How do you tell which head gasket is bad?

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Pinball
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How do you tell which head gasket is bad?

#1

Post by Pinball »

I believe I have a bad head gasket on my son's 77 GL1000. We had an overheating problem about a year and a half ago due to a bad water pump and I needed to replace the right head gasket. I knew it was the right one because of the white smoke that came out of that side of the exhaust. Well I think one of them is bad again but this time there is no smoke! The bike will fill the coolant reserve tank and never pull anything back in. When the bike is up to temp and you lift the cap off of the reserve tank you can see it pushing air into the tank...bubbles. I ran a compression test and got #1- 122, #2-118, #3- 121 and #4- 116 . I would tend to think it was the left one just because of the lower compression but it doesn't seem like a big difference. I am just tired of guessing on things. Would you just replace both of them? Is there a way to tell for sure which one is bad? Could I put air to the cylinder after it warms up to see if I get bubbles? Thanks for any help.
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Whiskerfish
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Re: How do you tell which head gasket is bad?

#2

Post by Whiskerfish »

Typically as the bike warms after the thermostat opens the fluid level in the overflow tank will increase. Then after the bike is shut down for a few hours and things cool off it will draw that fluid back into the Radiator. Seeing bubbles in the line going down into the overflow tank is not uncommon at all.

Just curious but when you did your comp tests did you have all the plugs out and the throttle wide open? It makes a big difference.
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Re: How do you tell which head gasket is bad?

#3

Post by Explorer »

I bought an '82 GL1100 in '84 and it told me it had a bad head gasket by squirting a stream of coolant up into the air about as high as the handlebars and back down again onto my right leg.
So I knew it was the right side, if it's not as obvious as that, change them both out.
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Re: How do you tell which head gasket is bad?

#4

Post by robin1731 »

Do a leakdown on it.

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Sidecar Bob
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Re: How do you tell which head gasket is bad?

#5

Post by Sidecar Bob »

OK, so the level in the coolant recovery tank increases as the engine warms up but doesn't decrease when the engine cools. What happens if, after letting it cool, you remove the rad cap? If you hear a whooshing sound try replacing the rad cap.

Rad caps have 2 spring loaded pressure valves, one to let coolant move to the recovery tank when the engine warms and the coolant expands and the other to allow coolant to flow back into the rad & engine when the engine cools and the coolant contracts. It is possible that the second valve isn't opening.

It is also possible for a leak in the hose between the rad's filler neck and the recovery tank to allow the cooling system to draw air into the rad instead of drawing coolant from the tank.

Failing either of those possibilities, if you are sure the problem is a head gasket (I had the same thing once on my 650), you can check for leaks with air pressure:
Make up a tool for the test by cleaning out the ceramic part of a spark plug, tapping the metal part so you can screw a pipe nipple in and putting an airline fitting on the other end (I also replaced the spark plug's gasket with an o-ring - see pic below).

To test, remove the rad cap and fill it up as full as possible, set one cylinder to TDC compression, screw the tool into that cylinder's plug hole, apply 80 PSI and watch for bubbles. If no bubbles appear in 10 minutes set another cylinder to TDC compression and repeat. If a head gasket is leaking you should see bubbles.
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Re: How do you tell which head gasket is bad?

#6

Post by flyday58 »

Sidecar Bob wrote:OK, so the level in the coolant recovery tank increases as the engine warms up but doesn't decrease when the engine cools. What happens if, after letting it cool, you remove the rad cap? If you hear a whooshing sound try replacing the rad cap.

Rad caps have 2 spring loaded pressure valves, one to let coolant move to the recovery tank when the engine warms and the coolant expands and the other to allow coolant to flow back into the rad & engine when the engine cools and the coolant contracts. It is possible that the second valve isn't opening.

It is also possible for a leak in the hose between the rad's filler neck and the recovery tank to allow the cooling system to draw air into the rad instead of drawing
1st explanation I’ve ever seen of this, great stuff. Thanks.
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Sidecar Bob
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Re: How do you tell which head gasket is bad?

#7

Post by Sidecar Bob »

Thanks. I hope it helps him.
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Re: How do you tell which head gasket is bad?

#8

Post by tlbranth »

SC Bob - was gonna say just that but you beat me to it and put it very well.
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Re: How do you tell which head gasket is bad?

#9

Post by Pinball »

Well I replaced both of the head gaskets and it looks like the problem is solved. I replaced the left one first but the problem continued so the right one was then replaced. I took it for a nice ride today and didn't have the coolant move so it looks like it is time to start working on the next little thing. I will be making one of those air tools to keep just in case...thanks for all the input on this.
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