[Jon Salem]

I have a 71 lemans sport 400/4bbl/T-41 option , no AC. The dash gage was occasionally indicating a hot condition (>250F), though it never boiled over, etc on shut off. And the gage would sometimes read normal when restarted immediately. So I replaced the thermostat and flushed the cooling system and did some wire wiggling. It seems to run a little cooler but occasinally the gage reads high despite it seeming to be normal.

Do the sensors read high when bad? Guess I'll replace it next and check the dash wires..............

[mattgouras]

I had an intermittent temp reading in a Toyota truck once. Turned out to be a bad radiator cap. Don't ask me how or why this caused the condition.

[gtohunter]

Sounds more like an electrical glitch with wiring or sending unit?? Given that it "reads normal when restarted immediately" makes you believe it's not physically running that hot.

[smagnotti]

When this happens make sure the alt is putting out no more than 14.7 volts to the battery, if its over this voltage the gauges will read high.
I might also pay to get a cheap drop in type temp gauge as used for cooking to truly confirm the coolant temp.

[surfnuke9]

this points to an intermittent high resistance condition in your temp gage wiring/sending unit.

[Jon Salem]

I've been meaning to check out the charging circuit, so the 14.7V output suggestion is good timing.

The cap pressure may affect the heat transfer rate (steam boundary layer insulates - sorry I'm a rocket scientist) and could thus cause an issue, so another interesting lead. And breaking a steam pocket could cause a quick temp drop. A faint glycol odor does occur when parked in the garage hot, but the rate of coolant loss is very very low. The manual does indicate cap problems when it actually boils over.

I'll check the resistance on the sensor and see if the manual gives a spec. Also, I want to see which way the gage banks when the circuit is broken or shorted.

I did put in a clutch fan that I picked out of a bone yard 29 years ago. Do the clutches go bad/slip? It really should have a 4 blade or a 5 blade flex fan (GTO) according to the manual. So, I’ll revert when replacing the cracked shroud.

Good ideas, Thanks.

[Jon Salem]

I'm getting 13 volts engine off. 14.5 volts on idle, so it does not seem to be a high voltage.

The sensor gives 660 ohms cold and 240 ohms in boiling water, so high resistance in that part of the the circuit is out, though if it is a bridge circuit, it could still be an issue.

Cap seems to hold pressure.

New sensor reads a little higher.

The fan clutch may be bad....but it seems to want to run hot even on the high way.....

Do water pumps stop circulating water sufficiently over time? I cannot see any flow with the cap off. It is the origional pump with 101,00 miles.

Will try cleaning the bugs from the radiator next.

[george kujanski]

I'd say intermittent reading like that indicate wiring glitches.

George

[chiphead]

Now you know why most of these cars have an aftermarket analog capillary-bulb gauge hung under the dash. After 40 years, the factory wiring gets a little buggy. As I recall, the electrical temp circuit goes thru the engine harness, thru the bulkhead connector, and up into the dash harness and into the cluster. Lots of junctions for corrosion, loose connections, frayed wires, etc to cause a problem. A real engine temp of 250F would cause pinging, dieseling, blown head gaskets, blown hoses, etc. I once got a P350 up to 230F. It was gurgling, running rough, blowing steam. When I shut it off, it made those evil TINK TINK TINK metal-expansion noises. You would KNOW something was bad wrong.

chip

[Geeto-Tiger]

Originally Posted by Jon Salem

I have a 71 lemans sport 400/4bbl/T-41 option , no AC. The dash gage was occasionally indicating a hot condition (>250F), though it never boiled over, etc on shut off. And the gage would sometimes read normal when restarted immediately. So I replaced the thermostat and flushed the cooling system and did some wire wiggling. It seems to run a little cooler but occasinally the gage reads high despite it seeming to be normal.

Do the sensors read high when bad? Guess I'll replace it next and check the dash wires..............

I am not sure if I understand your conditions but my car (409 Chevy) does a similar thing. I always attributed it to a short duration, temperature overshoot after ignition off. The temp gauge is at a steady state value of 210 F with the engine running. The engine is generating a much higher temperature but because there are many kWatts of heat exchange occurring, it stabilizes at 210 F. At key off, the initial core engine temp tends to overshoot and approach its higher value because there is no longer any heat exchange. The gauge will then reflect the actual 'non-heat exchanged' value. Many smaller vehicles in the 90's had an electro-drive powered fan delay following key-off for several minutes to try to resolve temp issues. I am not sure if this is completely logical and will look for your feedback.

Regarding the sensor, it is a thermistor. Chances are, if it works properly with a cold engine and while driving, it is likely giving you an accurate reading with eng off. Thermistor failure modes are usually hard failures and not intermittent. If you want to check it to make sure, the next time it goes to 250 F, disconnect the wire and get a resistance reading from the terminal to the base. We can look it up on a chart that I am expecting to give to old goat 67 (one of these days!).

[Jon Salem]

Geeto,

Thanks for the feedback. The 71 factory manual only has a little info on the sensors (page 6D-5)

It turns out the system has two sensors, both on the fluid cross over between the heads. The driver’s side sensor checks out (660 ohms cold, 90 ohms at 212F). It has a single pin terminal. I compared old and new sensors. The new responds faster and is lighter, but they give similar readings in boiling water.

The passenger side sensor is called a TCS switch (transmission controlled spark). It has three terminals stakes and four wires, one going to the other sensor. I get an open circuit between ground and all three terminals, so it may be bad. Terminal to terminal, two sets act as open circuits and the third gives < 0.3 ohms, so it seems to be shorted. The contacts are in poor shape, so I cleaned them to no avail. So, if anyone has specs on what it should give, please let me know.

Too make sure I'm not frying the engine, I shoved a K thermocouple in the system next to the driver (single term) sensor, with the thermostat removed. It gave 185F after a long idle, while the gage gave 250+. With the thermostat in, it have slightly higher readings on the exit-radiator side. The thermostat checks out and was new. The NAPA boys could not look find the TCS switch, so if anyone knows where to get one, let me know.

The system also has a TCS transmission pressure switch. It all ties into timing advance, which does occur in park when I rev the engine. I can sacn the info if anyone wants it. Picts attached.