Fuel Pressure Test

[Jason_25]

I'm gonna run one today to see if I can get rid of my problem. What should I be looking for exactly? A fluctuation in the pressure? I know it should be 36-39 at sea level, warm, at idle. Should I drive it around with the gauge inside the truck? I'm thinking since I have a rough idle I should be able to see the problem just at idle. Does anyone have any suggestions before I start?

 

[MrShorty]

a fairly complete pressure test involves...
1. pressure comes up when the key is turned on before the engine is started.
2. pressure is within specs at idle
3. pressure stays within specs when the accelerator is pressed (can the pump keep up). the manual said that this test may be more meaningful if you can set up to do it under load.
4. does the pressure stay within specs for several minutes after the key is shut off (checks for leaks/leaky injectors).

I'm going off of memory, but I think that should give you a fairly complete feel for any fuel delivery problems.

 

[Jason_25]

Ok here's the story. When primed, pressure stays at 38 PSI. When started and running at idle, pressure stays at 29 PSI. When FPR is disconnected from vacuum, pressure jumps to 38PSI. After turned off, pressure stays around 29 PSI but after 2 hours has leaked down to 17.5 PSI.

Does this seem normal? Or could something be wrong?

 

[Jason_25]

Also, what are the specs for what the fuel pressure should be doing while revving?

 

[SteveVB]

from my 91 FSm

Jason, from my 91 FSM :

Engine running 30-45Psi

Key On Engine Off 35-45psi

Leak down test they give a 5psi drop 60 seconds after shut down for the regulator, and 5 psi after 30 seconds with the return plugged for the fuel pump. The first test the regulator may be bad, the second the pump may be bad.

one more regulator test- vacuum test the regulator. Apply 20 in hg to the fuel return tube, and if the vacuum drops below 10 in hg in 10 sec replace the regulator.

hope that may help you.

 

[Jason_25]

WOOO HOOOO

I have found the problem everyone. My FPR is bad. I did the "squeeze test" to the return line and the pressure spiked to 45 PSI from only 29 PSI. I'm going to search for another one tomorrow.

 

[Jason_25]

Wait a minute......before I drop the money into this thing, does this sound reasonable? I ruled out the fuel pump because pressure spiked to 45 PSI so I know it's running strong. The symptoms I have appear to be a "sticky" FPR. Pressure is ONLY 29 PSI at idle. The Haynes manual says it should be atleast 30.

Does all this make sense? Thanks for any help you can give.

 

[oceanworker]

How accurate is your gauge? If the specs say 30 and you get 29, that is not much of a problem. How is the vacuum line running to the FPR? Also have you changed your fuel filter lately? How about running a can of injector cleaner thru on you next gas fillup.

 

[Jason_25]

Well I bought another FPR. Same damn thing. Apparently the fuel pressure is normal, just a little low.

Symptoms:Rough idle, terrible acceleration in high RPM's, almost cutting off when stopping

I'm gonna come right out and say it. I'm beginning to hate this POS 4 Liter. No matter how much money I drop into it, it still won't run right. If anyone has any suggestions for me to check something, please let me know. I am on my last nerve.

 

[SteveVB]

what have you done so far?

always start with the TPS- check it out.

also the IAC clean it and the Throttle body.

check your vacuum lines

symptoms get worse with the A/C on?

 

[Jason_25]

Thanks for replying steve.

Ok vacuum pressure is steady at 18.5.

IAC is new as of last year. Both o2's are new. TPS is new.

IAC is clean, throttle body, and intake is clean. Symptoms do not get worse with the a/c on. If anything, they get better.

 

[Brainsaw]

I had a similair problem. Engine ran rough and got only 100miles to the tank highway. I ran diagnostics, came up with bad "Oxygen Sensor" on the exhaust system. I changed it, didnt make a difference! Took the X to the shop, they changed the fuel pressure regulator and the "oxygen sensor" again cuz it was fouled with black soot. The bad fuel pressure regulator was fouling the "oxygen sensor".


Change oxygen sensor after fuel pressure regulator. Maybe this will help?

 

[Jason_25]

Thanks for the response. Don't think that's my problem though. FPR tested good. I get 20MPG highway so my gas mileage is great.

I have run the CEL codes if that's what you mean, they were clear, no codes.

 

[Brainsaw]

Did you clean your mass air flow sensor? I noticed the snap on post up top had bout the same symptoms.

 

[Jason_25]

Yep, mass air flow sensor is bright and shiny.

 

[SteveVB]

basics

Uh its probably something basic - sounds an awful lot like a vacuum leak still. I would check around the manifold, power brake booster grommet, the line to the FPR, the vacuum tee at the rear of th engine - my thought is that low vacuum would give you the low pressure on the old and new regulator you installed. Test the TPS even if its new, make sure voltage changes through out the range no spikes. MAF while your at it.
I dont know if 18 in hg is low or not I thought 20s but i havent checked mine in ages so I dont remember. Anyone take a reading lately?

 

[Jason_25]

Ok thanks Steve. Is it possible to test vacuum as I drive?

Ok guys I just did a little test. I disconnected the MAF connector completely. Then drove the truck. This pretty much solved the problem, but the exhuast smelled like it was rich when I did this. What gives?

 

[Jason_25]

Did another vaccum test. Stayed right at 21 in HG. On gauge, that showed to be right in the middle of normal. No fluctuations.

 

[SteveVB]

Jasson, the second vacuum test was that with the MAF disconnected. Or did you reconnect it, and if so how much running time after you reconnected? Any other changes?

Idea being that the computer read no MAF signal so the fuel curve went to the pre set map in the comp- no closed loop with O2 input. If the higher vac reading was with the MAF disconnected, or the comp still running open loop then the O2s would be suspect, or the MAF itself needs some looking at. Check the Fuel pressure with the MAF disconnected and the higher vacuum. Check the vacuum at the FPR.

The open loop ran richer and solved your problems- question is why is it runnig lean with everything connected? MAF, O2s come to mind. Get you volt meter check the MAF signals .5-.7v at idle. I cant remember the rest - should be in a manual Ill look later if I get time. Got to go someones at the door.

 

[Jason_25]

Steve, the vacuum won't change when the MAF is disconnected. Not sure why I said 18 at first. Definitely staying at 21 now.

Update:
I tested the MAF Voltage and I'm getting .85-.87 at idle and when I rev it up I get 1.5-2.0. It appears the maf may be defective?