Coolant Loss: Radiator Cap ?

Yes, eminently possible for a pressurized cap.

Possible BUT most of the overflow systems collect the coolant from the cap anyway. If it was a bad cap the jug would fill up and overflow, not go down.

I would bet on it being a small coolant leak.

Hit one of the parts stores and borrow a cooling system pressure test set. Install it and put the pressure up to the caps max (usually something like 10-15 psi.) then let it set and see if it loses pressure. If it does you have a leak somewhere. Grab a GOOD UV lamp and glasses and look for the leak externally. If that doesn't show anything have someone with an emission tester stick the probe in the radiator overflow jug with the engine running. ANYTHING over the baseline numbers in free air means a gasket problem.

I'm not certain as to the application's system, but he clearly stated radiator cap, not coolant recovery tank. And good luck locating and borrowing a pressure tester. In so far as I'm aware, even parts stores that provide loanable tools don't offer those, freon leak detectors, manifold gauges, UV lamps, carbon pile rheostats ... for testing purposes.

I know AutoZone offers the testers and UV lamps. I borrowed one a while back. Not sure on the A/C stuff although they sell it.

Perhaps you'd care to inform as to where, I'd be interested in verifying that such a policy varies with location. I advised a friend in Colorado (I'm in a nearby state) to try both O'Reilly and Auto Zone for a pressure tester to test for an issue with his cooling system. He informed me that he was told that neither had such a tool available for loan. Since that time, I've personally inquired about the remainder of those I mentioned in my vicinity and received the same response.

Kindly disregard, I just phoned one of the local suppliers and was informed that they now DO loan some of those, which I am gratified to hear. Time marches on. Thanx for the update.

O'Reilly's, Houston, Texas has radiator pressure testers for free loan. I did not ask about the other stuff however I have borrowed all sorts of things from them in the past.

Most vehicles have radiator overflow tanks. When the radiator cap valve either opens or leaks, the fluid normally flows to the overflow tank. I don't mean catastrophic open where someone turns it a full turn.

Yes, I was aware that they had a variety of tools for loan, just not those (in the past, anyway) that I mentioned. Is anyone aware of a listing, either online or elsewhere, where one can keep abreast of their current offerings?

And some of those tanks are themselves pressurized, with a pressure cap atop, while others are not, should you not be aware.

Could be wrong but I believe a defective head gasket can be responsible for loss of coolent--it goe out the tail pipe. If so, one sign is bubbles in the raidiator due to exhaust gas leaking into the radiator. MLD

Thanks! I did not know that.

Thinking back... Yea, my Pontiac Grand Am had a pressurized overflow tank. I replaced the tank once due to a pin hole.

Following up on that. Two tests. Take a sniff of the exhaust after the car is warmed up and the cooling system is up to pressure. If you smell coolant in the exhaust, you probably have a leaky head gasket. Second test: on a morning when the windshield is nice and cold, let the car warm up and turn on the defroster with full heat and high fan. If the windshield fogs up or you smell coolant, you have a leaky heater core. Unfortunately, either of these is a big PITA to fix and usually mucho dinero to the mechanic department.

Bob wrote in news:kbfgfv$qmv$ snipped-for-privacy@dont-email.me:

No. This is not the cap.

Is the fluid in the expansion tank slightly oily or discolored? Is there an oily film on the sides of the tank?

Your garage should do a hydrocarbon test on the coolant: You may have a failing head gasket.

Secondarily, CAREFULLY inspect the bottom tank of the radiator. This will mean having the car on a hoist, the splash-shields removed, and a strong flashlight. Sometimes the seam between rad-core and rad-tank goes bad and leaks under pressure only, and it can be very difficult to see the leak on account of the surrounding structure.

you can't say that tegger because it /is/ a frequent source of coolant loss. there are three seals on an accord radiator cap:

1. the high pressure primary seal to the outside.
2. the high pressure /to/ the expansion bottle - the 0.9/1.1/1.3 bar pressure regulator.
3. the low pressure /from/ the expansion.

if #1 is leaking, then the cap will definitely lose coolant.

yup, that's telltale head gasket. but check the cheap stuff first. the cap is a frequently neglected and common source of low level coolant loss.

so did the new cap fix it?