Take foot off gas... car won't stop

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When I take my foot off the gas, the vehicle will slow down about 5 mph and then keep going at that speed even with my foot off the gas. In town I typically go about 30 mph on the roads here. If I take my foot off the gas, it slows down to 25 and will keep going that speed until I brake. This morning I was going 25, took my foot off the gas and it (slowly) slowed down to 20 and stuck there.

Vehicle is a 2003 GMC Envoy with the 4.2, 108k miles. Has a lot of new parts and fluids, and I cleaned the throttle body and put in new plugs a month ago. I had a shop put in a new valve cover gasket at the same time (maybe there is a leak there?). This is something I just noticed, but it might have been doing it since I've owned it without me noticing (bought it at the end of July). I don't go on the hwy much, but last time I did the vehicle slowed down while coasting from hwy speeds to the point where I'd normally brake (around 40 mph).

To keep going a given speed it needs air and fuel. Any ideas?
 
Just a guess but, some engines have a "little shock absorber so the engine will not return to idle speed too fast (emissions). That absorber may be hanging up. I'd locate it and try to see if it is the cause. Ed
 
lube throttle cable/body,
I had one that would stick, try it with the vehicle turn off have someone push gas pedal and check if throttle goes back to resting position when foot is off it.
check idle speed in neutral/park
push gas pedal and let go and recheck idle speed

your cold start high idle thing may be faulty - not sure of technical term
 
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Originally Posted By: Eddie
Just a guess but, some engines have a "little shock absorber so the engine will not return to idle speed too fast (emissions). That absorber may be hanging up. I'd locate it and try to see if it is the cause. Ed


Are you talking about a dash pot?

If it does have a dash pot, give it some silicone spray.
 
Could be the throttle blade sticking in the bore. Might need to take the throttle body completely off, and give it a good soaking.
 
Originally Posted By: stockrex
lube throttle cable/body,
I had one that would stick, try it with the vehicle turn off have someone push gas pedal and check if throttle goes back to resting position when foot is off it.
check idle speed in neutral/park
push gas pedal and let go and recheck idle speed

your cold start high idle thing may be faulty - not sure of technical term


Cold start high idle could be idle air control valve; usually will be accompanied by oscillating revs though.
 
I don't think OP is complaining of a high idle, just that the vehicle doesn't continue to slow down on it's own. My experience is most modern vehicles will coast in drive for a long time at a decent speed without any throttle input.

The days of the 3 speed auto that you had to constantly give throttle to maintain momentum are gone.
 
Yeah, be happy that it does that. Not sure why you're driving that slow anyway. Manufacturers do all they can to reduce friction to improve gas mileage. You don't need much power to maintain speed. It's usually a double of speed requires a square of the power or something to that effect, so going 100mph requires a lot more power than just 30 or 40.
 
Electric throttle? It will stay partially open until you hit the brake or coast below some pre-set speed. Something to do with economy and emissions.
 
Pop it into N(eutral) and see if engine revs up past say 1500-1700.

I think something got almost MOOBAR at your last service. Look for air leaks in the complete intake stream, PCV circuit, look for tears or breaks the intake boot past the metering ( if any). I had a 91 Mazda 626 split an intake pipe in the winter and you didn't see the crack unless you REMOVED the pipe ( rubber accordion hose) and bent it backward a tad. Engine torqueing exercised that pipe a lot.
 
Thanks for the input.

I looked at the throttle body yesterday (as it was getting dark, though), and it doesn't seem to be closing completely. When started from cold, the RPMs are about 1500 for a minute or so and the engine cooling fan in the front by the radiator is on very high. RPMs do seem to be a little high when the vehicle is coasting with my foot off the gas, and even at rest in park or neutral.

Could it be the idle control valve? I don't even know if this vehicle has one or if it is integrated into the throttle body.

Rock Auto has a remanufactured throttle body by Cardone for $117 after the core charge. I can get a new one under $200 after AAP discount codes.

I could also try lubing the cable, but don't think that is it since it is protected by a plastic sheath, so I doubt it is damaged.

Thoughts?
 
clean the throttle body, very common to get gunked up on trailblazers/envoys while cleaning throttle body (remove throttle body, clean both sides well, with throttle body cleaner) disconnect battery to reset everything. owned an envoy and this really helped fuel economy and idle quality

the throttle body contains idle air control and tps and is typically replaced as a whole assembly - it's not user serviceable. if it's bad, easy to find at junkyard for cheap, not a very common failure though from what i've read
 
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It's got a fan clutch, they lock up usually when they go bad, they were very problematic in the first few years of the Trailblazer, Envoy, etc.

Time to replace the fan clutch, need to get it from the dealer.

I wonder if the fan clutch is locked up and the drag of the fan is making the car compensate and give it extra throttle.
 
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