1998 Mercruiser 5.7 TBI Backfiring Under Load & Loss of Power

[nola mike]

I really want to see compression numbers. Rough idle in a 5.7 makes me think you're misfiring or dropping cylinders. At idle I'd also putting a timing light on each plug lead to make sure they're all firing consistently

 

[98BaronMag]

Well folks, I think we've identified a major problem here. Did a compression test today and the results are as follows. Cylinder numbers are basic chevy 350 pattern.
Cyl 1 200psi
Cyl 3 185psi
Cyl 5 190psi
Cyl 7 195 psi
Cyl 2 198 psi
Cyl 4 35 psi
Cyl 6 0 psi
Cyl 8 200 psi
Seems like we're looking at at least a partial teardown, but I'd like some ideas as far as common causes of these types of issues and what I'm looking at as far as tear down/repair...or if the situation seems so severe repair isn't really feasible. Thanks
-Marcus

 

[ESGWheel]

Looks like a blown head gasket, perhaps from an overheat in the past…

You are looking at pulling the heads at a minimum and if you have led a good life you are only doing head gasket replacement (needs to be marine). Pull the heads and post lots of pics of your findings…

 

[98BaronMag]

Sounds good. I plan to drain the oil when I get back to it and check for water. Then I'll start engine disassembly. Might be a week or so before I can get back to the project, but I'll keep you all updated once I get there.

 

[nola mike]

Sounds good. I plan to drain the oil when I get back to it and check for water. Then I'll start engine disassembly. Might be a week or so before I can get back to the project, but I'll keep you all updated once I get there.

Might want to blow some air in the cylinders to see where your leak is occurring, although the head is coming off regardless. Did you pull the valve covers to make sure all the valves are functioning in those cylinders?