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rpm help!!!


fisherbear

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ok... so I just changed out the fan clutch because the temp gauge was getting a little high for my comfort... I replaced it with one that has "supercool".... basically a fan clutch on steroids that weighs twice as much as a normal one... after installing this one the temp is where it's supposed to be but the RPMs are much higher than normal... around 1200 when they are usually around 800... that's a big jump... could the increased weight in the fan clutch cause increased rpms? I've debated changing it out for the regular one but it's a pain to get off... thanks, Johnny
 


bucko

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Seems backwards...when a clutch fan goes bad, it slips, and the RPM's would go up at idle. When working, the clutch places a load on the engine (slight), which could reduce the RPM's a bit.

So if you installed a fan thats heavy duty, I'd think the RPM's would have dropped, not increased.
 

Rearanger

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I just changed out the fan clutch because the temp gauge was getting a little high for my comfort... I replaced it with one that has "supercool".... basically a fan clutch on steroids that weighs twice as much as a normal one
I think you were trying to fix the symptom and not the cause. That "steroid" fan probably has a higher CFM. I would have looked at the rad condition first, water pump flow, maybe even replaced the stat.

after installing this one the temp is where it's supposed to be but the RPMs are much higher than normal... around 1200
The idle air control (IAC) sets rpm through the computer. It's hard to see how a higher CFM fan would cause the computer to call for higher rpm. You might test the engine coolant temp sensor (ECT) (not temp gauge switch) to see if it is falsely telling the computer the engine is cooler than operating, as in cold start-up, which does call for higher rpm.

You can also clean the IAC with some spray solvent as well to see if it's sticking open.
 

RonD

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Cold engine RPMs about 1,200
Warm engine RPMs 800(auto), 700(manual)

No, as said, good or bad fan clutch won't change RPM.
On fuel injected engines a computer controls RPMs since there are no "jets".
Computer uses a controlled "vacuum leak", the IAC valve, to control idle by letting more or less air into the intake.
Computer bases "warm" or "cold" engine from reading coolant temp via the ECT sensor, a TWO wire sensor only used by the computer, it looks similar to the ONE wire Sender used for the dash board temp gauge, but the two are not connected or the same.


A failing fan clutch will cause temp gauge to rise when idling then drop to normal when driving, this is because the fan is mostly used to circulate air thru rad when truck is not moving.

If temp rises when driving but is normal at idle then it would be a circulation issue, which could be water pump, blockages in rad, or not fully open thermostat
 

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