Hesitating, stumbling...

[csxtrainman]

First time posting. And ill try to explain my problem the best I can...

I have a 91 XLT 4WD. When I try to move with a cold engine, or at least one thats been sitting for about 10+ minutes, the truck hesitates badly, almost to the point of wanting to cut off, but I let off the gas and the RPM's jump back up, same thing happens when I reapply the gas. Now it wont do this if I let it warm up for about a minute or so. It will do this for about 30 seconds, then finally she runs right. And other times I wont let off the gas, and the truck will go, then stumble, the go, and stumble again. Never have gotten a CEL. Extremely irrating.
The other thing its doing is when I get out on the interstate (65mph+) it will run fine, then will stumble badly enough that I have to get outta the gas to get her to go again.
Ive already replaced the FPR and the fuel filter. That didnt do it, so next time gonna clean the MAF, and see if that does it.

I had the truck scanned at Autozone 2 days ago, and it came back as O2 showing lean. Thats when I replaced the FPR....
Okay, so its yalls turn

 

[dogfriend]

Auto trans or manual trans?

Does it idle at normal speed, or is the idle lower than normal, like 500rpm?

I can think of 3 different problems that might cause this (Im sure there are others as well)

1. IAC valve problem. This may be accompanied by low idle speed.

2. Low Fuel Pressure. Clogged filter, bad fuel pump, bad fuel pump relay, etc.

3. Auto trans problem.

 

[Bashman]

I would check/replace plugs and wires. If you clean the MAF, remember to disconnect the battery to reset the computer.

[EDIT] Possible TPS?

 

[Luv2explorer]

I agrre with bashman, try the plugs. Number 5 likes to go really bad, and cause all kinds of hesitation problems.

 

[dogfriend]

Originally posted by Bashman


[EDIT] Possible TPS?

Yes, that might cause it too.

 

[csxtrainman]

Its a A4LD auto. The transmission will have to be replaced probably around spring of next year, as I dont have 1200 to drop at this time.

The plugs and wires were replaced about 5,000 miles ago, but im not happy with the Bosch Platniums, so im gonna go back with the stock motorcraft that I have in everything else I own.

 

[csxtrainman]

As for idle speed, it can idle normal at 800, but at times it can drop to 500 to the point of being barely lit. After I replaced the fuel filter, the idle hasnt given me any problem. The engine itsself does have a slightly rough idle, barely noticable on the tach, but it can be felt.

 

[dogfriend]

The idle speed dropping to 500rpm is a symptom of a sticking or non-working IAC valve. The IAC should open to keep the idle speed from dropping when you put the trans in R or D. Check out these threads to see if the symptoms sound familiar. It may happen at a certain temp but not necessarily with warm engine.

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The code of running lean could be due to low fuel pressure, vacuum leak or a host of other problems. This also goes along with your symptoms of rough idle. Check out this thread where we talk about everything that could concieveably cause rough idle:

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Finally the TPS could be a culprit in the hesitation. You could test for this with an analog voltmeter. As you turn the throttle, the signal should vary smoothly with no dropouts or jumps in voltage. If it isn't smooth, that would indicate a worn spot on the track that the wiper moves across. This will mess with the computers mind.