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2005 v6 silverado high idle

05grey43v6

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05grey43v6 Enthusiast

05grey43v6

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2005 silverado reg cab short bed 2wd 4.3 v6 5 speed manual 63,000 miles

 

Driving on the highway I downshifted while braking for a ramp and all the sudden my engine idle got stuck between 3800 and 4300 rpms. Limped home with the truck under what seemed like constant 20 percent throttle while in gear by itself. take it out of gear and it revs up to 4000 again, incredibly hard to put in gear and kinda dangerous to drive. No cel and nothing stored in my code reader, wondering where to start on fixing this?

 

Pulled on throttle cable and seems free, spring is good and throttle moves. Can I replace a solenoid? IAC?

 

Brake booster blow a vacuum line?

 

Thoughts on this would be appreciated.

05grey43v6 Enthusiast

05grey43v6

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Also, in gear like 5th going 70 mph if I take my foot off the gas pedal I hear a hissing noise from behind the hvac area. Hvac and fan was off. Push on the gas and after like 20 percent down it goes away only to come back when I let off again.

Motor City Rick Enthusiast

Motor City Rick

Posted
Posted

I hope you inspected that your throttle door was completely closed.

 

To get 4000 rpm's, you have to have one humongous vacuum leak.

 

Start looking at major vacuum fittings on the intake manifold, or the manifold has a major leak / crack etc. With the IAC in full open position, that "might" give you lower 2000 rpm's, not 4,000 rpm.

Supreme Pizza Senior Enthusiast

Supreme Pizza

Posted
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The engine vacuum is the highest during a high speed deceleration with the throttle closed (while in gear). Any vacuum leaks inside the passenger compartment would be more prominent at that time. Many vehicles use vacuum lines in the HVAC system. I would start with any vacuum lines that pass through the firewall. Like MCR said, it would probably not raise the RPM's that high.

05grey43v6 Enthusiast

05grey43v6

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Replaced the idle air control valve and took the intake tubing off, throttle body cleaner the inside of the body. There was some grime and carbon but not alot. Did the idle relearn procedure and truck was fine idling and a little revving while parked.

 

Drove it today and 10 min in it slammed back up to 4000 rpms. This time while coming to stop on a downhill from 30 mph. First time it did it was on a downhill too.

 

Other than the big vacuum line off the brake booster what other lines are there? All I see are pcv lines or lines from top of cyl heads into the intake.

Motor City Rick Enthusiast

Motor City Rick

Posted
Posted

Next time this high rpm's situation happens at stop light / sign, place the truck in "neutral" or "park", whatever is safer as fast as you can, turn "off" the motor for about 5 to 10 seconds, then turn the key "on" wait 5 seconds, then restart the motor. DO NOT touch the gas pedal at all after you get ready to shut down the motor and restart.

 

Was the high rpm's still present???

 

Report back on this.

05grey43v6 Enthusiast

05grey43v6

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Ended up taking it to a shop. Frayed or cut wire from the IAC to the main harness, guess it was rubbing on the motor and finally gave way. Repaired and running fine now.

 

Now I just have a shimmy at 60-70 mph you can feel in the seat. Tires just balanced, thinking u joints?

Motor City Rick Enthusiast

Motor City Rick

Posted
Posted

In your last statement "Now I just have a shimmy at 60-70 mph you can feel in the seat. Tires just balanced, thinking u joints?".

 

Easy step method:

 

Jack up one side of the rear axle (if you don't have limited slip / locking differential, if you do, than jack up complete rear axle) spin the wheel (s) up to 45 mph.

 

If there is vibration, it's a balance issue. Take it to 70 mph, did the vibration reduce then come back near 70 mph??? If so, rebalance tire(s) if either rear tire has this vibration issue. A good balance will be very little to no vibration and only hearing the tire(s) whooshing in air. Most balance on lasts up to 7,500 to 10,000 miles

 

Now to separate whether it's a driveshaft issue or tire balance.

 

Remove both rear wheels on jacked up rear axle. Install 2 lug nuts opposite of each other to secure brake drum / rotor securely in place.

Slowly spin rear axle up to 70 mph. Physically look at the drive shaft spinning at maximum vibration (need two people for this procedure). Do you see any lateral movement???

 

If any vibration is present, the drive shaft has to be serviced either for u-joint replacement, lateral run out issues or simple driveshaft re-balance.

Motor City Rick Enthusiast

Motor City Rick

Posted
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I've done complex diagnostics for the car manufactures in a 36 year span. Seen it all, heard it all, and cleaned up all the bull chit "guesstimate repairs" that people do. It's like looking at kids trying figure out how to turn a door knob.

  • 7 months later...
Maxine New Member

Maxine

Posted
Posted

Sounds similar to another high idle, surge problem in the forums and it was the TAC (throttle actuator control ) but I don't know if your vehicle has one.  It wouldn't hurt to check. 

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