5.0 GM compression problem update

[littlebookworm]

I'm getting a 1983 5.0/305 GM ready to install in my 1986 SunRunner. Compression in 7 cylinders 145-150. #6 showed 120 even with oil sprayed in. Leakdown showed air loss through intake. I suspected a bad valve or lifter. Pulled the intake and lifter; lifter OK. Pulled head. Someone rebuilt this engine. There was red goop all over the outside of the head and intake. More importantly, everything inside is spotless. There are no ridges at the top of the cylinders. The valves are clean and perfect. I couldn't figure out where the compression loss was until I checked the head gasket. There is evidence of blowby between the intakes on cylinders 4 and 6. Thinking back to the head removal, the center bolts there seemed easier to remove than the others. My thought is that the head wasn't torqued down correctly. Tomorrow, I'll clean everything up and check the head and the block with a straight edge and a feeler gauge. Hopefully there's no warp. I'm going to pull the other head as well and check it. Any suggestions as to what else I should do? Thanks. Hy

 

[erikgreen]

Re: 5.0 GM compression problem update

Yes... if improper torquing is all it was, you should celebrate and have a cold one

 

[Silvertip]

Re: 5.0 GM compression problem update

Even if the head was warped a little, most of the better auto stores (NAPA for example) will true up the heads and it doesn't cost a fortune. I would also make sure the bolt holes in that area weren't stripped by overtorqueing.

 

[dcg9381]

Re: 5.0 GM compression problem update

I'd have the head on that side checked via a machine shop - have them make sure that it's flat and maybe pressure test it. Cracks (which I doubt you have) can be hair line and very very hard to see... Not sure I'd be able to spot good/worn valves without removing the valves either.

Dunno V8s very well, but the motors that I work on - the head has to be properly torqued and then retorqued after 500 miles or so.

 

[Bt Doctur]

Re: 5.0 GM compression problem update

Dont forget to clean the head bolt threads and retap all the head bolt threads in the block ,reduces the chance of improper torque readings.
Most important to remember the thread sealant.

 

[littlebookworm]

Re: 5.0 GM compression problem update

Thanks, guys. Will definitely clean out all threads. The reason I don't suspect the valves are bad is how clean they are. Every valve job I've ever done began with valves loaded with carbon around their tops or under the crowns. When I look into the exhaust ports at the valves and when I look at the valve heads and squish area, all is clean. The thickness of the valve edges is uniform all around the 8 valves. The tops of the pistons are clean. The cylinders are clean. The ridges than form at the tops of the cylinders from normal wear are not there. The cylinder tops are slightly cut back just the way a ridge reamer would leave them. This, and the sloppy reinstallation of head bolts, and the almost new exhaust manifolds with grease still on the bolts, leads me to conclude that the engine was recently rebuilt, though a little sloppily for my taste. I'll check it out further tomorrow. Might as well pull some valves just to be positive. Hy

 

[JCF350]

Re: 5.0 GM compression problem update

Dunno V8s very well, but the motors that I work on - the head has to be properly torqued and then retorqued after 500 miles or so.

Re-torquing depends on the gaskets you use for all iron engines. Bimetal engines would require a "re-torque" unless your dealing with "torque to yield" style bolts.

I was glad when they started making the "no re-torque" head gaskets, on some boats it was darn near impossible to tighten the head bolts without adapters and "doin sum cypherun".

 

[rodbolt]

Re: 5.0 GM compression problem update

during the leak down the only way you would have air in the intake is through the intake valve, head gasket sealing is irrelavent.
you could bolt the intake to the head and put all the air you wish on the combustion chamber and if the valve seal is good no air will be in the intake.
keep hunting for the issue.
but the only path from the cylinder to the intake is via the valve.

 

[Bondo]

Re: 5.0 GM compression problem update

during the leak down the only way you would have air in the intake is through the intake valve, head gasket sealing is irrelavent.
you could bolt the intake to the head and put all the air you wish on the combustion chamber and if the valve seal is good no air will be in the intake.
keep hunting for the issue.
but the only path from the cylinder to the intake is via the valve.

Ayuh,....

Unless,......
The intake valve is open in the cylinder which is recieving the blow thru from the low cylinder.....

In other words,.....
If the intake valve is open in cylinder #4, while you blowing air into #6,.....
It'll sound out thru the carb......

Btw,......
Regardless of how Clean the heads look,....
I'd send'em to the Machine Shop for a Freshening.......
That,+ a valve grind Gasket set,.... Your top-end will be all Rebuilt.......

 

[littlebookworm]

Re: 5.0 GM compression problem update

Thanks, Bond-o. That's what I was figuring. Interestingly, as I was moving things around last night I dropped a magnetic screwdriver and it stuck to the metal ring on the cylinder head gasketthe section which surrounds the cylinder opening. The gasket is a composite material but the edge ring around each of the cylinder openings is regular iron, not stainless. The part that shows blowby between cylinders 4 and 6 is also iron. Is the use of regular iron around the cylinder openings typical in marine gaskets, or are these auto gaskets that were used instead? I checked the "freeze-out" plugs. They're brass, not iron, and appear to be original. They still have factory paint on them. Hy.

 

[Bondo]

Re: 5.0 GM compression problem update

Ayuh,.....

Most Marine head gaskets are advertized as SSteel fire rings,....
But,......
Many grades of SSteel are still Magnetic....
So,.....
Unless you can find a part # stamped on it,.... It's only conjecture as to whether it's an automotive or Marine gasket......
Napa should be able to get you a set of Fel-pro Marine head gaskets.....