1999 Jeep Wrangler, 4.0L Misfire Code - but found low compressio

Automotive
#1

At my sister’s house in OK. Brother-in-laws 1999 Jeep Wrangler 4.0L engine ~ 230,000 miles had the Check Engine Light come on. Code showed Misfire #1 cylinder.

Spark test for coil bar showed all good. Plugs all looked the same (mine with this same code error type had plug that looked very different from others).

So ran compression tests. Cylinders 2-6 were all within 10<psi of each other, approx 130-138 psi, BUT #1 was about 45 psi, did check several times. One check was to inject oil to see if rings would seal better and raise compression indicating bad rings. No change.

He’s putting it all back together now, my guess is it is valve. When back together, going to remove oil filler cap and see it there is some blow-back, indicating possible rings especially if it pulses. No blow back appears to be on intake so I’m guessing if valve it is exhaust valve.

Anyone experienced this: Misfire code indicating misfire due to low compression in a cylinder where plug is firing? Because the Check Engine Light came on now it seems odd that it would wait until there was 80~90 psi difference. Definitely a problem here.

Thanks

#2

Yes. The “misfire detection” occurs when contribution from a particular cylinder is below threshold.

Here’s someone else’s explanation:

Wet compression test should clear the rings, leaving valves as the culprit. Leakdown is the way to find out. Place on TDC (which should actually match up with the indicators, being #1 cylinder) and pressurize (carefully!) with compressed air. You should hear/feel the leak coming from the exhaust or the intake.
If you don’t have compressed air, it’s a guessing game. really, but at the end of it all, if nothing mechanical can be found (flattened cam lobe leaving the valves closed, never opening, for example), it’s a teardown for accurate estimate.

Good luck on it!

#3

Thanks - I’ll let my BiL know. This is a new technique to me, make perfect sense.

This guy does not drive the Jeep hard. I couldn’t believe it when he replaced the clutch at 155K+ on it. My foot and shifting is a tad harder on it.

#4

Ya just never know. My BiL had a dead hole on an otherwise flawless car @ 80k.
Fixed a burnt exh. valve and it worked another decade + and another 100k+miles.

As always, Eric the Car Guy is more thorough (and more accurate) than I:

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