Just wanted to share this with everybody..

I know about the valve/seat "gunk" could be a problem, but just wanted to know if anyone as ever seen a problem like this on the LNF in the past?

Yesterday, out of the blue - I had some trouble with misfiring, it would misfire once in awhile, and it only happens when driving around 20-35 mph - seems odd - but it's not constantly? Checked for codes found the MAF sensor was faulty but no misfire codes? So I pulled the plugs [AC/Delco] and each one was brown and the porcelain is white/tan. So I ordered new plugs and MAF sensor to see if this could resolve it [not to turn it into parts monster], but has anyone seen anything like this before? I checked the gaps on all of them, all are at 30..

Also, since the spark plugs were out, I figured why not do a compression test, found Cyl#1 @ 114 and the rest Cyl#2 to 4 were at 125.. [yes, each cyl is at TDC] By the book it should be at 120 to 128, without the 20% rule.. Has anyone found any issues with the piston rings or the head gasket in the past with similar compression test results on the LNF?

I ask because - I have found on a few Ecotec's I've worked on already, the rings have been "eaten" from either hot spots, worn ring or landings with cyl wall scarring or ECM running the engine in a rich condition b/c of a faulty MAF sensor - even though the test results shows MAF sensor works, but the return signal voltage was off by .06mV compared to brand new MAF sensor.. and only once, I found the head gasket to be "eaten" too from a hot spot which caused low compression reading.. odd stuff..

 

Well.....

I broke ringlands on two pistons. My compression test was like this.

146, 126, 138, 30
30 is obvious. The 126 that I bolded because it's in your range that you listed also had a broken ringland. The piece hadn't come out yet. When the piston was removed it was found to just be floating there between the rings.

 

Those numbers seem very low to me. I have 175 psi across the board. hot compression

 

It sounds as if you have more than valve gunk. Mid-120's across the board?! Sounds extremely low if you ask me.

 

damn, i need to do a compression test to.

most people seem to have between 160-180psi. it depends on model tester you use.

 

Yeah, I would say average would be 150-160 (i.e., pretty healthy). As long as you don't have [more than] around 10% psi variance between cylinders, you're pretty straight.

 

Yeah, 150-160 was what all of mine read on my last compression test and I'm just a few hundred feet above sea level.

 

Someone should make a Cobalt SS/TC compression test how to video so we can point people there regularly haha.

That said, I need to do one, if I remember correctly a year ago I was at 155, 155, 155, 155 and I'm 6000ft above sea level

 

yep, I check again this afternoon, #1 @ 116 and the rest of the cyl's at 128.. hopefully next week I'll get to drop & strip the block clean and get a few nice goodies, since it's going to be out of the car..

i'm at 300 feet above sea level..

 

160-165 on my built motor

 

runaway detonation kills the ring lands and breaks rings. do a leak down test way better but u will hear the compression in the base . piston rings and lands are fubard. life sucks sometimes I would say.

 

i will do the leak down tomorrow, and yes i'm ****'d either way I look at it..

but atleast i have a few options now..

 

floating pieces - nice, can't wait to see what mine looks like .. mines probably in a million pieces..

thank god - i have a good sense of humor to all of this...

 

I used the snap-on comp tester.. the GM comp tester in the shop isn't accurate [it's a little beat up]..

 

After my rebuild which was just a .020 over bore and forged pistons I was 165-169 across the board.