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American Idle - rough performance


January 30, 2006
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City, State
Lynnwood, WA
Year, Model & Trim Level
'92 XLT
My rough idle saga continues. Sometimes, the idle on my '92 is quite steady, although never perfect. Most of the time, I hear a periodic "miss" and the RPM's go up and down within 50 to 100 rpm. Backing down a hill while in reverse gear brings the idle down to 500 or less and the voltage decreases too. This has been going on for years. Coming to a stop after driving, while transmission is still in drive, the idle is steady for a few seconds, then dips to 500, then jumps up again.

Last week, one day I had severe surging a few seconds after coming to a stop - idle started to dip to 400, surge to 1000, and repeat unless I put it in neutral and held the idle at above 1000. It hasn't done that since, but it scared me enough to take it into the shop, after I carefully checked over the vacuum lines.

Since the start of my idle troubles, I have: Re-set computer by disconnecting battery, cleaned MAF, replaced air filter, checked all vacuum lines (shop did too), replaced temp sender, replaced thermostat, replaced IAC module (all three of these had failed), ran Techron thru the tank - each repair, things got a tiny bit better for a while, but then worse. BTW, I've NEVER had the CEL illuminate, and the only time I've ever had a complete stall was before I replaced the IAC. Car starts, stops and drives perfectly, and I get about 14mpg around town.

I know Dora the garage queen Explorer loves getting new parts, but I don't want to just throw new parts to try to fix the problem. I've read thru many, many rough idle threads here, so I'm trying to come up with a plan of attack.
The shop yesterday checked compression, vacuum, fuel pressure, stored codes, condition of plugs & wires, and everything was up to spec - they couldn't find the problem.

I've seen the following things suggested: Injector cleaner run directly thru system, replace plug wires, replace fuel filter, replace PCV valve, replace intake manifold gasket, replace transmission modulator valve (where's that?), replace fuel pressure regulator, replace coil pack, run SeaFoam thru the intake, replace EGR sensor and DPFE sensor (does that even apply to '92's? what is the DPFE?), replace vacuum lines.

So my question is....what should I try first? My inclination is to do plugs & wires first as it's due in about 8,000 miles anyway. Next would be fuel filter & PCV valve as they're probably due, too. If the fuel pressure tested normally, could the regulator still be bad? Any ideas, comments?

'92 XLT, Auto - "Dora"