idles rough then dies

[BOWLING91]

i got 92 sport explorer and when i let it idle in park it will idle fine for a while then start running on a couple cylinders then it will eventually die. but when it is in drive it will idle fine forever. i have checked idle air, tp, mass air, it has a new coil pack, new ect. fuel pressure at idle slowly goes from 20 to 30 psi. it will also die if i flat foot it every once and awhile. what could be wrong? and when it was in the negatives one morning wouldnot even hit once, cranked good just would not hit. when i came back when it was warmer it took a couple tries then fired right up. HELP PLZ!!!!!!

 

[Joushou]

Being a n00b and all (I don't even have a license yet... ), I'll place my bet on spark plugs and oxygen sensors.
I had a similar issue with my '98 explorer, and when I pulled the plugs out, it looked like the previous owner had been a little bit ignorant of those - they looked like the factory plugs. It causes unstable and aggressive idling, as well as issues even starting up in cold weather. Replace with new good quality plugs with the proper gap.

If that doesn't work, you might have issues with the oxygen sensors being old and clogged, causing bad feedback on the combustion, resulting in either too rich or too poor fuel mixture, which in the end can become problematic.

 

[kars85]

Being a n00b and all (I don't even have a license yet... ), I'll place my bet on spark plugs and oxygen sensors.
I had a similar issue with my '98 explorer, and when I pulled the plugs out, it looked like the previous owner had been a little bit ignorant of those - they looked like the factory plugs. It causes unstable and aggressive idling, as well as issues even starting up in cold weather. Replace with new good quality plugs with the proper gap.

If that doesn't work, you might have issues with the oxygen sensors being old and clogged, causing bad feedback on the combustion, resulting in either too rich or too poor fuel mixture, which in the end can become problematic.

My understanding here, so take it FWIW. When a O2 sensor fails, I thought that it operated in a closed loop, meaning the PCM stopped utilizing input and ran forever on the last good readings until it gets replaced...Check your vacuum lines. At 19 years old, that very well could be the issue...

 

[Joushou]

My understanding here, so take it FWIW. When a O2 sensor fails, I thought that it operated in a closed loop, meaning the PCM stopped utilizing input and ran forever on the last good readings until it gets replaced...Check your vacuum lines. At 19 years old, that very well could be the issue...

Technically, it does, but it has no clue which sensor is right, nor is there a chance that *any* of his sensors are new or properly cleaned. (These sensors cost $200 a piece in Denmark, so they're rarely touched, but are often the culprit when your idle is unstable, as it will cause the engine to either starve or be choked.)

I just know that my '98 had the exact issue described, and that's due to a combination of the O2 sensors and very very worn spark plugs, both things that he said he haven't checked, but I'm far from a mechanical genius, so take everything I say with a grain of salt. (Raised by a an autodidact mechanic, technically I'm an electrician...)

 

[kars85]

Technically, it does, but it has no clue which sensor is right, nor is there a chance that *any* of his sensors are new or properly cleaned. (These sensors cost $200 a piece in Denmark, so they're rarely touched, but are often the culprit when your idle is unstable, as it will cause the engine to either starve or be choked.)

I just know that my '98 had the exact issue described, and that's due to a combination of the O2 sensors and very very worn spark plugs, both things that he said he haven't checked, but I'm far from a mechanical genius, so take everything I say with a grain of salt. (Raised by a an autodidact mechanic, technically I'm an electrician...)

Right on, I get what you're saying. I guess as sort of a continuation of what you're going off of, would you recommend he just replace the two upstream O2 sensors and save some $$? In reality, those are the only ones dictating anything relevant to his issues, right?

 

[Joushou]

Well, first of all, I'll recommend verifying that all spark plugs are in good shape, and if the '92 is anything as bad as a '98 4.0 SOHC with A/C, he'd just want to swap them all for new ones, instead of putting them back in, just to have to change them after a few more miles. Also make sure to set the gap right.

If that doesn't work, he'd want to verify the O2 sensors, which can be done with an oscilloscope, or true RMS multimeter. I use a Fluke Scopemeter 125/S, but they're kinda expensive. If you're using a simple multimeter, remember that it HAS to be true RMS, otherwise the readings will be off. With an oscilloscope, you should verify that it cycles between 0.8v and 0.2v, around 0.45v average. With an RMS multimeter, you should just verify the reading to be around 0.45v. The sensor can get "stuck" in either state if broke, and this should become pretty clear using both methods, and require replacement. The sensor can also become clogged, causing incorrect readings. In this case, you can clean the sensors, which will improve performance.

Note that this testing has to be while the engine is running, and the exhaust pipe is *hot*, as you're pulling the plug and therefore disabling the heating element in the sensor. (The sensors doesn't work when cold, so make sure not to burn yourself on the exhaust.)

... And, as I said, the system should be able to survive with one bad sensor, but practically this doesn't work very well, and from experience, at least on Japanese and European cars, one bad lambda sensor is enough to make it choke/starve itself in idle. (The initial runtime may be fine, but that's because it's using a preset while the sensors are heating up. When the sensors are ready, it will stop using the "safe mode", and run by the sensor readings, which also gives worn down spark plugs more of an issue, due to the limits being approached.)

Resumé: Do the spark plugs, then start looking into the O2-sensors. It's bothersome to verify them, but seeing we were on the topic of "save $$", he might just as well give it a try.