Low idle with non revving engine

[myfirstls]

Aloha guys. It's been a while since I posted anything here. Which mean my LS WAS running perfectly. Well, almost perfect.

Anyway, I'm having this problem which occurs very rarely, last time it happened was about year ago, of rough low idle and when this happens, engine will rev really rough and very slowly to maybe 2200 rpm but not anything above that. It will idle at about 500 rpm, and it will run very rough. Press on the gas and it will rev up like diesel engine up to around 2200 rpm. Symptom are random and doesn't seem like it affected by weather or outside temp. Besides, because of where I live, we don't get that much of temperature swing anyway.

Symptom usually goes away if you shut engine off and turn it back on few times, giving few minutes before re-start, but sometimes it take several tries before normal.

Here are some fact about the car. Almost 80k on the clock. It does have intake tube and K&N cone filter (yeah, I just to have one. Still have stock box, so I may change it back to stock later), all COP and plugs has been changed about 5 years ago (I got the COP from Tasca). All plastics, including degas bottle has been changed (All OEM Ford parts). Other parts has been changed too, but I think it's non-related so I'll skip (like suspension or interior). Battery just been changed week ago (Previous battery was 5 years old ).

As of now, I don't have any code except the gas cap code (I forgot the actual code #) but this occurred even before this code.

I also did some search on the forum, but using rough idle didn't turn up what I was looking for, and won't rev didn't come up with anything. maybe I should've use something else.

What this could be? I can't hear any vacuum leak at the moment.

 

[joegr]

An intake vacuum leak could cause both the idle problems and the evap code.
You could try unplugging the MAF and see if that improves things.

 

[myfirstls]

As always, thank you Joe.

I will definitely try it and see if it improve and post the result.

 

[myfirstls]

OK. It finally(?) did it again. This time, I drove it roughly 45 minutes before shutting down the engine.

Shut down time for engine was like 10 minutes or so. No CEL or code (previous Evap code was cleared to see if it come back, but it didn't). Everything was normal, such as coolant temp. At that time, I happened to have bluetooth code reader and tried both forscan and Torque lite version but no code came up. Disconnected MAF and engine dies out almost immediately. Reconnected the MAF and engine ran but rough. Listen for vacuum leak noise but couldn't hear any while engine ran rough. Also tried wiggling various vacuum hoses but nothing changed. Didn't get any better or worse.

Restart was tried between 3-5minutes. About 30 minutes later, engine started fine and also drove fine. Only been few days since it did it the last time, so still waiting(?) for it to happen again.

What could it be??