Wavering idle.

[Tony8186]

I have a 2006 Apex MTX with an MPI supercharger, TJ's under tunnel exhaust, New plugs gapped to .018 and a new battery that has an extremely rough idle. For the first 10 seconds or so it idles smooth at about 1700 RPM. After about 10 seconds the idle starts wavering from about 800-1400 RPM. Just as it sounds like it's going sputter out and die it jumps back up. If you press the throttle in it dies immediately. If you slowly work the throttle in it spotters and runs like absolute garbage. You can work the throttle past this point and it seems to spot on at higher RPM, then you let off the throttle it dies. My wideband also bounces around from 12-16 with the wavering idle. Any help our ideas world be greatly appreciated.

 

[kinger]

Vacuum leak! I had this on my MPI apex for 2 years, I never found it. On a whim I decided to replace all of my vacuum hoses on the throttle bodies. Some are pre-bent at a 90 degree angle and wouldn't you know there was a pin hole and I mean pin hole leak in one of them. Replaced it and bam smooth as silk ever since. Never knew what caused the pin hole it was on the the inside of a 90 with no earthly business having a hole. Trying to find this was impossible. That is when I decided to just replace them all. One of the sled strippers on the site took them off a new sled they were parting and that is how I got the vacuum lines as a complete system.

 

[Tony8186]

Kinger, thank you for the reply. I will definitely look at all the vacuum lines again. I've gone over them several times, but if I have a pinhole leak my efforts may be futile. Tonight when I got home I disconnected the gems, disconnected the blower and ran it as stock. It idled a little low, I adjusted the idle up to 1500 RPM. It would still sputter and choke with the throttle. My wideband read rich at idle. I popped the plugs and they were charcoal black. Cleaned them up, re-gaped and placed them back in. I played with the CO adjustments. I'm at -4. At idle it is now a real smooth purr at 1500 rpm. Still a little stumble and throttle hesitation, but a vast improvement from where I began. Keep in mind all this is running as stock. Do you think it's still a vacuum leak? Would this cause the stumble at the bottom end of the throttle?

 

[kinger]

I have heard that a stuck injector could cause something like that. Was just one cylinder really black or were they all? When you playing with it in the garage its important to remember that when you start the sled you should let it idle until its hot and not start stop, that can cause the black plugs as well. I went down this same exact road, playing with CO settings, etc and in the end it was a tiny tiny hole. There is a picture of it in my garage blog. I would say there is a 90% chance you have a vacuum leak. I wouldn't waste any more time, find some new take off hoses and swap it out. Then you know your good. If its still occurring its fuel/ecm related.

Good luck!

 

[Tony8186]

Hey, thanks Asian for the reply and the info. I'll look into new lines. Unfortunately it wasn't just one plug. It was all off them. The a/f gauge it's in the low 13s at idle. Even without the gauge, you can tell it's rich, you can smell the fuel in the exhaust.

 

[Tony8186]

KINGER, l had a buddy block off the exhaust white it was running. It didn't kill the engine, it barely slowed it down. Exhaust fumes stayed coming up around the tank. Turned it off and let it sit for a minute then stuffed rags back into the exhaust. It started fine, idled a little low, and the same issue with the fumes. I definitely need to change the donuts... Would an exhaust leak contribute to a rich idle?

 

[kinger]

Not really it would cause backfiring though. That is good to fix.


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[TomKat]

Exhaust leaks (especially at the header area) can cause this issue. Check your exhaust flex tubes and the doughnuts that seal them to the exhaust. My flex tubes were shot. Once I replaced them, good as new... Good luck!