Coolant Temp Low, Thermostat or ECT?

[ConfederateTyrant]

My brother has a 2001 Chevrolet Impala, with the 3.4L V-6. He just recently had his BCM replaced as the car was generating a lot of electrical issues, such as obnoxious clicking, radio not working, interior lights flickering, keyless entry not working, guages registering wrongly, and other **** like that.

He had that fixed last week and today his check engine light came on. I told him to go to Auto Zone and get it scanned, which he did, but the guy didn't tell him a code number, just the description. The description was that the coolant temperature was lower than specifications, and the guy said that it is either the coolant sensor (ECT) or the thermostat. My brother then noticed that his temperature guage was reading in the cold area and not near the normal lines. He has been driving about 40-50 miles on the highway by the way.

When he got home he felt the upper coolant hose and said it was hot, not scalding hot, but hot. That seems to me that the thermostat is ok and that the sensor may be at fault. Also, he mentioned that after the engine was shut off, under the hood there is the sound of something going and he feels air blowing, so I am assuming it's the coolant fans, someone correct me if I'm wrong please.

Does that sound like a bad ECT or a bad thermostat?

Thanks in advance.

 

[porcupine73]

If the thermostat failed open I don't think there'd be much change in the gauge then. Anyways, I don't know how hard it is to do the thermostat on that vehicle, but if it's ready for a coolant change it would be a cheap thing to replace proactively anyway.

Well I don't know how Chevy's handle it, but on Subaru's there are two engine coolant temp sensors: one for the gauge, near the water pump (so that the gauge doens't flucuate a lot), and the other for the computer near the block return to radiator (used for fuel map, cooling fans, etc.).

 

[swalve]

Might have to do an old school thermostat test- start the car cold and feel the upper hose for the surge of hot when the thermostat opens. Or watch for flow in the radiator. Or test the temperature of the coolant with a thermostat.

 

[eljefino]

an exact POxxx style code is important.

The newest cars are smart enough to throw codes if they've been running X minutes at X throttle angle with X intake air temp... but still cooler than Y coolant temp. This assumes a bad thermostat but could be an out-of-rance ECTS.

Any car since the 80's will throw a different code if the wiring connector to the ECTS is open or shorted.

With the fans running, I suspect a fail-safe mode has been tripped, more likely with the second scenario than the first one. This should coincide with the CEL lighting, though it may be tough to remember if the fan had always run like that.

If one has access to real-time data, they can scan the car after an overnight sit and see if computer-reported coolant temp is very close to ambient.

A hot upper rad hose could simply be engine heat being "wasted" to the rad, and not retained by the thermostat, so that's rather inconclusive.

A good thermostat, will, on a cold start, allow interior heat within a couple minutes. A bad thermostat will have lousy heat 15 minutes later.

If you don't already have the diagnostic tools, it might be cheaper and simpler to do both parts and an antifreeze flush all at the same time.

 

[ConfederateTyrant]

Thanks everyone, I let my brother know, but he told me that the light went off on it's own, and the temperature guage was reading in the normal area like it should be.

Stupid OBDII systems.

 

[eljefino]

Should still investigate this, as it can be an intermittent problem that'll come back with a vengeance.

For example a sticky thermostat could overheat the motor 5 minutes after a cold start. A low-reading cts will never activate the radiator fan in unusually thick traffic.

 

[unDummy]

Yep, future problem brewing. Change the thermostat now. And, make sure coolant is fully bled and topped off.
Bad sensor, bad thermostat, bad waterpump or its belt, or an air pocket can cause the problem.
I don't see OBDII as stupid. Many techs are clueless without it.

New thermostat and rad cap, new coolant, and new belts, are all very cheap insurance for a 5 year oil car, if none have ever been serviced.

 

[ConfederateTyrant]

I'll let him know, has anyone thought that the BCM replacement could cause this?