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vballspen 12-08-2009 08:56 AM

high idle after cat back
i installed a dynomax exhaust cat back 2 weeks ago and now my 97 4.0 idles at 1500 rpms. I replaced the iac motor and i got a little better but it is still intermittent. Sometimes it will idle at 750 when in gear but when i take it out it jumps back up to 1500 every time. the jeep and motor have 203000 miles and is developing a slight knock. I plan on a full rebuild in the next couple of months, but in the mean time what can i do to adjust the idle, could this be caused by the new cat back?

s3nt3nc3d 12-08-2009 09:05 AM

I think I've read the most common cause of a high idle is a broken/disconnected vacuum that's prolly the best place to start looking. Maybe you snagged/cut one during the install.

vballspen 12-08-2009 09:21 AM

thanks for the info. i will go over them today weather permiting. I know idle is controlled by the computer but would the new exhaust throw this off? or can it be adjusted?

s3nt3nc3d 12-08-2009 09:25 AM

Exhaust shouldn't have had that kind of affect on it...your computer may have to relearn the exhaust a LITTLE bit, but nothing to that extreme. The relearning would mostly relate to slight adjustments to the fuel/air mix to compensate for the extra air flow through the engine, but the changes would be so minor, I don't think it would be noticeable at all other than to the computer.

vballspen 12-14-2009 09:13 AM

i checked over everything and sprayed carb cleaner on all the vacuum lines no change. but as it sat there idling it would stay at 1500 then kick down to around 750 just for a couple seconds and then go back to 1500. i tried with or without air and made no difference. any other options?

s3nt3nc3d 12-16-2009 12:49 AM

Figures after I reply to this mine started doing it too...I have yet to find a for sure solution. What I did to stop mine was I pumped up my brakes about 5 or 6 times real fast and on the last pump I held the pedal down. I'm not sure exactly what that does (something with the brake booster), but I noticed a long time ago that would drop the idle so low it'd nearly stall a vehicle out. When I did that, my idle dropped to normal and stayed there.

The ONLY thing I've changed recently that might be related to that is I bought a new TPS you might be able to try that. They cost around $50.

vballspen 12-20-2009 11:08 PM

and the search continues. its weird i will have it in park to open my gates and it will idle high, then drive 100 more feet and it will idle were it should. could a vacuum leak be this sparatic or do i have bigger issues

s3nt3nc3d 12-21-2009 12:15 AM

Mine's only done it the one time since I posted about it in here... I wouldn't think a vacuum leak would be that sporadic. I'm still leaning toward the TPS sensor...but I honestly am stumped as well.

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