924Board.org

Dutch924-racer · Joined: 23 Jul 2007 Posts: 1077 Location: The Netherlands

my 931 M31.01 was a perfect running car. I gave it a big service, new:

- spark plugs
- leads and distributor cap/ rotor
- oil/filter
- fuel filter
- cambelt
- valve adjustment
- boost controller
- fuel relay

After getting it all back to getter the car backfires and is hesitating while revving

O f course I tripple checked the cambelt timing and the ignition timing but that all seems te be good (checked with timing light).

What could it be ?
_________________
937 trackday car
Series 2 engine
951S turbo and intercooler
Fast road camshaft
Forge recirculation valve + boost controller

morghen · Joined: 21 Jan 2005 Posts: 8880 Location: Romania

vacum leak
_________________
https://www.the924.com

jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

If you're positive about cam and ignition timing, possibly a leak in the intake path. Charge pipe seal leak? Boost controller or line leak?
_________________
'81/'81/'82

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11723 Location: PacNW

Agree with Morghen, sounds like it's lean.
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

bear924 · Posted: Wed Mar 23, 2016 6:40 pm Post subject:

I had an issue for a while where my car would hesitate and miss while revving under load, however not while revving without load. My problem was a faulty over boost switch.

Carrera RSR · Joined: 08 Jan 2010 Posts: 2309 Location: Somerset, UK

What boost you now running Joost? Anything over 1.0bar and you could be bumping into the overboost switch threshold which momentarily cuts fuel relay. You will get a major misfire from this which scares the life out of me when it happened a few times. I thought I had blown the engine or turbo. I disconnected/grounded the over boost switch

When the switch failed some years ago, it caused complete loss of fuel relay and therefore no power or acceleration at all.

A blocked fuel filter also made the car hesitant at the top end of the rev range, starving the engine of fuel.

As also mentioned, check every elbow and vac pipes. A leak may be opening up under full load and not seen when stationary.

Also had problem with a dizzy rotor wobbling at high revs and hitting the dizzy cap. Had to buy another brand of rotor to stop this happening. This caused a stutter at higher rpm
_________________
1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11723 Location: PacNW

Dutch924-racer · Joined: 23 Jul 2007 Posts: 1077 Location: The Netherlands

Problem solved, it was the ignition switch/ control unit (under the left headlamp).

Looked like somthing burned inside it. Just replaced it with another one, and car run great again

Thank you all for the support!
_________________
937 trackday car
Series 2 engine
951S turbo and intercooler
Fast road camshaft
Forge recirculation valve + boost controller

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11723 Location: PacNW

I'd sure inspect the harness and make sure the unit didn't perish from "external" causes...
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3