6g74 Timing Belt Troubles. Low compression on one bank

[macdrew77]

My trusty 1997 SR 3.5L failed California smog last year which opened up a massive can of worms. It was leaking oil from god knows how many places and decided to pull the heads and do a top end rebuild: refaced the valves, lapped them in lightly, vacuum tested good, all new seals and gaskets everywhere. Here's the trouble. I've got the heads back on and while doing a compression test, one bank is much lower than the other. And the part that has me puzzled is the numbers don't really match up with my leakdown test numbers. The leakage on that bank is pretty similar to the opposite bank which has good compression numbers.
Driver's Side:
150psi 77% leakdown
165psi 88%
170psi 93%
Passenger
85psi 95% leakdown
80psi 86%
85psi 83%

Most of the leakage is going through the rings which is to be expected. 160k miles on the shortblock, and been sitting for 6+ months during this god forsaken project. The refreshed heads came from a 2001 Montero 3.5l but as far as my research indicated they share the same part number for just about everything other than cam sensor and driver side timing pulley.
Any ideas on why one bank would be lower than the other? Timing marks don't seem to line up 100% dead on (like half a tooth off maybe) between all 3 pulleys and it's been so long since my last timing belt job I can't remember if that is just the way it is.

 

[MontySquareo]

My trusty 1997 SR 3.5L failed California smog last year which opened up a massive can of worms. It was leaking oil from god knows how many places and decided to pull the heads and do a top end rebuild: refaced the valves, lapped them in lightly, vacuum tested good, all new seals and gaskets everywhere. Here's the trouble. I've got the heads back on and while doing a compression test, one bank is much lower than the other. And the part that has me puzzled is the numbers don't really match up with my leakdown test numbers. The leakage on that bank is pretty similar to the opposite bank which has good compression numbers.
Driver's Side:
150psi 77% leakdown
165psi 88%
170psi 93%
Passenger
85psi 95% leakdown
80psi 86%
85psi 83%

Most of the leakage is going through the rings which is to be expected. 160k miles on the shortblock, and been sitting for 6+ months during this god forsaken project. The refreshed heads came from a 2001 Montero 3.5l but as far as my research indicated they share the same part number for just about everything other than cam sensor and driver side timing pulley.
Any ideas on why one bank would be lower than the other? Timing marks don't seem to line up 100% dead on (like half a tooth off maybe) between all 3 pulleys and it's been so long since my last timing belt job I can't remember if that is just the way it is.

Half a tooth off is pretty normal. If the cam pulley has a mark in a different spot that could cause low compression because the timing would be off. Did you use the valve covers from the 01 or the 97?
Which bank is the different pulley on, and which bank has low compression.
Bank 1 is the passenger side of the engine (you probably already know this)

 

[macdrew77]

I'm using 1997 valve covers. Drivers side requires the pulley to be swapped because of the cam sensor location changing from back of the head to front of the head. That's the bank that has good compression though. I'm gonna compare the1997 cams and pulleys more closely, but they looked identical to the 2001 parts (other than cam sensor) on my initial inspection.

 

[Salonika]

How many times did you repeat your measurements? Assuming you had some struggles getting the hose threaded into the plug hole, or you used an extender…….I remember that my technique for installing and tightening made a difference on my readings. It took several rounds of readings before I became confident that they were even consistent, and that the error I was seeing was actually from the engine and not from my technique.

 

[macdrew77]

Alright just to follow up I found what the problem is. I remember I found a knick in my camshaft and ended up grabbing a replacement from a junkyard 2002 Montero 3.5. Turns out the camshaft is very different from the 1997, its about 90* off and I didn't compare them close enough before I put the engine back together. The parts interchange charts on the Mitsubishi parts websites are completely wrong, they all state that 3.5l camshafts are the same throughout all the years which is not the case. I did find one post that mentioned different specs for cams up to 2001, and cams from 2002 onwards. I'll have to head to the junkyard again and compare my 1997 cam to one from a 2001 Montero 3.5

 

[Salonika]

That won’t change anything since you are supposed to make sure the valves are closed before you run your test anyway…….it wouldn’t matter if your cam was made of squirrel wood. As long as the valves are in the closed position, you can check compression.

 

[macdrew77]

Are you confusing compression test with leakdown? Leakdown test, yes the valves will all be closed. Compression test the engine is spinning over, the intake valves need to open at the correct time to let in the maximum amount of air before closing as the piston compresses it.

 

[macdrew77]

Ok, turns out all gen 3 3.5l passenger side cams are different than the earlier cams. The valve covers are different, and their timing marks are in a different place. Instead of having a different cam pulley, they put the dowel in the cam in a different spot, roughly 90* offset. I was able to find a 2002 Montero Sport that had the identical camshaft to what I need. Hopefully this might save someone else from making the same mistakes I did.