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?

 

[98BaronMag]

Alright, I got back to it today. Removed everything in the way and removed the valve cover and exhaust manifold. Cranked it to verify full/proper motion. Everything seemed in order. Then, I removed wiring and distributor and pulled the intake manifold and the starboard side head. Head gasket had a large gap between cylinders 4 and 6 - right where I'm having low/no compression. I also noticed a rather large amount of rust chips in the coolant passages of the head and somewhat less in the block. Is there a procedure to clean those passages out? Also, is resurfacing absolutely necessary or potentially avoidable before reinstalling the head and new gasket? Pictures of block, head, head gasket, and coolant passages to follow.

 

[ESGWheel]

How far you have to go depends on how much damage there is. To determine that need to clean the heck out of the head and block surfaces. From there can better determine is the blown head gasket caused any erosion between those two cylinders necessitating resurfacing. It does not look like it did fm pics, but need to clean them up to really tell. Assuming no erosion then need to determine if flat. This requires a machinist straight edge like this link. Lay it across the various matings surfaces like the heads and intake manifold looking for warpage. See this video for how to on heads (link); similar for block. And if all good, can go the extra step to see if any cracks with DYI dye penetrant test.

 

[ESGWheel]

Follow up: you are already probably aware, but stuff all the cylinders and other holes, ports with paper towels / rags to protect getting the scrapings in places you do not want them. And no power or other tools that can remove the base metal.

 

[Bondo]

Ayuh,..... Yer doing both heads, Right,..??