New thermostat, erratic temperature fluctuations, no heat in the cab


Asked by Jan 26, 2015 at 11:03 AM about the 1999 GMC Safari 3 Dr SLE AWD Passenger Van Extended

Question type: Maintenance & Repair

1999 GMC Safari 120k miles
New Thermostat, battery and starter (yesterday)...
Recent Oil change, Normal Coolant Level, Less than 2 year old water pump and coolant system flush.

Changed the Thermostat because there was erratic heat in the cab with erratic temperature fluctuations. The temperature would jump to 240 within the first minutes of starting the engine but then immediately drop back to normal temperatures. It's cold here, a temperature jump from 0 to 240 like this when the outside temps are less than 30 degrees seems completely unreasonable. I don't think that the van ever really overheated, but I could be wrong. . .

The old thermostat was only a year old. It was covered in sludge when we pulled it out.
Drove my son to school this morning. Outside temp was -5, let the van "warm up" before we drove. After about 5 minutes, the windshield still had frost. NO cab heat what-so-ever. Now, when I drive, the temp gauge goes to 185 then raises to 240 then back to 180. All the while the heater is blowing cold air. I have a relatively short drive, less than 10 minutes.
Someone says the head gasket is blown, but I have no oil in the water or vice-versa.
What causes the engine temperature gauge to fluctuate like this?

3 Answers


in most cases with these old vans is the main flow valve switch gets stuck after a while and it either blocks the flow to the heater or just leaves the heat on in the cab. Also there is a chip in the front of the enviromental controls box jsut under where the fan is that tends to blow out and cause the heat to stop working. There is a replacement circuit to fix it if this is the issue. the circuit board looks like the image bellow should be located on the following image. hopefully this is the issue and will fix it. if not your digging for the flow valve

Best Answer

Location of the control board

7,345 This link will take you to a forums board that will help replace the flow valve if this is the issue.

Your Answer

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