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Why does my truck idle fine when cold, yet rough when warmed up.

(1990 Ford Ranger)
in Bernville, PA on April 01, 2011
I purchased a 2000 Ford Ranger XLT, 4WD with 94,000 miles from a local used car lot. Two days after taking possesion of the vehicle the check engine light came on. I took the vehicle back, and thier mechanic said it was an injector, at which point he did something that seemed to solve the problem however, I could, on occaision smell gasoline. About 2 weeks after the injector repair, gasoline began spewing from the engine. I had the truck towed back to the dealer, where again, the mechanic said it was injectors. After about a week I got the truck back, I drove it off the lot, and about 10 minutes later, while at a red light, the truck started idling very rough with rpms going from nearly 0 to 1500 ppm.There was no check engine light on. I turned around and returned to the dealer. The mechanic put the scope on it, and said because there were no codes, he couldnt fix it. I uttered a few choice words and left figuring I would get the problem solved elsewhere.

Boy, was I wrong.

First, a detailed description of the problem. When starting the truck cold, the truck runs great and continues to run great until the engine reaches operating temperature. Once the vehicle reaches operating temperature, whenever I sit in idle at a red light or stop sign or with the vehicle in park, the rpm's go from nearly 0 to 1500 rpm's. There is no check engine light on and the truck runs fine while traveling. The problem is while in park or idle, and only after the engine has reached operating temperature. I have had the truck to 2 local mechanic shops, and 2 Ford dealerships. The truck was at each ford dealership for a week. These guys have replaced every sensor associated with the fuel delivery system and also replaced the computer. They have been in contact with Ford Central, they tell me the analysis of the exhaust shows no problem whatsoever yet the problem persists. Are there any creative thinkers out there who might be able to offer insight?

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Popular Answer
I can't believe the dealer does not have a better understanding of this truck. Its hard for me to even guess without being able to do some basic diagnostic tests as to what could be wrong. I would look at the engine looking at sensor values on a scan tool, better professional grade scantools allow you bi directional control, allow you to look at sensor values without having to intrusively test each senor. If you don't get this problem sorted it will if it hasn't already damage the catalytic converter.
There is a professional service called Identafix. They only deal with professional repair shops. They are in the Minnesota 651-633-8007. Your auto repair shop do all their diagnosis tests, and discuss your problem with a team of professional technicians great service cheap and very bright people. If the repair shop that are repairing your car are stumped this is a great service. This company only deal with automotive repair shops and don't answer questions for the general public but are an amazing knowledge base.
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