low compression on cylinder 2, not providing power.

[goliath]

i just spent all day tearing into the truck and trying to figure out this problem.

my question is this...

I have about 150 psi(+-5psi) compression in all cylinders except cylinder 2 which is 130psi. my problem is cylinder 2 is missing, not contributing to the power of the engine. I disconnect the plug wire and the idle stays the same. engine has a real pronounced miss at idle, obviously not running on cylinder 2. I replaced coil, wires, plugs, verified spark, replaced the injector, and verified voltage to the injector. I still have no go on cylinder 2. I unplug the injector or plug wire and it doesnt affect the miss or idle so that cylinder still isnt working?!?

Im running out of things to change. Any ideas, or is my engine toast due to the low compression and i am in denial about it?

thanks
-joshua

 

[jimbo74]

i would further look into the coil pack or the ign module.... if nothing is happening if it is disconnected that means nothing is happening to it at all..... i think the 20 pound pressure difference is tolerable-not sure off the top of my head though as to whats allowed... however, it does seem odd about that cylinder....

 

[Glacier991]

Curious. If you have accurately verified spark, that's ruled out. You have replaced the injector, so we know THAT isn't bad. FUEL?AIR?SPARK?...hmmm fuel rail clog? Beats me, the best test now to ferret out the pressure issue would be a cylinder leak down test. (Air in introduced into the cylinder near TDC and the loss rate is measured. Can often pinpoint valves/rings etc.) 130lbs should not result in no ignition - may be a clue somehow, but that isn't it, IMHO. Rule 1: Suspect mechanical last.

When you pull the plug what does it look like? dry? wet? I'm still thinking we lack the fuel part of the equation somehow. This is NOT my area of expertise though.

 

[goliath]

thanks for the replies guys, i think the head is toast. my best guess is burnt exhaust valve on cyl 2.

I took out the fuel rail and all injectors, cleaned them and moved them around(keeping track of the suspect one). checked the fuel rail for clogs, and reassembled. the problem is still on cyl 2 even though that injector now sits on cyl 6. I can feel the injector pulsing by touch, so the electronics are working. i pull the wire off the coil and can see a big arc when i hold it near the coil, so there is juice there, and the coil is new also. the plug looks like its firing, no abnormal buildup on it, and its not wet. Air, fuel, spark, the basics are covered.

The miss is a rythmic extra powerful exhaust pulse from the exhaust when idling and the engine shakes like it has a big cam. I dont think the exhaust valve is sealing any more, letting part of the combustion race down the exhaust. this is a progressive problem, it has gotten worse over the last 10k miles.

Short of some miracle sensor problem, this is sounding like mechanical to me.

The low compression has me bothered also. I redid the compression test more accurately by following the directions and on the first stroke to TDC cyl 3 is 148, cyl 1 is 149, and cyl 2 is 120. it seems awfully low to me.

thanks
-joshua

 

[Robb]

I would think that your problem lies in the low compression, not in the fuel or spark to the cylinder. If you aren't creating consistent compression in that cylinder, then it isn't going to create combustion, or the combustion is going to be hit or miss. Even if that cylinder recieved perfect fuel mixture and spark, if it has low compression..............it has low compression.

I would look into a leak down test. Sounds to me like the valve seals are going bad, or a valve is sticking.

edit: doh! I was a little late in my reply there

 

[goliath]

thanks everyone, looking for heads now.

-joshua