compression issues

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I've got a 89 s-15 with a 2.5 4cyl. Had it for a couple years, It always lopes while idling and lacks power, in the fall it started back firing thru the throttle body under certain cruising conditions. I've ran codes and replaced a few sensors along the way(O2, egr solenoid, ignition parts) nothing seems to really help. I checked the compression and found #2 cyl was about 5.5:1 while the rest were 8.5:1 (figured this may be the root of the problem). The truck doesn't consume any oil or coolant. I quit driving it for now and decided to pull the head and see whats up. All I see so far is more carbon in the #2 and #3 cylinders(egr related?). Poured oil on top of #2 and compared to #3 to see if rings leak past, and #2 holding as good as #3?. The head gasket ripped apart on removal but I couldn't really see evidence of blowout any where around #2. That's where I'm at right now. Any advice or ideas?

thanks, jwxr7

posted by  jwxr7

How did you manage to work out your static compression ratios were different? Your test should have resulted in a pressure figure. Pouring oil on the rings to see if gravity feed will show a leakdown is wishing a bit much.

Backfiring is generally ignition or valve timing and both will give you loss of power and a lope. That's not to say it isn't something else.

posted by  Wally

Oops, I forgot to adjust for the atmospheric pressure when I did my estimated conversion. It should have read 4.5:1 for #2 and 7.5:1 for the rest. 4.5:1 = 4.5 atmospheres, 1 atmosphere= 14.696 psi. So 80psi minus atmospheric pressure,divided by14.696 = 4.44:1. I don't even know if you can convert like that, but every motor that I've seen specs for worked out like that. My motor is rated at 8.5:1 and 140 psi when checking compression by the book (this works with my conversion).
Either way my readings in psi were; #2 was about 80 psi and the others were around 125 psi without adding oil. After adding oil and retesting they all went up a bit (#2 to 90 psi others to 130-140 psi) so I know the rings are worn. But #2 must have other causes for leakage too (a bad valve)?.
I've already exhausted myself working on the ignition (timing, plugs wires, ig module, distributor etc), The cam lobe lift was within specs but I don't know about the valve timing.
I figured that if I fix the compression issue I could possibly eliminate some of my problems. My so called leakdown was nothing more than guessing at this point, that's why I'm on here asking questions. I'm no expert on internal engine problems like this. My main goal is to get the compression balanced out, I don't want to do a full rebuild really.

posted by  jwxr7

backfire thru intake? i woulda been leaning toward a flat cam, but you say the lift is fine?

posted by  BanffAutoSpa_ap

Yeah, lift looked fine. I'm thinking I may have a messed up valve. If the intake valve on #2 leaked or stuck it could cause a backfire right?

posted by  jwxr7

yeah if it's stuck and your piston tries to build compressionit will blow that through the intake

posted by  BanffAutoSpa_ap

I finally got a valve spring compressor and pulled the intake valve from cyl#2. I found two areas on the seat area of the valve that are eroded away and evidence of blow by up into the intake port. The exhaust valve looks fine and the head looks okay. I'm hoping I can just lap a new intake valve in and put it back together. I will check the other valves too.

posted by  jwxr7

I figured I should write a follow up. The head was cracked thru the intake valve seat. That's why the valve was messed up where it seats in those areas. I ended up replacing the head with a remanufactured one. I could tell that the reman unit had been fixed in the same place where mine failed. All the cylinder pressures evened out with the remanufactured head.

posted by  jwxr7

Thanks for the follow up! It makes a pleasant change from those who disappear never to be seen again! :laughing:

I´ll move this to the R&M section...I know it´s done with now, but it just belongs there, lol...Blame the OCD :laughing:

posted by  Cliffy

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