A "thumbs up" and a rough idle question.

[Swamp Gas]

Following the advice found here, I was able to change ALL the spark plugs(and wires )in my kid's 180,000 mile 98 Explorer. The driver's side, no problem. The passenger's side sucked. I removed the front tire and removed the transmission dipstick and knocked it out. Now, on the rough idle...

When cold, it idles fine. After driving a while, it will idle very roughly. It doesn't cut off, but it would like to. Since it's not wires and plugs, I think it may be an o2 sensor. Your thoughts?

 

[SoNic67]

It might be the EGR valve (it starts working only on a warm engine).
The O2 sensors would trow CEL off.

 

[budwich]

try cleaning the iac.

 

[Swamp Gas]

I went to Advance Auto Parts and got them to use their code reader. It indicated the right side upstream 02 sensor was bad. I bought another, installed it without issue but it runs worse than ever! Rough idle, no power when first accelerating, then it "catches up." I haven't cleaned the IAC yet, but I was expecting a positive gain in performance. This sucks.

 

[budwich]

there are basically no codes that say "your x.. o2 xensor is bad".... what was the actual code that was read? That will help the forum a lot more than "an interpretative analysis". After changing a sensor, you might have to disconnect the battery to get the system to "relearn" settings which will take a bit of driving thereafter.

 

[boominXplorer]

What engine? If its a SOHC then I bet you need new intake gaskets. Its a common problem with them when they run hot they idle low because air leaks in between the lower gasket and the cylinder head.

Oh and swap back the O2 sensor because if you lost performance, something isn't right with it.

 

[Swamp Gas]

After it cooled, I disconnected the battery and let it reset. It fired up and sounded great. When it got warm it went back to running really bad.

 

[2000StreetRod]

Which engine?

As boominXplorer posted, it is helpful to know which engine is in the vehicle. The choices are: OHV V6, SOHC V6 or OHV V8.

As budwich posted, it is helpful to know the current actual diagnostic trouble codes (DTCs) so that they can be looked up and deciphered.

Did the engine run rough when cold before changing the spark plugs and wires? If not, and you have the V6 check the firing order. A common error is to interchange the wires for #5 and #6 on the coil pack. Also, some members have had misfire problems with Bosch platinum plugs. Do you have any misfire codes? Did you examine the old spark plugs when you removed them? Were there signs of the engine running rich? Does black smoke emit from the tailpipe when the engine is warm? Running OK when cold and rough when warm could be due to a contaminated MAF sensor element. Have you tried cleaning the MAF sensor with MAF sensor cleaner? Running rich when warm can also be due to a defective engine coolant temperature sensor. Is the engine coolant at the proper level? If low enough to expose the temperature sensor the PCM will think the engine is still cool when warm.

 

[Swamp Gas]

It is the 4.0 SOHC. As for plug order, I always do one plug and one wire at a time so I won't get them out of order. The old plugs weren't fouled and looked pretty good for a high mileage engine. It didn't idle rough when it was cold before the plug change. The replacement plugs are Autolite. The change was not really noticeable over the old plugs and wires.

The diagnostic tool Advance used indicated misfires in 5 and 6, but under load it seemed to run ok. Now there are misses under load, rough idle, and it smells like it is running rich. BTW, coolant level is good.

 

[2000StreetRod]

firing order

The photo below shows the coil and cylinder numbers.

Please note #5 and #6 on the coil. The connections may have been wrong before you replaced the wires and plugs.

 

[Swamp Gas]

Cleaned the MAF sensor and the throttle body. No change. Runs fine at start, misses at idle, bucks/jumps at highway speed. Check engine light yet to come back on.

 

[RomeovilleIL]

The diagnostic tool Advance used indicated misfires in 5 and 6, but under load it seemed to run ok. Now there are misses under load, rough idle, and it smells like it is running rich.

Check what 2000StreetRod said about the 5/6 firing order. Chances are very good the plugs and wires were replaced by the previous owner. The clue is you comment that the plugs all looked pretty good for the number of miles on the engine and all came out without breaking wire boots or plugs (though are a pain to reach).

Since you are getting the 5/6 miss and running rough, take a moment to swap the 5/6 wires and see how it runs. Cost is zero and can be easily switched back if it doesn't fix the problem.

 

[Swamp Gas]

Well, plug wire order is correct. After rechecking everything I have done, I declared defeat. I have taken it to a local shop.

 

[Swamp Gas]

Very bad news. The shop called and said more than likely the timing chain(s) are toast. Because it's got over 150,000 miles they won't touch it. They said they could swap the engine for a used or remanufactured engine, but it costs more than the thing is worth.

 

[Swamp Gas]

I took it to another shop, one I have used in the past. I would have taken it there first but I thought they were out of business. Shop was closed, no sign, no answering machine. Turns out they were on vacation for a couple of weeks. The contact there was pretty upbeat about it. Hopefully, it can be salvaged.

 

[RockyMtnJohn]

Very bad news. The shop called and said more than likely the timing chain(s) are toast. Because it's got over 150,000 miles they won't touch it. They said they could swap the engine for a used or remanufactured engine, but it costs more than the thing is worth.

I'm no genius when it comes to things mechanical, so could someone please explain how a bad timing chain could cause an engine to run ok when cold but run badly when warmed up. To my mind, it would seem that a timing chain problem would be evident all the time. Or am I just misunderstanding the post?

 

[my98nnj]

RockyMtnJohn - Your understanding is indeed correct. If the timing chain or guides have failed the problem would be evident no matter what temperature the engine is.