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95 4.0 Rough and random idle

mxboy349

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I have a 4.0 that idles high, 1100-1200ish rpm, then will drop and rough idle 800-900rpm where it will almost die. I have replaced all vac lines with new, new plugs and wires, new Idle control valve, no dash lights. Where should I look next? Truck has plenty of power, no real hesitation, just rough and random idle when at a stop light or left sitting while idling. One thing I have noticed is my temp gauge doesn't seem to get very warm and I am suspecting a stuck thermostat. Would this cause a random idle issue?
 


franklin2

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Some of the 4.0's had a chronic intake gasket problem. There might even be a old TSB out on it.
 

RonD

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All gasoline engines need rich mix and higher idle when cold, called Choke mode in fuel injection systems
This is done based on coolant temp
So yes, if your thermostat is stuck open and coolant is not warming up and staying up, 190degF, then computer could be changing mix richer and setting idle too high

Computer doesn't use the dash board temp gauge/sender for coolant temp, that's a separate 12volt system
Computer uses the ECT sensor, a 2 wire 5volt sensor, which can also act up but wouldn't effect the dash temp gauge

Just as a test
Warm up engine or after getting home don't shut off engine
Let it idle
Unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or so, or engine my even stall, either is GOOD, it means no air leaks, test is done

If RPMs do not drop then there are leaks, and IAC Valve/computer can not set lower idle RPMs because of that
Leave IAC Valve unplugged
Pull off vacuum hoses from intake and plug intake port with finger, repeat with next hose, ect.......RPMs should drop if a leaky system is found
Start with power brake booster and PCV Valve hoses
 
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mxboy349

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Unplugged the IAC when I got home after a 20 minute stop and go drive home. Engine was well warm. The idle definitely changed and was rough, but not down to 500, when I plugged back in the idle smoothed out. I repeated the unplug again after a minute or two and this time it died. Temperature gauge was all the way down to the first mark above the C and on the way home the temperature gauge never really got "warm". I think I am going to try new thermostat along with some new belts and hoses. Can't hurt even if it doesn't fix the issue.
 

RonD

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So no Vacuum leak

Yes, new thermostat, dash temp gauge has 200degF as center, operating temp is 190deg, so just below 1/2 is what to expect
Just above 1/2 on a long uphill grade or pulling a load

Correct thermostat is 190-195degF, and should have a "jiggle valve" and yes that is its actual name, lol, its a small hole in the thermostat plate to let air out from the engine side, it has a loose metal pin in it that will keep the hole clean of debris, because it "jiggles"
When installing the thermostat the jiggle valve hole should be at 12:00, where trapped air would be
 
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mxboy349

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Well thermostat didn't solve the issue, but it didn't hurt to replace everything as hoses were pretty soft. I am pretty sure heater hoses were original. Still idles rough and randomly drops to 500rpm and almost dies. I am going to do some vac line checking and EGR system checking next.
 

RonD

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Start with power brake booster and PCV valve, for vacuum leaks
 

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