Overheating with Cold Air

Maintenance/Repairs
1

Dear Car Aficionados,

I desperately need your help.

I am driving a 2002 VW Golf GL and my temperature light is flashing on my dashboard.

It was on for about 2 weeks before the temp gauge actually started rising.
I then took it to “my” mechanic and they said it was a faulty temp gauge that wasn’t opening and closing correctly. Fine. Done and Done. Sensor and gauge a few hundred bucks. Done. Right? Wrong!

Not more than 2 weeks later the light came on again with no temp rising.
Just yesterday I was driving and the temp started rising. I quickly flipped on the heat to alleviate some of the hot air in the engine and cold air blew out of the vents. I double checked to make sure the A/C was not on and still cold air. Plus, the temp kept rising. I did a few more tests, stopping, starting and driving quickly and still no change in the air temperature blowing from my vents, HOWEVER, the faster I drove the faster the temp gauge would return to normal. It was when I was idling at a red-light or stuck on traffic that the temp rises and when i’m cruising the temp goes down. It appears that this is the only factor in my temp remaining at normal.

Please if there is any guidance you might have for me I would greatly appreciate it. I clearly must take it back to the mechanic, but I would love to be armed with more details on what the problem might be, especially since it appears they did not look much deeper than the temp gauge.

Many thanks,
S.A.D.

P.S. The Mechanics did tell me there was NO leak in the line.

2

I’m guessing some, but I suspect that what he actually told you was that it was a faulty thermostat that wasn’t opening and closing properly.

I also suspect that he did no further tests except perhaps testing the cooling system’s ability to hold pressure. If he did not pressure test the system, he needs to. If he did, and the system passed, and my other guess that the cooling system is now low on coolant is correct, he needs to determine where the coolant is going.

I guessed that the system is low on coolant again because
(1) your engine is overheating again,
(2) there’s apparently no hot coolant in the heater core to put out heat, and
(3) the temp gage doesn’t seem to be responding properly, which suggests that it’s measuring the temperature in an air pocket rather than in the coolant in which it should be immersed.

Okay, so where is your coolant going? Here are the likely possibilities:
(a) it’s leaking from an as-yet-unfound leak
(b) it’s blowing past a weak radiator cap when it’s hot, where it’s accumulating in and blowing out of the coolant reservoir overflow until at some point there’s not enough left for the engine to draw back into the cooling system when it cools, so it’s drawing air in
© it’s being drawn through a breech in your headgasket into one of your cylinders during its intake stroke, vaporized, and blown out with the exhaust gasses.

If it were me, the first thing I’d want to check if a system leak test passed is the integrity of the cylinders to hold pressure. It’s called a “cylinder leakdown test”. However, I should add that if he fills the cooling system, purges the air out, and then sees bubbles coming out the radiator fill hole when the engine is running, or if he tests the coolant for the presence of hydrocarbons and gets a “positive”, the leakdown test won’t be necessary… you’ll have a blown headgasket.

Unfortunately, my money is on possibility ©, a blown headgasket. And, since the engine has now been repeatedly overheated, I’d bet you also have a warped engine head. But I’ve been wrong many times, so the tests need to be done.

Post back. We do care.

3

Thanks Mr. or Mrs. the same mountainbike,

You have a very interesting hypothesis and I am tracking your thinking.
I will get more testing and ask about a cylinder leakdown test.

Let’s just say, hypothetically speaking, there is no way I would be able to replace a blown headgasket. What might you suggest I do next?

Can I continually replace the coolant, drive fast all the time, not idle for long periods and continue to drive the car? Or, will I eventually start on fire?

Thanks!

4

If a bad headgasket does turn out to be the cause, it can be replaced. If the head is warped, it can be “milled” (machined) flat again.

You absolutely should replace the coolant, but whether you do or not, the engine will continue to get worse if you continue to drive it like this. At some point, of there is a headgasket leak, the gasses being blown through the leak will erode pathways into the top of the block and the cost of repair will skyrocket. That too would then need to be machined. You’d be talking about an engine removal and total rebuild then. Considering the age of the car, if that comes to pass you might want to explore other options like a used or rebuilt motor. Work with the mechanic on this.

5

“It was when I was idling at a red-light or stuck on traffic that the temp rises and when i’m cruising the temp goes down”

I would check the fan clutch and the fan belt for slippage. The fan is more important at low speeds.

6

I was thinking an intermittent radiator fan motor that was overlooked or didn’t fail while in the shop but the second post suggests that the cooling system is low.

Is the cooling system low? At the first sign of trouble the engine should be shut off and allowed to cool so that the coolant level can be checked. Operating low on coolant leads to failed head gaskets.

7

Thanks Everybody for your input. It was very helpful. I will take all this knowledge to my mechanic and pretend I know what I am talking about!

8

In the meantime, carry a gallon of 50/50 coolant with you and keep the reservoir topped off. Add to COOL engine only! I believe your reservoir is pressurized. The rate at which the coolant disappears is a clue. If quickly (say, a quart every week or so), and no puddles are appearing, that points to the head gasket.