Rough idle at start, but only occasionally

[mhallack]

Ok this problem happened twice with me this past month. When I start the truck (94 XLT) it idles very rough for about 15 seconds, then dies. The first time it only happened once. After about a minute I tried again, and started fine.

Yesterday it did the same thing at the grocery store parking lot. Except after a minute I tried again and still ran rough, then died. I quickly tried again and it wouldn't start at all. I popped the hood to see if there was anything amiss (like I'd know what to do) After several minutes of sitting I tried again and it started and ran normal. And today the whole day it's been running fine and normal. Can anyone figure what the heck is going on, and can this be leading to something more serious?

 

[MrShorty]

I'm sure someone can figure out, but I doubt they'd figure it out based solely on the information given (unless they were really lucky).

In the few times it has acted up, would you say it was always on warm starts or cold starts or both?
If:
warm start: look at the FPR. hard warm starts are often related to the FPR diaphragm leaking into the intake, causing the engine to "flood." Easy to check, simply pull the vacuum line to the FPR and check for gasoline. Gas in that vacuum line=bad FPR.

cold start: put a fuel pressure gauge on it and see how long it takes for the fuel pressure to come to spec, and how quickly the pressure bleeds off. hard cold starts are often associated with a bad check valve in the pump, making it take longer than normal for the pump to generate sufficient pressure.

both: I would probably still start with a fuel delivery question. With the intermittent nature of the problem, it may take a few tries to get a good check, but next time it stumbles on start up, hook up a fuel pressure gauge and see if the pressure is where it should be when the engine stumbles.

I would probably also pull codes from the computer in case there are any clues in the computer's memory.

 

[mhallack]

It's only happened on warm starts. The way I described is the best way I can, I know it's a little vague. But do appreciate the help!!

 

[mhallack]

Oh, what is FPR anyway??

 

[MrShorty]

FPR=Fuel Pressure Regulator. Located at the front end of the fuel rail.

 

[Roadrunner777]

This does sound a lot like fuel. I'd get a pressure gauge on it and try and get a reading when it is acting up. You know what I might like for this problem, if I had to pick something now: An intermittent fuel pump relay. It's a cheap part, and if it isn't the problem, it's a great spare because it fits all the other relay positions.

 

[mhallack]

Update: Well it decided to conk out at a stop sign, then when I tried to start, it pulled the same rough idle crap. 30 seconds later it started right up as nothing happened. Besides the FPR, can it also be bad fuel too?

 

[mr cribb]

Have you cleaned the MAF (Mass Air Flow sensor) lately?

It's really hard to say w/o more info.

 

[mhallack]

I have checked that recently, along with the air filter too, both are good. I'm not sure what else to state about it, cause I've described everything that the truck has done.

I'm going to get the fuel filter replaced, along with plugs and plug wires. Who knows when they were last done. (I've only had the truck since March)