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Misfire

Donkeychote
Posts: 162
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
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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, Auto (yes I know, I know, but you'd be surprised, I was !)
1989 White and Navy Blue V.W. Golf Cabriolet (G.T.i.)
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Donkeychote
Posts: 162
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: 238
Joined: Wed Jul 12, 2017 3:53 pm
Location: Pagam 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,in this order.
1986 BMW M535i
1985 BMW 525e
1972 Vauxhall victor FE2300
1966 Ford cortina estate
1961 Renault dauphine Gordini
1978 Range Rover
1976 LR 101 forward control
1968 S2 lightweight Land Rover
1975 Range Rover
1973 Range Rover
1972 series 3 Land Rover SWB

NMV 1E
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Posts: 764
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, Auto (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.) For Sale !

Donkeychote
Posts: 162
Joined: Fri Jun 20, 2014 1:17 pm

Re: Misfire

Postby Donkeychote » Mon Aug 20, 2018 5:33 am

The new arm is also around 1k, so it seems right.
I was also thinking in the direction of heat, since the misfire gets worse when it warms up. I'll need to find time to go to the local jaguar dealership to get a new one, that way I'll be sure I get a correct one.
1988 3.6 Sovereign Talisman Silver Metallic

Donkeychote
Posts: 162
Joined: Fri Jun 20, 2014 1:17 pm

Re: Misfire

Postby Donkeychote » Sun Sep 02, 2018 3:36 pm

So I finally got a new rotor arm from the local dealer. At idle it now runs beautifully, but once warmed up and under load, it still misfires badly.
With all the usual suspects now replaced, i'll have to look at more unlikely things. Is it possible that this is an ecu problem?
Or maybe the distributor isn't installed correctly, but I think I would have noticed that earlier, since I've done more than 40000km with it already.
1988 3.6 Sovereign Talisman Silver Metallic

naki_k
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Re: Misfire

Postby naki_k » Fri Sep 07, 2018 1:35 am

There’s only so many things it could be.

Crank sensor is of course always a prime suspect.

Any fault codes on the VCM?
88 3.6 XJ6 PDH/CEM
90 Range Rover Vogue SE 3.9 V8 LPG
91 3.2 XJ6 SportPack Manual
91 4.0 XJ6 Manual
92 4.0 XJR Manual
92 4.0 XJR
93 3.2s - 4.0 Manual conv
93 4.0 Sovereign
93 6.0 XJ81 DD6
93 6.0 XJ81 XJ12
94 4.0 XJR
96 4.0 X306 XJR
96 4.0 X300 D6
97 4.0 X300 Sport Manual
2000 LR Discovery ES TD5 A
Breaking 40/ 300

Donkeychote
Posts: 162
Joined: Fri Jun 20, 2014 1:17 pm

Re: Misfire

Postby Donkeychote » Fri Sep 07, 2018 5:54 am

I didn't think of the CPS. I'll get a new one, see if that helps. If it doesn't, I'll just have a spare :)
1988 3.6 Sovereign Talisman Silver Metallic

katar83
Distinguished Mbr
Posts: 2502
Joined: Tue Aug 23, 2011 11:17 am
Location: Cambridge

Re: Misfire

Postby katar83 » Fri Sep 07, 2018 7:11 am

It's still surprising to find out how many of us here drive their Jags with no spare CPS in the boot at all times :shock:
So many kamikazes on the road, willing to risk a 12 hours wait for AA or RAC :lol:

I'd be looking at other ignition items too here. That hole as you said was probably a symptom.
Last edited by katar83 on Fri Sep 07, 2018 9:50 am, edited 1 time in total.
1997, 4.0 LWB X300 - Unlucky car but still a daily driver.
1988, 3.6S XJS - Never ending restoration project.
1994, 3.2S X300 - Accident damaged, October 2015. Gone!
1994, 3.2S XJ40 - Long gone.


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