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Misfiring 1990 964

Flahp1

Member
Joined
1 Jul 2017
Messages
12
Looking for advice on solving an intermittent misfire on my 964. Car starts and idles fine, no misfire with hi revs at stand still. However when accelerating hard, there is an intermittent Miss fire and popping from exhaust. At the same time, I get some flickering of some of the gauges (electrical fault?)

I have renewed the DME relay, battery and earth lead on battery recently.

Looking for advice on where to look for this fault next, not sure where to start!
 
The battery connections and earth lead have been rechecked and also did a hard reset. No change, intermittent misfiring still there..
 
Distributor caps/leads :dont know: Try unplugging one dizzy at a time to see if it still runs to elminate the belt
 
Hi Zingari - thanks for the help. Can you explain a bit more on what I'm looking for? Do I pull one lead off at a time while it's idling and see if what happens? I assume you mean the belt for the distibutor ? If it's loose, will that give me intermittent misfiring?
 
Take one dizzy main lead off and start the engine then do the same with the other dizzy.

I'm not saying the dizzy belt is the problem but you'll eliminate it from the equation.

Are the HT leads all in good condition? Recent spark plug change? However your symptons sound more like throttle body/injector if you get a flatspot when you want to give it the beans :dont know:
 
had a very similar problem years ago, solved by fitting a new O2 sensor. Chances are if you connect the Bosche diagnostic tool it will show the o2 cyling ok but also show a fault alarm. They dont last forever.
 
Thanks Zingari - I'll give it a go. HT leads look ok, no cracking etc.. been a while since plugs were changed... the misfiring happens at hi revs and even low revs sometimes, but tends to hit just as you put the power on, but then it can be ok right up to the red zone.
 
Thanks Ras62, that sounds like a quick and simple job that might be a good thing to do anyway!
 
Not really quick or simple unless you have garage equipment. Its one best left for the pro's. They require lots of heat and are usually very very tight so the hole needs a new thread.
 
Hi Zingari - looking at the engine, I have 2 belts up to the fan unit for the alternator, 1 is a bit more loose than the other, but both look tight enough. Could there be a fault on the coils?
 
Hi Zingari - looking at the engine, I have 2 belts up to the fan unit for the alternator, 1 is a bit more loose than the other, but both look tight enough. Could there be a fault on the coils?
 
The belts to the alternator I doubt will cause this problem. It could be the coils but I'd expect it more frequently.

It would be worth doing a diagnostic scan to see if that points you anywhere.
 
I've fitted the new O2 sensor, bit of a job to get it free, needed some heat and a pipe wrench. Car running very smooth. I'm convinced I've got an earth problem, the flickering of the gauges when the engine misfires looks like something on the -ve side. I've put some jump leads across the frame onto engine block and the misfiring seems to have a stopped.
 
Probably worth doing a system adaption now the O2 sensor is replaced. The engine has two negative connections onto the top of the crankcase halfs behind the fan and alternator so its unlikely to be an earth issue. The gearbox also has and earth strap. Are you still getting a misfire?
Some jobs worth checking. Voltage at the battery over 12.5v and on tickover above 13.5 volts shows the alternator is working. These cars hate low voltages! The other is making sure the air flow meter connection isn't rubbing on the pipes in the rear of the engine bay, there was a tech service bullitin to tie wrap the wiring to stop the chaffing which caused a misfire/hesitation. Third, reseat all the fuse and relays in the central electrics.
 

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