I have a 1999 Ford Ranger 4WD 4.0L w/86,000 miles. I just bought it so don't know its history, but whenever I'm stopped with the car in Park or I come to a stop at a light the engine misses or studders about every 5-10 seconds. The check engine light is not on.
I have read about cleaning the MAF and changing the plugs and/or plug wires, but was wondering if anyone has any suggestions as to what to do first or if it could be something else.
Thanks this is a great forum

 

I'm cautiously going to say that I suspect you have one plug wire going bad. However, the situation would be more noticible with a cold engine rather than a warm one. I would suggest pulling the codes, if any exist. As for the plugs, you may want to look at one or two to get a feeling for the condition of the rest. But really, if the plugs are aging, you can expect power loss, poor mileage, and harder starts, especially when cold. Move the MAF to the very bottom of the list as a dirty MAF generally results in engine knock.

BTW, I have seen engines with severe knocking problems that have broke away portions of the porceloin in the plug at the center electrode, resulting in the condition you are describing. Are you having any problems with engine knock?

On edit:

If your condition is such that the engine is actually trying to die momentarily, I'd have the IAC solenoid on my mind (cleaning).

 

Thanks for the reply, no the engine is not knocking. I will try to get the codes pulled and see if that suggests anything. I will also clean the IAC. If those don't help I will look at the plugs and plug wires as a solution.

Thanks again

 

Another possibility is a vacuum leak. Put a vacuum guage on it to eliminate that question.

 

Update
Took it to Autozone to get the codes pulled and nothing showed up on the meter. Then took it to a tune up shop and they ran tests and could not find anything wrong. They said the spark plugs, wires and everything was OK. They aso cleaned the MAF though they said it wasn't very dirty. Had then replaced the fuel filter also. But my problem still exists and what is strange is that the mechanic said that the little hesitation or studder is somewhat common as Ford runs these engines very lean to get the most fuel mileage out of them.
Does this sound right? Anybody got any other ideas as to what it could be. I did clean the IAC value, but would replacing it help at all.
Thanks for your help.

 

John, did you ever find out what was causing your rough idle? I have the same problem.

 

My rough idle turned out to be bad compression on one cylinder. Was most noticeable at idle while warm. I tried changing plug wires and plugs first. Wouldn't hurt to change plugs after a compression check.

 

No I haven't found my idle problem yet. Did replaced the IAC and that did not help. The other night I looked at the engine in the dark to see if I could see any arcing on the wires or plugs. I did notice a very faint glow or flicker on the wire/plug on cylinder 4 (driver side front). Should the arcing be more perceivable? So I thought I would replace the wires and plugs, but have not got around to it yet maybe tomorrow.
Will update after that is done.

 

You should see no arcing or glow of any kind.

 

Well I replaced the spark plugs and wires last night and that has so far corrected my studder problem. When I pulled the plugs they all looked OK except the gap was .044 which I thought was strange. The new plugs I got (motorcraft) were also pre-gapped at .044 which I also thought was strange. The sticker above the radiator says .052 -.056, so I reset all the plugs to .054. So far the truck runs great and the idle is smooth. Keeping my fingers crossed that I found my problem. Will update in about week or two.
One other note when I pulled the wire on cylinder 6 the connector inside the boot stayed on the plug all the rest came off OK, so I don't know if that wire was a problem or it was just me pulling to hard.
Thanks for everyone's input. Great forum :

 

I am working on the same problem, I have a 99 Ranger 4.0, same stumble rough idle as described; While I do most my own mechanical work, I replaced the lower intake gasket as most my plugs had oil on their threads and I still had the problem (compression tests showed good compression) after I did that I still had the same results so I ended up taking it to a shop and they diagnosed fuel injectors and being bad, I replaced them myself rather than pay them the $300 they wanted for labor and the I am good now.

I payed $40 each for new Motocraft injectors which wasn't too bad, I have had terrible fuel mileage so I hope to recoup the costs of the injectors by getting better MPG, just did it so we will see....

After all this I now however getting a coolant leak from the timing cover but I praying that it's just that and nothing more serious....

 

My '00 4.0L OHV w/97K is doing the same thing.
The idle is normally very smooth, dead smooth, then there will be a single miss, run clean, another miss, etc.
It has good days and bad days, with no noticeable connection to the weather. It's autumn here in New England.
Parked in the driveway every night.
Not a lick of trouble since the new IAC about 10K ago.
Plugs about 20K old. Wires? I don't remember if I changed them on this truck yet or not. I'll have to look. Owned it since new. Runs great anywhere but at idle, in gear.

 

Well, changed the plugs today (Bosch out, Autolite DPs in). The driver's side front cylinder is leaking coolant into the cylinder. Time for head gaskets.