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Intermittent rough idle when engine fully warmed up

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Hi everyone, I have done some research around this topic but cant seem to find the same symptoms as I currently have. So I have a 2000 2.7 tip with 95k miles, it had a new AOS about 2 years ago and a new MAF 1500 miles ago. Car runs fine apart from a rough idle but only when its fully warmed up. If I drive 20 miles and leave it for a hour, when I start it up again idle is fine until it gets fully up to temp. Any start from cold and its fine initially. About a year ago after a service the mechanic started it up and it did the same thing but cold,  hasnt done anything like this until the last couple of weeks. Oil filler cap comes off easily. Had the inspection cover off and cant hear any vacuum leaks. Cleaned the throttle body about 2 years ago when the AOS went bad, but to be honest it wasnt very dirty. I had it running with the cover off got it up to about 80c close to running temp and it seemed fine, took it for a spin and the fault returned. It does not stall.


so any ideas would be appreciated.

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Thanks, its a bit of a trek for me to get the codes read but if that's what I have to do so be it. Was just hoping someone had come across this temp related issue before. Just got back from a 100 mile trip car is fine until fully warmed up then idle stumbles and is slow.


Being an E gas car am I correct in thinking it doesn't have an Idle Control Valve.

Edited by timbo
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...Oil filler cap comes off easily.   Had the inspection cover off and cant hear any vacuum leaks....




During normal running conditions, the car should idle around 680/700 RPM. If then you remove the oil filler cap, that should create a vacuum leak in the crankshaft, and the car should start running rough. The cap should always fit tight when in place.


I also don’t think you will be able to hear most vacuum leaks, sometimes they are very small, but enough for having low or no vacuum formed in the system. And you also have 2 systems to troubleshoot: the crankshaft and the intake manifold. This is also where a friend’s smoke machine comes very handy.



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Right, e-gas has no idle control valve. You may want to try a throttle body recalibration. Turn the key to the last position before cranking and let it sit there for 2min without touching the gas pedal. Then remove key and it's done.

It will also be good to verify the coolant temp reading (via an obdii scanner) when the engine is rough.

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Update, so after a number of longish runs it seems to have settled down now. My theory is that Ive been doing a lot of short runs, the butterfly valve wont have completely closed each time allowing some crud to build up, so when completely warmed up the crud got in the way, the long runs may have cleared it away. coupled with a possible oil overfill. Dip stick says full Dash stick says half full, I think I made the mistake of going by the dash stick.  Changed the oil and filter and went by the Dip stick. Does my theory seem plausible?

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Only if you're extremely lucky.

If you have a smart phone or iPad, a very cheap app is available called Dash Command. $10 via Apple Store. Then a obd2 to plug into the port. I bought a specific one on Amazon that comunacated with my model phone via wi-fi, not blue tooth. I was having some issues and this proved invaluable for a total of $ 40. It also shows a lot of engine info while running. I just kept clearing the code P1126 and trying different things. Good luck.

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