Not all cylinders firing properly
#1
08-15-2009, 03:45 PM
Guest
Posts: n/a
Not all cylinders firing properly
Hello,
The title was going read 'engine missing' but didn't want to mislead anyone into thinking it had been misplaced/stolen...
But thats the problem, so was hoping for some suggestions for what to try next.
Its a 1991 XJ Soveriegn 3.2 and the engine generally misses regardless of what RPM, and not in a regular pattern.
A couple of months ago the crank position sensor failed and I replaced it with a second hand unit from a scrap yard, but the problem didnt spring up till a few weeks after. I also get the impression that when this goes it goes completely so Im not really suspecting a dodgy CPS.
The rotor arm and dissy cap have both had a damn good clean and I cant see any cracks in the cap. The leads look fine.
I'm sort of thinking maybe the coil is on its way out, the missing is getting slightly worse as the days go on. The Jag still drives ok with the problem, I've done over a 1000 miles in it with the engine missing...
It's probably also worth mentioning it is LPG converted, but the problem occurs on either fuel. Although I get the feeling it does it to less of a degree when on petrol...?
Can anyone suggest anything? I would appriciate any help!
Thanks very much
Oli
The title was going read 'engine missing' but didn't want to mislead anyone into thinking it had been misplaced/stolen...
But thats the problem, so was hoping for some suggestions for what to try next.
Its a 1991 XJ Soveriegn 3.2 and the engine generally misses regardless of what RPM, and not in a regular pattern.
A couple of months ago the crank position sensor failed and I replaced it with a second hand unit from a scrap yard, but the problem didnt spring up till a few weeks after. I also get the impression that when this goes it goes completely so Im not really suspecting a dodgy CPS.
The rotor arm and dissy cap have both had a damn good clean and I cant see any cracks in the cap. The leads look fine.
I'm sort of thinking maybe the coil is on its way out, the missing is getting slightly worse as the days go on. The Jag still drives ok with the problem, I've done over a 1000 miles in it with the engine missing...
It's probably also worth mentioning it is LPG converted, but the problem occurs on either fuel. Although I get the feeling it does it to less of a degree when on petrol...?
Can anyone suggest anything? I would appriciate any help!
Thanks very much
Oli
Last edited by Oli; 08-15-2009 at 03:51 PM.
#2
08-15-2009, 06:28 PM
#3
08-16-2009, 08:25 AM
Veteran Member
You don't state what miles are on the engine.
Misfires in General;
First Eliminate the obvious. You state "Not in a regular pattern", presumably means not always the same cylinder/cylinders. With the engine running have you tried removing and replacing the plug leads one by one. If you do this when you arrive at the offending cylinder you will not notice a change of engine beat. Try replacing the lead to this cylinder with one of your other good ones (remember to also swap the other end at the dizzy).
Repeat the above and if you notice that you now have a different "bad"cylinder it's most likely you have a broken down lead and have found the problem. Replace it. Better replace all, because if one is on the way out the others may follow.
If you still have the same bad cylinder, then look at the plugs, are you getting a spark. Check by removing plug, reconnecting the lead and have someone crank the engine whilst touching the electode to the block. BE CAREFUL THAT YOU ONLY HANDLE THE INSULATED PART OF THE LEAD YOU CAN GET ONE HECK OF A JOLT, do not attempt to start the engine just look for a spark accross the plug gap. If you do get a SHOCK replace the lead. If no spark, replace plug and repeat.
If the problem remains and you have random different bad cylinders go back to the dizzy cap.
Although the dizzy cap might look good, there can be hairline arcing between the carbon electrodes that you may not be able to see. Try a swap with a known good one. Same with the leads, they breakdown from the inside out.
You say that the rotor has had a damn good clean, are you sure that you have not over ground it? Put in a new one.
If you have replaced all of the above for NEW service items, then think about the coil.
Also check all vacuum hoses for cracks or leaks or blockages in a running vehicle this could give the symptoms of a misfire eg. a lumpy engine.
In other words Eliminate the obvious, before looking at anything more major.
I would also NOT recommend driving for over 1000 miles, whilst ignoring a misfire. Not only do you waste fuel, but you take a big risk in fouling the valves, head, oil, bearings etc by having all that unburnt fuel sloshing around.
When you have cured your misfire do an oil and filter change, because your oil will be contaminated.
That's about all I can offer, just General advice.
Cheers,
Misfires in General;
First Eliminate the obvious. You state "Not in a regular pattern", presumably means not always the same cylinder/cylinders. With the engine running have you tried removing and replacing the plug leads one by one. If you do this when you arrive at the offending cylinder you will not notice a change of engine beat. Try replacing the lead to this cylinder with one of your other good ones (remember to also swap the other end at the dizzy).
Repeat the above and if you notice that you now have a different "bad"cylinder it's most likely you have a broken down lead and have found the problem. Replace it. Better replace all, because if one is on the way out the others may follow.
If you still have the same bad cylinder, then look at the plugs, are you getting a spark. Check by removing plug, reconnecting the lead and have someone crank the engine whilst touching the electode to the block. BE CAREFUL THAT YOU ONLY HANDLE THE INSULATED PART OF THE LEAD YOU CAN GET ONE HECK OF A JOLT, do not attempt to start the engine just look for a spark accross the plug gap. If you do get a SHOCK replace the lead. If no spark, replace plug and repeat.
If the problem remains and you have random different bad cylinders go back to the dizzy cap.
Although the dizzy cap might look good, there can be hairline arcing between the carbon electrodes that you may not be able to see. Try a swap with a known good one. Same with the leads, they breakdown from the inside out.
You say that the rotor has had a damn good clean, are you sure that you have not over ground it? Put in a new one.
If you have replaced all of the above for NEW service items, then think about the coil.
Also check all vacuum hoses for cracks or leaks or blockages in a running vehicle this could give the symptoms of a misfire eg. a lumpy engine.
In other words Eliminate the obvious, before looking at anything more major.
I would also NOT recommend driving for over 1000 miles, whilst ignoring a misfire. Not only do you waste fuel, but you take a big risk in fouling the valves, head, oil, bearings etc by having all that unburnt fuel sloshing around.
When you have cured your misfire do an oil and filter change, because your oil will be contaminated.
That's about all I can offer, just General advice.
Cheers,
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