I have a 1986 chevy truck, 350, just had a rebuilt motor put in about 6 or 8 months ago.

I noticed the other day that when I accelerate the temp gauge goes up to about 240 degrees, when I slow down and idle it is at normal temp, around 210 degrees or below 210. The temp gauge literally goes up like an RPM gauge, that fast. With A/C on or off, does the same thing.

I was a little low on antifreeze but I have added some and it still does the same thing. Since having the engine replaced I have had the heater core replaced and the alternator. In fact I replaced the alternator last weekend and it just started raising the temp this week. Can the belt be too tight? The fan is run from two belts, one from the alternator. Is it just a coincidence? It runs fine at idle.

Any ideas?

If, literally, the guage goes up like a tach there is something wrong. You just can't heat a huge hunk of metal in 5 seconds. You should start by checking the timing first. Go for 8 degrees advance at idle with no advance plugged in, for a normal small block 350.

Chuck

Does the temp gauge come down as quick as it goes up?

Dan

Yes, the gauge goes down as quickly as it goes up.

Dan

Yes, the gauge goes down as quickly as it goes up.

I have a 1986 chevy truck, 350, just had a rebuilt motor put in about 6 or 8 months ago.

Since having the engine replaced I have had the heater core replaced and the alternator. In fact I replaced the alternator last weekend and it just started raising the temp this week. Can the belt be too tight? The fan is run from two belts, one from the alternator. Is it just a coincidence? It runs fine at idle.

Any ideas?


I would be suspect of the alternator. Does this truck have an volt meter? If so, does it increase and decrease with rpm. A simple way to eliminate the alternator as the cause would be to either unplug the alternator wires or jump the belt off of alternator. Then take it for a short test drive and see if the problem goes away.

Check the voltage at the battery during acelleration and decereration and see it it is stable at about 12 to 14 Volts? It seems impossible for the engine to heat up and cool that fast so my guess it that this is an electrical problem. You should have plus voltage to the guage and ground comes through the sensor based on the temperature, higher temp equals less resistance to ground and the meter moves accordingly. We know that the engine can't instantly heat and cool so my suggestion is to eliminate electrical problems as the culprit first, then look at the cooling system.

Someone double check me on this but, if you ground the sensor lead the temp meter should go to hot (all the way up). If you do that and rev the engine, it should stay at hot. If it stays hot try replacing the temp sensor that is in the head or mainfold. Don't do this test procedure until someone else confirms. I am not sure about the meter movement direction.

Chuck

yup- ground out the wire, and it pegs to full HOT.

You could have a faulty temp gauge or wiring. You might want to connect a mechanical temp gauge, either as a second gauge or as a temporary measure. That would give you a better idea of what is going on.

Another point someone has not mentioned on the electrical front. While I think you are going in the right direction here recommended by others... often many unknowing mechanics will use some sort of sealing tape, putty or sealer on the sensor. This is a no-no whereas it should be in full contact with the hosting threads or it will serve as a resistor to ground! The fit is an interference fit and should seal without any type of sealant or tape (usually teflon).

check gage readings against a i/r gun ...i never trust gages till comfirmed by another source such as a i/r gun ... compair the 2..just my thinking any who

Thanks to all that posted. I was driving down the road and my engine stopped running and wouldn't start again. Had battery power, the starter was turning the engine but it wasn't firing. Replaced the ignition module and now my truck doesn't overheat.