1999 Pontiac Montana Overheating

[pontiacguy102]

The temperature gauge on my 1999 Montana with 103k miles became erratic a couple of months ago (fluctuating from normal to red line) to the point where I had to do something.

In an attempt to diagnose the problem, I noted that the upper radiator hose was cool to the touch when the temp gauge was approaching the top limit of the range, indicating a faulty thermostat. After it was replaced, the gauge continued to fluctuate for the next couple of weeks. I monitored the antifreeze level and constantly removed air from the system. A pressure test was performed and no problems noted. The radiator cap was also changed. The temp gauge finally stabilized and remained solid in the normal range for several weeks. Then the gauge without warning began to fluctuate again.

Given that the temp gauge would rise when the vehicle was slowing and fall during acceleration, it was suspected that the water pump was the problem and was subsequently replaced. Unfortunately, this did not resolve the problem.

After limping home today with the gauge fluctuating between normal and red line, I tried to determine if there was a circulation problem as in a blocked radiator. The top radiator hose was hot to the touch but it seemed like the radiator itself was not hot.

I 'm wondering if the radiator may be blocked. The lower intake gaskets were replaced at 60k miles and the cooling system flushed by the dealer. Although anything is possible, it seems hard to understand why the radiator would be blocked. I have not replaced the sensor.

Appreciate any help or similar experience.

[Cressidaadr]

Welcome to the forum...

I'm afraid it is looking like you have a bad head gasket letting pressure into the coolant system. I am in the same situation and will be starting a new thread about head gaskets in the Venture forum. However if you will search on these and the other sister van forum you will get a number of post about the head gaskets.

Here is what I would do next. Start to get used to opening the hood often - - after you have driven a couple of miles, and then when you stop before shutting the engine off and then shut the engine off and jump out and watch for a minute. You need to look at the overflow reserve and watch for the steady bubbles to appear. They may be there sometimes and sometimes not but I bet you will see about dime sized bubbles coming out over the next few days if you will start to watch for them. The rate will be about 2- 3 per second and since it is just starting they may not always occur each time you look.

Also you are about due for your second IMG failure so at least it can get done at the same time you do the heads. My 98 van has 136k miles now and the IMG was replaced at 40k, 85k and again a few weeks ago. I had coolant in the oil and my guess is you may have also - assuming the IMG has failed right on time thanks to Dexcool IMO. (You cannot tell coolant is in the oil by looking unless it is really bad.)

You know how I am feeling now having to go right back into the engine again for the darn headgaskets because I just ignored the bubbles "hoping" they would clear up after doing the IMG. - I was wrong

Post back as you learn as it helps all of us.

[rkvons]

[quote=pontiacguy102]
After limping home today with the gauge fluctuating between normal and red line, I tried to determine if there was a circulation problem as in a blocked radiator. The top radiator hose was hot to the touch but it seemed like the radiator itself was not hot.[/quote

If you are driving and you pull over and immediately check your radiator, it might not be hot. I would think the test would be to have it sit running in your driveway and when the fans kick on to cool it, is the air coming off of the radiator hot? If it is, it is cooling. You should see the temperature gauge climb up until the fans kick on, and then the temperature should drop back down.