91 XLT Strange Missing Issue

[grihn]

I did a search of the forum but didn't find anything that really matches my issue, so forgive me if this has been asked before.

I've got a '91 XLT (2wd if it matters) at about 110k that I'm experiencing a strange issue with it missing on occasion. Sometimes when I first start the engine, it seems to be missing, sometimes so much so that the entire vehicle shutters, sometimes it's barely noticeable. However, if I just start driving, or even sometimes shut it off and start it again, the miss goes away. It happens completely at random, ambient temperature isn't a factor, and sometimes it'll go for months without doing it. Sometimes it will happen after the vehicle has been sitting un-driven for a day or two, but sometimes I'll drive to the store without issue and when I come back out and start the truck it'll happen. I keep a regular maintenance schedule with oil changes and the such, it has relatively new spark plugs and wires (I only remember this happening after those were changed, but not immediately so, and I can't say for sure that it wasn't doing it before then...I did put OEM wires, but the plugs are Bosch's with the dual ground electrodes, so I've considered trying OEM plugs). I replaced the fuel filter last year (this issue has been going on for longer), and I run fuel treatment stuff through about once a month. I've never replaced the injectors, though the fuel treatment is supposed to clean them. The truck does have an aftermarket air filter, but the MAF is still in place, as is the stock intake hose and throttle body...basically I just took out the filter box and replaced it with a cone, and followed instructions that came with the filter on "reprogramming" the MAF. I can say for certain that the issue was happening before I replaced that, so I don't believe it to be a factor. Everything on the motor aside from the air filter and spark plugs is original or I've replaced it with OEM parts...nothing internal has been replaced, just things that shouldn't have anything to do with this problem (power steering pump, starter, water pump, radiator/hoses, etc). Unfortunately since the problem is intermittent and doesn't seem to be getting any worse, I don't have much hope for being able to track it down, but if someone has seen this before and/or has ideas on something I should do, I'd be thrilled to hear from you. Thanks for your help!

 

[Ramblinwilly]

Hope this check list helps.
1. Pull CEL codes even if the light is not on. Post the codes here if there are any. Codes are easy to pull and it doesn't require any special tools. Very easy to do. This is the first step.

2.

"it has relatively new spark plugs and wires (I only remember this happening after those were changed, but not immediately so, and I can't say for sure that it wasn't doing it before then".

Check each boot on each plug. Make sure there is good connection. One boot may have come undone. This is only my opinion but most others would agree. These vehicles can be somewhat picky about plug brands and since your issue started here, start diagnosis here as well. I would install a set of Autolite plugs that are properly gapped. Gap them yourself to be sure.

3. If 1 & 2 above don't shed any light or don't solve your problem then it's time to start testing the other sensors/stuff one by one. If you don't have a shop manual like Haynes, get one. They are an invaluable resource in diagnosing.
a. Fuel pressure.
b. Fuel Pressure Regulator
c. MAF
d. IAT
e. Fuel Filter

Issues like yours can be any number of things. Take your time, be methodical and accept the fact there is no substitute for proper diagnostic techniques.

Good Luck. Let us know how you made out.

 

[grihn]

Thanks, I see if I can make time to get the codes tonight.

 

[2stroke]

I had the exact same thing happen on my 94. One thing I noticed is that it was worse on hot days. What I think it was, was a weak fuel pump. If I'm not mistaken, fuel pressure should be above 30 psi key on engine off. I had mid 20's. A simple fuel pressure test can tell you about that. Since you recently replaced your fuel filter, I doubt its that. Also, I have never had a problem with a certain brand of spark plug in these engines. I've run everything from Champion to Motorcraft, and prefer NGK, and they all have worked for me. I doubt a bad plug could cause this problem. I bad connection with the plug wires is possible, just use a little dielectric grease and make sure they are snapped on tight. Fuel pressure regulator is a prime suspect, and there is a simple test to find out if its bad. Pull the vacuum hose off and look for gas in it. Its a good idea to wait a minute with it off, and the engine will probably run bad with it un plugged. IAT and MAF are also possible, although harder to test. You can clean a MAF, but you really have to wait for a CEL to know they are bad. There are some tests with a DMM you can do. To be perfectly honest, I would probably not worry about it unless it starts to affect you when driving, or gas mileage.