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Ignition goes off when accelerating from idle, 928 S MJ 84

 
Old 06-10-2019, 08:45 AM
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_Thomas_
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Default Ignition goes off when accelerating from idle, 928 S MJ 84

Hello


I have a strange problem with my Porsche 928 S (MJ 84). Engine starts properly, but then goes out when I depress the accelerator pedal quickly (most likely due to no ignition, but may be not enough gas?). If I press the accelerator very slowly the engine does not go out and it accelerates properly; at about 3000 rpm I can engage the clutch and start driving. While driving everything is OK above about 2500 rpm, at about 1000 - 2500 rpm the engine sometimes stutters, especially if I accelerate (quickly).
The red (!) on the dashboard does not go out (but all warning lights go off).
Does anyone have any idea why this might be? Possibly alternator / Voltage regulator? (Charge works in principle, display 14V, but sometimes falls down when accelerating, not sure, it is difficult to reproduce or measure ...) or not enough fuel (I think not, though, I got explosion in exhaust when the ignition was back when I let go of the accelerator pedal ...)

Thomas
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Old 06-10-2019, 08:02 PM
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Majestic Moose
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Can you give us some maintenance history? How old are the spark plugs and wires, distributor cap and rotor? Is this a euro '84 with LH jet and twin distributors?

When did the problem begin and have you done any work just before?
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Old 06-11-2019, 02:12 AM
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Can you give us some maintenance history? How old are the spark plugs and wires, distributor cap and rotor?
> all many years old, but the problem started suddenly, not gradually (see below)

>Is this a euro '84 with LH jet and twin distributors?
yes

> When did the problem begin and have you done any work just before?
Sorry I should have mentioned it: the problem started suddenly, after I left the car parked in the garage for about 2 weeks. Although the red (!) on the dashboard was already not going off 1-2 days before I parked it, but there was no noticeable problem
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Old 06-11-2019, 07:19 AM
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Check the timing of the distributor rotors

remove the caps put engine to tdc the rotor tips should be centered at the hash marks on the dizzy body
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Old 06-11-2019, 02:19 PM
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Bad bat ground cable can cause it.
They corrode under the plastic shroud.
Cant be cleaned.

Last edited by Landseer; 06-11-2019 at 04:04 PM.
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Old 06-11-2019, 04:48 PM
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You mean the main battery cable in the trunk? Will check that.
Bad grounds make sense... Are there other grounds which likely affect the ignition?
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Old 06-11-2019, 05:03 PM
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Originally Posted by Mrmerlin View Post
Check the timing of the distributor rotors

remove the caps put engine to tdc the rotor tips should be centered at the hash marks on the dizzy body

Yes I will check as much if the ignition as I can while I am in there
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Old 06-12-2019, 07:26 AM
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Bad bat main ground is one of the tricky little problems we've seen take these cars down. Can be particularly intermittent.

Other really key grounds on 84 are two that ground computers and injectors and sensors. Both are on passenger cam cover on 84. Look closely, one of them is buried under the emissions valve that mounts to the cover. You need to remove that, it's actually not as difficult as it looks.(ok, that's for USA, just caught yours is euro, easier still, no valve)

Routine when I drag one home is first clean grounds and replace cable. May not be your cars problem, but easily could be.

Crank position sensor connector, also twisted stressed maf connector, or maf itself on euro are other places to look.

Last edited by Landseer; 06-12-2019 at 07:48 AM.
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Old 06-13-2019, 02:46 PM
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I checked the battery connections and indeed the thinner of the + connection cables was not screwed on tightly; after tightening the battery connections the (!) on the dashboard went out, but the motor is still staggering. The staggering is the worst if I accelerate from about 1800 rpm to 2500 rpm. I also checked the ground connections on the cam cover and most other connectors incl. MAF etc., but that did not improve anything.

The question is now, could the electronics of the car (MAF, LH jet electronics, voltage regulator) have been damaged because the battery connection was not good/intermittent?
(Or just bad coincidence, and I should overhauld the ignition, replace the distributor caps (they are rather old) and check spark plug gaps ...)
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Old 06-14-2019, 03:41 AM
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Clean the electrodes under the caps, or replace ( they are 944 parts actually ).
Check for corrosion in end of coils.
Maybe a nick or cut in coil wire or plug wire?

Its a long list of possibilities, slowly methodically work-thru.

It would be nice if you could swap MAF units to try a known-good one.

Mr Merlin has a lot of experience with the Euro also, and he mentioned the coordination of the distributors.

Probably not situation where there is damage to controllers, but if all else fails there is John Speakes in UK who is pro with controllers.
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Old 06-15-2019, 08:49 AM
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Sounds as though possibly the MAF output has gone and the car is in 'limp home' mode. Once you get the rpm high enough the injector open time doubles and
with a 5 speed is can run quite well.
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Old 06-15-2019, 11:53 AM
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Hi John
I think you were right, the MAF has no output voltage, I measured it according to the instructions on your home page: 0 Volts at pin 7 (and 12 V at pin 9) of the LH computer plug.
Regards
Thomas
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