Strange cooling system problem

[brianl703]

Car is a 1988 Ford Mustang 5.0.

The problem is that sometimes the cooling system doesn't pressurize. That is, I can drive the car and when I get home and check the upper radiator hose, it's hot but there's no pressure. When I squeeze it it's very obvious that the valve in the radiator cap has not closed.

Other times it pressurizes just fine.

You'd think it's the radiator cap..except that I've already replaced the radiator cap and the problem still happens. But wait..I did some research and here is what I found:

The Stant cap for this car according to all of the application guides is 11331 (for the Lev-R-Vent one).

The Stant cap for 84 and previous Mustangs is 11330, according to the same application guide.

The Motorcraft cap for both applications is RS-80.

So Stant lists two different caps when Motorcraft lists one.

Is it possible that the Stant cap isn't the right one and I should get the Motorcraft cap??

 

[slalom44]

Radiator caps are designed to "open" (let pressure out) when the pressure exceeds its specified pressure. When cooling down, the cap opens up, pulling fluid from the reservoir and preventing your system from going into a vacuum.

If you have no pressure but the hose feels hot, either you have a faulty cap or your system isn't hot enough to allow it to build pressure. If your temp gauge is showing that your engine takes a long time to get warm, it is possible that your thermostat is stuck open.

Squeezing the hose isn't a good test of whether your system is working properly. If your temperature gauge is showing the engine getting to temp normally and not overheating and you aren't going through coolant, you don't have a problem.

 

[westex39]

I have noticed this too on more than one vehicle. I think it is pressurizing during the initial warm up period. When you reach full operating temperture and shut the engine off for a few minutes the water temp rises and causes the coolant to expand and push the coolant into the overflow tank. When you restart, the coolant returns to the normal running temp but since there is some pressure left in the system the coolant that was pushed into the overflow tank while the engine was off does not return to the radiator. If the engine temp were to rise beyond the point it was while it was shut off. Then it would begin to pressurize to the point the cap would release the pressure at the specified pressure.
If you want to check it, start with a cool engine and start it. Monitor the pressure build up in the hose as the engine comes up to full operating temp. You should have the specified pressure then.
Hope this helps.

westex

 

[brianl703]

The last time the system was not pressurized, I had just made a 13-mile drive home from work, and I checked it with the engine running just as I pulled into the driveway.

The temperature gauge does indicate that the system is heating up to normal temperature.

The reason I am concerned about this is that it IS losing a little bit of coolant. I added UV dye to the coolant to figure out where it was going and the only place I can find where it's being lost is the top of the overflow bottle! If the system doesn't pressurize due to a radiator cap problem couldn't the overflow bottle get overfilled and start dumping coolant? Especially if a vapor pocket forms due to the lack of pressure?

No coolant in the oil so it's not going there. (Did you know that something in most engine oil glows under a UV light? The Exxon Superflo in it now glows a light yellowish-orange under the UV light)

 

[Kestas]

I find that the green coolant glows brightly under UV light WITHOUT dye.

I was diagnosing a/c leaks and was in a bit of a pickle because the a/c compressor was sitting below the water pump, who's weep hole was in a blind area. I didn't know if the glow was from a leaky a/c shaft seal or a leaky water pump.

 

[brianl703]

I went to Wal-Mart and looked at the 11330 and 11331.

The difference between the two is the metal flap on the very bottom.

On the 11331, it is not spring-loaded and apparently relies on the force of coolant rushing past it to close.

On the 11330, it is spring-loaded and basically a one-way valve. The only way coolant will get into the overflow tank with this one is when the pressure builds up enough (to lift the rubber flap off the seal). When it cools down, the vacuum pulls the metal flap down against the force of the spring, allowing coolant to flow back into the radiator from the overflow tank.

If I continue to have a problem with the cooling system not pressurizing I will just try the 11330 cap. I wish there were some place besides the Ford dealer to look at the Motorcraft cap to see how that one works.

 

[brianl703]

Now this is strange.

The cooling system again did not pressurize on the way home from work today.

But it has pressurized every time I checked it when I drove it this weekend, in slow-speed city-type traffic.

The only thing I can think of is that my drive home from work involves a 1/3 mile slow speed drive from work to the traffic light where I make a left-turn onto a highway.

The engine has not fully warmed up when the light turns green and and when it does fully warm up the car is going 60MPH or so.

I wonder if the thermostat has crapped out in a way that, at highway speeds, the cooling system does not heat up fast enough to cause the coolant to rush out at a velocity which will close the flap under the radiator cap.

One thing I did notice is that the temperature gauge shows that the temperature goes up as the engine is heating up and then drops down. Presumably the thermostat has opened when the temperature drops down. It's about a one or two tic-mark decrease, too.

Is that normal? I haven't ever noticed it on any other car before.

Maybe I should replace this thermostat. I have no idea if it's ever been replaced. It might be the original 17-year-old thermostat

 

[Drew99GT]

That's normal for the thermostat to show a drop on the guage right after it first opens. Maybe you should pressure test the cooling system and end the speculation? I've found when radiator caps are bad, the coolent will lightly boil in the overflow and you'll lose coolent.

 

[brianl703]

Thought I'd update this thread. I found a leak at the timing cover with the UV light. I also changed the radiator cap to a full-pressure cap (with the one-way spring loaded valve between the overflow tank and the radiator). It's the equivalent to the Stant 11330 cap.

As expected this made the leak worse due to the system fully pressurizing every time.

I added some Bar's Leaks Gold Seal (the powder that comes in a tube) and that stopped the leak. (This is the same stuff that the GM coolant pellets are made out of, so I believe it to be safe for the cooling system).

Prior to that, I had drained a sample of the oil and sent it off for analysis. The results are posted in the used oil analysis forum and show no coolant contamination at all.

It seems like fuel economy is slightly up..maybe the engine is running at proper temperature now that it's pressurizing all the time.

I still don't understand why it wasn't pressurizing all the time with the old partial-pressure cap. It wasn't due to the leak, as I first noticed that it sometimes wasn't pressurizing over a year ago.