No compression...

[JT1]

Compression testing yourself is a good idea, however, if it were my engine I would perform a leakdown test.

For those unfamiliar with a leakdown test, it is as simple as pressurizing the suspect cylinder with shop air via a leakdown kit and observing whether the cylinder holds the pressure.

A leakdown test will allow you to actually hear where the leakage is coming from if there is any. Leakage from the intake valve will obviously result in hissing from the intake, Exhaust valve from the exhaust, and if the piston rings are at fault you'll hear hissing from the oil cap.

 

[Cause4Alarm]

Hey....I don't tell anyone what to or not to talk about so unless you're a mod or admin don't do it to me.....the engine in question is a 60deg GM so it is in topic, and while the rocker bolt is a good theory at this point it just that....theory

 

[mechanicx]

That, a leakdown test, is exactly what the shop that diagnosed the low compression should've done ideally. It would've been a lot more useful than just a compression test. If the cylinder had low compression and normal leak down we'd know it would mean the intake valve is probably not opening, or if leak down was high we'd know that wasn't the issue and where the the air was going. It'd probably be the easiest way to find the general source of the problem, if you have the test kit and a compressor.

 

[onion]

A cheap and quick method of figuring out that lack of compression is to do a little tinkering and listening. If that was my vehicle, I would first listen to the engine and see if I heard any unusual noise. Also pull the plug of the suspect cylinder and confirm low compression. If compression is in fact low, take the air cleaner loose. Try idling the engine and listening at the intake and exhaust- if there's a burned valve, you'll often hear it popping at the intake or exhaust. Also you can disconnect the ignition module and listen while cranking the engine. If you hear obvious popping, you likely have a burnt or otherwise damaged valve. Also while the engine is idling, remove the oil cap. If you can feel and see blowby coming out of the crankcase- expecially if it's noticeably 'puffing', then you likely have a scored cylinder, cracked piston, or the like.

If you do all these checks and don't see, hear, or feel any obvious problems other than a miss... and your compression is in fact low... then you're likely looking at a valve-train problem. Pull the valve cover and have a look. Like others have said, these engines have a known issue with rocker bolt threads pulling out of the cylinder head. Also the rocker arm pivots occasionally come apart, allowing the rocker to be loose and off center, not opening the valve correctly. Either should be obvious enough when you remove the valve cover.

If rocker bolt threads are the problem, this CAN be a cheap and simple fix... if you can install helicoils. But be careful with those if you haven't installed them before. It ain't rocket science... but lots of people manage to screw them up.

 

[cchase]

It seems like it would be fairly straightforward to convert a compression gauge to a leakdown tester if I am thinking about this correctly.

Of course the only "issue" would be finding TDC for cylinder 5.

But, the other side of this is that it really doesn't matter whether it's an intake valve or exhaust valve leaking because the solution regardless would involve removing the cylinder head.

I sincerely, at this point, doubt it's a head gasket. There simply hasn't been any indication. Cylinder 3 next door isn't misfiring and the coolant and oil look good, and there are no hissing noises coming from the engine compartment while the engine is running. Of course it's too soon to make any kind of conclusion.

As far as cracked pistons or dead rings, the reality is that this engine hasn't been used in such a way that I could conceive of that being a problem. Again, too soon to confirm anything but I would have to believe it would be exceedingly unlikely to be something like this.

At this point I'm going to confirm injector operation, run the compression test, then pull the valve covers since that's a fairly simple operation and should be able to tell me if the bolt has pulled out of the head. If that fails, then I think I'd concentrate on a leakdown test and concern myself with other possibilities.

Any thoughts or ideas concerning the problem and my plan of attack welcome.

 

[eljefino]

FWIW I flew with my toolbox, of course checked it as baggage. Was unlocked.

I made a list and put my name and address on it and listed every single tool that was in the kit, and threw that in the box, and saved a copy for myself on my computer.

I know they opened the thing and spread it all out on the table, because all my stuff was in there, except a ball point pen, which I assume they assumed was theirs.

Also the box was waiting for me in the "lost baggage" section, not the carousel, but was not delayed.

Don't know if this makes you feel better or not.

 

[mechanicx]

Originally Posted By: cchase
At this point I'm going to confirm injector operation, run the compression test, then pull the valve covers since that's a fairly simple operation and should be able to tell me if the bolt has pulled out of the head. If that fails, then I think I'd concentrate on a leakdown test and concern myself with other possibilities.

Any thoughts or ideas concerning the problem and my plan of attack welcome.


I think that sounds like the best plan of attack for the circumstances. As an initial step along with confirming injector operation, I would throw in checking the spark again just to be sure.

 

[cchase]

Originally Posted By: mechanicx
Originally Posted By: cchase
At this point I'm going to confirm injector operation, run the compression test, then pull the valve covers since that's a fairly simple operation and should be able to tell me if the bolt has pulled out of the head. If that fails, then I think I'd concentrate on a leakdown test and concern myself with other possibilities.

Any thoughts or ideas concerning the problem and my plan of attack welcome.


I think that sounds like the best plan of attack for the circumstances. As an initial step along with confirming injector operation, I would throw in checking the spark again just to be sure.


Good idea...

 

[cchase]

Originally Posted By: eljefino
FWIW I flew with my toolbox, of course checked it as baggage. Was unlocked.

I made a list and put my name and address on it and listed every single tool that was in the kit, and threw that in the box, and saved a copy for myself on my computer.

I know they opened the thing and spread it all out on the table, because all my stuff was in there, except a ball point pen, which I assume they assumed was theirs.

Also the box was waiting for me in the "lost baggage" section, not the carousel, but was not delayed.

Don't know if this makes you feel better or not.

It absolutely does not. I don't have a cheap torque wrench...

 

[Vikas]

I am sure you can find Harbor Freight close by