engine temp all over the place
 

I have a new motor radiator water pump. The new thermostat is 180 degrees. The temp goes up and down from 110 to 180 and never settles or stays at 180. I replaced the thermostat with another new one; with the same results. Any ideas as to what is causing this? Oh by the way I used a laser remote sensor to verify the temp variations. It's not gauge related.

is the temp fluctuating at idle or when driving . Im wondering about air flow or water flow .

This isn`t seen on a gauge but with a laser temp reader. Okay. Think of it this way. The coolant in the engine gets up to temp, the T-stat opens at that temp, the hot coolant goes into the radiator.
The coolant in the radiator, which is considerably cooler, is being circulated into the engine, when the cooled coolant reaches the T-stat it closes it, the engine builds back up to temp which kicks open the T-stat and the process starts all over again. That`s why you keep seeing the temp go up and down as the "cooled" coolant from the radiator circulates in the engine cooling it down only to build it back up again so the process can start over. Something else many don`t understand is when they get a mechanical temp gauge is why the temp rises when the engine is shut off, it gets higher because the coolant is no longer circulating.

I see the temp fluctuation's on the gauges and checked it with the laser temp tool. the gauges show up and down fluctuations at idle and while running, either in town or on the highway. The engine does not overheat but goes to the 180 temp of the thermostat and back to 110.

That's a pretty wide Temp swing. Where is the Sensor for your gauge located?

That's a good question. Also, Where and on what are you shooting the laser.

Is this a big block Chev????
A missing bypass hose will do this-----small hose from intake to waterpump.

A plugged heater core on a small block that does not have an AC water valve will do the same thing.

Wide temp swings untill all of the coolant temps have stabilized.

Thanks for the replys. First I use the Laser on the radiator, manifold in several spots, the thermostat housing. Even after extended driving over 35 miles the temp still goes from 110 to 180. When it reaches 180 it drops fairly fast to 110. I am going to the dealer today to get a GM thermostat and see if that will help. It seems that the thermostat opens fully when at normal operating temp of 180 but does not close until it reaches 110. Since this has happened with two new thermostats I have been thinking it may be the brand of thermostat.

The most consistant spot I've found to shoot the laser is the thermostat housing. My temp. sending unit is mounted on the crossover next to the housing on my Edelbrock perf. rpm intake, not the head. :D

You may need to drill a hole in the t-stat. If you dont have a heater internal circulation pressure can hold the stat open or closed.

OK, I'm lost in all this; are the temp swings with gauge like the type that would be installed in or on the dash, or are these numbers you're getting off the laser?

Lasers are sensitive angle of impact and return the beam makes to the part tested. Basically the temp read cannot be trusted on a hand held device because of this.

If you're seeing this on a mechanical or electrical gauge it means there are several things to look for:

- Sender, wiring, bourdon tube, or gauge is faulty.

- Thermostat faulty

- Lack of bypass, Chevy small blocks in particular are sensitive to this. For air-conditioned models there must be a three-way valve in the system that bypasses the heater circuit when the AC is switched on, this has plenty of failure modes of its own resulting from whether the AC controls on the dash are working as well as the bypass valve working as commanded. Lack of bypass causes pump cavitation which puts vapor in the cooling system, tears up the impeller and can result in pump bearing failure as it shakes the daylights out of the impeller shaft.

- The excessive use of RTV rubber sealants, this stuff breaks off and circulates till it cuts a passage off which then makes steam out of what inadequate amounts of coolant get to it.

- Leaking head gasket putting compression and combustion gases into the cooling system that end up insulating the thermostat from the coolant.

- Cooling system not fully filled.

- Water pump faulty
- Wrong rotation pump for type of belt drive. Chevy uses the same casting with a different impeller for Vbelt or Serpentine belt, common to see the packaging ID for the wrong pump inside.
- Impeller not turning with the input shaft.
- Foreign object in the pump blocking flow.

- Inlet hose collapses with pump suction.

- Foreign object or crud in the radiator though this usually results in a constant overheat.

- Pump driven fan has incorrect rotation for belt drive used, like the pump the end user is 100% dependent that the correct part is in the packaging.

- Failure of the pressure cap, loss of system pressure will result in local boiling even though the bulk coolant temps may be in a "normal" range. The resulting steam becomes trapped at the thermostat screwing with its temperature sensing abilities.

- Missing shroud, results in uneven cooling on radiator face.

You get to check all this out.

Bogie

The first thing I'd look at is the coolant level. If you have air trapped in the system, the thermocouple that senses the coolant temps will fluctuate considerably. It can not sense any temp in air and will yield a low temp until some coolant passed over it to then yield a higher temp.

I'd look at modifying or swapping out the thermostat second. Some new thermostats come with the relief hole already there. A 1/8" hole is sufficient.

Without the hole the gauge reading will fluctuate quite a bit until the coolant temps stabilize. The hole allows some coolant to flow thru even when the thermostat is closed or only partially open. This will show a more even temp rise without all the fluctuations.

Isn't that what the factory bypass is for? The extra hole in the water pump?

My by-pass goes from the intake to the heater core and back to the pump. I always assumed it was like that to get the temp. up quicker for the heater in cold weather and help with warming up the engine before the thermostat opened. All cooling systems are not the same though. Old trick for racing street/strip car's in hot weather was running the heater blower to help cool the coolant circulating. :D

Thanks for the reply.