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Old 07-25-2020, 10:17 PM   #1
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Location: Boston
Default '92 240 GL: Idle low when starting (every time)

Hi all, the short and sweet of it is that every time I start my '92 240 GL 5spd the rpms go to the normal 950rpm while the starter spins it up but then immediately dips to 400-500rpms and slowly creeps up to about 750 after 15-30 seconds, then is perfectly normal at 900-950 once I'm driving it or it's sat for a little bit. This happens whether or not the engine is hot or cold upon start up. If i tap the gas while it's struggling at 400-500 it doesn't get help it stay past anything more than 650ish until some more time has passed.

Thoughts on the cause:
-fuel pump(s) not behaving
-dirty IAC
-air intake leak
-bad spark plugs
-weak battery

Going to dive into in the next couple days, but wanted to have a focused to-do list before i did.
thanks in advance
1992 240 GL (4dr, B230F, M47, LH 3.1)
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Old 07-26-2020, 02:58 AM   #2
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Location: Sydney, Australia

clean throttle body
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Old 07-26-2020, 03:36 AM   #3
Otto Mattik
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Location: Upper Midwest..more or less

Vacuum leaks commonly cause a high idle issue.

Desperate need of elf assistant for interior work - Cash/Beer option $

Feedback thread: http://forums.turbobricks.com/showthread.php?t=323689
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Old 07-26-2020, 11:19 AM   #4
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i've noticed some 5xx ecus have a post-start rough idle/dip for a second or three, (that throttle also doesn't help) but 15-30 seconds sounds excessive
1991 244 LH2.4 m46 330k? miles
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Old 07-26-2020, 08:50 PM   #5
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Location: Kansas, USA

Originally Posted by seanile View Post
-fuel pump(s) not behaving
Or maybe fuel filter should be changed out first...if long overdue.

Unknown...but in general, ECUs run on a default/learned settings until O2/etc are "online."

If yours has a cold start injector, this might explain 900-950 rpm

If ECU's engine temp sensor has not be replaced, I'd replace it. Same with O2 sensor
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Old 07-27-2020, 08:47 PM   #6
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Location: Boston

Hey all, thanks for the input.
This a new issue that it didnt do before, and it came on kind of quickly. I like the intake leak and fuel filter ideas in particular. Ill look around the engine for any gasket leaks and if not ill spray some carb cleaner near the intake to see if she revvs.

Ill look around for any spare sensors i may have too. I also have a variety of fixes on the agenda so the throttle body will be cleaned when I’m doing IAC, MAF, and PCV TLC.

I took a video so it should demonstrate everything better than i can describe it. When it leaps to 1100 it’s because i gently tapped the gas. This happened after driving for 5 minutes So the engine was hot, but it did the same thing when i started it cold 5 mins prior. I was surprised to see that tapping the gas helped, it hasn’t consistently in the past. Where it settled/@950 is normal conditions for it.
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