Engine misfire when warm

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13 Jul 2011 07:10 #5532 by Sharkhead
I have an issue with my 85 928 S... She starts, idles and runs great till I get some heat in the motor. It then beings to miss and backfire with rpm's on 2000.
This only occurs one I have been drive for about 15 minutes or so.
Even hot, the car idles fine but cant rev above 2000 rpm’s or so without shaking, cutting out and backfire.
I have replaced the battery, did a visual inspection of engine bay, checked spark plug wires in dark for aching, found nothing. I feel it may be a intake manifold issue but not sure how to diagnose that.
Any suggestions or feedback work be greatly appreciated.

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13 Jul 2011 08:02 #5533 by srshaw3
Replied by srshaw3 on topic Engine misfire when warm
It could be a vacuum leak, which is common.

Perhaps look through these tips, in particular the vacuum leak ones near the bottom: nichols.nu/cat03.htm

Also, it could be something simple like bad gas.
Are the cap, rotor & plugs recent?
How about the ignition wires?

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13 Jul 2011 10:07 #5540 by 928mac
Replied by 928mac on topic Engine misfire when warm
I am not convinced that it is a lean mixture (Intake leak)

When a module or even a coil gets hot it can start to fail.

I coil will miss fire or just quit.
A module will do the above and anything else it feels like doing, including firing at the wrong time.
I do not have a lot of experience with 928 computers and you need a scan tool or scope to see what is happening.

Do you know anyone in your area with a sharktuner or a scope?

Brad

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13 Jul 2011 10:13 #5542 by Sharkhead
Replied by Sharkhead on topic Engine misfire when warm
Thanks for the reply.

I will review the document and check vacuum lines.
Fuel was recent... Yes, newer cap, rotor, plugs and wires... maybe (2yrs) with under 300 miles since replaced. And again, only happens with warmed to normal operation temperatures.

thanks again

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13 Jul 2011 10:16 #5543 by 928mac
Replied by 928mac on topic Engine misfire when warm
See if you can find anyone close to your area on here

forums.928oc.org/vbulletin/forumdisplay.php?f=15

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13 Jul 2011 10:19 #5545 by srshaw3
Replied by srshaw3 on topic Engine misfire when warm
There is a part called a temp II sensor on the intake, perhaps it has malfunctioned.... www.electronikrepair.com/page9.html

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21 Jul 2011 21:44 #5674 by Kiln_Red
Replied by Kiln_Red on topic Engine misfire when warm
I am thinking along the same lines as Brad on this issue. Although you say you've had a recent ignition tune-up, this sounds like a classic case of a troublesome ignition. Usually, when the temp sensor for the fuel brain fails it will cause poor, hard starting as a result of a overly rich fuel mixture. This is caused because the resistor fails thus sending an incorrect input back to the brain indicating it needs additional fuel in volumes necessary for sub 0 temps. Once at temperature, the car resumes normal operation.

Try replacing both coils. They're inexpensive, and if nothing else, at least you'll have two new coils and you will have eliminated one likely culprit.

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21 Jul 2011 22:00 #5677 by 928mac
Replied by 928mac on topic Engine misfire when warm
The only other way is if you have a shop close by that has a scope.
When a scope it hooked up to the ignition it shows what is happening when the spark fires. A good mechanic can tell ignition issues or lean fuel from a vacuum leak. should not be that much to hook a scope up to our old ignition systems as it is not a computer diagnostic.

Good luck and let us know what you decide or find.

Brad

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