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low compression in 2&4

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haddixjl

15+ Year Contributor
77
0
Jul 27, 2006
orangevale, California
I'll start from the beginning I got a 91 that I was told had low compression. So I checked the timing belt and it was off a tooth each cam. I pulled the head and got a different one rebuilt from a board member. Put the car back together and same deal low compression. The short block was rebuilt before I got it by sloboy racing with Manley pistons so I figured the previous owner bent the valves when he started it and the rings didn't seat in. I took the engine too the machine shop and he measured up everything re hones the cylinders and I put it back together with file fit right gs from Manley. Now I have put a few hundred miles on the car and just checked compression again and 1&3 are 150 and 2 is about 75 and 4 is 50 or so. So is there something else that can cause low compression to occur on the same cylinders? Maybe egr leak or intake or something as easy as that? I can't imagine that the rings just wouldn't seat again.

Cliff notes
Engine rebuilt head rebuilt and checked out again by diff machine shop all checked out good I re ringed the motor and still bad compression in same two holes
1=150
2=75
3=150
4=50

Thanks for any ideas on what to check :ohdamn:
 
Yes it was re honed and completely checked out by a machine shop. I bought the new rings from Manley and file fit them
 
Hmmm... this is a perplexing one.
Do a leak down test on each cylinder to see where the air is escaping to.
Cylinder one and three are good but test them anyways to see if they might give you a hint on why the other two cylinders failed
 
any ideas on which one would be a good one to get? they are generic for use?
 
The ring gaps were set per manleys spec and they are their rings.
I just did a leak down and got 4-5% on number 4 and 10% on number 2 but it was a pain to get a reading out of 2 it kept trying to turn over with the pressure hooked up. I checked the compression again and 4 was closer to 100 and 2 was about 75. Any way it needs more run time to break in?
 
It was warm not to the point of cycling the fans. It only has 170 miles on it so far. I used Joe Gibbs break in oil in it. I had planned on changing it at 300 miles. So still on the first oil change so far. I plan to drive it to work this week and get some more miles on it and see what happens.
 
Well you might be SOL bro, your rings should seal up perfect withiny the first 20 miles if they don't before that they probably never will! But who knows you may get lucky and just need more drive time on it! What breakin method are you using have you have it any boost yet? Have you talked to your builder yet?
 
I put it together. I just fired it up and ran it at 2k rpms for 20 mins. It has seen boost at this point. It is just odd its the same two holes as before
 
2 holes? What holes?

Why did you break it in that way. You should make sure everything is ready for a drive and fire it up and drive it right away. Sitting in the driveway free revving the motor is the worst thing you can do! And if it has seen boost how much? Got a log file or anything we can look at?
 
Last edited:
2&4 I broke it in that way because that what I was used to at least with other motors. I know usually its for the cam break in but these were stock cams. It is just odd that it would be the same two that it was before.
 
I was a tooth off when I did my timing belt for the first time on each cam and after
Fixing it I still have great compression. Sounds like improper honing or maybe ring install.
 
If I'm reading correctly, you've got poor compression test results but the leakdown numbers are ok?

This is a tough one, makes no sense. Decent leakdown results indicates that the rings and valves are sealed. This, combined with the low compression numbers leads me to believe that it's a timing issue. BUT if it were a timing issue then all four cylinders should show low compression numbers. You may want to redo your tests.
 
If I'm reading correctly, you've got poor compression test results but the leakdown numbers are ok?

This is a tough one, makes no sense. Decent leakdown results indicates that the rings and valves are sealed. This, combined with the low compression numbers leads me to believe that it's a timing issue. BUT if it were a timing issue then all four cylinders should show low compression numbers. You may want to redo your tests.


This^
 
Is there anything inherant with 2&4 in the tming? the wrong sprockets? could it be wrong camshafts or something like that? I have a spare set from a different motor that I could try. The car idles sorts funny almost sounds like a subaru boxer engine at idle but when its crusing up past 3000 it runs smooth like an inline engine.

how much would a motor with bad rings leakdown?
I will try to rerun it tonight when I get home. Is there a vfaq for running the leakdown tests?
 
The no-load free revving at 2k rpms sounds like one hell of a bad idea. No offense, of course. Perhaps it worked for you before. Like stated above, the rings seat very very fast. Let it warm up then either 1: change oil immediately, refill, and take it for a good "spirited" drive or 2: Let it warm and just take off when its up to ~150* or more.
The motoman method is the most widely used on this site. Most do that. Have you looked in to it?

Still, even if the break in method wasnt up to snuff, why only 2 and 4??? Im with ya, i dont get it bro. Sorry for it happening though. Im in a similar spot.
 
these cars make them hard to love, I hate getting beat by a car. What is wierd about it is that even after a fresh hone and new rings its the same 2 holes. Thats why I was wondering about the EGR valve or any other things that might cause this. I almost wonder if it could be the cams. I will check timing again but if it was timing I would think it would run bad or choppy through the entire rpm range.
 
I would think if those two were just to far out of spec than the leakdown test would have been alot worse? Ill check the timing again and see what I can come up with. Is it possible that some other camshaft like from an NA motor or something could be different and causing the problem?
The motor is already .020 over and the rings were all able to be filed to fit
 
What are the chances of the camshaft lobes being ground wrong on cylinders 2 and 4? I mean it sounds weird but if timing is spot on and leak down is fine... I can't imagine anything that could cause low compression at that point.

I usually only do a leak down test if my compression numbers are bad and always the leak down test shows the reason why... never has it shown me less than 10% loss after bad compression numbers. It's the reason we do it.

Other than that all I can think of is bad thread seal on 2+4 with your compression tester.
Have you put boost on it? At 75psi on two cylinders it should be running like dogshit. If it pulls fine I'd try a different compression tester.
 
the same compression tester gets 150 on 1&3. I have put boost on it at this point full throttle pulls from 2nd through 4th and it feels good, but I havent been in one of these cars for years and my point of refernece is skewed as my actual race car is a 9.0 second car and my daily driver is a corolla so kind of opposite. I have another set of cams I can compare with and I will check the timing again this weekend.

Is 11% leakdown excessive? On the Vfaq it seemed to be reasonable
on cyl 4 I only got 4% which is great

on boost it doesnt stumble or doing anything wierd but I am currently running a cat and the stock air filter and housing so its pretty restricted at the moment.
I will probably order a wideband also and install it this weekend to also be able to see how it is running.
 
11% Is not excessive. I was being a tad facetious.
The idea behind the cams is really far out there. I still think you need to run a leak down test again. Something is not right. A good leak down test can't coincide with a bad compression test. It just doesn't make sense.
 
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