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'96 Mazda B2300 (Ford Ranger) idles fast, sometimes

I have a '96 Mazda B2300 with 142,000 miles that has a fast idle problem. It is intermittent in nature and seems to occur most frequently during the morning drive to work. It happens rarely on the drive home from work. Under these conditions, even when fully, fully warm, the idle often seems to stick at about 2000 - 2500 rpm even at a complete stop. Sometimes I’ll load the engine by letting out the clutch to slow the engine down to 1000 rpm and sometimes it stays down and sometimes it won’t. By how it’s acting it appears to me that the ECM is telling the IAC to open when it shouldn’t. It also has a slightly rough idle, but this may just be common with the Lima 2.3 engine.

Things I’ve done and checked. I’ve been using a ScanGauge to monitor the ECM, not necessarily in sequential order:

Codes: None.

PCV valve: Replaced.

IAC valve: Cleaned and lubricated the original one, it wasn’t very dirty. Eventually got frustrated and replaced it. No change. Unplugged it while at fast idle, immediately idle drops to about 600 rpm.

TPS: Reads consistently at 17 while at idle. Throttle body removed and thoroughly cleaned, not too dirty, adjusted throttle plate to ? turn out from stop. Checked for binding or hang up of throttle cable also when at fast idle and all is fine.

Coolant temperature: Consistently between 188 and 203*F. Dash gauge is garbage, varies between the lower mark and half way, but never over half way.

Air Temp sensor: Consistently a couple degrees above ambient temps, and rises when not moving.

Ignition advance: Can be all over the place at idle. It can be at 8 at 1100 rpm or 25 at 1100 rpm idle. Don’t know if it should be consistent, or what factors play into the ECM’s determination of the ignition advance at idle.

Power steering switch: Unplugged while at fast idle with no change.

A/C switch: Have to find this but the A/C was removed years ago before this was noticed as a problem.

Injectors: Ultrasonically cleaned and “rebuilt”.

Plugs: Replaced with stock Autolites.

MAF: Replaced with new due to pinging problem. Reduced pinging to nearly nothing, but fast idle persists.

Vacuum leaks: I checked for leaks on all hoses leading to the manifold and do not find any. Hose condition is very good on all. Diaphrams do not leak for EGR solenoid, brake, fuel pressure, HVAC.

EGR Differential Pressure Switch: Validated with vacuum tool, vacuum gauge and ohm meter. All within spec.

System Voltage: Varies between 13.5 and 14.5, usually about 14.4.

Fuel pressure: 35 at idle, 45 at lower vacuum. It was within spec of the Chiltons shop manual.

Battery replaced about a year ago.

Reset ECM several times during this process by leaving the battery disconnected for >20 minutes and reconnecting. No change.

As there is a loose correlation between temperature and possibly high humidity I suspect an electrical problem somewhere. What else should I check or recheck? I’d rather diagnose properly rather than throw parts at the problem. Thanks in advance for your ear on this!!!

Garth (Stumped) LaComb

Olympia, WA

So, anyone want to take a stab at this one?