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Losing a full tank of coolant every 450 miles, no symptoms...yet

Not necessarily. I had a weird coolant loss on my LT1 Z28. I would never see any signs of a leak when parked, yet, it would lose coolant while driving it. Hooked up to a pressure tester it lost one PSI over a 14 hour period of sitting pressurized. My friend had the bright idea of letting all the pressure off the system and then starting the car with the tester hooked up. About 1 minute after the car started the pressure started to rise until it hit 25 psi when I shut it off. It wasn't even warmed up. That all happened in a minute or two. I turned out that it had a head gasket leak or crack somewhere that was essentially a one way valve. No coolant into the engine where it didn't belong. Combustion gasses were flowing into the cooling system once the engine warmed up enough for the crack/gasket to open up. The cooling system was pressurizing to the point of pushing coolant out of the cap/overflow tank while driving. I'm betting this car has a similar issue. I replaced a customer's head gasket on a 940 Turbo that was doing the same thing. It passed several pressure tests. We finally found the problem because of the white trail down the block where coolant would leak out the side of the head gasket behind the exhaust manifold. The coolant dye is a good idea to sort out if something like this is happening.

Still coolant loss is a symptom of the problem. Regardless of where, it shouldn?t be losing or consuming coolant. If it hasnt been properly pressure tested it should be.
 
Still coolant loss is a symptom of the problem. Regardless of where, it shouldn?t be losing or consuming coolant. If it hasnt been properly pressure tested it should be.

I agree 100%. My point was simply a pressure test may not find the problem. The same thing can occur if the overflow bottle cap is the problem. You replace the cap with your pressure tester and the results say everything is fine. Meanwhile, the actual problem is sitting on the work bench while you are running the test.
 
Thank you all for your time to respond. Sorry it's been a little slow my end, with Christmas, the new year, working hours, and assembling the things to troubleshoot your recommendations, it took a bit longer than expected.

Pull the spark plugs looking for a steam cleaned cylinder. Easy enough to eliminate the head gasket.

I've pulled the spark plugs, and all look the same (good), no white deposits. Perhaps i should take to dealership with a snake-camera to make doubly certain?

^^^good advice

Put a pressure tester on it and put the car on a lift and see where it's leaking out. There is likely an external leak.

I use a motive power bleeder for my brakes which uses the same cap/thread as the reservoir, so i hooked this up to the reservoir and pressurized to 21 psi (same a the green cap). At first i thought it was holding firm, but then noticed a drop of about 1PSI every 90 seconds or so. I had run some UV dye through the system beforehand, and was unable to see any external leaks with the UV light.

a bit of a side note: you should never top up the expansion tan fully to the brim. Because as the name suggests, it is there to allow expansion of the coolant when it goes from cold to hot.

Indeed, i just top up to the ring running on the outside of the reservoir, about 2/3 - 3/4 of the way up.

That said, the cooling system is designed as a closed pressurised system. It means the system needs to hold pressure. Even if you pressure test the system and you found it to be holding pressure just fine you could still have a problem. Because when you do the pressure test you connect the tester to the reservoir. To be able to do that you have to leave off the normal cap. And that cap needs to alsobe able to hold pressure (and act as a safety valve in case of OVERpressure). So this cap can be faulty as well. (not at all uncommon!) In which case the system can't build pressure and the expansion tank may overflow which would mean the system loses coolant.

In short: try a different known good cap,


I bought a new cap from the dealership, but the issue persists. Perhaps this new one is a dud too? I'm waiting to get an appropriate fitment to test the cap on its own.


i read the OP again:
if the water pump has been changed then the cooling transfer pipe from the back of the engine to the pump had to have been taken off. That means the seal might not seal correctly anymore. When i did my water pump i couldn't make that pipe seal so is used some RTV under and around the seal, then i connected the pipe to the pump. Let he RTV dry before you fill the system back up.
Also, the water pump has a rubber seal between pump and cilinder head. This seal can fail but it could also be that the water pump has been installed incorrectly.
It needs to be push upward against the head in order to make that rubber seal do it's sealing job. (the pump casting has slotted holes for this exact purpose. The pump needs to be levered up against the head. then while holding the pump levered in place you need to tighten up the fastners. That should do the trick.

The water pump was changed when the head gasket was done, with new seals. I didn't use RTV, but did follow the rest of your advice vis-a-vis pushing up against the head before tightening the fasteners. As mentioned to ZVOLV above, there is UV dye in the coolant, and after the pressure test could find no external leak around either of the seals.


I had a 85 245t leak on the exhaust port floor from a pinhole. That was fun to find. Radiator crimps good on end tanks? Hose nipples not cracked? Youd see rhose rhough i would think. Hose ends that go into firewall? Valve for heater ok? Outer edges of head gasket behind exhaust manifold?

Interesting, how did you diagnose that one? Yes, radiator crimps all good, as are hose nipples. All the radiator hoses are new, Volvo. Seal at firewall is good. Heater control valve is also quite new and leak free.

No symptoms? Uh, no. You have a symptom, coolant loss. Now find the problem.

Pressure test the system, you will find your leak.

As above, ran the pressure test. Extremely slow drop in PSI.

You might want to put dye in the coolant tank, run it awhile, then turn it off and using UV light look for evidence of leaks (the dye would be obvious to see).

I use the Autopro product.

Using the dye was the only way I was able to find the source of the bad head gasket on a friends 90 wagon. It was using coolant but was passing every test such as compression test, exhaust gas in the coolant test and a cooling system pressure test. Then after adding the dye the coolant loss source was revealed. It was coming out the back of the head only while driving at speed and left only a small trail as it ran down the back of the engine and came off the bell housing.

Whoever first put UV dye in coolant for diagnostics was a genius! Unfortunately, for this case, it doesn't yet seem to be helping in finding the issue.

You have eliminated each and every one of the recommended issues with test or observations except for the head gasket. There the feedback was that it was replaced 2500 miles ago.

That alone is enough to make me question it. Why don't you pull the spark plugs as suggested and completely eliminate that.

As above, pulled the plugs and all look fine.

Not necessarily. I had a weird coolant loss on my LT1 Z28. I would never see any signs of a leak when parked, yet, it would lose coolant while driving it. Hooked up to a pressure tester it lost one PSI over a 14 hour period of sitting pressurized. My friend had the bright idea of letting all the pressure off the system and then starting the car with the tester hooked up. About 1 minute after the car started the pressure started to rise until it hit 25 psi when I shut it off. It wasn't even warmed up. That all happened in a minute or two. I turned out that it had a head gasket leak or crack somewhere that was essentially a one way valve. No coolant into the engine where it didn't belong. Combustion gasses were flowing into the cooling system once the engine warmed up enough for the crack/gasket to open up. The cooling system was pressurizing to the point of pushing coolant out of the cap/overflow tank while driving. I'm betting this car has a similar issue. I replaced a customer's head gasket on a 940 Turbo that was doing the same thing. It passed several pressure tests. We finally found the problem because of the white trail down the block where coolant would leak out the side of the head gasket behind the exhaust manifold. The coolant dye is a good idea to sort out if something like this is happening.

This is a very interesting case study, thank you, and your friends idea is a bright one. I'll try the same today. As mentioned above, the reservoir cap is new and - assumed - good, until at least i can check this on the bench.

The original head on my car was cracked and leaking directly into the exhaust.

that sounds like a fun one. How did you diagnose it?

i had a heater shutoff valve on my 88 765 that didn't leak at idle.

Assuming this is the same as the heater control valve, this is new and showing no leaks under pressure.

I agree 100%. My point was simply a pressure test may not find the problem. The same thing can occur if the overflow bottle cap is the problem. You replace the cap with your pressure tester and the results say everything is fine. Meanwhile, the actual problem is sitting on the work bench while you are running the test.

The cap is new, but I'll test it separately once i find an appropriate fitment.

Still coolant loss is a symptom of the problem. Regardless of where, it shouldn?t be losing or consuming coolant. If it hasn't been properly pressure tested it should be.

I think the test i performed was a good one. Only potential for the very slow drop in PSI from the pressurizing system was at the Cap. I sprayed some soapy water on there, when it was connected and at full pressure, but didn't see any bubbling. The seal on the bottle-end of the Motive power bleeder is also good.

----

Once again, thanks to all who shared some insights or troubleshooting tips on this. Its a slippery issue, and I'm very grateful for the experience is helping me back it into a corner.

I was thinking to try a hot vs cold pressure test next, to see if there are any discrepancies between the two (expanded vs contracted engine?). I'll report back.

Thanks again, and happy new year.
 
Great follow-up!

The compression into the coolant is a good bet. There is an exhaust gas test kit for coolant available at auto part stores. Cheaper than a dealer test.
 
"At first i thought it was holding firm, but then noticed a drop of about 1PSI every 90 seconds or so."

"As above, ran the pressure test. Extremely slow drop in PSI."

If you meant what you wrote above, that is not an extremely slow drop in pressure. One psi in 90 minutes would be a slow drop. If you are seeing that drop in 90 seconds you should be able to find the leak with a visual inspection. It could be your heater core leaking. Then, draining out the drain tube while you are driving. That would keep you from being able to see the dye.
 
that sounds like a fun one. How did you diagnose it?

It was consistently overheating & using coolant. You could smell antifreeze with it running. I thought it was a blown HG. When I went to pull the exhaust manifold off, there was a gush of coolant that came out of cyl #3. When I pulled the head off, the piston crown was still crusty, so it wasn't leaking into the combustion chamber. The only other logical conclusion was that the head was cracked directly into the exhaust port. That was the only way the coolant could get there without steam cleaning the piston top.
 
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