Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Overheating and Rough Idle!?

So heres the deal. My 94 Toyota Pickup is having a strange intermittent issue. Engine will run hot, like the thermostat is stuck periodically. Does not overheat, just runs much closer to the red than usual. Again this only happens sometimes. It is worth noting that it is winter here in Northern Sonoma County, CA, so it may actually get into the red if I were driving around in the summer. I have also not taken it on trips over 30 miles when it was doing this.

When this problem occurs (e.g. the engine has been running hot) and I shut the truck off and say run into the store, then come back to a heat soaked engine it idles very rough when started (like it’s missing on one cylinder). Once I put it in gear and start driving the idle smooths out and I have no problems. Problem has been going on for about a week.

Sticking thermostat seems obvious but that does not explain the rough idle on start up. Or does it. If so why?


It might. An engine running hot can run erratically.

First check your coolant level, then change your T-stat. And if you haven’t flushed your coolant out with fresh coolant for a few years, you might want to do that when you change your T-stat.

Also, when was the last good tuneup? Are the sparkplug wires original? Rotor? Distributor cap?

Wires cap & rotor were replaced about 2 years ago. Along with plugs. Coolant level checked already. I had coolant changed and radiator flushed before I towed a trailer here from oregon last summer.

If you notice that the truck tends to run hotter when going uphill or over 60 mph, then replace your radiator cap.

You might be a candidate for a fresh radiator too based on the age. After a while they can get coated on the inside and lose their ability to transfer heat.

FYI: 94 Pickup V6 are known to blow head gasket. Check the oil dipstick to see if the oil is milky. Or better off, change the oil and see if the oil is milky. If the oil is fine then I’m leaning towards the thermostat.

I think some combination of a coolant flush, new radiator cap, and new thermostat will likely fix this. One thing I do to check the t-stat on my car if I suspect it is not working is to observe how much coolant is flowing through the top radiator hose. When the engine coolant gets hot enough, the t-stat opens, and quite a bit of coolant should be flowing through the radiator. If it’s skimpy flow, then the t-stat is probably stuck.

The other likely possibility is that the water pump is failing.

I like to start simple, a pressure test would be first, and I would throw money away on a new radiator cap and thermostat without hesitation. A flush as suggested above would have already been done in my book but like above in case you missed the manual recommendation!

@Jeremymm check for proper operation of the fan clutch

Check your timing too

I am having similar issue with my 92 Landcruiser (3FE). I have replaced water pump, t-stat, fan clutch, flushed radiator, pressure tested system, vacuum bled system, new temp sensor. It also just had the head removed and was found to be warped. The water jets were pretty clogged too. runs fine now except when idling in hot weather for more than 15 minutes. Idle starts to jump up and down and temp creeps up until I shut it off or drive it, to cool it back down. Radiator is 10 years old and was replaced 40k ago.

input much appreciated, I am at my wits end. Thanks

The idle speed is designed to be partially controlled by the coolant temperature, that’s so the car idles faster on cold starts, then goes back to normal idle rpm as the coolant heats up. So your uneven idle rpm may be caused by the cooling system not maintaining a constant temperature or you have air in the cooling system. Or the gadget that controls the idle speed vs coolant temp isn’t working. On my Corolla of the same year but 4afe engine, that device is located in the throttle body. And isn’t replaceable, except by replacing the entire throttle body. Coolant is routed through that area. It works sort of like a thermostat, wax changes w/temperature to open an air passage in the throttle body to increase or decrease the idle rpm. On my Corolla that device failed. I had to block that air passageway off and just live with the fact the engine idles a little too slow when cold.

However this wouldn’t explain why your coolant temperature is creeping up. For that I suspect from among

  • the cooling system isn’t holding pressure, like the radiator cap seal has failed, head gasket leaking, etc.
  • Engine compartment fan isn’t spinning when it should, or isn’t spinning fast enough. Make sure the fan is blowing in the right direction too, pulling air in from the front.
  • Radiator needs to be replaced
  • Air in cooling system