Erratic/fluctuating idle while in P & N | Ford Explorer - Ford Ranger Forums - Serious Explorations

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Erratic/fluctuating idle while in P & N


New Member
December 17, 2008
Reaction score
City, State
Brandon, Manitoba
Year, Model & Trim Level
1992 XLT
Our '92 Explorer 4x4 4.0L just started doing this and there has been no work done on it recently. Cold start is good (and by cold I mean -30 to -40 C) but after 5-10 minutes of warming up, the idle starts to rhymthically fluctuate from about 600 to 900 RPM. When it's put into gear, it will stall but once we're moving everything is good. After about 10 minutes of running time, the "check engine" light will come on but within 10-15 minutes, it clears. When at idle in gear (eg. at a stop sign/light) the idle evens out at about 600 RPM.

The codes read 173 (O2 sensor not switching-system is or was rich-single,right or rear HO2S-fuel control) & 179 (fuel system was rich at part throttle-single, right or rear HO2S-fuel control) but I'm thinking the sensors are reading it correctly and the problem is farther up. Also, I'm hearing a whining sound from near the left side of the tranny but given our current weather, haven't gotten under it to pinpoint the origin.

From what I've been reading on the forum here, I have seen codes 172 & 173 coming up together but not the ones I'm getting paired. I'm leaning towards the TPS or the MAF. Am I on the right track or should I be starting elsewhere. Thanks in advance.


Moderator-Stock 91-94
Elite Explorer
Moderator Emeritus
February 9, 1999
Reaction score
City, State
Vail, Arizona
Year, Model & Trim Level
1992 XLT 4x4
If it is indeed running really rich, check the Fuel Pressure Regulator. Pull the vaccum line off of it.. If there is gas in there the diaphram has ruptured and its pulling fuel into the engine making it run really rich..

It is also possible the FPR is just giving really high pressure. After our engine rebuild the motor was still running odd.. I finally checked the fuel pressure and found we had 60+psi instead of the normal 32-35ish...

Its at least worth looking into. Testing the fpr is easy to check with a fuel pressure tester which is a $20 tool (for a cheap one).