Low rpm while coasting/coming to a stop - LS1TECH

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Low rpm while coasting/coming to a stop

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Old 07-09-2012, 10:31 AM   #1
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Default Low rpm while coasting/coming to a stop

When I'm coming to a stop or slowing down to turn my rpm drops to +/-500. the car hasn't stalled on me yet but it seems like it's going to.

I know I have a small vacuum leak and I'm having an issue with the transmission also but i don't think it's related.

The car is an 86 regal with a mild cam 5.3 and a 4l60e.

Any ideas?
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Old 07-09-2012, 11:51 AM   #2
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How small is the vacuum leak? I would fix this first, then see how the issue is, before spending time trying to find other possibile causes.
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Old 07-09-2012, 12:05 PM   #3
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It's pretty small, it's from where the evap used to be, I've got it plugged but It's half-***. I can still hear it squealing. I couldn't find anything good to plug it with.
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Old 07-09-2012, 12:33 PM   #4
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Fixed the vacuum leak temporarily with a bolt and some butyl, still have the problem.
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Old 07-09-2012, 12:36 PM   #5
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I noticed a couple of other things too, the vacuum hose coming off of the back of the intake going to the brake booster is squeezing in on itself.

Also if I put the car in drive and give it no throttle and let it pull itself forward at idle the rpm drop steadily and the car acts like it's going to stall.
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Old 07-11-2012, 07:48 PM   #6
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Any other ideas?
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Old 07-12-2012, 05:48 PM   #7
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My car has the same problem. The last time this happened to me it was a gunked up idle air control valve located on the throttle body. Going to try that first. If i remember, they are about $15-$20. It is what controls the amount of air is fed into the idle circuit. When they get cruddy they dont adjust quick enough and can cause that low idle on decel and it straightens out after a second.
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Old 07-13-2012, 09:02 PM   #8
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I took mine out and cleaned it with carb cleaner but it didn't really help.

I'm wondering now if there's a misfire or something??? I'm not much of a mechanic...

If the iac is that cheap I should probably just replace it.
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Old 07-15-2012, 05:10 PM   #9
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I'm really surprised this doesn't seem to be a common problem, the car runs great other than this issue.
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Old 07-15-2012, 08:43 PM   #10
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Could any of this be related to a burnt valve? Or maybe a bad torque converter?
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Old 07-19-2012, 11:53 AM   #11
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Quote:
Originally Posted by TZ2884 View Post
When I'm coming to a stop or slowing down to turn my rpm drops to +/-500. the car hasn't stalled on me yet but it seems like it's going to.

I know I have a small vacuum leak and I'm having an issue with the transmission also but i don't think it's related.

The car is an 86 regal with a mild cam 5.3 and a 4l60e.

Any ideas?
It could be several things. It sounds like you need to check/fix all the little leaks you have going on. If the O2's are seeing more air than the engine is actually getting it will throw the IAC count off.

Also, are you running a stock PCM or has it been tuned by HP tuners or something similar? If it has been tuned that might be the problem or at least part of it.

There are several things in the tune that will control how fast/slow the idle drops off when you get off the gas.

The only way I could see it being a torque convertor issue is if its not unlocking and is pulling the engine speed down.
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Old 07-19-2012, 12:59 PM   #12
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Thank you for the helpful reply. The car has been tuned but this issue just started recently.
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Old 07-19-2012, 07:38 PM   #13
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Ok, I hope I'm on to something here, I unplugged the map sensor and the problem seems to have gone away.
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Old 03-10-2013, 09:46 PM   #14
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Ok, I hope I'm on to something here, I unplugged the map sensor and the problem seems to have gone away.
Well?
I know it's an old thread but I wanna know.
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Old 03-15-2013, 03:11 AM   #15
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Don't rule out bad O2 sensors! Experienced this first hand while knowing for a fact that it was my O2 sensors being fowled. In addition to the RPM dipping when coming to stop, there was also rough idling when hot operating temp was reached (not sure why not when cold (random misfires P0300--not actually misfires, but rather imbalanced firing as confirmed by a Snap-On scanner)). But my sensors were severed fowled, so a mildly bad sensor might just give the low RPM you're experiencing.

Keep in mind also that, if it is/was your sensors, unplugging other sensors around the intake might make it appear as if it fixed the problem when, in reality, you've only bypassed it.
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