Low idle when hot

[Litton]

Now that the summer temps are about 105 and my AC is working, I have uncovered an interesting little proplem with my engine idle.

Everything works fine when the car is started.....idle is high and drops to ~740 as the engine temp reaches 175 to 185. Idle increases when the AC is turned on jjust as it should.

However, as the engine temp gets up to ~200 while driving in city traffic, the idle drops some more to the point that it won't run without adding a little gas. The idle is smooth but drops to less about 200 and just sort of stops. Starts right back up as long as I give it some gas.

I wonder if my thermo switch for the idle control is givin me a bit of trouble or should I be looking elswhere? The other thought is if the ECU is smart enough to reduce timing as the engine temp goes up, like the later models.

[psfred]

First, I'd bet you have a bad visco clutch. You shouldn't be getting that hot.

Second, replace (or at least check) the coolant temp sensor and air temp sensor. If one of them is bad, you could be going way lean at idle.

You should also check the idle mixture hot -- you may need to richen it up a bit (or lean it out, could be bad either way).

I would also boost the idle speed a bit -- 750 is kinda low, I'd go for 800 minimum. You should also be getting idle (ignition) advance by that temp too, about the time the electric fan switches on. Check the fuses, switch, and relay for the aux fan -- on all of mine, the fuse was corroeded to the point the fan didn't run when the switch closed.

Peter

[Litton]

I have been wondering about the clutch but it temp has been rock solid at 185 until it really got over 100 outside. The Aux. fan comes on at 212 (or thereabouts) pretty much as it should. However, even so, I wouldn't have expected the idle speed to drop so far with a little bit more temp increase.

I should be able to check the air temp sensor easily enough by simply unpluging it.

As I think about it, one more item comes to mind.....the idle does not change with the AC on or off at this temp. Works fine at 185 but not at 212. (I am assuming the mark above the 175 line is 212).

Tks for the comments, I will check a couple of those items when I get home.

[Litton]

I have been wondering about the clutch but it temp has been rock solid at 185 until it really got over 100 outside. The Aux. fan comes on at 212 (or thereabouts) pretty much as it should. However, even so, I wouldn't have expected the idle speed to drop so far with a little bit more temp increase.

I should be able to check the air temp sensor easily enough by simply unpluging it.

As I think about it, one more item comes to mind.....the idle does not change with the AC on or off at this temp. Works fine at 185 but not at 212. (I am assuming the mark above the 175 line is 212).

Tks for the comments, I will check a couple of those items w

[psfred]

The reason the idle speed doesn't change with AC on hot is that it it already "fast" -- you have something else going on.

It's also possible that the aux air valve is failing to close until it really gets hot, with the result that the idle is way too low when it does close. Check by blocking the hose from the aux air valve.

you need to check the resistance of the temp sensors.

Peter