69 cutlass s, 350 4bbl, HEI, recently restored entire drivetrain/suspension (except engine, which i only changed a few gaskets)

lately, only when parked in driveway after being out and about, "hot" idiot light will flicker or come on solid. it has never come on while out for drive. Previous owner replaced radiator/water pump (though not sure how recent) and i replaced thermostat and flushed/changed fluid. also during resto, i installed new wiring harnesses and changed temp sending unit. Fan clutch is not leaking and fan is operating and shroud in place. water pump is also not leaking.
I have an hei w/ timing set at 16

wierd thing is when in park, light flickers w/ idle, but when in drive and idle settles down, light comes on solid. again only when parked in driveway after a drive. engile idle is a touch lopey as engine has never been rebuilt and prob is a little tired.

is this nothing to worry about. figure next step is to get reading on actual temp of engine, without a temp gauge, what other options could i use

thanks

 

Best thing is to buy or borrow an infrared thermometer to take the engine's temp. It is a non-contact, 3 second test.

If less than 230*F, then the temp switch may be going flakey on you.
The temp switches can be either around 230* or 250*.
Since it is flickering, I suspect an electrical fault.
When it is flickering, remove the lead from the temp switch and connect right to the engine block. If it comes on solid, then replace the temp switch.

 

The light should only operate with the key in the RUN position. You're not saying that the light comes on when the car is turned off, are you?

The first thing I'd check for is low coolant level. Air pockets in the cooling system can cause the sender to do strange things.

 

car is running, has happened both times after a drive when i have left it idle in drive while i popped hood to check carb adjustment or look over engine. like i said, no leaks/drips/ steam and i just replaced sensor last summer during chassis/drivetrain resto. seems wierd it doesn't come on sitting at traffic light, but comes on in driveway idling w/ hood up.

 

Also, with the light on, disconnect the wire from the sender and do not allow it to contact anything metal. If light does not go out, you have an short elsewhere in the circuit.
Do not run the engine with the light on for long, just in case it is a true overheat condition.
An IR thermometer can be had at most tool stores, even Harbor Freight (for $10) - a must for every toolbox.

New senders have been known to fail early, some DOA right out of the box. Cannot rule that out.

 

Is the driveway sloped? If it is, then it's possible that an air pocket is forming around the sender only when parked at that angle.

 

I've had several temp senders go wacky. What would happen is the sensing bulb would get a little off centered to the switch body and didn't take much for it to swing over and ground against the body, giving a false hot indication. Flickering makes me suspect this.

Sometimes even a piece of rust will get lodged between the sensing bulb and body and conduct just enough to ground the switch and give hot indication.

 

GM service manual tells 253 degrees for temp light activation. if the cooling system was reaching that temp you would hear the radiator rumbling like Old Faithfull just before it spews.

switches are cheap. siphon a gallon of coolant from the radiator and change it after the wire off test

 

i bought an ir thermometer and highest reading i received was 196 idling in driveway, but light was activating when thermometer read in 180s (pointing at coolant crossover on front of intake. i also increased idle to 650 (i had it around 600). when i gave it gas, light went out
wierd that it just began going off last few times out and wierd it doesn't go off when idling at traffic light

 

quick question, how do you test if fan clutch is operating correctly, i was wondering since increasing rpms while in park makes light go out

 

Just to add to all of this temperature conjecture.....
Just because the clutch does not leak oil, does not mean it is a good unit!
Just had to replace my clutch fan on my 90K Dakota-
Looked good, ran hot in 110 deg stop and go driving. Ran fine on the freeway.
Yeah, it looked good. It just would not move any amount of air when it got to 205.
That fan needs to pull a massive amount of air when it gets warm. By your description, it sounds like when it is rolling, it stays cool.
The best way to test is to run ANOTHER unit in place of the first. Mine seemed fine...I changed it, it stays cool now.
Get a real gauge on it, think about your clutch fan.
Jim

 

Happened to me last month. Read the service manual (took 3 minutes) and deduced the problem was the temp sending unit. Bought it new at O'reilly (quite cheap), and in 5 minutes problem was solved. Hopefully yours is that easy as well.

 

If the light does not light up when the wire is pulled from it and isolated, then change the sender.. Engine vibration and harmonics could be causing a bad switch to do that. At only 200* or less, that light should be off completely.

When idling and engine / coolant temp rises above 195*, the fan clutch should engage. There should be an audible ROAR and LOTS of air being moved. Your hair will be blowing in the wind. When it cools below 190 or so, the clutch will disengage.
Keep in mind clutch engagement / disengagement cycles are gradual and take several seconds - not instantly like a modern AC compressor clutch.

 

sender appears to check out okay, i examined the clutch and noticed the front was stained around spring. i purchased a replacement fan clutch and diff. in fan speed is noticable and light has remained off. thanks for help.