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Weird idle after stopping


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Sorry for posting this in the newbie section, I still can't post in the virtual mechanic 🤨

I have a weird idle issue after coming to a stop at lights or stop signs. When going into neutral with the engine ~2000 - 2500 rpm the idle will drop to ~500 - 600 and it after a bit it might go back up to 700 but until I tap the throttle a little it'll stay lower and doesn't quite sound like it's running healthy. I've been able to get around this by just giving it a little gas while braking and then it doesn't have problems keeping idle after I get to a stop. This only happens when using the brakes at a lower rpm or when coasting in neutral. Another situation where the idle drops is when backing out of my driveway in reverse and using the brakes when it's just idling with the clutch in.

I suspected it might be a bad brake booster so I tried a few tests that are supposed to tell you if you have either a vacuum leak in the lines going up to the booster or the booster itself, and it seemed like everything was functioning normally. I have some new ( specifically reinforced brake vacuum booster vacuum line ) coming so I can replace the old ones and rule out it being a vacuum line.

Could it possibly be something with any of the emissions equipment going bad after 40 years?

If anyone has any ideas or pointers to the right directions anything is appreciated.

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First thing to try is a TPS/ISC reset outlined here, you actually set the idle RPM per the procedure:
https://www.starquestclub.com/forum/index.php?/forums/forum/65-fuelecu-system/
If you do everything right, get your voltage readings in order, and it's still acting up, I would look for more vacuum leaks. I had a very similar issue on mine that ended up being a "bad" blow-off valve that would leak vacuum at idle, swapping it out with a good one fixed the problem.

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  • 2 weeks later...

Alright so I tried a TPS reset and magically the weird idle problems have been vanquished! Thank you!

However :( now when I give it any throttle above maybe 30 - 40% it "falls on its face" and cuts out. The rpm won't go any higher than where it was at before the stumble and the engine makes an obviously bad sound, like it's either running waay to rich or lean. I unfortunately don't have an AF gauge to confirm or deny this but that's what my suspicions are anyway.

The voltages from my TPS indicate .5V at no throttle and ~4.7 - 4.8 at full throttle. I know you're supposed to be getting a full 5V with the throttle fully depressed but would that really cause such a stumble if it's only off by .2V?

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Posted (edited)

I believe I have fixed my issue, but I'm not 100% sure yet, I'll post an update when I take it for an actual drive.

I was all geared up to try and clock my TPS again so I simply loosened my throttle cable, which lead me to have the idea of trying to rev the engine some to see if it was still doing the weird sputter and it worked completely fine! It seems I had my throttle cable adjusted at an odd angle after the initial TPS/ISC reset and as soon as I got it back into its original position the sputter was gone!

I'm off to test if it'll work under actual load because so far it's only been in my garage.

 

edit: everything's working right 👍

Edited by The_Ginger_Stig
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