi that the pump doesn't correctly, or efficiently, advance the timing. Never really checked into this - does anybody know?? I always say no more than 20 and no less than 15.

 

[steved]

From what I can gleen is the VP44 is calibrated to 13. 5 psi. The there is some sort of tolerance from that. Generally thought to be +-3psi due to the intermediate stage diaphram seals.



Bob Weis

I have read the VP44 wants 14psi, +-2psi (or in other words 12 psi to 16 psi)... so you info is close to what I have read as well...



But, I think the Carter LP is probably not even capable of those readings (for any length of time) as built. I know all of mine started at 12-15psi and soon after installation were only holding 10psi at idle...



And I have read about guys force feeding the VP44 25psi before... but I'm not sure if those are modded or OE VP44s...



steved

 

[rweis]

There is another thread "VP44 failed today" where II addressed some of the things they see when they get failed VP44's. One of the thread responses is as follows:

The actual supply pressure should be 13. 5psi under load and 15-16psi would be ok at idle. 20=21psi will not make too much problem at idle. The problem with raising the pressure more and more is it changes the calibration of the pump. The biggest problem with excesive supply pressure is that the VP44 runs transfer pressure up to 100-120psi at full load. Any additional supply pressure increase will result in more transfer pressure over the 100-120psi. Since the VP44 is a rotory pump the head and rotor will seize or break due to too much hydraulic side loading. The seizure point from too much transfer pressure would be in the 150=160 range. Stuck trans. pressure regulator piston can cause high pressure and seizure. This is caused by contamination. If you want exact numbers for pressure I can get those while at work. If I can get off the phone. Thanks, Brady

This is where I get the impression that the closer to idle you are the more tolerant the deviation from 13. 5 psi is. The closer to 2K (cruise, or whatever rpm you normally are at most of the time) the less tolerant the deviation from 13. 5 psi (calibration psi) because of the hydraulic loading.



I think the transfer pressures are what run the things like the advance piston and hence the comment from AFarmer about too low a input pressure (10 psi or less) may not function the advance piston correctly. I can understand why it would not and can accept the concept of too low a input psi does not generate enough transfer pressure to cause the advance piston to function correctly.



I can also accept your comment about the narrower the range from idle to criuse rpm probably the better as it is closer to the calibration psi in all instances. However guys do have much much higher input psi's and seem to run that way for very very many 10,000's of miles to even 100,000's of miles.



I run a RASP and target 11 - 15 psi, trying to keep sufficient transfer presures for the "granny" way I drive (ie slow accelerations from idle) until I get the input psi up to at least 13. 5 (which is about 1200 rpm) and I know the transfer pressures are high enough to function the VP44.



I read that on the high side psi the limit is the tolerance of the transfer diaphram. It is designed to flex . 5mm (I think) and not distort it's seals. What this equates to in input pressures I do not know. The quote above implies there is not much allowed transfer pressures variation (100 - 120 psi), but what input deviation from 13. 5 input psi is that range of transfer pressures.



I sure hope some of the rebuilders will chime in and help us understand the psi relationships better.



Bob Weis