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GLV305 Compression question

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bobphoenix
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GLV305 Compression question

Post by bobphoenix » Wed Oct 09, 2019 4:32 pm

Hello All,
I have recently picked up a 1978 17' Lancer Stinger with a GM based 305 motor. (1977 according to the block casting numbers).
According to the last owner, he was told when he bought the boat in 2012, that the boat was sitting for a couple of years before he bought it because the owner was advised that one of the cylinders was low on compression and he didn't want to fix it. My seller didn't do anything to the boat prior to my purchase either.

As I'm sorting out my new boat, I did a dry compression check across all cylinders and sure enough, number 5 is sitting at 100 pounds with all the rest at 140, 150 and numbers 3 & 8 at 160..! So, I shoot some 30 weight oil into all the cylinder (three squirts each from and oil can then turn the motor over a few revs before the next compression check...) and then check the numbers again... Number 5 builds right up to 150 but now, #8 and #6 dive down to 120... What gives?

First where do I begin with #5 is the building pressure from the oil a sign of rings or top end problems? And,
numbers 6 & 8...? Could I have blown the head gasket by putting the oil into the cylinders?

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Bilge Rat
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Re: GLV305 Compression question

Post by Bilge Rat » Wed Oct 09, 2019 8:13 pm

Did you put the plugs in after adding the oil and then crank it over? That could cause some problems as the oil doesn't want to compress.

Compression tests are best done on a warmed up engine, throttle fully open, all plugs out. Keeping all the plugs out might show poor compression between 2 adjoining cylinders if a head gasket was blown .
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bobphoenix
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Re: GLV305 Compression question

Post by bobphoenix » Thu Oct 10, 2019 3:30 pm

Thanks for the reply Bilge Rat...

No, I put the oil in and left the plugs out... But, I didn't have the throttle wide open either... Another inconsistency was in the test itself...
For the dry test, I used the push in attachment on the tester (it has a rubber stopper on the end), and for the wet test, I used the screw-in adapter that goes into the plug hole...

I can't warm up the engine before the test because I still haven't gotten a spark out the distributor yet, either.

jim g
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Re: GLV305 Compression question

Post by jim g » Thu Oct 10, 2019 7:06 pm

bobphoenix wrote:
Wed Oct 09, 2019 4:32 pm


As I'm sorting out my new boat, I did a dry compression check across all cylinders and sure enough, number 5 is sitting at 100 pounds with all the rest at 140, 150 and numbers 3 & 8 at 160..! So, I shoot some 30 weight oil into all the cylinder (three squirts each from and oil can then turn the motor over a few revs before the next compression check...) and then check the numbers again... Number 5 builds right up to 150 but now, #8 and #6 dive down to 120...

What gives?
Its the hydraulic lifters for the valves. They have bleed down over the years the engine been sitting.

The 2 choice you have to get a good compression reading are

Pull the distributor and prime the oil system or file the points to get a spark and get the engine running.

The difference between a hot and cold compression test is minimal.

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