I have an Evo 9, and lately after the car hits operating temp and then driving the car for more than 10-15 minutes, it will stumble at idle. I can let the car sit and idle all day long, and it will never stumble unless I drive it. Rpms will drop to about 6-700 at idle, and eventually it will bring it back up to about 900-1000, but I can still feel a miss/stumble in there. Fuel trims are about normal at all ranges, less than 4%, and it isn't setting any codes, and doesn't seem to be reading any misfires. I'm thinking I've got a sticking valve or actuator that actuates during driving, like PCV, Evap Purge or maybe MIVEC, since it won't happen if the car isn't actually driven. Anybody seen anything like this before?

 

Routine maintenance check your plugs your plug gap and wires. Then have a tuner check your tune.

 

I do see this a lot. What are your mods and are you tuned for them?

I agree with Impact Blue. Do all the maintenance, boost leak check, etc and get it tuned if it hasn't been for your mods.

LMK if I can help.

 

Maintenance is up to date. Plugs done about 10k ago. Vehicle was tuned about a year ago and no mods have been done since then. I have hks intake, ams turbo back exhaust, boost controller, 850cc injectors and walbro 255 pump. This started about a month ago.

I'm doubtful of a vacuum leak seeing as it only does it after driving, even if vehicle is hot it will not do it if it just sits and idles. Also fuel trims are pretty close to 0.

 

It could be a boost leak. What kind of BOV do you have and is it VTA or recirc?

Try a boost leak test. I've gotten pretty quick with it, about 10 minutes, but IIRC it used to take a 1/2 hour.

Anyhow, nothing has changed since getting tuned and this problem pops up. I can tune the idle tables for when you're driving/coasting and coming to a stop. But if you have been driving on it for a year and all of a sudden this happens, then it probably isn't in the tune and/or MIVEC.

Boost leaks happen all the time. Just try it as it's the simpliest thing to do.

 

OEM Recirc valve. I'll check for a boost leak next week. I just find it really strange that it only happens after driving, and the ecm can't seem to work around the stumble. I drove it to work today, and gave it full throttle, vehicle builds boost, and it didn't have the issue at all during the drive. I haven't dealt with a boost leak on the car before, so I'm not really sure what the symptoms would be.

 

I notice my car at 87K doing this occasionally especially with the A/C on. I think the air valve is getting slow. If on E85 you might be getting a little vapor lock. Back in 1979, fuel with lots of alcohol is what pushed the automakers into putting the pump in the tank. And, if the EGR valve is sticking open that would do it too.

 

Not running e85, makes no difference AC or not. Also considered EGR sticking. With totally useless mixture coming in, fuel can't burn, and o2 can't compensate. Considering waiting for it to happen then crimping off the EGR line if possible to see if it stops. Need to look and see where the EGR is on the car.