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Vacuum leak?


defflepp1
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1994 lumina LQ1, it currently has a weird idle. on start up(cold or hot doesn't matter) it revs to about 2000 then slowly comes down to about 800. ANY slight tap of the gas and it revs up to about 1500 then slowly back down. at first i thought it was a vacuum leak but, 1 i replace all vacuum lines with new hoses and 2 using the carb cleaner method i didn't find any leaks. so then i unplugged the IAC valve and that seemed to fix the problem. with it unplugged it doesn't spike up with a tap of the gas and the idle seems more stable. now i just replaced the IAC last november could it have gone bad that quick? between now and then i don't think its been driven but a couple hundred miles. is there a way to test the IAC? any help would be really appreciated

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Throttle Position Sensor? Starter fluid might work better than carb cleaner. Or pump smoke into the plenum, I used a cigar. Any leaks in the top of the engine will have smoke coming out if you can pump enough in.

 

Is there a lot of build up on the IAC? Another possibility is it could be a defective unit, some brands of auto parts work better than others.

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so today i went and pulled the IAC and TPS off of a z34 in the junkyard. installed both idle was HORRIBLE, didn't want to come down from 3000. put my old IAC back in and sprayed some carb cleaner in the port to try and clean it some what, and it seemed to run better than before. it still idled high after start up but comes down quicker now, and it still spikes up with a tap on the throttle. so I'm thinking to replace the IAC again(the one i got in november was supposedly AcDelco) and liberally clean that port. if that doesn't fix it ill just leave, i can live with the high idle and its still getting almost 19MPG delivering pizza.

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A high idle is usually a gasket leaking in air, the carb cleaner/starter fluid method isn't always accurate I've heard. Stock air box, no additives in the fuel? A bad seal around either sensor could also attribute to a vacuum leak.

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I'm kinda betting that it's going to be your lower intake manifold gasket. When I bought mine, it did the same thing, until one day mine would cut off after spiking to 3000rpm after a start. I would have to hold the gas to keep it running. Then once it was warm, it would run fine and could run all day.

 

The LIM job on the LQ1 isn't hard at all. It's daunting if it's your first time but it's a piece of cake to put it back together. The hardest part is getting the coolant hose under the throttle body to get back on since it's held on by alligator clips.

 

Try spraying the carb cleaner around the LIM gasket. You can see it in a corner by taking off the plastic that says "3.4 DOHC TWIN CAM". It's held on by 2 10mm nuts. The LIM area is very close to the valve cover, but inside of that hole where the fuel rail runs. If you get white smoke, and a strong and horrid burnt carb cleaner smell, it's going through your LIM.

 

Also, check your MAF. You didn't mention swapping or cleaning your MAF. Mine idled a bit better after the swap, but still shut off.

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stock air box, running 87 octane ethanol free gas. i just replaced the UIM and LIM so i hope thats not it. i have not cleaned/ replaced the MAF yet, ill try to clean it tomorrow and see if that makes any difference.

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Try the smoke test listed above with a cigar. Put a glove over the TB, pull the brake booster hose off of the booster (with the check valve still attached to the hose), then put a clean hose on the check valve. Spark up a cigar\cigarette, and blow in the clean hose. I was chasing a vacuum leak for a LONG time, and it ended up being my valve cover. Ether or carb cleaner won't show you all leaks, smoke will show you a lot more.

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stock air box, running 87 octane ethanol free gas. i just replaced the UIM and LIM so i hope thats not it. i have not cleaned/ replaced the MAF yet, ill try to clean it tomorrow and see if that makes any difference.

 

How long ago did you replace? Did you use RTV on them?

 

Those LIM gaskets are known to go bad, especially considering that they're made of plastic. I used RTV on mine, problem went away.

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i tried the smoke test and got nothing. so the fact that it doesn't do this when the IAC is unplugged leads me to believe that is the problem, so I'm going to replace that and see how it goes

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