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High Idle


 
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gene
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Joined: 05 Oct 2005
Location: Colorado Springs

PostPosted: Sat Oct 22, 2005 9:51 pm    Post subject: High Idle Reply with quote

The saga so far:

1988 4.0 idles high only at operating temperature. If started cold, engine will idle at ~750 with the IAC valve/motor kicking in if RPMs get too low. Once engine warms up, RPMs start to climb. Typical idle @ 210F is ~1250. It turned out old IAC was stuck shut, which did not prevent this issue. I installed new one and looked for vacuum leaks. Majority of vacuum lines leaving the intake manifold have been replaced. This excludes the hose to brake booster ( getting it tomorrow ) & and a vacuum line which goes over the top of engine block and then goes to a T and then heads out to the grill ( could not trace it past passenger headlight assembly ). The only other thing I can think of is the throttle body gasket leaking...but not sure how likely that is.

Any help would be greatly appreciated!
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JeepinNH
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Joined: 13 Sep 2005
Location: New Hampshire

PostPosted: Sun Oct 23, 2005 9:43 am    Post subject: fast idle Reply with quote

I had an 86 once that was idling badly. I also noticed that the vacuum lines for the heater controls were not working. I found the vacuum ball behind the passenger side front bumper had a crack in it making for a huge vacuum leak. Just look from underneath the bumper. You can plug the hose in the engine compartment to make sure it's not that, sounds like it may be the one you were tracing by the headlight. I've seen people spray carb cleaner on the outside of the throttle body and hoses to check for leaks, the rpm's will change if it gets sucked in. This isn't the best route because you have to be careful of sparks with the flamable spray but sometimes you get desperate. Good luck.
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waggy
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Joined: 03 Nov 2005
Location: toronto

PostPosted: Thu Nov 03, 2005 7:29 pm    Post subject: tps Reply with quote

try cleaning your tps also . did you replace to O-ring on the iac ? mine was all cracked . take out the iac and the tps and clean out the whole tb
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ajpulley
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Joined: 07 Nov 2005
Location: Massachusetts

PostPosted: Wed Nov 09, 2005 6:04 pm    Post subject: Re: High Idle Reply with quote

gene wrote:
The saga so far:

1988 4.0 idles high only at operating temperature. If started cold, engine will idle at ~750 with the IAC valve/motor kicking in if RPMs get too low. Once engine warms up, RPMs start to climb. Typical idle @ 210F is ~1250. It turned out old IAC was stuck shut, which did not prevent this issue. I installed new one and looked for vacuum leaks. Majority of vacuum lines leaving the intake manifold have been replaced. This excludes the hose to brake booster ( getting it tomorrow ) & and a vacuum line which goes over the top of engine block and then goes to a T and then heads out to the grill ( could not trace it past passenger headlight assembly ). The only other thing I can think of is the throttle body gasket leaking...but not sure how likely that is.

Any help would be greatly appreciated!

I had a very similar problem. It took me two years to figure it out. It was more pronounced when I cleaned the throtle body with a can of carb cleaner. Turned out that the throtle body plate needed to be re-adjusted so that it would fit perfectly when closed. Seemed as the aluminum expanded from engine heat, the misaligned plate wouldn't close completely causing an abnormal idle, usually about 1100-1400. And, when I cleaned away the built up carbon, it just made the air flow past the edge of the plate better. Loosen the two screws that hold the plate (not too much)and open the thrtle just enough to center it in the bore. I believe you have the three bolt throtle body (it was an AMC thing). I also turned back the throtle plate stop screw just a tad. Now, the computer keeps the idle about 500-700. I hope this helps.
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87 XJ 4.0 Borla header, K&N, MAP adj., 3" (Skyjacker front, Rusty's springs and shackles), 31s, Skids
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ajpulley
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Joined: 07 Nov 2005
Location: Massachusetts

PostPosted: Sun Nov 13, 2005 6:23 am    Post subject: Gene... Reply with quote

Gene, let me know if this solved your problem.
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87 XJ 4.0 Borla header, K&N, MAP adj., 3" (Skyjacker front, Rusty's springs and shackles), 31s, Skids
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gene
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Joined: 05 Oct 2005
Location: Colorado Springs

PostPosted: Sun Dec 11, 2005 9:47 pm    Post subject: Reply with quote

Thanks! It worked for me. Although the throttle body plate adjustment brought me down to ~1100 RPM. I followed the vacuum lines to the vacuum ball and found them to be heavily cracked. All lines replaced, engine idles between ~600-900. Idle still fluctuates, but does not go much over 1K.
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dizzymac
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Joined: 06 Aug 2005
Location: New Hampshire

PostPosted: Fri Dec 16, 2005 5:15 am    Post subject: Reply with quote

Is it an AT or MT. if AT did you check the TPS voltage.if its out of adj. it will change idle RPM's and shifting if goes altogether. Adjusting the throttle plate should be a last resort if everything else is in spec. That vacuum line goes to the vacuum reservoir good idea to check it for cracks too.
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gene
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Joined: 05 Oct 2005
Location: Colorado Springs

PostPosted: Fri Dec 16, 2005 4:14 pm    Post subject: Reply with quote

Its M/T. TPS voltage is in specs.
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