94 Blazer Coolant Temperature Fluctuating

[Samuel Churchill]

Hello I have a 94 Blazer that has an unusual problem of the gauge jumping up to 260 degrees and then after a couple of seconds dropping back down to 210. This will happen at cruising speed, in town, and even at idle. I replaced the thermostat as well as the coolant sending unit in the head. The last thing I can think of would be the gauge itself is defective.

I have seen a few other people having the same problem but never end up finding a solution. Any help would be apprecieated.

 

[454cid]

A vehicle that is low on coolant will show very hot, and then suddenly drop, but usually the drop is below normal.

 

[Samuel Churchill]

The radiator is full and the reservoir is within range.

 

[RawbDidIt]

The radiator is full and the reservoir is within range.

Might be air in the system, try burping the upper radiator hose with the reservoir cap open. Listen for bubbles or liquid flowing, if you hear bubbles keep going until you hear just liquid flowing, refill reservoir to correct level and test by going for a city drive. Low speed high load (like stop/go) puts the most stress on the system so if the problem is there that's where you'll notice it first. If there's no air in the system, check your serpentine belt and tension, belt slip can cause issues with coolant flow. May only be slipping with increased load on the system such as an additional electrical load on the alternator or the A/C compressor. If the belt and tensioner check out, remove belt and check pulleys for additional resistance or lateral movement.

Sent from my SM-N976V using Tapatalk

 

[east302]

The last thing I can think of would be the gauge itself is defective.
.

I’m not sure if the TBI uses an ECT sensor to report coolant temperature to the computer, but a scanner would show what temperature is being reported.

 

[PlayingWithTBI]

I’m not sure if the TBI uses an ECT sensor to report coolant temperature to the computer, but a scanner would show what temperature is being reported.

Yes it does, the CTS (Coolant Temp Sensor on TBIs, ECT on Vortecs ha ha) is a 2 wire sensor next to the T-Stat housing, The one wire in the head between cylinders 1 & 3 feed the gauge. Yes, a scanner will give you the actual feedback the PCM uses.

 

[Samuel Churchill]

I don't have a scanner except for the u shaped paper clip code reader. Tomorrow I will purchase an infrared thermometer but I have a sneaking suspicion the temps are fine.

Would a failing gauge motor cause the needle to start rising slowly to a point then drop back down quickly to the actual temp? It does this over and over.

 

[alpinecrick]

Both of my vans did that. Flushing the system with a flush kit minimized the fluctuation but did not entirely eliminate it. They only do it--even after being warmed up--in the mornings when I start down the road.

 

[Erik the Awful]

The guaranteed way to get rid of air pockets:

Buy one of these:
https://www.amazon.com/Lisle-24680-Spill-Free-Funnel/dp/B00A6AS6LY

Park your truck with the front end raised, or put the front end up on jackstands.

Use the funnel above to top off your coolant and keep filling until coolant is at the top of the tapered section of the funnel. Fire up the engine and run it until the thermostat opens, keeping the fluid in the funnel filled to the top of the tapered section. When the thermostat opens, the upper radiator hose gets hot, and the funnel quits burping bubbles, use the handy no-spill feature of the funnel to remove it, cap the radiator, then put the excess coolant back in the jug.

Easy.

 

[Foos]

I just went thru the same thing and I replaced the water pump and that took care of the problem