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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.
  • edited January 2013
    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
This discussion has been closed.