95 OHV Fuel delivery issues

[south59]

Symptoms: 1) Occasional warm no-start condition and 2) Engine will stumble at idle leading to engine cutting off.

-Fuel pressure at idle is ~29 PSI steady (Low in my opinion)
-Fuel pressure at idle with vacuum disconnected from FPR ~39 PSI steady

This is the weird part that I'm having trouble figuring out- Fuel pressure will increase from 29 PSI at idle to 40 PSI after ignition is turned off. This also occurs after the engine dies on its own. This increase is almost instantaneous. I've had other trucks decrease pressure after ignition is turned off(bad check valve) but ive never had the pressure increase on me.

What can I do to troubleshoot this further? Fuel relay has been swapped, no other electrical components checked yet. I plan on replacing the fuel filter tomorrow, but I don't want to have to drop the tank unless I have ruled out the FPR or any other components.

 

[MrShorty]

29 psi KOER with engine vacuum applied might be a little low, but not necessarily out of spec. 39 psi KOER at idle without engine vacuum applied is right in spec. The fact that the fuel pressure jumps back to 39 when the engine stall/shuts off doesn't bother me. I think the next thing I would do would be to see if fuel pressure stays at 29 as engine speed/load increase or if it drops further. If the fuel pressure drops further, that could indicate that fuel delivery can't keep up with engine demand. If fuel pressure stays at or above 29 psi under all conditions, then I would start looking elsewhere for the problem.

 

[south59]

Thanks for your help MrShorty, I've got a pretty substantial update to the problem. I replaced the fuel filter, and as I suspected, the truck still continued to stall in situations when the engine was returning to an idle from a higher RPM range. The fuel pressure also did not change under load.

I suspected a vacuum leak and started probing around the engine. Around the lower battery area, I found 2 disconnected vacuum lines whose plastic connectors had substantially deteriorated. I was able to trace one of the lines back to the throttle body and also noticed that one of the connectors had about 5 fresh layers of electrical tape wrapped around it connector. Lets just say that our mechanic who recently claimed to have fixed a hissing problem is in a bunch of trouble.

Would a substantial vacuum leak like this cause the truck to stumble and die at idle?

 

[MrShorty]

That certainly is a possibility. Fix the vacuum leak and see what happens.

 

[south59]

I fixed the vacuum leaks, but the truck is still stumbling at idle, and randomly dying. I finally was able to reproduce the no-start condition and was unable to hear the fuel pump prime after cycling the keys to the on position. On the 6th time, I could hear the pump prime.

This has got me even more confused than I was when I began....I'm now leaning back towards a fuel delivery issue. I swapped the fuel pump relay with another relay from the dist. box and was wondering if i should go ahead and buy a new relay before I start considering replacing the fuel pump?

Whats been done so far:

-New Iac
-New MAF
-New Intake Gaskets
-New Fuel Filter
-TPS has been adjusted to .96v
-Seafoamed

The truck is right at 130,000 miles and the fuel pump has never been replaced.

Update: New fuel relay did not fix the problem

 

[MrShorty]

When my '92 had an intermittent no start condition, I diangosed it as follows:

1) Listened for fuel pump when it wouldn't start. Checked fuel pressure (0). Determined that fuel pump wasn't running. (but it was intermittent so, as soon as it started up, I had to wait until the next time it wouldn't start to continue).
2) Started carrying a voltmeter and wiring diagram around so I could immediately get under the hood and poke around. My approach to the fuel pump circuit started at the battery and worked my way down the fuel pump circuit. At the point where the voltage dropped was where the problem was. In my case it didn't take long to find it because the bad connection was between the fuse and the relay. spliced in a new wire and I've been going fine since. Again, once the pump started working, I had to wait until the next time to continue the diagnosis.

 

[budwich]

a meter is your best friend as was pointed out. Just changing out parts isn't necessarily a good approach to resolving alot of these issues (which there are). You should also try measuring the resistance of the motor.... there should be some as it shouldn't a short nor an open circuit.

 

[south59]

Well of course, the truck has been running perfectly since my last post and I sure hope it stays that way. Thanks for the advice guys; I'll be prepared with the multimeter and chilton manual if the problem decides to resurface.