I bought a 1993 Ford Explorer XLT that has a rough idle when it's cold. The idle isn't particularly fast, but is accompanied by a distinct lack of power. The vehicle may stall when attempting to drive it in this condition, and I had to press the gas pedal to the floor to get it restarted after it stalled on me. It will also eventually get power if you floor the gas pedal when attempting to drive in this state. This clears up after a few minutes of idling or driving, presumably once it warms up, and the vehicle then drives fine. It accelerates fine, doesn't want to die at stop lights, etc. The problem only presents on a cold start.

I checked the codes with a jumper wire, both key-in engine on and key-in engine off. Both times it only gave me 111, which I think indicates no error codes.

I started with changing plugs and wires. The plugs had been in there for a long time, and were gap'd way too large (around .075). They were white and fuzzy, but didn't indicate any obvious issues. This obviously did not solve the problem.

I tried turning the key a few times to let the fuel pump charge the system before starting, but don't think it made a difference.

The vehicle does not have an EGR or a MAP.
The Mass Air Flow Sensor looks clean, although I have not attempted to clean it. The idle did improve when I unplugged the MAF, but I don't know if that proves anything.
The Idle Air Controller was replaced with a used one pulled from a parts yard by the previous owner. I have not done anything else with it yet, reasoning that the vehicle would idle rough at stop lights if the IAC was bad.
I wasn't able to find anything wrong with the vacuum lines, but it's possible I missed something. Any hints on where, specifically, I should look?
I have not yet replaced the O2 sensors. Is there a way to check them before swapping them out?

I am thinking about starting with the O2 sensors, but would certainly take any advice before I start blindly replacing parts.

Thanks in advance. This one has me stumped.

 

Considering the issue only seems to be on a cold engine, I'd check the Engine Coolant Temperature sensor to see if the readings are appropriate for the engine temperature. The easiest way to do that is with a higher end diagnostic tool compatible with your 1993 model. That's before OBD-II was mandated and I'm not sure what version of diagnostics were in the 1993 Explorer. Also, if your Explorer pre-dates the use of heated oxygen sensors, the PCM would be ignoring O2 sensor values when in open loop so I would not be worried about O2 sensors quite yet.

-Rod

 

Thank you for the reply shorod. I have an update on this.
The seller had bolted on a replacement IAC, A used one he had pulled from the parts yard, and last night I was wondering if he had reset the computer afterwards. I reset it, started it up and it ran sort of like normal (poor idle normal), so I let it warm up and then drove it to the store and back. This morning, however, it fired up strong. RPM's jumped to 1100 for a few seconds, then settled down into a nice smooth idle.

I was going to wait a few days to verify the idle issue had actually been resolved before posting, but...maybe?

 

I hope you get that lucky!

-Rod

 

Well, no one is that lucky, including myself.

It was running like crap again this morning for about 5 minutes. It started with a low idle again. I left the house and was on the road trying to get it up to speed. It didn't want to go over 15mph with it's lack of power. I got the RPM's up to around 2800 and it started surging. power, no power, power, no power. It finally threw a code and the check engine light came on. A few minutes later it was running better and the check engine light light went off a few minutes after that. I am hoping there is a problem code still in the system and it didn't clear itself. I will try the self diagnostic again tonight to see if it tells me anything.

If there is no code, I will start with the Engine Coolant Temperature sensor. They aren't terribly expensive, so I will probably just replace it and see if it changes anything.

Any other thoughts with the new symptom?

 

It's been cold here since I purchased the vehicle. Rarely getting above 30F. When the problem didn't manifest yesterday, It was lunchtime and the ambient temps were close to 40F. I don't know if that's important information or not.

Thanks again for the advice.

 

 

Based on the code and symptoms I'd still start with the Engine Coolant Temperature sensor. Be sure to get the ECT sensor though as I think your 1993 probably has the ECT sensor for the engine management and a second coolant temperature sensor that is only used for the gauge in the instrument cluster.

-Rod

 

Rod,
I found the 2 wire temp sensor over lunch. I was not able to check the volt readings, but did a resistance test. The car had been started and run for a minute, so the coolant may have been close to 100F already. It's almost 60F ambient air temp here, and the car started and idled fine of course. I set the voltmeter to 20k ohm and received a value of 17,400 (17.4xx on the meter).
17k seems to be a valid range, based on this chart:

TemperatureResistance (K Ω)Voltage Drop

50° F (10° C)58.75 Ω3.51 V DC
68° F (20° C)37.30 Ω3.07 V DC
86° F (30° C)24.27 Ω2.60 V DC
104° F (40° C)16.15 Ω2.13 V DC
122° F (50° C)10.97 Ω1.7 V DC
140° F (60° C) 7.70 Ω1.33 V DC
158° F (70° C) 5.37 Ω1.02 V DC
176° F (80° C) 3.84 Ω0.78 V DC
194° F (90° C) 2.80 Ω0.60 V DC
212° F (100° C) 2.07 Ω0.46 V DC

From this page:
https://troubleshootmyvehicle.com/fo...d-p0118-page-1


I plan to pull the sensor, place it in my freezer to chill it down to near zero, and then test the ohms. Double check me here, but that should be a valid test, right? Or should I just pony up the $30 to get a new Motorcraft sensor?


Edit to add, you are correct that it also has a 1 wire sensor for the temp gauge.

 

Air temp was around 50F this morning, and the Coolant Temp Sensor gave me a reading of 57k ohms. It appears to be working.

I can still take the sensor out and put it in the freezer to get it below 30 degrees for an Ohm test, but it doesn't appear that I need to.

Any other suggestions? Do you still recommend replacing the part?

 

I checked the values on the IAC with a voltmeter this morning. I had a resistance reading of 9 0hms across the terminals IAC terminals, and 5.8 volts with the engine on and harness plugged in. I had .13 volts with key on engine off.

Those numbers match what the IAC on my 1990 F150 inline 6 gave me. So they seem good.

50 degrees, started fine again. This appears to be solely related to freezing temps. I'm stumped.

edit to add. Resistance reading on the Coolan Temp Sensor on the f150 matched what I got from the problem Explorer.

 

 

It seems quite unlikely that the ECT sensor would have a discontinuity only during cold temperatures, so I don't think there's anything to be gained from further testing of the old ECT sensor.

A manifold gasket issue that only causes a vacuum leak when the engine is cold is not a bad suspicion. I'm trying to think if there's a good way to test that with a vacuum gauge or other item. Since the engine runs rough when cold one might expect he vacuum to be sporadic already, which turns in to a bit of the chicken versus egg scenario - is the roughness due to the vacuum or is the vacuum reading due to the roughness? Without concurrently monitoring for ignition misfires that could be a bit tricky to nail down. Unfortunately the misfire monitors in 1993 are not as good as they are with OBD-II.

-Rod

 

 

Have you checked fuel pressure? Didn't you say something about a lean code? The fuel pressure isn't monitored electronically, just controlled by the vacuum actuated regulator. May not be getting enough fuel at cold temps. I had that problem with my '90 F250 460. Started and ran fine at moderate or warmer temps. Would start fine cold on one tank, but not the other, which is what led me to check fuel pressure.
ETA: on the f250, my coolant temp sensor threw a code, so I replaced it, didn't help the start issue, so I dug deeper.