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missing at WOT

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#1
Icedawg

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My Spec Miata, 99, starts to miss at about 6500 rpm, in any gear, at WOT.  Not all the time, but often enough.
It may be heat related, OK early in a run, then developing as the engine bay gets hotter, but it is not always that clear, and it does not always happen.

I replaced the coil pack with another used one, and it stopped for two weekends,  but now it is back.
The engine is freshly rebuilt.  The fuel filter was changed at the start of last season.

What are the other possible culprits, are there some well known ones?



#2
Ron Alan

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Search this site for Cam angle sensor harness plug fix! Classic symptoms!


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#3
Tim Wright

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Search this site for Cam angle sensor harness plug fix! Classic symptoms!

Fun story, I spent an entire weekend chasing a random miss i with a new motor. Missed only in 4th??

After changing everything, turned out to be an injector wire wasn't clicked in and would only shake loose at higher under hood pressures.

Bummer weekend.



#4
Martinracing98

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In Spec Racer Ford someone was experimenting with how thin of oil they could run. With thin oil and high RPM the lifter was not capable of lifting the valve correctly. Since the valve did not open well, they would not get full charge of air and the car would have a "miss". It would be worth confirming you have good oil pressure when the engine is hot 



#5
chris haldeman

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1999 Miata is a solid lifter engine. Focus on cam sensor and or crank sensor air gap. Classic Cam sensor failure is heat related as magnet looses power with heat and is typically around 6200 rpm
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#6
Ron Alan

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1999 Miata is a solid lifter engine. Focus on cam sensor and or crank sensor air gap. Classic Cam sensor failure is heat related as magnet looses power with heat and is typically around 6200 rpm

I thought it has to do with harness plug to the cam sensor craping out? Or do we see both?

 

I know the original cam sensor that came with my car is still in it...because the 1st time we started having the high rpm miss at the 25hr one of my volunteers(EE major)pulled out his handy little propane pocket solder iron and we soldered the wires on the harness directly to the cam sensor and wrapped it up! Been like that for 3 years :) Cam Sensor is part of my harness now! Been in 3 different motors with no issues! Maybe I've been lucky it still works?


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#7
chris haldeman

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My solid lifter comment was aimed at Martinracing post suggesting a lifter might have something to do with it.
Problem is 99.9% cam sensor connection
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#8
Martinracing98

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1999 Miata is a solid lifter engine. Focus on cam sensor and or crank sensor air gap. Classic Cam sensor failure is heat related as magnet looses power with heat and is typically around 6200 rpm

Learn something new every day



#9
Icedawg

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Well, thought I would complete the discussion.

Replaced coils, not so bad but still there.

Replaced cam angle sensor with the fancy pig-tailed version referenced above, improved, but once in a while a hiccup.

Replaced spark plugs, problem solved.

 

Sadly, then had to replace engine, due to thrust bearing failure.

Went with new coils and cam angle sensor, but used old dirty plugs during break in.

Once in a while a miss around 6500.

Replaced plugs, seems to be gone.

At least the new engine pulls nicely.


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