This is a loaded question but I have a 1997 Ranger with a ROUGH Idleing 4.0 V-6 what is the most common reason ? it has 197K miles on it, not throwing any codes. The previous owner says it started ideling rough after he pressure washed the motor. I bought it and I think he was just telling me a story. The aamco dealer checked for codes and didnt see any. Could it be Vaccum, Wires, EFI ???

I'm not trying to smart off at you, but with those miles and unknown maintenance, I'd start with The Usual Suspects. I'd check the vac hoses as those are easy and cheap.

If that didn't change much, I'd go for the plugs, wires, etc. I don't know if by '97 there are plug wires, but you get my drift. Age and miles can be hell on ignition parts.

It might...might...be an ignition module but that's not at the top of my list. Others to look at are the MAF sensor (may be able to be cleaned with MAF cleaner), the Idle Air Control (if so equipped), etc.

If those don't work, go up the food chain. Is the timing adjustable? I don't think it is but there is the cam position sensor that can be off and would effectively alter the timing. I'll defer to those who own later models than I do (a '91) on how to get to that sensor and what's involved.

I know this answer isn't exactly consoling, and for that I apologize. But the miles leave all kinds of possibilities for problems. If you get totally bored you might try a cylinder pressure test to make sure there isn't something seriously wrong. Normally the 4.0's are very long lived but that can rule out a major cylinder loss.

Hope this helps. Keep us posted.

Brian

id check the idle control adjuster does it stall if you dont have your foot on the gas?

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97 ford ranger extended cab 4X4 4.0[sold]
1999 ford f150 extended cab 4.6 3 inch body lift 35 inch tall goodyear wranglers 4X4 straight pipe
1998 ford mustang V6 3.8l automatic
mainly stock except high performance headers, performance chip,flowmaster delta 40 series muffler