Puzzling misfire - advice please
Discussion
My car has a constant misfire. You can hear it while driving along, and feel it through the steering wheel. No variation, always there. Try accelerating from 30 in 4th and it becomes more noticeable, but still even. And I could smell fuel while driving.
So I checked my extenders (brand new) and leads (2 years old) were all on tight and showing nice brassy contacts. The plugs are all new. Car ran fine after the new plugs and extenders were installed.
Then I pulled out all 8 plugs, looking for the blackened, wet one. All spotless.
I am baffled. I want to fix this myself and have a track day next Thursday.
Suggestions as to what it might be, and why my plugs all look fine? All have a 25 thou gap, BTW.
So I checked my extenders (brand new) and leads (2 years old) were all on tight and showing nice brassy contacts. The plugs are all new. Car ran fine after the new plugs and extenders were installed.
Then I pulled out all 8 plugs, looking for the blackened, wet one. All spotless.
I am baffled. I want to fix this myself and have a track day next Thursday.
Suggestions as to what it might be, and why my plugs all look fine? All have a 25 thou gap, BTW.
Yep I'd go for electrical issue or stepper motor or dodgy lambda giving Ecu iffy readings,,,
How's your idle at start up etc, if that's steady then you can probably discount the stepper, maybe the coil breaking down, try pulling each plug lead off when it's running and see off here's an obvious difference to try to narrow down which pot is playing up Anthony. Al
How's your idle at start up etc, if that's steady then you can probably discount the stepper, maybe the coil breaking down, try pulling each plug lead off when it's running and see off here's an obvious difference to try to narrow down which pot is playing up Anthony. Al
Hi Anthony,,
If you can't find someone with a crypton tuner, my mate has one - Jcn 8 of the M25. My last car was sensitive to the cross over on the ignition leads. Have you tidied them up when you changed the extenders?
Also, I think you still have the Lucas system, so have you got any fault codes or tried an ECU reset?
If you can't find someone with a crypton tuner, my mate has one - Jcn 8 of the M25. My last car was sensitive to the cross over on the ignition leads. Have you tidied them up when you changed the extenders?
Also, I think you still have the Lucas system, so have you got any fault codes or tried an ECU reset?
Hi All
I am going to be a busy boy today
Thanks for all those suggestions guys.
I will try tidying the plug leads, they are not the tidiest I have seen, though when Peter was helping me yesterday at Curborough he didn't see that as an issue.
He started pulling plug leads, but I will continue that exercise.
I have a spare coil and a spare set of leads, so I will try changing those (one at a time!)
And I will have a look at the ignition amplifier.
The misfire is there at idle, but it idles fine most of the time (the misfire is causing a little fluctuation, but not enough to bother me on the 60 mile drive back yesterday), so I am ruling out the stepper motor. I will check the lambda wiring just in case.
Thanks for your offer, Graham. My local garage just might have a Crypton tuner, so he will be my first port of call for outside help.
He's an interesting guy.....heading for retirement, scruffy oil stained office, always some car or other in bits in the garage, but his pride and joy, a Humber Hawk, sits gleaming in the workshop, and when you get him talking you find he did a 9.9 second quarter back in the day at Santa Pod.
Onwards and sideways......
I am going to be a busy boy today
Thanks for all those suggestions guys.
I will try tidying the plug leads, they are not the tidiest I have seen, though when Peter was helping me yesterday at Curborough he didn't see that as an issue.
He started pulling plug leads, but I will continue that exercise.
I have a spare coil and a spare set of leads, so I will try changing those (one at a time!)
And I will have a look at the ignition amplifier.
The misfire is there at idle, but it idles fine most of the time (the misfire is causing a little fluctuation, but not enough to bother me on the 60 mile drive back yesterday), so I am ruling out the stepper motor. I will check the lambda wiring just in case.
Thanks for your offer, Graham. My local garage just might have a Crypton tuner, so he will be my first port of call for outside help.
He's an interesting guy.....heading for retirement, scruffy oil stained office, always some car or other in bits in the garage, but his pride and joy, a Humber Hawk, sits gleaming in the workshop, and when you get him talking you find he did a 9.9 second quarter back in the day at Santa Pod.
Onwards and sideways......
Simply pull one lead off at a time. The misfire will be more pronounced when you remove the lead from a good cylinder and will stay the same with a bad one. Only remove the lead for a brief time if you have cats fitted as unburnt fuel can damage them.
If the fault is not HT related you can do the same with the injectors.
If the fault is not HT related you can do the same with the injectors.
There was a batch of pattern faulty rotor arms around a few years ago that would short out and cause misfiring. With a fault like this, it did not cause a misfire on any specfic plug lead, so you would not see it as plug specific. Also Plug leads should not be grouped together, they should be spaced apart, and in a specific route to prevent crossfiring between the leads.
You can test a rotor fairly simply, they usually fail by jumping to earth through the Bakelite to the distributor shaft so if you remove the cap, pull the coil lead (or king lead) out of the cap and hold it 1/4" away from the center of the rotor with some plastic pliers and have someone crank the starter there shouldn't be any spark present. If it sparks to the rotor then the rotor is faulty.
Update time.
Thanks guys for all the suggestions, all good. That's the problem with a misfire, it could be so many different things.
I tried my spare set of leads, and they did seem to improve matters, but not completely - still an unpleasant chugging when accelerating from 30 in 4th.
Thinking ahead, I called Rob Robertson (V8 Developments) to ask him what injectors he had fitted two years ago, in case I needed to order some. He answered the phone third ring, and asked what the issue was. He then told me what my injectors are, but said he didn't think that was the problem. He knows I live about 30 miles from him, so he suggested I drop in to see him this morning for a diagnosis.
So I drove over there, and he listened to the engine, decided it definitely was still misfiring, checked that i definitely had a good spark, then started testing exhaust manifold temps the old fashioned way (cable tie!) to see which cylinder wasn't firing, on the assumption that I had fuel to all cylinders. Sure enough, there was one not firing, and there was evidence of wetness around the manifold at that point. It fired fine without the extender, no spark with the extender. Extender changed, problem fixed.
I just want to say a big thanks to Rob for his friendly approach - inviting me over, stopping working on the V8 Marcos that was in pieces, testing my car and fixing it, all because I had bought an engine off him two years ago. Top bloke and much appreciated.
The extenders had been changed three weeks ago, and were from a reputable source and a good make, so beware guys - I had two out of eight fail, and in different ways.
I will now go and tidy my plug leads!!
Thanks guys for all the suggestions, all good. That's the problem with a misfire, it could be so many different things.
I tried my spare set of leads, and they did seem to improve matters, but not completely - still an unpleasant chugging when accelerating from 30 in 4th.
Thinking ahead, I called Rob Robertson (V8 Developments) to ask him what injectors he had fitted two years ago, in case I needed to order some. He answered the phone third ring, and asked what the issue was. He then told me what my injectors are, but said he didn't think that was the problem. He knows I live about 30 miles from him, so he suggested I drop in to see him this morning for a diagnosis.
So I drove over there, and he listened to the engine, decided it definitely was still misfiring, checked that i definitely had a good spark, then started testing exhaust manifold temps the old fashioned way (cable tie!) to see which cylinder wasn't firing, on the assumption that I had fuel to all cylinders. Sure enough, there was one not firing, and there was evidence of wetness around the manifold at that point. It fired fine without the extender, no spark with the extender. Extender changed, problem fixed.
I just want to say a big thanks to Rob for his friendly approach - inviting me over, stopping working on the V8 Marcos that was in pieces, testing my car and fixing it, all because I had bought an engine off him two years ago. Top bloke and much appreciated.
The extenders had been changed three weeks ago, and were from a reputable source and a good make, so beware guys - I had two out of eight fail, and in different ways.
I will now go and tidy my plug leads!!
QBee said:
Update time.
Thanks guys for all the suggestions, all good. That's the problem with a misfire, it could be so many different things.
I tried my spare set of leads, and they did seem to improve matters, but not completely - still an unpleasant chugging when accelerating from 30 in 4th.
Thinking ahead, I called Rob Robertson (V8 Developments) to ask him what injectors he had fitted two years ago, in case I needed to order some. He answered the phone third ring, and asked what the issue was. He then told me what my injectors are, but said he didn't think that was the problem. He knows I live about 30 miles from him, so he suggested I drop in to see him this morning for a diagnosis.
So I drove over there, and he listened to the engine, decided it definitely was still misfiring, checked that i definitely had a good spark, then started testing exhaust manifold temps the old fashioned way (cable tie!) to see which cylinder wasn't firing, on the assumption that I had fuel to all cylinders. Sure enough, there was one not firing, and there was evidence of wetness around the manifold at that point. It fired fine without the extender, no spark with the extender. Extender changed, problem fixed.
I just want to say a big thanks to Rob for his friendly approach - inviting me over, stopping working on the V8 Marcos that was in pieces, testing my car and fixing it, all because I had bought an engine off him two years ago. Top bloke and much appreciated.
The extenders had been changed three weeks ago, and were from a reputable source and a good make, so beware guys - I had two out of eight fail, and in different ways.
I will now go and tidy my plug leads!!
Good result!! It's not what you know, it more who you know..............but you knew that LOL!!Thanks guys for all the suggestions, all good. That's the problem with a misfire, it could be so many different things.
I tried my spare set of leads, and they did seem to improve matters, but not completely - still an unpleasant chugging when accelerating from 30 in 4th.
Thinking ahead, I called Rob Robertson (V8 Developments) to ask him what injectors he had fitted two years ago, in case I needed to order some. He answered the phone third ring, and asked what the issue was. He then told me what my injectors are, but said he didn't think that was the problem. He knows I live about 30 miles from him, so he suggested I drop in to see him this morning for a diagnosis.
So I drove over there, and he listened to the engine, decided it definitely was still misfiring, checked that i definitely had a good spark, then started testing exhaust manifold temps the old fashioned way (cable tie!) to see which cylinder wasn't firing, on the assumption that I had fuel to all cylinders. Sure enough, there was one not firing, and there was evidence of wetness around the manifold at that point. It fired fine without the extender, no spark with the extender. Extender changed, problem fixed.
I just want to say a big thanks to Rob for his friendly approach - inviting me over, stopping working on the V8 Marcos that was in pieces, testing my car and fixing it, all because I had bought an engine off him two years ago. Top bloke and much appreciated.
The extenders had been changed three weeks ago, and were from a reputable source and a good make, so beware guys - I had two out of eight fail, and in different ways.
I will now go and tidy my plug leads!!
Trev
It was quicker than getting gadgets out......made me laugh too, but worked. Tried it myself when I got home.....!
I think you aren't the only one who appreciates that an old Sun/Crypton tuner gives you so much more information when dealing with our antique sheds.
Rob had the advantage of being with the car. I am quite sure you would have diagnosed it as quickly (less than 10 minutes).
I think you aren't the only one who appreciates that an old Sun/Crypton tuner gives you so much more information when dealing with our antique sheds.
Rob had the advantage of being with the car. I am quite sure you would have diagnosed it as quickly (less than 10 minutes).
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