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1988 3.2 Carrera Intermittent "Stutter"


menaporsche

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Hi everyone.

My '88 Carrera 3.2 has developed an intermittent "stutter" or "stumble".
The symptom is a momentary cut in power whilst driving.
It can happen at any speed but is normally above 2000 rpm.
I think that it's most likely to occur just as you back out of the throttle, but not on complete overrun.
Normally the car will recover almost immediately but on one occasion, at low speed in traffic, the engine stopped completely but I was able to restart on the key after a few seconds. There was a slight smell of fuel when this occurred, something I'd not noticed before.

The car will idle quite happily seemingly for ever, and will respond normally to a throttle input whilst stationary.

I've no idea whether this is relevant but it started happening soon after the car had been in for a clutch change and the replacement of fuel system (tank, pump, feed & return lines).

The following actions have taken place but without improving the situation.

Distributor cap & rotor arm replaced (hair line crack in the original)
DME relay and both flywheel sensors replaced.
All power and ground lines to engine confirmed OK.
Cold and warm fuel pressure at idle and fast idle confirmed ok.
The ECU, both fuel regulators and air MAF have been substituted from a fully functioning donor car.

I've owned the car since 2009 and other than the inevitable DME failure, this is the first time that I have had an engine issue of any sort.

The car is completely standard apart from an after-market alarm and immobilizer which has been on the car since 2015.

Does anyone have any suggestions please?

Cheers
Mike

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Hi Mike,

Have you checked the idle stabilisation valve is not sticking? I don’t think it will be this, however it’s worth checking.

I did have have something similar on my previous 86 car, I eventually traced that back to a broken wire in the back of the MAF wiring harness connector block. I’d suggest checking the harness / engine connections as the intermittent symptoms sound electrical!

Good luck!

phil

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I believe as the engine speed increases, the ignition will advance - therein allowing the spark plugs to fire earlier, giving time for the fuel/air mixture to ignite before the piston reaches top-dead- centre.   On mechanical distributors this is achieved by centrifugal weights in the lower part of the distributor.  I know my 3.2 has these weights - but unsure if they do anything , or if the advance is made by pure electronics. I’m sure someone else will contribute that knows far more than me, but this may be an area you can research.

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21 hours ago, Mobydick said:

 I know my 3.2 has these weights - but unsure if they do anything , or if the advance is made by pure electronics.

All ignition advance in a 3.2 is electronic.

The distributor weights are to keep the rotor tip aligned with the distributor cap contacts.

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8 hours ago, SilverWT said:

All ignition advance in a 3.2 is electronic.

The distributor weights are to keep the rotor tip aligned with the distributor cap contacts.

The weights and springs were there originally to advance the ignition using centrifugal force , to move the top part of the shaft [which either opened points, or passed by a magnetic pick up] independently of the lower part of the shaft, so not sure why they would still be relevant if advance is now done completely in the ECU ? arguably  the top part of the shaft  can/should be fixed solid i.e.; no movement, if I have miss-understood you, apologies in advance.

Edited by Ant7
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27 minutes ago, Ant7 said:

 so not sure why they would still be relevant if advance is now done completely in the ECU ? arguably  the top part of the shaft  can/should be fixed solid i.e.; no movement, if I have miss-understood you, apologies in advance.

As the ECU advances the timing, the spark current could be connected to the rotor arm before the rotor arm has reached the correct contact "pin" in the distributor cap. Potentially causing a weak spark/miss-fire. 

So malfunctioning 3.2 distributor springs/weights could potentially course a problem, even though the ECU controls the ignition timing.

Mark

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Hi Everyone,

Thanks for your comments.

I had a look at the connections at the weekend, couldn’t find anything. I’ll bear the other suggestions in mind over the next few weeks - got a lot on from now until October.

Interestingly, I’ve been out for a relatively gentle 10 mile drive today and the car behaved impeccably!

Cheers,

Mike

 

 

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