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3k miles on motor, low compression.

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gstbuford

15+ Year Contributor
98
0
Jan 10, 2008
Prairievlle, Louisiana
I recently decided to buy a 1g awd from a close friend of mine. He had it rebuilt with all mitsu parts by a mitsu technician around 3 months ago. Since then he has only put about 3000 miles on it. He is in a financial bind, so he decided to sell the car to me. Now the issue. The car starts up and runs good. Drives fine and boosts well (8 psi). I did a compression test and the results were 90 psi across the board. The car has not been run hard. Never launched, rarely boosted. The balance shafts were removed so its kind of rough at idle, but should the car run as good as it does with this compression? Thanks in advance.
 
Silly question but was your compression tester seated correctly? (hand-tightened all the way) and did you do the compression test with the throttle plate fully open.
 
And you had the throttle blocked/held wide open, you cranked it over enough times, and your compression tester is good? If so, I'd say that's just too low for sure.:hmm: 90 psi across the board...:hmm:

If the car runs good though, I'd drive it a while and note any oil consumption and/or smoke and check the compression again later.
 
Well, if you have set up the compression tester correctly and it's reading accurately I would advise doing a wet compression test rather than a static compression test to see if your results change. Let me know..
 
Since the idle is a little rough, I will check the timing tomorrow mourning. Would being a tooth off time cause a low compression reading across the board?
 
I would use a known good/calibrated gauge. I have seen compression gauges read 100psi off. If you are loosing that much comression you would know. There would be lots of oil coming from the pcv vent into the intake pipe and the oil dipstick would pop out and spray oil everywhere.
 
I used two different compression tester. Both different brands. And both would not go over 90 psi. I did a compression test with a cap full of oil in each cylinder and the compression jumped up to 160.
 
I'd drive the car and see if it get's any better.

On a side note though.... bomper04 your avatar just FREAKED ME OUT!:|
I saw your car in your avatar and then started to read your post and then BAM... Your picture just jumped out at me.WTF It was really weird. It's been a long week and I'm going to bed now.LOL

Good luck with the car gstbuford:thumb:
 
Hahah, apologies Gravedigger, I was not aware that I have that effect on people haha jk!
Anyways to clear up your compression test results it appears that the most likely cause would be poor piston ring sealing. A difference from static and wet readings usually indicates sealing issues..but your car as stated earlier runs "fine." In addition, those readings you got (wet), is that for 1 cylinder or pretty much across the board? If it's about the same give or take 5-10PSI on each cylinder, we can pretty much conclude that your mechanical timing is okay, I would still check it just to be certain, however.
 
I wish I could wait it out and see if anything gets better, but I havent bought the car yet due to this issue and he cant really wait on me too much longer, as he needs the money and has another potential buyer. I really want this car as it has a good beginning platform (Jacks Transmission, ACT 2600 and Flywheel, 16g Turbo, 3 inch exhaust) but the compression numbers really out weigh the other mods.
 
If your time frame to check out this car is limited, I would honestly suggest passing this up IMO. But who am I to stop you from making the decision. Without good compression numbers, especially since we saw that 70 some odd PSI increase from the wet test we can conclude that there are some underlying issue(s)
with the engine mechanically. Play it safe, if have a bad feeling about it, just let it go..there will be more along the way. Wish I could help more but given your situation we can't really do much more..
 
If your time frame to check out this car is limited, I would honestly suggest passing this up IMO. But who am I to stop you from making the decision. Without good compression numbers, especially since we saw that 70 some odd PSI increase from the wet test we can conclude that there are some underlying issue(s)
with the engine mechanically. Play it safe, if have a bad feeling about it, just let it go..there will be more along the way. Wish I could help more but given your situation we can't really do much more..

Thanks man I appreciate it, I will check timing tomorrow, but if timing is spot on I will pass and continue my hunt for another one.
 
Not sure if this mattersbut only 3k on the motor? Thats not even completly broken in yet. Doesnt it take around 5k to properly break in a rebuild motor and for everything toseal/seat correctly? Also what kinda oil was used since rebuild?
 
Eh, that's a really debatable topic to be honest. Break-in periods are really...personal? I don't know if that's the word to use but I guess what the point I'm trying to convey is that everyone believes in a different method for properly breaking in an engine. Some say to take it easy for XXXX amount of miles and then dropping the hammer after you pass that magic number then everything is "broken in." While I along with my colleagues believe in the 30-50 method to properly wear-in piston rings..after all they need to conform to the shape and surface of the cylinder wall..and they do this by the combustion pressure during the power stroke when the pressure forces the compression rings to the bottom of the ring groove and help them make full contact with the cylinder walls. WTF
 
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