New to the boards, any help is appreciated!

My 94 3.4 has a sporadic overheating problem. The problem only happens once in a while (once a week). It starts overheating within 5 minutes of starting the car. Sometimes I can shut down and restart and it seems to start correcting itself. Other times the temp gauge shows the temp rising and falling - 1 minute it's >260, the next it has dropped down to 240, then back up past 260...etc.

I checked the fan and it is working. I replace the thermostat last weekend thinking it might be sticking. Any idea what should be next on my list?

Thanks!

 

Check the sending unit for the gauge, located on the drivers side on the block. If it's bad then it'll give your gauge innaccurate readings.

 

Please forgive my mechanical ignorance...Any idea how I can tell if the sending unit is bad, or is it cheap enough that I should just go ahead and replace it?

Thanks!

 

voltage tester will give you a hint on how well it's working

 

Giving some more thought to the issue. I don't think it is the sending unit (unless it somehow controls the heater) because when the guage is indicating that the car is overheating I am getting no hot air through the heater...?

 

260 degrees? How does it run when its like this? If its running at 260 degrees...Its gonna run like TOTAL crap. You wont get any acceleration, and I'd be surprised if it actually ran. Are you sure its not the sending unit, it sounds like it. If it cools down from 260 to 240 at idle, and fluctuates THAT much, then its got to be the sending unit. While idling, I seriouslly doubt it can cool down 20 degrees in a minute or two. Check your gauge connections. Im wondering whether it is TRUELY overheating, and not the gauge giving you the wrong reading.

 

An easy way to check to see if its the sending unit:

Open the radiator cap when the engine is cool
Stick a cheap meat theromometer in the coolant.
Start the car, and as it heats up, compair the temprature on the therometer to that of the gage. They should be close. Don't try to open the radiator cap when hot!


EDIT: This will only work if your coolant is moving... If your thermostat isn't opening, and coolant wont make it to the radiator, it obviously won't be accurate.

 

Its running fine, none of the problems I would expect while overheating, so maybe it is the sending unit. I'll definitely check it out.
You are right, the temp is bouncing around a lot and way too quickly. The only other thing that I can think of is that there is something clogging up the cooling system sporadically letting the coolant through...

 

Okay, FIRST off when your temp gauge is at 260, are you getting a "Check Gauges" light? If not, then your sending unit IS bad because the Check Gauges light I believe runs off a different sensor. If you're getting a check gauges light then you might be overheating, but don't numb the fact below out.

If your car was at 260 degrees, you WOULD be smoking like crazy. Trust me the same thing happened on my Z28. When we were moving even at highway speeds, the gauge would only go to the half mark yet it was full of antifreeze. Now as soon as I left the highway, the first red light I got, the temp gauge went STRAIGHT to redline and I would ALWAYS turn my engine off until I got a green light thinking I was over heating. Here's how we figured out the problem.

We took it to a mechanic and he hooked it up to another computer and ran the engine in idle and revved it a few times. The gauge definately went up past the half mark and then it went to redline. This is when the mechanic told us that the gauge was actually 75 - 100 degrees off because his computer, was actually reading 180 degrees, or opperating temperatures. Yet our gauge was redlined into 260. I'm sitting here wondering why the fan isn't coming on, and it's simply because in reality, the engine wasn't hot enough for the Hi-Fans to kick in. He proved it too by revving the engine and heating it up, when his computer finally reached like 220 - 240, our needle was burried and we FINALLY got a check gauges light. The rememedy was a simple sending unit plug. It's really cheap too and plugs into the driverside of the block near the front of the engine. You just gatta reach for it. Oh, and DO NOT attempt changing this while your engine is hot. You WILL get burned!!! I repeat, you WILL get burned!!!

Make sure you get a good quality or ATLEAST a GM one too, or you'll find yourself replacing it again in no later than two weeks.