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Advice needed after compression test
#760475 May 08th 2020 1:59 pm
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Yesterday I had no idea what a compression test was but watched a few youtube videos and it went pretty smooth except for one cylinder.

1 - 130
2 - 135
3 - 124
4 - 132
5 - 130
6 - 130
7 - 105 Dry and 110 Wet
8 - 135

I was noticing some coolant loss and wanted to see what was going on. I did an oil change it it looked like straight oil. After removing the air filter today I saw what looks like some dried coolant on top of the engine and you can see in one of the images there is a plug with 2 plastic holes where two hoses would hook up to. Maybe going to the AC that was ripped out? Also there is no coolant reservoir.

The intake manifold gasket looks like it is leaking as well as the valve cover gaskets. I have never done this type of job but I am thinking I could follow a few tutorials and get it done. Should I also replace the exhaust manifold gasket and what other ones should I be looking at doing?

If I open it up what would I be looking for with cylinder 7?

Also I included a picture of the transmission and I am assuming this was a rebuilt one installed at one time?

Attached Files 20200508_114452.jpg20200508_114222.jpg20200508_114214.jpg20200508_114142.jpg20200508_114128.jpg20200508_114113.jpg

1977 Chevy G20 with 350
Re: Advice needed after compression test
jtozier406 #760476 May 08th 2020 2:00 pm
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Also it is a 77' G20 with a 350


1977 Chevy G20 with 350
Re: Advice needed after compression test
jtozier406 #760483 May 08th 2020 3:22 pm
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carpal tunnel
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What did #7 spark plug look like compared to the rest? If a cylinder is leaking water it will flash to steam during combustion. This will cause that plug to look like it's been steam-cleaned (which it has sorta).

I've never cared that much for compression test. A cylinder leakage test and a good plug read speaks volumes.

That "plug" on top of the thermostat housing with the broken hose connections is a thermostatic valve and was part of the old vacuum advance.

The exhaust manifolds don't have gaskets at the heads.

Last edited by CatFish; May 08th 2020 3:30 pm.
Re: Advice needed after compression test
CatFish #760484 May 08th 2020 3:45 pm
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Originally Posted by CatFish
What did #7 spark plug look like compared to the rest? If a cylinder is leaking water it will flash to steam during combustion. This will cause that plug to look like it's been steam-cleaned (which it has sorta).

I've never cared that much for compression test. A cylinder leakage test and a good plug read speaks volumes.

That "plug" on top of the thermostat housing with the broken hose connections is a thermostatic valve and was part of the old vacuum advance.

The exhaust manifolds don't have gaskets at the heads.



Here are the pictures of the spark plugs. The previous owner changed them about 500-1000 miles ago so they are relatively new. The gap looks high and some are grey and some have an oil build up?

I can grab a leak down test kit from autozone tonight.

Was the thermostatic valve for the AC?

Basically seeing if I should move forward with gaskets in this engine or if a new crate engine sometime this year will be a better move.

Attached Files 20200508_133509.jpg20200508_133425.jpg20200508_133416.jpg20200508_133359.jpg

1977 Chevy G20 with 350
Re: Advice needed after compression test
jtozier406 #760487 May 08th 2020 4:19 pm
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carpal tunnel
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A new spark plug in cylinder that's consuming water will still look almost new at only 500 to a 1000 miles.
You'll need an air compressor for a cylinder leakage test.
If you can rent a coolant system pressure test kit you can pump the system up to operating pressure and look for water in #7 (and elsewhere) You might try that first as it will be easier and quicker and it's a hand pump operation. You can also leave the coolant system pumped up for a couple of hours if you don't see any water at first


Last edited by CatFish; May 08th 2020 4:21 pm.
Re: Advice needed after compression test
jtozier406 #760490 May 08th 2020 4:37 pm
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Do you think the spark plug looks like it has extra oil?

I have an air compressor for my nail gun that should be able to hook up to the unit. Do you know how to find 'top dead center' for each cylinder on the 350? That is one thing on these leak down tutorials that I am not sure about.


1977 Chevy G20 with 350
Re: Advice needed after compression test
jtozier406 #760491 May 08th 2020 4:48 pm
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carpal tunnel
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The spark plugs look a little dark but it's hard to tell if it's oil, running rich or just a cold plug.
For cylinder TDC:
Pull the valve cover so you can watch the rocker arms. With both valves closed the cylinder is as closed up as it's gonna get. The valves and the distributor rotor button will tell you when you are close to TDC on whichever cylinder you're checking.

Re: Advice needed after compression test
jtozier406 #760493 May 08th 2020 4:58 pm
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Thanks CatFish. I will try that out when I get some gaskets to install.

Curious if in the future I need to replace this engine, what would be your choice?


1977 Chevy G20 with 350
Re: Advice needed after compression test
jtozier406 #760494 May 08th 2020 5:06 pm
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The 350 is a good motor … if you get a new crate its the same block you have now with new gaskets...… I don't believe the motor is bad it just needs some lovin …

maintenance is always better than a catstroific failure ...


SUNSHINE VANS-VAN DIEGO
ADRENALIN BY THE GALLON & CHASIN RACIN
ONE FOR THE DIRT & ONE FOR THE STREETS
'93 CHEVY G30 454 4X4 SPORTVAN EXT 146" WB
'92 CHEVY G30 454 BEAUVILLE EXT 146" WB
Re: Advice needed after compression test
jtozier406 #760495 May 08th 2020 5:17 pm
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carpal tunnel
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These engines were prone to cam lobe wear. When you pull the valve covers you can measure and compare the rocker arm movement for a idea of how much lobe wear you have. I've seen guys crank down on the rocker arm adjustment on a cylinder with a bad lobe just to get it quiet enough to sell the vehicle. If the cam is worn, or if the rings and valves leak you'll have a better idea of the condition and past life of the engine and can make an informed decision on the next step. JustMy2%.
Replacement would depend on how long I was gonna keep it and the condition of the rest of the van. That would be a personal choice. We have a '94 GMC with well over 300K and it's part of the family. In fact I believe, sometimes, the wife likes it better than she does me. If it needs anything it gets it. Same with our '95 G20 but it just turned 65K.

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