fuel pressure regulator question

[Mic_]

RE 1994 Explorer V6 4.0l

I have a fuel pressure regulator question.


Having some not so frequent stumbles right after starting my truck, but not quite stalling I decided to check my fuel pressure.

At idle with vacuum line connected I get 30-31 psi.

If I disconnect the vacuum line it goes to 40 psi.

Seems a bit marginal to me…should I spend the dollars on a new regulator.

Fuel pump and filter were replaced about 8 months / 8.000 miles ago.

 

[MrShorty]

Those numbers are within specs. What does the fuel pressure do if you rev the engine?

 

[Opera House]

Put that gauge back on and see if it holds pressure after you turn the key off (not starting engine). You should be able to hold 30# pressure for 30 minutes. You may have a bubble in the rail that is causing the stumble. Any driving problems?

 

[Mic_]

When I rev the engine I do not see it fluctuate, but when I out a load on it (power brake) it increase the pressure by about 5 psi.

I left the gauge on for 10 mins and it did hold 30 psi.
I'll have to try for 30 minuutes today.

My thoughts were the psi numbers seemed a bit on the low side.
And I have haad no major driving problems, just infrequently I get a stumble on cold starts leaving the garage as I shift from P-R-D...

 

[MrShorty]

"When I rev the engine I do not see it fluctuate, but when I out a load on it (power brake) it increase the pressure by about 5 psi." -- This is normal. What is happening is that the fuel pressure is moderated by engine vacuum (a measure of engine load). As the vacuum decreases (indicating a heavier load on the engine), the fuel pressure increases. You'll really be able to see this if you have a vacuum gauge hooked up at the same time, or hook up a hand vacuum pump to the regulator.

"I left the gauge on for 10 mins and it did hold 30 psi.
I'll have to try for 30 minuutes today." -- Doesn't sound like there's any major leakdown in the system.

It would appear to me that fuel delivery is not the cause of your little stumble. I suppose to be absolutely sure, you could shift from P-R-D while watching fuel pressure to see if you can see an unexplained hiccup in the fuel pressure, but I wouldn't expect to see anything.

 

[Mic_]

Thanks.

I was going to check the IAC for carbon.

Has 128k on the truck, should I clean or buy new.



Also...why do we need vacuum to the FPR?

Why not unplug the hose and run 40+- psi all the time?

 

[MrShorty]

"Also...why do we need vacuum to the FPR? Why not unplug the hose and run 40+- psi all the time?" -- I'm not sure; you'd probably have to talk to one of the engineers who put the system together. I've accidently left the vacuum hose off my FPR before (plugged, of course) and couldn't tell any difference in the way the engine ran. The computer's fuel trim calculation seems able to handle that much variation in the running fuel pressure. Like I said, the idea is to have fuel pressure respond to engine load. Whether it's necessary, I can't say.

 

[Ira]

As for your FPR, when mine went bad, it was reading good...at least most of the time. I noticed my problem while sitting at at drive-up at the bank. It was idleing fine then dropped to the point of nearly stalling for a few seconds and then went back to normal. It did this a couple times while sitting there. I came home, installed a gauge and it checked out fine. I even let it sit for about an hour to see if pressure dropped but still fine. Thought maybe it was just a fluke. CEL started going off a few days later, and found that there is no code for FPR for my year. When I checked codes, it had O2 sensor. Changed that, and it was fine and ran fine...for about a week. Once again, CEL, checked and found same codes. Decided to leave gauge attached and let it idle. Did it for nearly an hour and noticed the stall began. I immediately checked pressure and it was around 60-70, from what I recall. Don't automatically assume that just because it is good for a couple mins while running test, that it is still good later. With mine, it took awhile before the symptoms started up again.