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Author Topic: Misfire Diagnosis  (Read 3088 times)

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Alex W

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Misfire Diagnosis
« on: 18 July 2011, 19:30:16 »
I need some help in the best procedure to further diagnose my misfiring V6.

I don't know if its the plugs, leads or Dis/Coil pack.

I have not been able to remove the coil pack but a limited torchlight inspection did not reveal any splitting like that shown in the maintenance guide photos.

I can't afford to just go replacing parts at random until I stumble upon the solution (I've already replaced the crank sensor, the breathers have been cleaned and a split breather pipe resolved).

Fault codes (19 & 21)have not cleared despite 20+ starts.

Can anyone give me some guidance as to the best way to proceed.?
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henryd

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Re: Misfire Diagnosis
« Reply #1 on: 18 July 2011, 19:36:33 »
have you lifted the plug leads from the plugs to check if there is any oil in the plug recesses?
« Last Edit: 18 July 2011, 19:37:52 by henryd123 »
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Alex W

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Re: Misfire Diagnosis
« Reply #2 on: 18 July 2011, 19:45:41 »
Cam cover gaskets are OK
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Re: Misfire Diagnosis
« Reply #3 on: 18 July 2011, 19:49:13 »
19 is crank sensor...it can cause misfiring when failing...if you have a code 19 you almost certainly have a failing crankshaft sensor...

Is the other code definitely 21?? Or 12 i.e. Start of sequence??
« Last Edit: 18 July 2011, 19:49:56 by ShieldOfSteel »
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Re: Misfire Diagnosis
« Reply #4 on: 18 July 2011, 19:54:25 »
Take the crank sensor back, tell them it is apparently faulty and you want another one. If its still showing 19 then imo the new sensor is a dud.Wont be the first time its happened. :y
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Alex W

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Re: Misfire Diagnosis
« Reply #5 on: 18 July 2011, 20:12:54 »
The Code 21 was caused by my failure to plug in the electrical connector to the throttle housing when I put the plenum back on ( I replaced the HBV as well). That was over 20 starts ago.
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TheBoy

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Re: Misfire Diagnosis
« Reply #6 on: 18 July 2011, 20:15:13 »
OK, we need to clear the codes manually, and see what comes back. I was desperately trying to save you the journey...
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Alex W

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Re: Misfire Diagnosis
« Reply #7 on: 18 July 2011, 21:05:01 »
Thanks Jaime, when would be convenient?
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Alex W

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Re: Misfire Diagnosis
« Reply #8 on: 19 July 2011, 19:28:55 »
I now suspect that as my fault codes still refuse to clear, despite many more than 20 starts, the code 19 that caused me to just replace my crank sensor may well have been the same fault code, still in the car from last year, from when the crank sensor really failed.

I have read some other forum posts about misfires and it seems that a misfiring engine would not necessarily generate an engine fault code at all, at least not in a car as old as mine (2000 W).

Can anyone confirm that?
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TheBoy

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Re: Misfire Diagnosis
« Reply #9 on: 19 July 2011, 19:31:34 »
Correct - misfires on the older engines are not detected, so HT issues often result in misfires, which will not flag codes.


Does it hunt at idle?
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Re: Misfire Diagnosis
« Reply #10 on: 19 July 2011, 20:03:03 »
I was getting 19 when I had a misfire , totally baffled me and resulted in a barny at the factors over the pattern c/sensor and the purchase of a completely unnescessary new sensor from Vaux, culprit turned out to be the DIS which looked perfect when removed.

I think the pattern DIS work ok unlike the pattern crank sensors.
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Re: Misfire Diagnosis
« Reply #11 on: 19 July 2011, 20:24:07 »
Quote
Correct - misfires on the older engines are not detected, so HT issues often result in misfires, which will not flag codes.


Does it hunt at idle?

I'm not sure what "hunt at idle" means,  but the revs are not steady at idle, constantly going up and down, you can feel the car vibrate from the driver seat. Sometimes the revs drop down and you think its going to stop but then it picks up again.
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Re: Misfire Diagnosis
« Reply #12 on: 19 July 2011, 20:25:47 »
Quote
Quote
Correct - misfires on the older engines are not detected, so HT issues often result in misfires, which will not flag codes.


Does it hunt at idle?

I'm not sure what "hunt at idle" means,  but the revs are not steady at idle, constantly going up and down, you can feel the car vibrate from the driver seat. Sometimes the revs drop down and you think its going to stop but then it picks up again.
Have you checked for vac leaks :question
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Alex W

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Re: Misfire Diagnosis
« Reply #13 on: 19 July 2011, 20:32:59 »
Quote
Quote
Quote
Correct - misfires on the older engines are not detected, so HT issues often result in misfires, which will not flag codes.


Does it hunt at idle?

I'm not sure what "hunt at idle" means,  but the revs are not steady at idle, constantly going up and down, you can feel the car vibrate from the driver seat. Sometimes the revs drop down and you think its going to stop but then it picks up again.
Have you checked for vac leaks :question
Yep, fixed a split plenum breather pipe, no change. Changed the spark plugs today (they were due anyway) no change.

Looks like it must be the DIS Pack or plug leads.

Is there any way of determining which it is without costly trial and error? I haven't had the dis pack out because of accessibility but a torchlight inspection revealed no obvious splits like in the guide photos.
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Re: Misfire Diagnosis
« Reply #14 on: 19 July 2011, 20:39:27 »
You can try getting hold of some known good leads and swapping them in pairs( two longest ones are easiest to do this with) but if the Dis is goosed it will be affecting prob 2 cylinders. If your delving deep you may as well change the DIS, you really dont want to be going in there twice :o 
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