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Location: Houston, Texas
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Question RPM Blip / Bounce + Bucking/Surging - Is this normal?

If you blip quickly the throttle in neutral to 2000-3000...should the RPM drop and dip just below idle, then bounce up to like 1100 and settle back to idle?

Same thing happens when you clutch in to shift.

Also it bucks/jerks/surges when you let off partial throttle while crawling in traffic/parking lot in gear. I am assuming it's the bounce in the RPM that causes the surge forward.

Car has around 50,000 miles.

Does this sound like the worn AFM wiper track issue?

I can upload a video if anyone needs more clarification.

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1987 Porsche Carrera Coupe
Old 05-11-2020, 03:38 PM
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In my experience there is some “normal” variation in the tachometer reading at least on the 3.2 in my car. It’s not smooth like a modern car, ever. Once after an autocross it’s stopped working on the way home, and I never found the exact cause except I ensured everything was connected properly and then it worked again and fortunately has been fine. Searching “tach bounce” located threads with actual informed content. Second thread has a video.
https://forums.pelicanparts.com/porsche-911-technical-forum/991120-bouncing-tachometer.html
https://forums.pelicanparts.com/porsche-911-technical-forum/964685-78sc-erratic-tach-behavior.html
Hope you find the fix!
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Old 05-12-2020, 04:02 AM
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I experienced the same issues with my 87 Carrera when I first got it but was able to resolve these problems by basically increasing the air flow meter spring tension and decreasing the idle air bypass. It now runs smooth as silk in any gear around town even at 1200 rpm. I'm so spoiled now, I never bother using 1st gear but take off in 2nd and find myself in 5th at 30mph with plenty of throttle response.

I never dreamed I would end up with this much fun when I first bought my second hand Carrera with 200k (250k currently) with such disappointing performance but it has taken me decades of trial and error to understand what the DME likes to see the way the factory does without a "Band Aid" approach and limitations of a chip.


Cheers,

Joe
87 Carrera running like Dr Porsche intended.

Last edited by stlrj; 05-12-2020 at 12:50 PM..
Old 05-12-2020, 06:46 AM
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Definitely not normal.

When my SC did this, the mechanic said it was the axles so I "rebuilt" all the CIS parts and that fixed the parking lot bronco syndrome. Yours is a Motronic but it could still be an air leak.
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Old 05-12-2020, 12:32 PM
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Well good news. I believe the issue is resolved. Took it for a short drive and it's behaving a lot better! I am excited to drive it tomm.

Replaced the 23 year old fuel filter. Didn't solve the issue.
Adjusted idle (it was running high). Didn't solve the issue.

Replaced the Air flow meter (AFM). Problem solved.

I was about to order one and do the whole core exchange deal but I saw a fellow PP member was selling one that was rebuilt by Bosch (still in box, at least that's what the ad said :-), it didn't arrive in the box sealed but looked "refreshed").

Hardest part of replacing it was getting the damn clips for the air filter box snapped in. Rear top clip was a pain.

Afterwards, I opened up the old one...and check out the issue. The bottom track is going off the black area.


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Old 05-13-2020, 07:31 PM
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