compression test results..

[UnionBlvd]

Ok so i'm a newb and I did a compression test on my 620 I recently bought, it has a l20b & w58 head. All cylinders were 150 except cylinder 2 which was 120. I thought maybe I didn't screw in the tester all the way, so I did it again and then it tested at 105? Then I did it again today and it was 150, wet compression test was at 170.

 

So umm...can anyone tell me why it keeps changing? Is it not the rings but something in the head? I'm hoping it's the head so I have an excuse to buy a better head.;)

 

 

If I buy a square port head...I'm assuming I also have to buy a different intake correct? Do I need a different exhaust manifold also?

 

Thanks~

[datzenmike]

The square port refers only to the exhaust ports. You will need any non W58 head that has square ports.

 

I see you have a '77? yet it has a W58 head? If the build date on the door jam says it was built after June '77 then it's a '78 model truck. I mention this because the '77 would normally have a U-67 head on it. A '78 will have disc brakes on the front.

 

 

Most if not all U-67 heads (car and truck) had intake and exhaust manifolds that were bolted together and used exhaust heat to warm the intake.

 

So tell me, does your intake and exhaust bolt together like this...

 

 

 

Try all the cylinders and record the results. If it happens randomly on other cylinders then the tester may be wrong. If it happens repeatedly on the one cylinder I say something is wrong there.

 

Do the threads on the spark plug hole look stripped or cross threaded?

 

Check the valve lash on that cylinder. A tight valve may hold the valve up off the seat and lower the compression. A cold motor would have higher compression than a hot one if this was the case.

[yello620]

Is the throttle blades in the carburetor being held open for the motor to actually get air to compress??

[zed]

I also have varying compression on one cylinder of my L18. Maybe a sticking valve? I've done the test with throttle open and closed - doesnt make much difference to the results.

[banzai510(hainz)]

UnionBlvd

Dont worry about it if your rig is running good

[UnionBlvd]

The square port refers only to the exhaust ports. You will need any non W58 head that has square ports.

 

I see you have a '77? yet it has a W58 head? If the build date on the door jam says it was built after June '77 then it's a '78 model truck. I mention this because the '77 would normally have a U-67 head on it. A '78 will have disc brakes on the front.

 

 

Most if not all U-67 heads (car and truck) had intake and exhaust manifolds that were bolted together and used exhaust heat to warm the intake.

 

So tell me, does your intake and exhaust bolt together like this...

 

 

 

Try all the cylinders and record the results. If it happens randomly on other cylinders then the tester may be wrong. If it happens repeatedly on the one cylinder I say something is wrong there.

 

Do the threads on the spark plug hole look stripped or cross threaded?

 

Check the valve lash on that cylinder. A tight valve may hold the valve up off the seat and lower the compression. A cold motor would have higher compression than a hot one if this was the case.

 

Nah, mines a 2/77. No disks, must be an newer motor. Sparks threads are good, and the other cylinders are always the same so I know it's not the tester. I guess a valve inspection and adjustment is in order.

 

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I figured if I change the head I would need another intake cause mine says w58 on it? It has a Weber on it btw...I probly should have mentioned that.

 

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UnionBlvd

Dont worry about it if your rig is running good

 

It runs fine but the other day I heard a crazy rattle a couple times on the freeway, I don't know where it came from so I thought I should figure it out before it got worse..if it's ok I'll just keep it.

 

Thanks guys

[MicroMachinery]

My tester has a pressure release that sometimes doesn't seat all the way, so it will give a false reading. Do the test a couple times.

 

It sounds like your engine is in decent shape if they all have 150. If it runs even and doesn't burn a bunch of oil, I'd rock it. Leave good enough alone.

[datzenmike]

I also have varying compression on one cylinder of my L18. Maybe a sticking valve? I've done the test with throttle open and closed - doesnt make much difference to the results.

 

If the throttle is closed it might take an extra turn to get max pressure on the gauge , I never open them. Check valve lash to be safe.

 

Don't forget this is a test at only cranking speeds. A running motor has way more compression seating the valves.

 

 

Yes, must be a newer '78 or '79 motor. Could even be from an '80 720. Maybe from a car too.

[banzai510(hainz)]

your intake is fine.

Its removing those exhaust manifolds thats a pain on those later years. if yours dont leak then dontk worry about it.

[datzenmike]

I wouldn't freak out about a tester that isn't giving reliable results. No need for another head yet. If it runs good drive it. But check the valve lash.