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Would a stuck open thermostat cause a slight rough idle?

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Old 10-19-2005, 05:59 PM   #1
porshapower
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Would a stuck open thermostat cause a slight rough idle?

I have been chasing a slight bad idle and have tried everything. One thing that struck my mind is that the temperature barely ever moves to the 1st line. Would a bad temp switch for gauge or thermostat or temp sending switch cause a slight bad idle?? 1996 explorer 4.0..ThanksNel
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Old 10-19-2005, 06:13 PM   #2
Vanquish502
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No. An open thermostat would just let the engine run cool all the time, making your heater useless.
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Old 10-19-2005, 06:33 PM   #3
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it will also not allow the computer to switch into open loop operation,, the truck would run rich all the time and lose performance.

But I would not suspect a bad thermostat to cause a miss.




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Old 10-20-2005, 07:57 PM   #4
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My thermostat stuck open a while back. It gave a fast idle, but it was smooth.

Since it's so easy, I'd replace the thermostat first. Those stock Motorcrafts are made in Germany, and seem to be failure-prone for some reason......? I replaced mine with a Stant.




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Old 10-21-2005, 08:48 AM   #5
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When my thermostat went not only was I not getting heat, but the idle was rough and my gas mileage went south. Changed the thermostat, got heat, smoother idle, and my gas mileage improved greatly. Its a 15 minute job. Good Luck!
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Old 10-21-2005, 10:09 AM   #6
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Quote:
Originally Posted by 410Fortune
it will also not allow the computer to switch into open loop operation,, the truck would run rich all the time and lose performance.

But I would not suspect a bad thermostat to cause a miss.
???????????
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Old 10-21-2005, 10:18 AM   #7
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Spinlecone, what 410 is saying is because the thermostat is stuck open the engine is not coming up to temperature. The ECC sees the engine is cold and enriches the fuel mixture (like a choke on the old carbs). However the engine never gets warm enough so the computer may never switch from open loop (running off predefined air/fuel etc) to closed loop (where it checks its sensors for feed back on air/fuel etc.).

I agree with him that it should not affect idle. Otherwise all cold vehicles would suffer bad idle. I've had a stuck open tstat before and it did not affect idle at all.




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Old 10-21-2005, 10:40 AM   #8
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Thats what I thought he was trying to say, open to closed loop
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Old 10-21-2005, 11:10 AM   #9
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the coolant temp sensor has nothing to do with idle, does it? Just tells the PCM when to switch from OPEN to CLOSED (hahaha sorry I always thought it was closed then to open, I had it backwards mr picky)

Man where is that flipoff smiley when you need it? LOL




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Old 10-21-2005, 11:21 AM   #10
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Actually, I don't believe coolant temp has anyting to do with open to closed loop, believe that is done by the 02 sensors.
My sensors are heated sensors, at cold start up am in open loop for maybe 5 secs, than it go's to closed loop
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Old 10-21-2005, 12:01 PM   #11
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No way its the coolant temp! At least it was on the EEC-IV....




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Old 10-21-2005, 01:02 PM   #12
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Originally Posted by 410Fortune
No way its the coolant temp! At least it was on the EEC-IV....
K
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Old 10-21-2005, 04:02 PM   #13
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When mine went I had had no heat rough idle, stalled a few times, really bad mileage etc.
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