Technical Compression problem

350 Chevy motor has low compression on #5 left bank and 0 compression on #8 right bank. New rebuild less than 100mi. On engine. Heads have no cracks.valves are seating. Cylinder walls are smooth. Compression is good on all other cylinders. All parts are new. What could be the cause? Thanks trucke

 

Open valve!

 

Valves Seem to be seating as should. Trucke

 

Flat cam? Pull the VC off that side and see what’s going on.

 

This would be first step, closely followed by airing up the cylinder to hear where the air is going IF the cam isn't flat and the valves are operating.

 

I think it is now time for a cylinder leakage test. That will tell you where the compression is going. Do you have it apart now? You're comments suggest you may have.

I'm betting that #6 has a valve seating problem, possibly even a bent valve. With 0 compression, #4 has a much more serious problem such as a ventilated piston.

 

???

 

I was wrong it's #5 and # 8 my screw up. Trucke

 

you should probably edit your first post to correct this info as many people will not read these responses as a linear conversation, and simply reply to your first post with it being 4 and 6.

You didn't list what made you think to run a compression test, but you did indicate it's a new build with less than 100 miles on it..... So if you've put air into it and not getting alot out of the intake, exhaust or crank case, then i'd look for a flat cam. Cant compress it if it's not coming in....

-rick

 

I put an vacuum gauge on port on intake. Needle was not constant on gauge. Trucke

 

Compression low on both banks. Heads and gaskets not shared. Crankshaft journal not shared.
Problem is either top end or bottom end. Simpler check is top end. Leakdown tester is quickest. Videos and instructions are here and all over online. If this is not available, pull valve covers on both sides and watch valves for affected cylinders. Do they open and close the same as unaffected cylinder valves?
Are the studs the same height and angle? Is there any visible difference on the affected cylinder's valvetrain compared to the others? If nothing can be seen, it's time to either talk to the engine builder or if that isn't an option, remove the heads for head, gasket, valve, cylinder and piston inspection.

 

Cylinder heads are off. Inspected heads ,block ,cylinder wall , pistons. Have not pulled valves yet. Trucke
.

 

Look at cam lobes and valve springs.
Cylinder heads can be checked for valve sealing with tap water down the ports.Just tilt head so port fills up with water. Valves should not leak. Blow off or dry in the sun.
0 compression should be fairly easy to spot.

 

well. you might as well start over... no way to find the problem now..... but one thing you can do is put plugs in the head, and put a liquid in the chambers and see if the valve are seated.... if they were still on the motor you could have backed the valves off to see if the compression comes up,... but like said,... do a leak down first... but that's all to late... my bet is the valves were too tight... lotta people do that mistake with lifter not pumped up, then go another turn... now the valve never shuts...

 

I sent u some vacuum gauge readings thru a p.m. but u have not responded. I hope it is of some value and u have more info to determine your exact problem.

 

Vacuum reading fluttering like h example. Thanks trucke

 

Now that it's all apart, have you tried to pull the lifters on the affected cylinders and inspect them for damage?

 

Vacuum is a useful tool to ascertain overall health.
You have 2 cylinders with low/no compression.
It ain't healthy, already confirmed.

 

How did the combustion chambers look on the heads? That is a good way to see what is going on, they should all look pretty much the same.

 

The low and no compression cylinders are clean as a whistle . Others have carbon residue on piston. Trucke

 

Since you have the heads off, the simplest thing to check first is the cam lobes and lifter bottoms on those 2 cylinders. If they are OK, then be sure to put the lifters for those two cylinders back in the same holes they came out of.

 

Really clean pistons can indicate head gasket problems. However, a bad head gasket should have been obvious during tear down.

 

@jaracer I'd agree if this was normal.
In this case, it's a fresh motor with less than 100 miles and zero compression on 2 cylinders.
Probably never had combustion in either since assembled.
OP has not provided a single picture, pulled the heads before diagnosing valvetrain and is giving incomplete answers.

 

There were a couple of head bolts that were not tight ,no coolant or oil visible though , head gasket didn't show any signs of leakage. Trucke

 

With that low amount of miles, evidence may be scant, especially if the gasket itself did not fail.

 

If you're glutton for punishment put the heads on the opposite side and see if the issue follows suit, then you know it's certain combustion chambers or certain lobes on cam.

 

Not Tight????

Well, heads not torqued down correctly or at all could be this whole problem.

 

It can be a lot easier to diagnose this stuff with the heads on. I had low compression on one hole on one of mine. My OTC compression tester has a separate hose that connects to the gauge with an air hose quick connect. I removed the schrader valve from the end of the hose, hooked my air hose to it with the regulator turned way down, and all the air was coming from the exhaust.

I had carbon between the valve and seat. It was hanging open, but visually it was really hard to tell. Can you check the valves are seating with liquid and/or air somehow? The loose head bolts sounds like a culprit but I'd want to confirm everything else before putting them back on.