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Flahp1
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Joined: 01 Jul 2017
Posts: 12



PostPosted: Sat Jul 01, 2017 2:11 pm    Post subject: Misfiring 1990 964 Reply with quote

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!
 
  
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Flahp1
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PostPosted: Sat Jul 01, 2017 2:20 pm    Post subject: Reply with quote

The battery connections and earth lead have been rechecked and also did a hard reset. No change, intermittent misfiring still there..
 
  
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Zingari
Shanghai
Shanghai


Joined: 25 Oct 2009
Posts: 11478
Location: Cheshire

1993 Porsche 964 Anniversary

PostPosted: Sat Jul 01, 2017 3:35 pm    Post subject: Reply with quote

Distributor caps/leads Dont know Try unplugging one dizzy at a time to see if it still runs to elminate the belt
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Flahp1
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PostPosted: Sat Jul 01, 2017 3:58 pm    Post subject: Reply with quote

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?
 
  
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Zingari
Shanghai
Shanghai


Joined: 25 Oct 2009
Posts: 11478
Location: Cheshire

1993 Porsche 964 Anniversary

PostPosted: Sat Jul 01, 2017 4:19 pm    Post subject: Reply with quote

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
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ras62
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Joined: 20 Jan 2012
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Location: Cheshire


PostPosted: Sat Jul 01, 2017 5:28 pm    Post subject: Reply with quote

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.
 
  
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Flahp1
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PostPosted: Sat Jul 01, 2017 7:40 pm    Post subject: Reply with quote

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.
 
  
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Flahp1
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PostPosted: Sat Jul 01, 2017 7:42 pm    Post subject: Reply with quote

Thanks Ras62, that sounds like a quick and simple job that might be a good thing to do anyway!
 
  
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ras62
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Joined: 20 Jan 2012
Posts: 570
Location: Cheshire


PostPosted: Sun Jul 02, 2017 9:45 am    Post subject: Reply with quote

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.
 
  
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Flahp1
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PostPosted: Mon Jul 03, 2017 7:37 am    Post subject: Reply with quote

Thanks for the advice Ras62!
 
  
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Flahp1
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PostPosted: Sat Jul 08, 2017 8:12 am    Post subject: Reply with quote

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?
 
  
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Flahp1
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PostPosted: Sat Jul 08, 2017 8:12 am    Post subject: Reply with quote

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?
 
  
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Zingari
Shanghai
Shanghai


Joined: 25 Oct 2009
Posts: 11478
Location: Cheshire

1993 Porsche 964 Anniversary

PostPosted: Sat Jul 08, 2017 9:39 am    Post subject: Reply with quote

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.
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Flahp1
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PostPosted: Sat Jul 08, 2017 1:55 pm    Post subject: Reply with quote

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.
 
  
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ras62
Montreal


Joined: 20 Jan 2012
Posts: 570
Location: Cheshire


PostPosted: Sat Jul 08, 2017 2:21 pm    Post subject: Reply with quote

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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Flahp1
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PostPosted: Sat Jul 08, 2017 2:43 pm    Post subject: Reply with quote

Lots of good advice there Ras62, thanks. I'll work through that lot in case I have a couple of things going on. Car is not misfiring at the moment. Car is also starting far better now with jump leads across the engine,so suspect earth strap on gear box is broken and was causing sluggish starter motor. Far better now.
 
  
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Flahp1
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PostPosted: Sat Jul 08, 2017 3:51 pm    Post subject: Reply with quote

Battery is 13V stopped and 13.25V when idling. Not too low, but not over 13.5V as you suggested.
 
  
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Zingari
Shanghai
Shanghai


Joined: 25 Oct 2009
Posts: 11478
Location: Cheshire

1993 Porsche 964 Anniversary

PostPosted: Sat Jul 08, 2017 4:41 pm    Post subject: Reply with quote

If you're still on the original twisted earth wire that connects to the battery and frame change it. It might look OK but a new one is cheap as chips plus clean the brass connectors where it attaches with other wires to the body earth point.

My car had a bad earth that I diagnosed to the battery connector. I'd leave the car and go back and the electrics would be completely dead including the clock. Then it would magically re-connect. A new lead fixed it Thumb
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Me? The 13th Duke of Wybourne? Here? In a sixth form girl's dormitory? At three o'clock in the morning? With my reputation? What were they thinking of?
 
  
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ras62
Montreal


Joined: 20 Jan 2012
Posts: 570
Location: Cheshire


PostPosted: Sat Jul 08, 2017 5:50 pm    Post subject: Reply with quote

Flahp1 wrote:
Battery is 13V stopped and 13.25V when idling. Not too low, but not over 13.5V as you suggested.


That's a bit low but the battery is fine by the sound of it. Worth checking the battery at say 1500rpm to see if it rises. Ideally it should be 13.5-14.5v. Any less isn't ideal.
It's certainly worth checking the engine/gearbox straps and also the main +ve connector at the starter motor. This is the main connection point to the battery and they do get neglected and corrode. This may explain the low tick over voltage. Before getting any spanners up there pls disconnect the battery +ve terminal or sparks will fly!
Hopefully the cutting out will now be solved with the new O2 sensor installed. I tried all sorts before I changed it on my car and was getting desperate. The problem is they dont have defined fail modes so can appear fine on the hammer tester.

One issue I should have mentioned is the clock. The joints internally do go dry and cause all manner of issues for instruments and even running. Do a google search for further explanations. Worth removing the clock and resoldering the main contacts inside.
 
  
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Flahp1
Newbie


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Posts: 12



PostPosted: Sun Jul 09, 2017 12:05 pm    Post subject: Reply with quote

Thanks again ras62. I was out in the car this morning, couple of hours with no problems whatsoever. Stopped a couple of times, engine turned over very quickly and fired up every time. Went out again later and when I went to start car up, absolutely nothing there, almost like there was no battery connected. Couldn't find anything obvious, fiddled around with the jump leads from the engine to the frame, swear I heard a click, it then fired up as good as ever. Very odd, any ideas? Relay? Starter motor was changed last year.
 
  
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