Weird rough idle problem

[Jzawadzki04]

Hello everyone,

I have posted about this before and I have found other threads about this so I apologize about posting again but I can't seem to figure this out and I'm baffled.

I have a 1998 Explorer XLT with the 5.0L and it has a really weird issue. If the engine is cold it starts fine (well I say fine. It still has to crank for a few seconds before it starts but once it starts it runs and idles fine.) But if I drive it around and the engine is warm and I turn it off then start it a few minutes later it idles really rough. It idles low and the RPMs fluctuate and it misfires and pops (which I assume is detonation but I'm not sure.) It only happens if the engine has time to cool down any. If I turn it off and then turn it back on immediately it starts fine. Also I've found that if I hold it at about 2500 RPMs for a minute or two or just drive it 100 feet or so it seems to fix itself. My first guess would be the crank or cam position sensor but hey, what do I know? Whatever it is, it seems to be a heat related issue.

I have changed the spark plugs and wires in the last 1000 miles but other than that I haven't done anything to it. The coils seem to be working properly because 80% of the time it runs like it was brand new but I don't want to just start throwing parts and money at it though considering it's got 250,000 miles and I paid $1200 bucks for it.

So far I've just been living with it but it's starting to get annoying and I'm working at an auto parts store for the next month and I want to take advantage of my employee discount. Any help or suggestions are greatly appreciated. Thanks for looking.

 

[Turdle]

Does the check engine light turn on when you turn on the key and turn off when the engine is running?

If not, I would do these things,

Clean the IAC valve

Check the ECT ( 2 wire coolant sensor)

 

[Jzawadzki04]

Does the check engine light turn on when you turn on the key and turn off when the engine is running?

If not, I would do these things,

Clean the IAC valve

Check the ECT ( 2 wire coolant sensor)

Thank you for the response Turdle. I actually just got done replacing the ECT and I had a fuel filter laying around so I went ahead and replaced that. It seems a little better but still idles rough. Any tips on how to clean the IAC?

 

[Jzawadzki04]

Oh also my CEL bulb is out so I have no idea.

 

[Turdle]

Oh also my CEL bulb is out so I have no idea.

Oh. In that case before doing anything please go have the truck scanned for problem codes. There might be something going on that a code will identify

 

[Jzawadzki04]

Oh. In that case before doing anything please go have the truck scanned for problem codes. There might be something going on that a code will identify

Hey sorry for the late reply, I've had a busy week at work. But I got codes P0171 and P0174 which both relate to running lean. I cleaned the MAF, reset the codes, and they haven't come back since. Also after changing the ECT sensor and the fuel filter I've had no improvement. I have a new fuel pressure regulator but for the life of me I can't find the damn thing under the hood to replace it! Can you offer some assistance as to where it is? I'm not sure it's the problem but I have it so I might as well put it on.

 

[Turdle]

Hey sorry for the late reply, I've had a busy week at work. But I got codes P0171 and P0174 which both relate to running lean. I cleaned the MAF, reset the codes, and they haven't come back since. Also after changing the ECT sensor and the fuel filter I've had no improvement. I have a new fuel pressure regulator but for the life of me I can't find the damn thing under the hood to replace it! Can you offer some assistance as to where it is? I'm not sure it's the problem but I have it so I might as well put it on.

I would check the fuel pressure first, a cheap dial air gauge will do in a pinch. You just have to maintain pressure on it.There is a shrader valve on the passenger side fuel rail. Beware a little fuel will trickle out

The regulator is on one of the fuel rails toward the rear.

However, I doubt the fuel pressure is your problem. I would look for a large vacuum leak. Main suspect is the PCV fitting where it goes into the lower intake manifold, or, a broken T fitting in the pcv vacuum lines.

also check the air intake tube for cracks. Somewhere there is non metered air getting in behind the maf sensor.

Here is a picture of the underside of the upper plennum. Note the larger hoses routed through an F shaped fitting. this is the pcv vacuum circuit, and you can see there are several places it could leak.

 

[Jzawadzki04]

I would look for a large vacuum leak.

It definitely is a possibility but I don't understand why it would only act that way when it's hot. And on top of that, if it's hot and I turn it off and immediately back on its okay, but if the engine cools off any it acts up. I'm not challenging you, I really appreciate you're help, and you obviusly know much more about this than i do. I just want to know how a vaccum leak could occur only under specific circumstances. Shouldn't a vaccum leak affect the performance of the car hot or cold?

 

[lobo411]

It definitely is a possibility but I don't understand why it would only act that way when it's hot. And on top of that, if it's hot and I turn it off and immediately back on its okay, but if the engine cools off any it acts up. I'm not challenging you, I really appreciate you're help, and you obviusly know much more about this than i do. I just want to know how a vaccum leak could occur only under specific circumstances. Shouldn't a vaccum leak affect the performance of the car hot or cold?

Just a guess, but when things get hot they tend to expand. It's possible that whatever is leaking expands/swells when hot and doesn't leak so badly/anymore.

 

[Jzawadzki04]

Okay so I pulled my intake manifold off today. There were a few cracked vaccum lines. But could somebody please tell me where these lines go to?

 

[Jzawadzki04]

Well the picture won't show but it's the two hoses that come out of the front from the underside of the intake manifold. I know one of the goes to the coolant lines but where does the other one go?

 

[Jzawadzki04]

http://postimg.org/image/9qs4cf8ld/

 

[gatzdon]

I know I'm late to the party, but just wanted to point out that changing the Engine Coolant Temperature Sensor doesn't fully rule that out.

You need to see what the PCM is reporting from the from ECT. If there is a corroded wire, the PCM will think that the engine is colder than it really is, but may not throw a DTC.

You can see what temp the PCM is reporting from the ECT without starting the vehicle, just turn the key on, engine off. The temp value should be close the outside temp.