I've got an '86 GT (2.8) that seems to have a coolant overflow issue. It runs fine besides a stumble off idle when cold....runs great when warm.

During a drive, coolant temp is normal....perhaps even on the cool side. As soon as the car is stopped, the water flow stops, and the water within the engine block boils, forcing water into the coolant overflow reservior. The problem is, that it seems to boil off too much, and the reservior lacks enough capacity to handle the amount of coolant flowing into it. So the reservior overflows, spilling all the coolant on the ground.

Once the engine cools, the water inside condenses, and sucks all the coolant from the overflow bottle back into the cooling system. Once the overflow bottle is dry, it starts sucking air. After 3 or 4 trips, air bubbles invade the cooling system, and the water pump starts cavitating. The temp gauge spikes, and I'm lucky to get it home.

Any ideas what the issue could be? I originally had a tiny little coolant reservior, but i've replaced it with a stock one and it still doesn't seem to have enough capacity. Thermostat has also been replaced. The engine runs well besides this nagging issue, and I'm tired of adding water to it every time I stop.

Maybe time for a new radiator cap.

First get a new rad cap. Have new one? maybe wrong one...
see my cave, radiator cap in coolant section.

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I have not replaced the cap, but it looks new(ish) and appears in good condition. I'll check it this afternoon and report back.

Thanks for the input.

So the radiator cap looks pretty darn good, and has a 16 psi rating on it. I don't believe that's the problem.

I'm going to pop the oil drain plug tonight just to see if it could possibly be a head gasket issue. I've noticed a very slight oil sheen in the coolant when it spilleld on the ground...not a serious amount, just enough to make a few rainbow colors on the surface.

I suppose the silver lining here is that if I can't get to the bottom of this relatively quickly, it's going to accelerate the timetable for my engine swap.

The only way for coolant to get to the overflow is for the cap to allow it to go there. Is the cap on all the way ? Sometimes it feels like it went on all the way, but needs pushed down and turned a little more to get to the tabs.

Is this a 'vented cap' problem?

Cap is definitely on all the way, turned until the metal tabs hit the stops on the radiator.

Perhaps it's possible it's just the wrong cap? The rubber seal of the cap is on a spring that protrudes down into the radiator...do they make different caps with longer or shorter springs?

I guess it wouldn't hurt to just replace the cap anyways. I'm pretty baffled here. Cooling systems are usually so simple and trouble free, that I didn't think there was much to go wrong.

This sounds like the exact problem I had with my '87 years ago. I second the thought of putting a new radiator cap on. On physical inspection, mine loked like new but was faulty. Put a new one on, fixed it instantly.

Go to the Ogre's cave and look up Radiator Caps. It gets complicated.

You really can't tell if a radiator cap is good by looking at it.

The top of the cap should read NON VENTED. If it says vented it is the wrong one.

quote Originally posted by Indiana_resto_guy: The top of the cap should read NON VENTED. If it says vented it is the wrong one.

X 2!!! Make sure the cap is NON VENTED otherwise the fluid gets pushed past the light vacuum spring and fills up the overflow bottle and pours out on the ground.

Thanks to all that responded. I just thought i'd get on here and give everyone an update on the fix.

I replaced the radiator cap last week, and have now driven 400 trouble free miles, with numerous starts and stops in the middle. The problem seems to be fixed.

The previous radiator cap was 16psi, NON-vented....exactly like the new one that replaced it. The top of the cap is a different shape, but the inner workings were identical. All the rubber seals appeared to be in good shape. There was no reason to believe the old rad cap was the problem, and yet here we are. Replacing the cap fixed it, although I don't understand why.

There is a thing called a spring that regulates the pressure in the cooing system. If that spring fails, then you have no pressure built up, and the coolant just flows out as it expands. That spring is in the cap and that spring fails on many occasions, that is why there are so many available to replace the old ones...

The spring on the old cap was in good condition. I don't have a fancy scale or anything to test the pressure applied by the spring, but just testing it with my thumb, the new cap and old cap springs seemed to have the same strength.

I'm convinced it must have been a rubber seal issue. Although the seal looks to be in good shape, that's no promise that it will actually hold pressure.

I have had the seals go bad too, but there are two springs, one for main pressure and one for return flow.
Can you see a little brass disk in the center of the assembly? That is the return flow valve. It should be closed till suction from the system draws water back. Sometimes that disk falls off and makes it a vented cap!

quote Originally posted by Chubbs: I've got an '86 GT (2.8) that seems to have a coolant overflow issue. It runs fine besides a stumble off idle when cold....runs great when warm. During a drive, coolant temp is normal....perhaps even on the cool side. As soon as the car is stopped, the water flow stops, and the water within the engine block boils, forcing water into the coolant overflow reservior. The problem is, that it seems to boil off too much, and the reservior lacks enough capacity to handle the amount of coolant flowing into it. So the reservior overflows, spilling all the coolant on the ground. Once the engine cools, the water inside condenses, and sucks all the coolant from the overflow bottle back into the cooling system. Once the overflow bottle is dry, it starts sucking air. After 3 or 4 trips, air bubbles invade the cooling system, and the water pump starts cavitating. The temp gauge spikes, and I'm lucky to get it home. Any ideas what the issue could be? I originally had a tiny little coolant reservior, but i've replaced it with a stock one and it still doesn't seem to have enough capacity. Thermostat has also been replaced. The engine runs well besides this nagging issue, and I'm tired of adding water to it every time I stop.

Replace your thermostat just because its only a few bucks. That could be the culprit. If it isn't the problem, it just helps you eliminate all possible solutions.