1985 XJS misfiring under load
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From: australia
1985 XJS misfiring under load
hi guys,
1 month in to ownership and thought I was going ok with minor tidy ups and fixes occurring but now engine is running rough under load.
the symptoms
Started without incident, idled fine and went for a drive. As soon as I came to the first hill (200m from start) engine lost power.
returned home and lifted bonnet, nothing obvious, checked distributor leads were all on and looked for obvious electrical leads disconnected and the like. Nothing obvious.
started again and ran revs up in park and neutral. Was running rough but difficult to tell if it was one cylinder missing or more than that.
Took it for a short drive. Stalled within 100m from home. Restarted after a bit of turning over and only drove 20m before stalling again. Took more to start it this time but returned home but running rough.
the last week of tinkering / operation
The last time I drove it was 5 days ago. Did about 80 km round trip. No issues. Filled up with fuel about 5 km from home but no issues getting it home.
was having an issue with the trip computer ( see my other post here) but fixed that 3 days ago by disconnecting to and reconnecting. During this process I pulled a few fuse some but replaced them too.
diagnosis
Have read the 'No start' thread which starts with fuel pressure checks. Haven't started This yet because it actually starts so not sure if I should follow the same procedure.
could the fuse thing have caused some other electrical gremlin?
could it just be dirty fuel ?
should I start with fuel filter and fuel pump checks / replacement or can I rule this out if it's actually starting and running?
also while it's idling it's hard to tell if it's running rough. Sounds ok. It's only when you try to increase revs that it sounds ordinary.
where should I be looking?
1 month in to ownership and thought I was going ok with minor tidy ups and fixes occurring but now engine is running rough under load.
the symptoms
Started without incident, idled fine and went for a drive. As soon as I came to the first hill (200m from start) engine lost power.
returned home and lifted bonnet, nothing obvious, checked distributor leads were all on and looked for obvious electrical leads disconnected and the like. Nothing obvious.
started again and ran revs up in park and neutral. Was running rough but difficult to tell if it was one cylinder missing or more than that.
Took it for a short drive. Stalled within 100m from home. Restarted after a bit of turning over and only drove 20m before stalling again. Took more to start it this time but returned home but running rough.
the last week of tinkering / operation
The last time I drove it was 5 days ago. Did about 80 km round trip. No issues. Filled up with fuel about 5 km from home but no issues getting it home.
was having an issue with the trip computer ( see my other post here) but fixed that 3 days ago by disconnecting to and reconnecting. During this process I pulled a few fuse some but replaced them too.
diagnosis
Have read the 'No start' thread which starts with fuel pressure checks. Haven't started This yet because it actually starts so not sure if I should follow the same procedure.
could the fuse thing have caused some other electrical gremlin?
could it just be dirty fuel ?
should I start with fuel filter and fuel pump checks / replacement or can I rule this out if it's actually starting and running?
also while it's idling it's hard to tell if it's running rough. Sounds ok. It's only when you try to increase revs that it sounds ordinary.
where should I be looking?
Joined: Feb 2010
Posts: 28,811
Likes: 11,280
From: Adelaide Stralia
Oh boy.
First is fuel, as in ya got some rubbish, and/or disturbed some in the main or sump tank.
Drain the fuel, via the spigot in the bottom of the sump tank, accessed thru a hole in the boot floor.
Remove the sump tank top plate, tap the ring around to do this, and look inside. Clean the plastic filter on the end of the suction pipe.
Replace the main fuel filter.
Fresh fuel.
Open the bonnet, remove a fuel supply line, RH side of the engine. Extend said line outside the engine bay into a can, and turn the Ign ON, and note the fuel, amount/pressure (is it a dribble or a spurt), and the colour.
Whilst you are at it, look under the RH side of the car. There is a metal pipe running rear to front, and it is the fuel supply pipe TO the engine. Tyre fitters generally crush it, as they are too lazy to use the supplied jacking points.
How old are the spark plugs????. Many V12's I have been involved with have 10 newish plugs and the 2 front are factory originals, FACT.
The list continues once you report back.
First is fuel, as in ya got some rubbish, and/or disturbed some in the main or sump tank.
Drain the fuel, via the spigot in the bottom of the sump tank, accessed thru a hole in the boot floor.
Remove the sump tank top plate, tap the ring around to do this, and look inside. Clean the plastic filter on the end of the suction pipe.
Replace the main fuel filter.
Fresh fuel.
Open the bonnet, remove a fuel supply line, RH side of the engine. Extend said line outside the engine bay into a can, and turn the Ign ON, and note the fuel, amount/pressure (is it a dribble or a spurt), and the colour.
Whilst you are at it, look under the RH side of the car. There is a metal pipe running rear to front, and it is the fuel supply pipe TO the engine. Tyre fitters generally crush it, as they are too lazy to use the supplied jacking points.
How old are the spark plugs????. Many V12's I have been involved with have 10 newish plugs and the 2 front are factory originals, FACT.
The list continues once you report back.
Last edited by Grant Francis; Apr 22, 2018 at 04:37 AM. Reason: spelling still sucks
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From: Pacific Northwest USA
Grant has you on the right track. I'll just add a couple tidbits.
What I always do is empty the fuel filter contents into a clean glass jar and then cut the filter open. What I see in the jar and the filter will dictate my next steps. If it's pristine I'll move on to other avenues. If cruddy, I'll address that problem.
Tracking down vague and/or intermittent faults is typically a matter of eliminating possibilities. Choose a logical jumping-off point (fuel system in this case), repair faults as you find them and, if no joy, move on to other possibilities.
Cheers
DD
What I always do is empty the fuel filter contents into a clean glass jar and then cut the filter open. What I see in the jar and the filter will dictate my next steps. If it's pristine I'll move on to other avenues. If cruddy, I'll address that problem.
Tracking down vague and/or intermittent faults is typically a matter of eliminating possibilities. Choose a logical jumping-off point (fuel system in this case), repair faults as you find them and, if no joy, move on to other possibilities.
Cheers
DD
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Joined: Sep 2017
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From: australia
Ok guys thought I'd report back at least to close the loop if anyone else has similar issues.
no issue with fuel pressure at the rail.
replaced all 12 plugs
replaced all HT leads
retained distributor cap but freed up advance weight which was stuck
Cleaned all injectors including cold start.
replaced fuel hoses
replaced injector harness
this has resulted in the car running again and not stalling under load
it does seem to still have a miss on 3A which may be unrelated but think it needs tappet adjustment.
have arranged for tappets to be done soon.
no issue with fuel pressure at the rail.
replaced all 12 plugs
replaced all HT leads
retained distributor cap but freed up advance weight which was stuck
Cleaned all injectors including cold start.
replaced fuel hoses
replaced injector harness
this has resulted in the car running again and not stalling under load
it does seem to still have a miss on 3A which may be unrelated but think it needs tappet adjustment.
have arranged for tappets to be done soon.
Last edited by Greg McDonald; Jun 10, 2018 at 08:27 AM. Reason: autocorrect aghhhh
Joined: Feb 2010
Posts: 28,811
Likes: 11,280
From: Adelaide Stralia
Well done.
I would not be doing tappets, and that is just my opinion.
They are a shim style and I have never seen them wear.
The amount of labour involved is horrendous, and that Mega $$$$ chain tensioner needs to be played with. NOT for the feint hearted.
The miss is probably a spark plug, or an Injector.
Pull the pug lead off the plug, insert one of the old ones in that lead, and start the car, note the spark, or lack of. You are looking for a "fat Blue cracker" of a spark, and anything else on our Aussie spec 12.5:1 comp engines will not cut it.
Plugs gapped at 0.025" is also a requirement.
If there be spark, then fuel not arriving is the next item, and a tad harder to test. That injector may well be "clicking", but not actually opening the pin seat for fuel.
3A plug lead is basically jammed up against the distributor, and sometimes the HT energy takes the easier path, and goes via the casing instead of jumping the plug gap.
I do trust that you replaced the fuel filter????
I would not be doing tappets, and that is just my opinion.
They are a shim style and I have never seen them wear.
The amount of labour involved is horrendous, and that Mega $$$$ chain tensioner needs to be played with. NOT for the feint hearted.
The miss is probably a spark plug, or an Injector.
Pull the pug lead off the plug, insert one of the old ones in that lead, and start the car, note the spark, or lack of. You are looking for a "fat Blue cracker" of a spark, and anything else on our Aussie spec 12.5:1 comp engines will not cut it.
Plugs gapped at 0.025" is also a requirement.
If there be spark, then fuel not arriving is the next item, and a tad harder to test. That injector may well be "clicking", but not actually opening the pin seat for fuel.
3A plug lead is basically jammed up against the distributor, and sometimes the HT energy takes the easier path, and goes via the casing instead of jumping the plug gap.
I do trust that you replaced the fuel filter????
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Member
Joined: Sep 2017
Posts: 49
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From: australia
I didn't do fuel filter. Fuel pressure at rail was good so I assumed good flow through filter. I did take filter off and shook it up and poured out fuel. It was clean from both sides. Not too technical I know but seemed unlikely as the cause.
In regard to spark, yep getting good spark at 3A. Pulled lead and it makes less difference than pulling other plugs - that's how we determined it was 3A with the miss. It is hard to tell though with one cylinder not running.
Compression on 3A was 135 psi which I gather is enough to get it to fire but not the 180 psi it should be hence the feeling that exhaust valve may not be sealing fully and tappets need adjusting.
In regard to spark, yep getting good spark at 3A. Pulled lead and it makes less difference than pulling other plugs - that's how we determined it was 3A with the miss. It is hard to tell though with one cylinder not running.
Compression on 3A was 135 psi which I gather is enough to get it to fire but not the 180 psi it should be hence the feeling that exhaust valve may not be sealing fully and tappets need adjusting.
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From: France
As Grant said, the tappets do not need adjusting. You may have a lump of crud on the valve seat though, or the injector is iffy. I suggest warning it up, stuffing it in second gear on the stick, and revving the hell out of it for half an hour at about 100 mph on a nice driving road.
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From: australia
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From: Adelaide Stralia
Yep, a tank full of our 98 fuel, and flat out across the Hay Plains, or even the Gunbarrel Highway will sort the beast.
If 3A is at 130psi, I agree also with the crud under the exhaust valve, and a good loooooong hard run is what they need. NOT a town engine by any means.
If the issue is not following the Injector, then that Injector is OK. Wiring and/or a connector issue. as mentioned, may also be at play.
Have a drink, think about it, drink some more, get into it, works every time.
If 3A is at 130psi, I agree also with the crud under the exhaust valve, and a good loooooong hard run is what they need. NOT a town engine by any means.
If the issue is not following the Injector, then that Injector is OK. Wiring and/or a connector issue. as mentioned, may also be at play.
Have a drink, think about it, drink some more, get into it, works every time.
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Joined: Jan 2014
Posts: 429
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From: NJ, USA
Try to 'catch' car when it runs 'rough'. Perhaps increase idle so the car does not die every time it is in 'rough' mode.
If you manage to do that - pull spark plug wires one at a time and see if it makes any difference. It it does not - you found offending cylinder and from there you can debug further if it is injector, or spark.
If pulling every wire make difference - you have 'engine-wide' issue. Could be something common like temperature sensor.
In my case it was connection to power resistor, when out of contact half of the cylinders would stop working. Took me a while to figure...
By the way, since it is 1985... do you have electronic ignition or points? If points - check them carefully. Carbon buildup, grain of sand, or loose point may have similar effect of lost power.
If you manage to do that - pull spark plug wires one at a time and see if it makes any difference. It it does not - you found offending cylinder and from there you can debug further if it is injector, or spark.
If pulling every wire make difference - you have 'engine-wide' issue. Could be something common like temperature sensor.
In my case it was connection to power resistor, when out of contact half of the cylinders would stop working. Took me a while to figure...
By the way, since it is 1985... do you have electronic ignition or points? If points - check them carefully. Carbon buildup, grain of sand, or loose point may have similar effect of lost power.
Last edited by NJ2003XJ8; Jun 11, 2018 at 01:46 PM.
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