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Old 11-25-2012, 03:21 PM   #1
Avitar408
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1996 SC1
Default Cylinder misfire leak-down results

This is the 4th engine misfire in the past 3 years , each with a completely different cause. The past causes (all fixed), in order:

- Bad EVAP purge solenoid.
- Burned Exhaust valve on #3 (this one built up over time)
- Bad ICM

See my previous posts for more details on above.

This time, #4 misfire. Compression test shows about 220/240 for dry/wet test on #'s 1-3, and 110/110 for #4.

I used the compression test hose to do a DIY leakdown test (drawback of no gauges of course) and at TDCC for #4, air was hissing into the exhaust manifold at a 20 PSI air input. Air hissed into the exhaust manifold throughout all 4 strokes as I was manually turning the engine over.

The engine misfires mostly when it is strained, particularly when in 4th gear at about 1,800-2,100 RPM and when accelerating in 4th and 5th gears. If I maintain about 2,100 RPMs or higher in 4th and 5th, the SES light stops blinking until I start to coast. I don't get the blinking SES light at idle or in the first 3 gears, but it is a little rougher then I would like at idle.

This happened pretty suddenly, the loss of power and subsequent misfire, and that made me wonder... Could this just be caused by a carbon buildup on the exhaust valve or seat? The car DOES burn about a quart of oil every 300 miles, although when I remember to throw in a quart of ATF after an oil change it goes to a quart every 600-800 or so miles.

Or does this mean either the exhaust valve or seat is trashed? I checked all the valves when I replaced the #3 exhaust valve about 7K miles ago and they were within spec and in really good condition, so I didn't bother replacing them. The seats were all in immaculate condition as well.

Any thoughts?
...
96 SC1 5 speed, 185K Miles
Owned since 2004/ 87K
Previously: 95 SL1 5 speed
Owned from 02-04/ 84K-145K then Totaled :-(
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Old 11-25-2012, 06:46 PM   #2
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1998 SC2
Default Re: Cylinder misfire leak-down results

#4 is the highest temperature cylinder due to the EGR passage. Why just depends on the previous failures. Exhaust leaks may adversely effect the mixture as well.
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