8Mud
NAXJA Forum User
- Location
- Central Germany
Just thinking out loud.
!. Air (or exhaust gasses) trapped in the block can cause localized coolant boiling (hot spots). As the steam builds the whole process goes run away pretty quickly. Impaired coolant flow can also cause hot spots, from restricted coolant flow and/or the insulating properties of a gas (slows heat exchange) and causes hot spots.
2. The system will eventually purge the air to the top (radiator) and purge it into the coolant recovery bottle (maybe a better spot for an exhaust gas test?). It may take a week to get a complete purge, most air gets purged during the first few hot-cold cycles.
3. Things that can impact the normal air purge. Are a pinhole leak (anyplace), a bad radiator cap (seal) or the sealing surfaces in the filler neck.
A). As the radiator is refilling from the recovery bottle it is also sucking air back into the system, through a tiny leak.
B). If the pinhole leak is up high it may be sitting in an air pocket and hardly noticeable.
C). If it does leak some coolant, it may be a small series if drips that evaporate before they collect into a noticeable stain. Behind the shroud, up high on the radiator where the cross over channels are mated to the end tank, seen it twice now on two different XJ's, both times on the passengers side.
D). Make sure the filler cap overflow nipple is clear and the tubing isn't clamped closed or collapsing (use vacuum or thick walled tubing, that will take the heat) between the nipple and the recovery tank. Might even be a good idea to blow through the recovery tank inlet/outlet and make sure it is open.
4. Coolant flow reduction, can cause hot spots and rapid temperature rise.
A). Partially plugged radiator.
B). Damaged radiator hoses.
1). The bottom hose collapses, often associated with a clogged radiator.
2). The hoses are layered, the inside layer often gets soft and baggy with age.
C). The wrong water pump or the impeller has come loose. The wrong impeller won't pump backwards, but the proper direction just very poorly.
D). The thermostat bypass (heater tubing) if it is blocked or restricted, the coolant next to the thermostat may heat a lot slower than the coolant down next to the cylinder walls. It will eventually open, but you may get steam through there as well as coolant.
Some things I've tried in the past.
A). Pump up the cooling system on a cold motor (engine off). I knock the bottom valve out of an old radiator cap for a test. A short piece of fairly thick tubing hooked to the coolant overflow nipple at the filler neck and a bicycle pump. Use the same tip you use for pumping up an air mattress. Use a cable tie to keep the tubing on the nipple from blowing off and a pair of vice grip pliers to squeeze the tubing closed to hold the pressure. Buy a spray can of leak tester (soap), spray the radiator up high, inside the end tanks and other likely spots, look for bubbles. I wouldn't pump the cooling system above around 20 PSI. at 30 PSI it starts splitting seams, especially if you have the plastic end tanks with the gasket.
B). Find a way to get air into the cylinders, best when that cylinder is top of the compression stroke. Buy the adaptor, make one out of an old spark plug (what I did) or sometimes you can take the check valve out of your compression tester and use that. I cut a couple of wooden wedges and jam the belt at the harmonic balancer. The engine may turn over around 30 PSI (or even lower), you want to avoid this. I usually pump it up to around 30 PSI and keep it that way for awhile, with the radiator cap off. If you have a head gasket leak into the cooling system, the air will eventually find it's way to the top of the filler neck, though it may take a while. It may be a few tiny bubbles, it may be a tiny leak in the head gasket. I have shop air, but prefer to use a foot power bicycle tire pump (with a built in pressure gauge), works for me.
C). The last head gasket leak I tried to chase down. Showed nothing with an exhaust gas test, nothing with the air into the cylinder test (same cylinder had a leaky valve), but did spew a few drops of coolant colored condensation out the exhaust pipe right after starting a the motor cold. I collected some condensation out of the exhaust with a white paper towel, it was blue tinted the same color as my coolant.
I tried to do this in outline form, because it is easier form me, that is how I think anyway. I still may have missed a few possibilities.
Something I forgot, coolant dripping into a cylinder will often blow (steam) a clean spot on the top of a piston, right down to the metal. You look down the sparkplug hole and you won't see the normal glaze or soot, you see metal. I use a good flashlight and move the piston up (or down) for a better view.
!. Air (or exhaust gasses) trapped in the block can cause localized coolant boiling (hot spots). As the steam builds the whole process goes run away pretty quickly. Impaired coolant flow can also cause hot spots, from restricted coolant flow and/or the insulating properties of a gas (slows heat exchange) and causes hot spots.
2. The system will eventually purge the air to the top (radiator) and purge it into the coolant recovery bottle (maybe a better spot for an exhaust gas test?). It may take a week to get a complete purge, most air gets purged during the first few hot-cold cycles.
3. Things that can impact the normal air purge. Are a pinhole leak (anyplace), a bad radiator cap (seal) or the sealing surfaces in the filler neck.
A). As the radiator is refilling from the recovery bottle it is also sucking air back into the system, through a tiny leak.
B). If the pinhole leak is up high it may be sitting in an air pocket and hardly noticeable.
C). If it does leak some coolant, it may be a small series if drips that evaporate before they collect into a noticeable stain. Behind the shroud, up high on the radiator where the cross over channels are mated to the end tank, seen it twice now on two different XJ's, both times on the passengers side.
D). Make sure the filler cap overflow nipple is clear and the tubing isn't clamped closed or collapsing (use vacuum or thick walled tubing, that will take the heat) between the nipple and the recovery tank. Might even be a good idea to blow through the recovery tank inlet/outlet and make sure it is open.
4. Coolant flow reduction, can cause hot spots and rapid temperature rise.
A). Partially plugged radiator.
B). Damaged radiator hoses.
1). The bottom hose collapses, often associated with a clogged radiator.
2). The hoses are layered, the inside layer often gets soft and baggy with age.
C). The wrong water pump or the impeller has come loose. The wrong impeller won't pump backwards, but the proper direction just very poorly.
D). The thermostat bypass (heater tubing) if it is blocked or restricted, the coolant next to the thermostat may heat a lot slower than the coolant down next to the cylinder walls. It will eventually open, but you may get steam through there as well as coolant.
Some things I've tried in the past.
A). Pump up the cooling system on a cold motor (engine off). I knock the bottom valve out of an old radiator cap for a test. A short piece of fairly thick tubing hooked to the coolant overflow nipple at the filler neck and a bicycle pump. Use the same tip you use for pumping up an air mattress. Use a cable tie to keep the tubing on the nipple from blowing off and a pair of vice grip pliers to squeeze the tubing closed to hold the pressure. Buy a spray can of leak tester (soap), spray the radiator up high, inside the end tanks and other likely spots, look for bubbles. I wouldn't pump the cooling system above around 20 PSI. at 30 PSI it starts splitting seams, especially if you have the plastic end tanks with the gasket.
B). Find a way to get air into the cylinders, best when that cylinder is top of the compression stroke. Buy the adaptor, make one out of an old spark plug (what I did) or sometimes you can take the check valve out of your compression tester and use that. I cut a couple of wooden wedges and jam the belt at the harmonic balancer. The engine may turn over around 30 PSI (or even lower), you want to avoid this. I usually pump it up to around 30 PSI and keep it that way for awhile, with the radiator cap off. If you have a head gasket leak into the cooling system, the air will eventually find it's way to the top of the filler neck, though it may take a while. It may be a few tiny bubbles, it may be a tiny leak in the head gasket. I have shop air, but prefer to use a foot power bicycle tire pump (with a built in pressure gauge), works for me.
C). The last head gasket leak I tried to chase down. Showed nothing with an exhaust gas test, nothing with the air into the cylinder test (same cylinder had a leaky valve), but did spew a few drops of coolant colored condensation out the exhaust pipe right after starting a the motor cold. I collected some condensation out of the exhaust with a white paper towel, it was blue tinted the same color as my coolant.
I tried to do this in outline form, because it is easier form me, that is how I think anyway. I still may have missed a few possibilities.
Something I forgot, coolant dripping into a cylinder will often blow (steam) a clean spot on the top of a piston, right down to the metal. You look down the sparkplug hole and you won't see the normal glaze or soot, you see metal. I use a good flashlight and move the piston up (or down) for a better view.
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