New ? On old car - temp gauge & idle

Hello all!
Asked about my 1992 paseo weird temp issue recently. No work has been done, I’ve been driving a new(er) car the last 3 weeks.
Today went to get a run in, knock the rust off the discs etc & went around the block.

Hard to start.
Had to blip the gas to keep running till it warmed up for 20 seconds or so.
While driving & after legit warmup - temp gauge was bouncing alllll over the place from 0 to “normal” when moving.
Sitting it hung out where is should.
The upper rad hose was warm-ish, lower cool, no overheat.
I’m thinking temp sensor going wonky.
Bad temp signal to ecu would cause the hard start/wonky cold idle.
Anyhoo - thoughts?

Have you checked the coolant level?


1 Like

Is the check engine light on?

fullll coolant, yes

no check engine, obd1

Find out the temp sensor spec and test yours. Ohms vary with temperature. There may be two sensors: some cars have one for the gauge and another for the computer.

OBD1 has a check engine light system. I’m guessing from your answer it is not on. Temp sensor then.

I’m aware.
think there’s 2 on this car - but what are the chances both are funky?
(idle & jumpy gauge)

I think there’s air trapped in the cooling system.

If air comes in contact with any of the coolant temp sensors, it’ll cause the temp gauge to read erratic, and the temp sensor for the computer will cause the computer to think the engine is cold. So the erratic idle.

Try this.

Get the engine up to operating temperature. While the engine is idling, loosen the upper radiator hose clamp and slip a small flat bladed screwdriver between the radiator hose and the hose neck.

Allow the engine to idle until any air in the cooling system is purged out.

Remove the screwdriver and tighten the hose clamp.


Tester is probably on the right track. I would only add that in some cars you need your heater control on Hot to open a valve between the engine and the heater core and get the whole system burped.

Did you check both the radiator and the coolant overflow bottle? If the coolant is ok, first thing I’d do w/that problem (esp given the car sat unused for 3 weeks) is make sure the battery voltage is ok (engine off appx 12.6 volts, engine running 13.5-15 volts). If that’s ok then I’ve remove the battery post connectors and clean and tighten the connection to the battery.

More than once every couple seconds?
It’s an electrical gremlin. Not uncommon on a VW.
Check for clean, tight connection to temp sensor.

Pretty sure it’s a Toyota.

Oops! Paseo, Passat, potato, potAHto.

Cars that sit in cold for a while, especially cars with older batteries, need to be driven and fully warmed up. If it were mine I’d do what the others have suggested, then, armed with a cell phone for calls if you get stranded, take the car for a ride and go at least 20 miles. When you get home let it cool down fully, like overnight, then check the coolant levels in the radiator and the overflow.

Okay guys I’m about to get on this car here and a couple questions:

Why would I do that funky screwdriver in the hose deal instead of just idling it with the radiator cap off? Also doing that in my not just going to get antifreeze coming out the gap?

It is a Toyota and the temperature gauge is literally flipping around like a pinball flipper. Never goes higher than whatever the current temperature is. Verified with an infrared thermometer well is verified as I can get from the car having an analog temp gauge

I’m getting some major cold start issues which leads me to believe that the car is actually getting a bad temperature reading to the ECU. Which could very well be because there’s air going by the sensor. Although it’s me it certainly seems like an electrical gremlin

Sunday I should have a chance to really get into things. I’ve got a lot of suggestions from everybody I think:

1 warm it up with the radiator cap off

2 fill up the overflow tank put the radiator cap on and go for a drive

3 see what happens

I don’t know if anybody remembers my prior post from about a month-and-a-half ago but I was having a weird temperature issue no heat from the dash a warm upper radiator hose and a cold lower radiator hose. The temp gauge was fine and the car never overheated and it did eventually get good Heat from the dash vents. At the time that suggestion was put a thermostat in which I have not done yet. Would anybody recommend the thermostat? It couldn’t hurt and there’s probably a ground on the thermostat housing I could check.

Thanks again everybody this community is great!!

Because with the radiator cap off, the cooling system doesn’t build any pressure to force the air out of the cooling system.


That makes perfect sense but what about you know hot pressurized antifreeze shooting out around the screwdriver?

The coolant is only under 12-14 PSI MAX.

The radiator cap controls the coolant pressure.

So, once the air is purged, the coolant just dribbles out.

It doesn’t shoot out.

I’ve been purging cooling systems of air for almost 50 years using this method.

Works every time.


Had to take the toyota today.
No start.

New cap & rotor, & wires dryer & she fired right up.

Did the air burp trick.

Driving, stopped, checked fluids etc (sat for a month)

2 hairline, invisible cracks in the radiator near the fill neck.

1/4 of a bottle of bars leak is looking like it did the trick for now, till my new radiator gets ordered.

Temp gauge is solid. And all issues are resolved / explained.