What is (likely) up with my '97 Pontiac Trans Sport?

For months and months now the needle on the temperature gauge has been fluctuating like mad, and in the last couple of months it’s been going up into the red (mostly, though not always, when I’m idling at a stoplight, say), and then going right back down to where it should be. It’s been taken to our very trustworthy, VERY reputable mechanic a few times, and everything has checked out (fan working, back this summer they replaced a hose [not sure which one, sorry] that was causing it to leak coolant), and the only thing they tell me is to bring it back in if it stays in the red.

Well, earlier this evening, I was stopped for an inordinately long time at a red light, and this time the needle went all the way into the red and stayed there whilst I was stopped. Of course, the warning light came on (this has happened several other times before when it’s gone into the red, but it goes off as soon as I start moving and the needle drops back down), and I was stopped so long this time due to heavy traffic, that when the light finally did change and I started moving again, the engine was very unresponsive for several seconds; it seemed to be stuck in lowest gear, and I was literally flooring the car for about four or five seconds and it was only going, say, 35 - 40 MPH; also, while this happened, the dash lights started to dim, as if I was draining the battery inordinately. But after four or five seconds, tops, it got right into the gear it was supposed to be in, and started driving normally (though, of course, and as usual, the temp gauge continued its fluctuation).

The only other bit of info I can add is that it seems when the transmission shifts gears during routine driving, the temp gauge needle goes up high (sometimes into the red, sometimes just on the verge of going into the red) for a second or three, then goes right back down.

Any ideas what this might be? And thanks for taking time to read this long post from a an admitted, and obvious, ignoramus concerning all things auto-related.

“everything has checked out”

Tell you what. With posts like this, there’s a really annoying little game that goes on. People respond with suggestions for things that need to be done/checked to sort out the problem and then the OP says - “oh, that was done already.” This can go on for quite a few iterations.

Call your mechanic and get an actual, specific list of things that s/he did, checked, looked at, etc. The first 10 things will be the first 10 things here. Save everyone the trouble.

Is the check engine light on? There are these very specific diagnostic codes that go along with such a light. If it has been on or is on also post the exact codes (format: “P1234”)

Is the check engine light on?

No. What does cycle on and off, though (and has since I first got the vehicle back in March), is the yellow “Low Coolant” light (not to be confused with the red temperature gauge light, which only comes on, as described in my OP, when the temp gauge needle goes into the red for more than a few seconds). But as I said previously (and I just checked this again this evening), the coolant levels are fine.

As far as what has been checked out previously: It’s not much, since the check engine light doesn’t/hasn’t come on. Basically, the mechanic has checked the coolant levels, checked to see if there were/are any leaks (the only leak found was repaired back this summer, as mentioned), and checked to see that the fan was/is running. I know for a fact that that’s all that’s been checked since the vehicle’s been in my possession, and I’m 97.5% sure that that’s all that was checked prior to my ownership of the vehicle. Again, because whatever the eff this is doesn’t cause the check engine light to engage.

If you idle it in your driveway, will it overheat? When it does, check to see if the fan is running. Also, will turning on the heater full blast bring the temp down? Has the thermostat been replaced? Has it had a coolant flush? If the water pump is driven by the serpentine belt, have the belt and pump been checked?

(I’m doing exactly what cigroller mentioned–asking the obvious that your mechanic should have checked and tried already) It doesn’t sound like your mechanic has done much except stand there and scratch his head with the hood open.

Keep in mind that if it IS overheating, you are slowly damaging your engine. Another few instances of the needle going in the red and you will no longer have the problem–you will be seeking a replacement engine. I would not drive the van like this.

*Your name is amusing. I love that movie.

The thermostat is probably bad or the radiator clogged. The low coolant light coming on is odd, recheck the coolant levels and keep an eye on it. Was the system bled of any air? Like said before the serious overheat is killing your engine. If you are stuck in traffic and can’t move, turn your heater on high, fan and temp, to try to blow some heat off.

If you idle it in your driveway, will it overheat?

Haven’t actually done this yet. I’m not one for “letting the engine warm up” during the winter.

When it does, check to see if the fan is running.

However, last time I had it in to my mechanic, this is what he did, and the fan was working.[/quote]

Also, will turning on the heater full blast bring the temp down? Has the thermostat been replaced?

Turning on the heater has exactly no effect on the fluctuating temp gauge needle. Never has. And the thermostat has not been replaced.[/quote]

Has it had a coolant flush? If the water pump is driven by the serpentine belt, have the belt and pump been checked?

It has not had a coolant flush, that I’m aware of. And I’m not sure if the water pump is driven by the serpentine belt, but I don’t think either the belt or the pump has been checked.

I’m going to call my mechanic right now and axe him all of these questions. Another post coming later.

*Your name is amusing. I love that movie.

Thanks. :slight_smile:

Based on your response to Cigroller, and assuming you described the problem in full to your mechanic, you need to start by replacing your very trustworthy, very reputable mechanic.

There are a number of possible causes for orepeated oevrheating and they can all be checked out. Here’s a list.

The system can be pressure tested for leaks.
The thermostat can be sticking, and yes, they can be tested.
the water pump impellars could be eroded, and yes, the pump can be flow tested
the radiator could be coated inside with gunk, with some of the tubes even blocked. Yes, this can be checked with a simple pyrometer, and it can also be flow checked.
A hose could be collapsing internally.
the fan(s) may not be properly working.
the radiator cap could be not holding pressure…yes, it can be tested.

Since this has been overheated repeatedly, I’d want to perform a pressure leakdown test also. You’ve probably at least blown the headgasket by now, and maybe warped the head.

Your mechanic should have at least checked the above items. Not having done so could have exascerbated the original overheat and done much more expensive damage.

I’ll be taking the van to the mechanic Monday, where they will be doing a coolant flush and replacing the thermostat. Hopefully this will take care of the problem

Fingers crossed!

I sincerely hope it does, but I’m very wary of anything correcting a problem until the cause of the problem is diagnosed.

Sincere best.

I will just add one thing to the list of things to check which is the coolant temperature sensor. This would go along with a test of the actual coolant temp. If the sensor goes bad it is possible for the gauge to go wacky while there may not really be a problem.

Other than that are you aware of the intake manifold gasket problems on these engines?