Aviator rough idle

[ponkotsu]

When I say rough, I don't mean terrible. Once it's good and warmed up it idles pretty smooth at 600RPM. Seems to hunt once in a while. Cold idle is a little rough. Sometimes when accelerating from a stop it has a very slight stumble/miss until it gets over 1500rpm. I think the average motorist wouldn't even notice.

It's got new plugs and boots. New air cleaner. I verified all the coils have correct resistance so are probably good. Cleaned the MAF sensor, IAC and EGR. PCV is good. No noticeable vacuum leaks under the hood. Fuel economy seems to be okay for the vehicle. (12 city 20hiway). Runs smooth. Goes through only about 1/2 qt of oil in 6k miles, so not bad for 150k. I've run a couple different quality fuel system cleaners through the tank with no effect. Other than the slightly rough idle, it runs and drives like new. I've never had any engine codes.

I heard Explorers have a plastic intake, but I believe the Aviator has an aluminum with 8 individual throttle plates.

Short of pulling off the intake and cleaning, does anyone have any suggestions? This is definitely not bad enough to warrant pulling the intake because that seems like a pretty intense job on an Aviator. The engine has 150k on it, so maybe that's just as good as it will get?

 

[mn12]

When I say rough, I don't mean terrible. Once it's good and warmed up it idles pretty smooth at 600RPM. Seems to hunt once in a while. Cold idle is a little rough. Sometimes when accelerating from a stop it has a very slight stumble/miss until it gets over 1500rpm. I think the average motorist wouldn't even notice.

It's got new plugs and boots. New air cleaner. I verified all the coils have correct resistance so are probably good. Cleaned the MAF sensor, IAC and EGR. PCV is good. No noticeable vacuum leaks under the hood. Fuel economy seems to be okay for the vehicle. (12 city 20hiway). Runs smooth. Goes through only about 1/2 qt of oil in 6k miles, so not bad for 150k. I've run a couple different quality fuel system cleaners through the tank with no effect. Other than the slightly rough idle, it runs and drives like new. I've never had any engine codes.

I heard Explorers have a plastic intake, but I believe the Aviator has an aluminum with 8 individual throttle plates.

Short of pulling off the intake and cleaning, does anyone have any suggestions? This is definitely not bad enough to warrant pulling the intake because that seems like a pretty intense job on an Aviator. The engine has 150k on it, so maybe that's just as good as it will get?

What type of plugs did you get. I installed Bosch in my 03 Aviator and it stumbled and misfired all over the place. Went with motorcraft and haven't had a hickup since. The DOHC modular seems to be pretty picky about plugs

 

[ponkotsu]

What type of plugs did you get. I installed Bosch in my 03 Aviator and it stumbled and misfired all over the place. Went with motorcraft and haven't had a hickup since. The DOHC modular seems to be pretty picky about plugs

Denso Platinum TT. Hard to believe that would cause it, but you never know. I didn't really notice any change in running after I changed to the Denso.

 

[mn12]

May not be the same thing. On mine it was bad enough to set a misfire code

 

[ponkotsu]

Had a coupon code so bought new Motorcraft plugs. I realized that the Denso's had a small gap, about .04", should be .054. I changed the plugs and while I was at it cleaned out the intake butterfly valve and PCV. Restarted and ran horrible. Realized one of the coil plugs was disconnected. Previous mechanic broke a couple of the plug locks so they are hard to keep on while getting the cover on. Anyway, it's running about the same now, maybe slightly better. Starting to think it won't get any better.

 

[ponkotsu]

Gonna check the alternator when I get time. I noticed that the heater blower fan was jumping around in speed. I've heard that a bad alternator can cause a rough/hunting idle.

 

[ponkotsu]

No question this is a misfire. If I hold the brake and give it light throttle I can get it to miss pretty good about 1k rpm. Anyone know if an ODB scan tool can help pinpoint the problem if I'm not getting any CEL codes?

 

[Albino 94LTD]

I'll put money on a coil even if the resistance reads normal.

It might not be dead but isn't sparking like it should.

 

[ponkotsu]

I'll put money on a coil even if the resistance reads normal.

It might not be dead but isn't sparking like it should.

It really does feel like an ignition miss. Short of buying at least 1 new coil and changing one at a time, not sure how to figure it out. Hate to buy new parts if they are necessary.

 

[Albino 94LTD]

Buy 1 new coil and changing one at a time

That's the only way I know of

 

[ponkotsu]

The two middle coils on the right side appear to have burned secondary terminal and are not OEM. Seems like the engine got rougher after going from the .035 to .054 spark gap which leads me to believe weak spark. I'm going to bite the bullet and replace those two coils. If I'm wrong at least I have spares.

 

[ponkotsu]

Replaced the two most suspect coils and didn't see any change. Last night I had the thing idling for about 15 minutes to stay warm while I was waiting for someone. The idle was jumping all over from 500 to 1100rpm. And was also getting rougher and rougher. Seems like maybe I have multiple problems. Bad IAC and dirty injectors?

 

[Albino 94LTD]

Replaced the two most suspect coils and didn't see any change. Last night I had the thing idling for about 15 minutes to stay warm while I was waiting for someone. The idle was jumping all over from 500 to 1100rpm. And was also getting rougher and rougher. Seems like maybe I have multiple problems. Bad IAC and dirty injectors?

Keep checking coils until you're sure they are all good

 

[ponkotsu]

Dang. Getting orange spark on all of them, even the new ones. Must be a low voltage problem.

 

[ponkotsu]

This has been quite a battle but I think I've got it pretty good. In the end the worst offender was a leaking intake manifold. I don't think it was so bad but this engine seems super sensitive. I smoke tested the heck out of it and never found the leak. I finally bought a scanner software, ScanXL and started to work on it. I studied a bunch of videos on how to diagnose with a scanner. Some of the best videos are by a guy called "ScannerDanner". He really knows his stuff and does a superb job of explaining. So despite not finding any vacuum leaks with all the normal checks, the scanner data was telling me that was the most likely cause. The only thing left was the intake so I bit the bullet and bought new gaskets and swapped it out. That made the biggest difference of anything I did. It's not perfect, but this engine does have over 150k miles on it so I wouldn't expect it to be.

Hoping this engine will now last me for at least another 50k. If I still have it by that time I'll likely rebuild the engine.