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Should a sparkplug discharge be audible? by Twilight Fenrir
Started on: 01-28-2015 06:17 PM
Replies: 4 (145 views)
Last post by: Patrick on 01-28-2015 10:46 PM
Twilight Fenrir
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Report this Post01-28-2015 06:17 PM Click Here to See the Profile for Twilight FenrirSend a Private Message to Twilight FenrirEdit/Delete MessageReply w/QuoteDirect Link to This Post
I've been working on figuring out what's wrong with my 86 2m4 for about a month now... It starts, idles fine, revs fine in park... but once i get going, it stops accelerating around 2K RPM, and won't go any higher no matter how hard I press the pedal.

I've got a whole 'nother thread going into details on this, but research has told me that a weak ignition coil CAN cause this sort of problem. Per service manual testing, the Ignition coil checks out. However, if the coil is internally shorted, it wouldn't necessarily fail the simple, and unspecific conditions the service manual suggests. The coil may still put out enough power to fire the plugs, and run the engine, but it may not have enough power to combust the air fuel mixture under load...

It was suggested to me, that placing a plug on the engine block, and listening for a loud "SNAP" would help identify if my coil was producing sufficient power. If it snaps loudly, it's good. If it's quite, or inaudible, it's bad. So, I used a pair of welding clamps and attached a plug to the block, pulled the fuel pump fuse, and cranked the engine. Can't hear it at all. I moved it over and clamped it onto the strut tower, so I could see/hear it better while cranking, and still no sound.

I'd just like some confirmation that this is a valid indicator. I couldn't find any other means of checking the output of the coil through a few hours of google-ing.

Also... I haven't seen enough spark plugs running to have developed much of an eye for it... but it LOOKS weak to me... for what that's worth.

[This message has been edited by Twilight Fenrir (edited 01-28-2015).]

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Patrick
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Report this Post01-28-2015 06:31 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Twilight Fenrir:

It was suggested to me, that placing a plug on the engine block, and listening for a loud "SNAP" would help identify if my coil was producing sufficient power.


I wouldn't even bother with using a spark plug, but... unless you've re-gapped the plug to about a quarter inch, that test is next to useless. A healthy ignition system will have no trouble producing a spark that will jump a quarter inch gap at atmospheric pressure.

 
quote
Originally posted by Patrick in This thread:

You want something metal sticking out of a spark plug boot that you can hold 1/4" away from an engine ground. I've done it by sticking the end of a screwdriver into the boot, and while holding its (well insulated) handle, rest the shaft of the screwdriver on something metal like an exhaust manifold. While the engine is being cranked, lift the screwdriver shaft 1/4" away from your ground. The spark should make a good loud "snap" as it jumps the 1/4" gap. And make sure your hand isn't too close to the screwdriver shaft, or you'll be cursing me as you involuntarily throw the screwdriver 50 feet into the neighbour's yard.

[This message has been edited by Patrick (edited 01-28-2015).]

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wftb
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Report this Post01-28-2015 10:24 PM Click Here to See the Profile for wftbClick Here to Email wftbSend a Private Message to wftbEdit/Delete MessageReply w/QuoteDirect Link to This Post
sounds like a fuel pump problem to me , maybe that has been flogged to death already and if you have tested the pressure already just ignore me .I had an 85 2m4 that was doing the same thing and I decided to put in a new pump but when I installed it all that was wrong was the hose from the pump to the outlet tube was rotted .It would flow just enough gas to idle and rev up but put it in gear and no power at all .
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Twilight Fenrir
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Report this Post01-28-2015 10:37 PM Click Here to See the Profile for Twilight FenrirSend a Private Message to Twilight FenrirEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by wftb:

sounds like a fuel pump problem to me , maybe that has been flogged to death already and if you have tested the pressure already just ignore me .I had an 85 2m4 that was doing the same thing and I decided to put in a new pump but when I installed it all that was wrong was the hose from the pump to the outlet tube was rotted .It would flow just enough gas to idle and rev up but put it in gear and no power at all .


Yeah, I checked the fuel pressure, it's good. Unless my injector is shot, it's not in my fuel system. Have run the car without an air filter, so it's not incoming air. Exhaust sounds normal, so it's very unlikely an obstruction in there.

The whole mess started when my ICM went south on me. Swapped it out, car ran great for a week, then broke down again. Spent several weeks systematically replacing everything in the distributor until I stripped the screws that kept the cap on, and just replaced the whole thing. I kept losing, and regaining spark throughout the process. Noticed the ignition coil to distributor boot was arcing to one of the smaller wires on the coil, so I replaced all the wires while I was at it.. But I got to thinking, that arcing like that was dumping huge voltages into the coil, which would run through it and get ramped up more, and probably short out a few coils, which might have first destroyed my original ICM.

But I've been searching all day and I can't find a definitive means of testing the actual output of the coil... I did find many references that say a weak output would produce exactly the problem I am having, I.E. Poor/No acceleration under load.

When I did perform the test in the original post... the spark was fairly dim, and yellow-ish, not brilliant and blue. So, maybe that is the definitive answer there... I wish the coil from my '84 would just bolt up so I could swap test it. But they have different connectors.
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Report this Post01-28-2015 10:46 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by wftb:

sounds like a fuel pump problem to me , maybe that has been flogged to death already...


Yes, it has.

 
quote
Originally posted by Twilight Fenrir:

But I've been searching all day and I can't find a definitive means of testing the actual output of the coil...


You don't believe the test I suggested will tell you enough?

[This message has been edited by Patrick (edited 01-28-2015).]

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