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Another mis-tery

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#1
Steve Scheifler

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See the short video clip linked below.

This is a 1999 former SM now Champ. Apparent misfire is intermittent, sometimes frequent as in this clip and sometimes no issue for an hour or more. Cam & crank sensors, coils and engine harness have all been replaced with new. Tach and AiM data log pretty much say it’s ignition/spark but I gather this is quite different from what multiple VVTs have been doing. Those are described as going completely dead for a second or more then catching again. This is more like actual misfire though it sounds like backfire as well. (There’s a large exhaust leak at the far down-pipe flange so if spark fails perhaps raw fuel enters the pipe, picks up air at the leak then ignites when the engine fires again.) But that’s probably just a distraction from the root cause.

I won’t itemize everything that’s been tried but this has been difficult to reproduce on the dyno and inconclusive on the rare occasions when it does happen. Has anyone dealt with something similar?

https://drive.google...ew?usp=drivesdk
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#2
bmarshall1

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Sounds exactly like my 99 when the crank sensor wire was rubbing on the belt and shorting out (prior to the belt wearing through the wire).  I've have had a bad coil pack and that kept the car from starting.  Wish I could offer more.



#3
Ron Alan

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crazy how many different things can produce a similar symptom. I was getting ready to cut my harness and install the new plug for the cam sensor fix on my 99. But the symptom was just enough different(stumble was at 5000 and a little more sustained). I also noticed my battery(old) seemed to not always hold a charge between sessions? I dont have voltage data...so apparently what was happening was an intermittent/failing voltage regulator on the alternator. Secondly, related or not, the battery read 12.4 volts but only had 95 cranking amps. Replaced both...problem solved. Very limited knowledge on electrical stuff but have been told the ecu may have been causing the problem when it wasn't seeing/getting voltage it liked? 

 

One other symptom...this never happened right out of the box...always 3-5 laps in? Temp triggered?

 

Not much help to you Steve as I'm sure you have looked but maybe someone else down the road with less data like myself. 


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#4
Steve Scheifler

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Thanks Ron and bmarshall, all input encouraged. As it happens the alternator was replaced just before the car was dropped off. If I hadn’t had a few brief but clear occurrences here on the dyno I’d be tempted to put some hope on that. Heck, I’m still tempted! But the battery seems fine and the charging system is functioning. They believe they had a bad coil at one point and replaced with one from Mazda but didn’t keep the old one so I can’t autopsy that. There could be an issue eating coils but the new one shows no sign of running hot and I’m not sure how else I’d know of a problem.
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#5
bmarshall1

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How often do cam sensors go bad? Aren't crank sensors the normal culprit? NVM, I see it has been replaced.

 

What about a loose connector or a bare wire grounding out, what was the last bit of work done to the car? Maybe work backwards.



#6
tylerbrown

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Was the Main Engine Relay replaced? Engine Harness? ECU?


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#7
Steve Scheifler

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How often do cam sensors go bad? Aren't crank sensors the normal culprit? NVM, I see it has been replaced.

What about a loose connector or a bare wire grounding out, what was the last bit of work done to the car? Maybe work backwards.


Most relevant wires are in the new engine harness but did visual and wiggle checks where possible. Nothing significant done prior to it starting but yeah, always start with the last thing touched. Played with crank sensor gap up to .065 trying to make it worse but it didn’t miss a beat at that.
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#8
Steve Scheifler

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Was the Main Engine Relay replaced? Engine Harness? ECU?

Relay yes, with new but didn’t help. Likewise the main engine harness. I have a spare ECU ready to plug in if I can get it acting up again but of course it’s not cooperating. That will be their first move if it starts again on track.

What hasn’t been replaced is the injector harness. We know it’s spark not fuel but I think it also has wires to the coils so that’s the other remaining suspect. I may need to remove and test it separately.
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#9
TylerQuance

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Sometimes the main relay high current contacts suck and will cause even a 10-ga jumper wire with good crimped ends to heat and lose contact. Check for an abnormally hot main relay after extended dyno runs with the relay box closed.

#10
Steve Scheifler

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Sometimes the main relay high current contacts suck and will cause even a 10-ga jumper wire with good crimped ends to heat and lose contact. Check for an abnormally hot main relay after extended dyno runs with the relay box closed.


Yes, that was one of the first things I checked. Contacts all look good, no indication of heat or corrosion. Thermal imaging showed it running surprisingly hot but after cleaning the contacts and tightening up each connector in the socket it was the same. I thought perhaps the fuel pump was drawing too much current but ruled that out. There was a ‘95 in the shop so I checked and its relay runs nearly the same temp. I certainly can’t 100% rule it out because it may sometimes get even hotter on track but it would be difficult to confirm without waiting for the problem to start again then quick jumper it and go back out.
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#11
Dave D.

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I would assume plugs and wires were done first? Any carbon tracking on plugs or coil terminals?



#12
bmarshall1

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Whatever the result is let us know, thx.



#13
Steve Scheifler

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On Saturday the car did five 20 minute sessions without a miss, then about 15 minutes into a 90 minute open session it started missing again. They came in and swapped ECU but the problem continued. Hmmm…
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#14
Steve Scheifler

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I would assume plugs and wires were done first? Any carbon tracking on plugs or coil terminals?


Yes, and no. They had replaced almost everything relevant before bringing it to me. Unfortunately it rarely acts up on the dyno which limits what more I can do.
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#15
Steve Scheifler

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I’ll have them pull the injector harness so we can inspect the wires to the coils more closely, but it may be time for a MegaSquirt or equivalent with full logging capabilities.
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