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Low compression and misfire.


Byronator

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So my engine was idling rough one day and the check engine light was on. It said there was a cylinder six misfire. I investigated my misfire and it’s not a spark issue. I tested the compression and as far as I know it’s low. Idek what the compression should be though? It was like 70 something I think. And cylinder six wasn’t the only one that was low lol. Anyways, I don’t really know what I should do to fix my misfire. I imagine my piston rings need replacing in order to fix the compression. I think the next thing I will check is my fuel injector. That could be causing the misfire right? So I’m looking for input as to what I should try to fix before I dive into replacing something that doesn’t really need to be replaced.
 


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Yes 70 is low and the reason for the misfire. Compression should be 150psi+

You need to find why the compression is low. There can be several reasons.

First, run the compression test again but this time add a tablespoon full of oil into the cylinder. If the compression rises then its worn rings. If it doesn't go up it's not the rings.

Next do "the glove test". @RonD has a great rundown on how to do it but in a nutshell, with the engine cold take the radiator cap off and put a rubber glove over the opening sealed up tight. Start the engine and watch the glove. If the glove dances around you have a blown head gasket causing the compression leak. You also wanna check the oil for signs of coolant (it will look like a milkshake).

If the glove doesn't move and the oil didn't help, chances are it's a bad valve. Cracked, burnt, bad seat etc so pulling the head would be the next step. If I recall the 3.0's are known to burn the #6 exhaust valve for some reason.
 
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Byronator

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Thank you. I’ll run those tests sometime. The oil wasn’t milky so my bet is that it’s the valve.
 

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Hey guys. Turns out it wasn’t the valves. I replaced the heads and I still have a misfire. Thoughts?
 

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And what are the compression numbers now?
 

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And what are the compression numbers now?
So the number of 70ish I gave earlier was wrong. I watched/helped a buddy of mine do the compression test. I had never done one before so I didn’t know any better. We did the test wrong, only on one or two cylinders with the spark plugs still in the other cylinders. I did the compression test again later a few day’s before I replaced the heads, with all the spark plugs removed and the throttle fully open. I got values from roughly 150 to 165ish. I don’t own a compression testing system so I haven’t had the chance to do one with the new heads installed yet
 

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Just today my check engine light came back on. I used my little code reader and it says o2 sensor and EGR “A” flow insufficient detected.
 

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Check out the rear of the affected O2 sensor - I had oil leach into mine and foul the porous backing enough to kill it a couple years ago(oil pressure switch blew and leaked all over, a little while later the sensor died as the remainder of what i couldn't reach flowed down onto it). Unscrew the EGR valve from where it sits by cyl 6 and examine for gunky crap inside. Give it a light shake, it should rattle around freely.

Since you replaced the heads - did you re-use the OEM exhaust manifolds or buy replacements? Frequently the 3rd party replacements don't have the right curve machined onto the manifold-pipe interface, and every time you hit a bump or something else in the road it will cause a momentary O2 leak - after about 30-40 cumulative miles of this happening the computer will register an O2 sensor-related code that *looks* like your sensor is done for or that you have some kind of compression problem/misfire going on.

I had to replace my head gaskets during summer and got a new 4/5/6 exhaust manifold(original one's stud popped before I bought it, so the prev. owner just drilled out a new hole rather than spending $10 on some PB Blaster and a bolt remover) and I started getting weird O2 sensor codes soon afterwards that all indicate, on the surface, that I was misfiring or my O2 sensor was toast. I spent a good amount of time on smooth roads trying to figure out why my O2 sensor graph was suddenly crashing down with no success, but when I had to take a detour down a crappy, bumpy road I was able to notice the relationship since the bumps were noticeable, and start looking in the right direction for information.

If you post the actual codes encountered it will be very helpful in helping to diagnose.
 

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