502MAG EFI motor issues
 

I have a 2000 Baja 272 with a 502MAG EFI motor that has low (90psi) compression on 2 cylinders. The cylinders are not next to each other but opposed from each other. The other 6 cylinders have 120psi. What is the normal compression PSI for a 502MAG? Any others seen reading like this? I have about 450 pampered hours on this motor and it has always been serviced well and never run hot. Any ideas on what I might have going on here?

I would do a leak down test to see if it is leaking out the intake valve or the ex. You may be ready for a valve job or a complete freshen.

Was the engine warmed up to operating temp before the comp test????

Even the 120's are low. Ideally, you'd want 140 or so.

Probably just a valve job, springs and head gaskets, but it has enough hours that the rings or bore could be wearing also. A leak down will tell you more.

What altitude are you at? High altitude could account for the low readings, but not the difference between cylinders. Sea level compression would read around 150 on that engine. Do the leak down test to determine where the compression is escaping. It probably just needs a valve job.

are they the 2 cylinders under the exhaust elbow. if so you probably have 2 bad elbows maybe rotting out dumping water down the exhaust and into those 2 cylinders causing rust on the valves and seats. your compression test results should all be within 10% from the highest #. cylinder leak down is the best test over compression. sounds to me that you will be tearing the heads off for a rebuild. exhaust elbows normally will only last about 5 years especially in saltwater use. i would never go any further than 5 years in saltwater use. you could also have to bad exhaust elbow to manifold gaskets leaking internally. very common depending on the type gasket mercury used cause they had a lot of problems with them. you will not always detect water intrusion if it is very little at a time getting in the exhaust.

I have stock exhaust and the 2 cylinders that are low on compression are right below the exhaust elbows on both sides. It's the third cylinder back on both sides that are low. Boat has never been in salt water and the elbows look OK. I took them off and we'll try to rig up something to pressure check them.
Could water get into the cylinders when boat is in reverse/backing up with the captions call exhaust opened up. I have seen several of me buddies back up and shove lots of water down their exhaust tips...understanding that there is baffeling in there to help protect but......

The baffles in the tips are pushed open by exhaust gases when the engines are running. A big slam of water will still cause water to enter the cylinders and will probably stall the engines.