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Misfire

Donkeychote
Posts: 151
Joined: Fri Jun 20, 2014 1:17 pm

Misfire

Postby Donkeychote » Sun Aug 12, 2018 8:19 pm

After a couple of months happy motoring, I finally have a problem again.
The idle hasn't been quite as smooth as it should be for a bit, which I put down to spark plugs. But after changing them last week, it remained, and today it started misfiring. By the time I got home it was getting really bad, I could barely touch the throttle pedal without at least one cylinder missing, and often more than one.
Checking out the ignition system, we found that there was a hole on the side of the center turret of the distributor cap:

Image


I still have the old cap and spark plugs (only changed them as a precaution, not because of a problem), but that didn't fix the misfire. Seems the hole was only a symptom instead of the cause.
I'll check the coil next, but don't have a spare one on hand. If that doesn't fix it, what could be causing this?
1988 3.6 Sovereign Talisman Silver Metallic

NMV 1E
Snr Member
Posts: 659
Joined: Fri Apr 17, 2009 9:26 pm
Location: Lydiate, Merseyside.

Re: Misfire

Postby NMV 1E » Sun Aug 12, 2018 9:32 pm

My basic understanding is the H.T. spark is seeking a path to earth, normally via the spark plug air gap. If it can't get to earth via the intended route ... I'd suspect the resistor in the rotor arm :?:
Life begins at Forty !... I was born with nothing, I seem to have most of it left.


2006 Baby Blue Hyundai Getz 1.4 CDX (yes I know, I know, but you'd be surprised, I was !)
1989 White and Navy Blue V.W. Golf Cabriolet (G.T.i.)
1992 Tornado Red V.W. Golf "tintop" 5 door (G.T.i.)

Donkeychote
Posts: 151
Joined: Fri Jun 20, 2014 1:17 pm

Re: Misfire

Postby Donkeychote » Sat Aug 18, 2018 9:50 pm

I measured the resistance across the rotor arm today, and it came out as 1k ohms. Do you know how much it should be?
I had ordered a new arm since they are pretty cheap, but got a wrong part delivered, so I'm not sure if I can compare with that.
1988 3.6 Sovereign Talisman Silver Metallic

Jaguarxj40
Snr Member
Posts: 200
Joined: Wed Jul 12, 2017 3:53 pm
Location: West Sussex

Re: Misfire

Postby Jaguarxj40 » Sun Aug 19, 2018 10:47 am

Hi,
I can relate to this problem, when I first purchased my 40 17 years ago it was not looked after very well and the guy who owned it previously ( a work collegue ) he had so many issues with it mainly non servicing he wanted shot of it and I brought it very cheaply.

Didn't test drive it before buying because it was so cheap, paid my money and oh my god it drove terrible and I had similar problems to you, could only drive it on very slight throttle. If I put my foot down it would misfire and almost cut out.

The original leads I conclude were sevicable and are still on the car :shock: but I renewed the cap and rotor arm and this cured the problem completely.

Gary
Current
1996 BMW Z3
1990 Jaguar XJ40 4.0

Previous,
1986 BMW M535i
1985 BMW 525e
1972 Vauxhall victor FE 2300
1966 Ford cortina estate
1961 Renault dauphine Gordini
1978 Range Rover
1976 Land Rover 101 forward control
1968 series 2 light weight Land Rover
1975 Range Rover
1973 Range Rover
1972 series 3 Land Rover SWB

NMV 1E
Snr Member
Posts: 659
Joined: Fri Apr 17, 2009 9:26 pm
Location: Lydiate, Merseyside.

Re: Misfire

Postby NMV 1E » Sun Aug 19, 2018 12:29 pm

Donkeychote wrote:I measured the resistance across the rotor arm today, and it came out as 1k ohms. Do you know how much it should be?
I had ordered a new arm since they are pretty cheap, but got a wrong part delivered, so I'm not sure if I can compare with that.


Sorry I don't, though 1k ohms doesn't seem that (too) high ... The average plug lead is about that (from memory). I'd check the reading against the new one just for interest's sake. Its possible the resistance of your original one could alter as the arm warms up (via the distributor drive spindle ??) :)
Life begins at Forty !... I was born with nothing, I seem to have most of it left.


2006 Baby Blue Hyundai Getz 1.4 CDX (yes I know, I know, but you'd be surprised, I was !)
1989 White and Navy Blue V.W. Golf Cabriolet (G.T.i.)
1992 Tornado Red V.W. Golf "tintop" 5 door (G.T.i.)


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