Jammed Throttle

July 23, 2007 12:26 PM

The throttle on my 2003 S10, V6 54K miles jammed open. Very fortunately it happen in a rural area where the driver was able get off the road and stall the engine by using both feet on the brakes. Has anyone on the list experienced runaway throttle on a S10 or any other car?

I told the family member who had borrowed the truck to have it repaired locally. The culprit was the idle control valve motor that appeared to have become fouled with carbon. The service manager at my local Chevrolet dealer never heard of this problem before and could not offer any maintenance procedure that might prevent this from ever happening again. I called Chevrolet and spoke with two people that appeared to be reading from menus. They were of no help and were only able to tell me to bring the (repaired) truck over to the dealer that had never heard of this problem and could not give any suggestions on how to prevent it from happening again. They also said that I should have brought it to a dealer for diagnosis and repair. Fat chance of being able to find one on a weekend in rural Maine. Any ideas on how to prevent carbon buildup in the air/fuel system? I have found that this is not a S10 specific problem. Other cars that use similar idle controls on the fuel injection (BMW for one example.) I do not want to hear about zombie juice gas additives. How can I get the word out that there may be a serious safety problem here? How can I raise some hell? I could have lost a family member if this had happened in traffic. Many thanks

posted by Raybun to travel & transportation (17 comments total)

The culprit was the idle control valve motor that appeared to have become fouled with carbon.

That's exceedingly odd and, for that particular piece, seems absurd on the face of it. Those little motors, or solenoids, absolutely can not pass enough air to cause something like a real runaway throttle. Not only that, it will cause the vehicle to run quite badly due to the unmetered airflow (EFI needs to know exactly how much air is going in so it can add the appropriate amount of fuel). Are you sure this guy/girl didn't just go goofing off in a field and feed you a cover up story? Is the pickup damaged in any way?
posted by IronLizard at 1:01 PM on July 23, 2007

Oh, and carbon fouling would shut off the added air and cause it to die/idle very low. Which leads me to ask how in the hell carbon made it's way up to this thing. They're located on wither the throttle body or the intake and combustion deposits don't ever go there unless there's something incredibly fucked up with your motor. Like it's running backwards.
posted by IronLizard at 1:07 PM on July 23, 2007

It doesn't really help, but in answer to your first question.. yes, I have, on a Mercedes 190. No idea what it was in the end (supposedly something to do with the *computer*) but it was enough to lead to the car being scrapped.

I've always wondered what to do in such a situation. In my case it resolved itself after a few miles of panic, but if it came down to having to actually shut the car off.. why not just turn it off? It'll screw up steering and braking on most modern cars, but in a rural area that's unlikely to be an issue? That, or just throw it into neutral?

Anyway, good luck with finding out the cause!
posted by wackybrit at 1:12 PM on July 23, 2007

I was driving a mid-90's audi, when the throttle became stuck full open. When it first got stuck, I was 200 yards behind another car, on a twisty mountain road, with no shoulder between the road and the 100+ foot cliffs. After attempting to unstick the throttle with a couple of full depressions of the gas pedal, I just rode the brakes until there was a safe place to pull over, and turned off the engine. Upon lifting the hood, it was immediately obvious, there was a piece of broken plastic that had gotten wedged into the throttle linkage. According to the mechanic, it was a piece of the cruise control, but since that whole system was already disabled, it was an easy fix to just pull the broken part out of the way and go on with life.

If your brakes on the truck can't stop it from accelerating at full throttle, you've got problems way beyond carbon buildup. I was able to maintain a safe distance between me and the car ahead for at least a mile. I'm sure the lifespan on my brakes took a toll, but there wasn't any point during the experience that I thought I couldn't stop the vehicle once it was safe to do so.

wackybrit, on a vehicle with auto-steering and power brakes, killing the engine is going to make control much more difficult. Then again, most american cars lock up the steering wheel when turned off.
posted by nomisxid at 1:19 PM on July 23, 2007

I had a similar issue with the gas not kicking down. IANAM, is that the same thing as a stuck throttle? It was like I had the gas to the floor although I did not even have my foot on the pedal. I could put it in 1st gear and travel UP (a steep) HILL with out even touching the gas pedal I was getting so much gas. My culprit was the IAC (internal air control) valve.
posted by goml at 1:30 PM on July 23, 2007

nomisxid writes "If your brakes on the truck can't stop it from accelerating at full throttle, you've got problems way beyond carbon buildup. "

There are lots of automobiles out there with, uh, more than adequate engines that can't be stopped by the brakes in this situation.

nomisxid writes "on a vehicle with auto-steering and power brakes, killing the engine is going to make control much more difficult. Then again, most american cars lock up the steering wheel when turned off."

Not until you remove the key. Having said that the best thing to do is dump the transmission in neutral so that vacuum and pressure are maintained and then turn off the engine once you are safely stopped.
posted by Mitheral at 1:46 PM on July 23, 2007

"...Then again, most american cars lock up the steering wheel when turned off."
posted by nomisxid at 4:19 PM on July 23

I've never seen a vehicle sold in North America that engaged the steering column lock before the ignition switch went through an "accessory" position. In any event, the U.S. Army suggested procedure for stuck accelerators is to shift into neutral, brake and turn off the roadway, before shutting off the engine. Modern production vehicles (including the OP's S10) with electronic engine management modules all have a built-in rev limiter that will keep the engine from being damaged by over-revving while a driver is doing this.

I've had several S10 pickups since the early 90s, and I've never heard of a stuck throttle problem on this model truck, although, in theory, it could happen on any individual vehicle due to parts wear or breakage, perhaps more easily on older carbureted engines. I doubt it is a problem with this vehicle in terms of being a recallable "design flaw," and the only federal recalls for that year/vehicle family are to have an air bag information sticker on the passenger side replaced with one that has correct wording. There have been recalls for Ford Ranger pickups with throttle problems in that same period.

If you feel strongly that there is a design flaw that contributed to your problem, you might want to search the NHTS database for similar complaints, investigations, and recall notices, or file a complaint with the NHTS. That's the most effective way of calling attention to the problem, and "getting the word out," if there is in fact some slowly surfacing problem with this model vehicle. However, the lack of similar complaints in the 4 years since this model was sold argues against it being a recallable defect.

Although you specifically state that you don't want to hear about fuel additives, as a fellow S10 owner, I feel compelled to draw your attention to the voluntary Top Tier Gasoline standard, and to point out that the very high detergent levels added to Shell and Chevron high octane gasolines can be quite effective in maintaining fuel system cleanliness. If your truck has exhibited varnish on the interior of the throttle body at 54,000 miles, you may want to start a routine of driving a tank of high detergent gas through it ever 3rd or 4th tank, at least.
posted by paulsc at 2:02 PM on July 23, 2007

There are lots of automobiles out there with, uh, more than adequate engines that can't be stopped by the brakes in this situation.

If your brakes are in decent condition, they should be able to slow/lock the wheels regardless of the throttle position. If there are exceptions to this, they're very few.
posted by jalexei at 2:52 PM on July 23, 2007

Mithreral, you must not drive many cars then. I've seen plenty where the only way to get to accessory is to turn the key 'backwards' from the off position. On my current '00 beetle, there is no such thing as an accessory position, because you can turn the radio off/on without the key; there is no key position where the engine is off, but the wheel isn't locked.
posted by nomisxid at 4:04 PM on July 23, 2007

er, not mithreal, paulsc. What I get for writing my response based on reading in lynx, then copy-pasting over to firefox to post. =p
posted by nomisxid at 4:05 PM on July 23, 2007

nomisxid - every single car that I've owned (40 years worth) had the ACC position. I never owned a VW, but the Audi I owned had it, too. Possibly paulsc isn't the one who hasn't driven many cars.
posted by Kirth Gerson at 4:15 PM on July 23, 2007

It happened to my mother. The culprit was the cruise control. If you have CC this might be worth getting investigated.
posted by chairface at 4:29 PM on July 23, 2007

My culprit was the IAC (internal air control) valve.

#1 It's actually a IDLE Air Control valve (or one of several other names.) This thing's only purpose in life is to control idle (accelerator off) speed. The majority have such a limiter flow capacity that some manufacturers use two or even THREE similar units on some older cars.
#2 There's a simple way to test this on nearly any car by using a nine volt battery. Disconnect the connector from the harness and jumper the two prongs to your battery. Your engine's idle will speed up somewhat (never, never ever to full throttle. more like up to 2000 rpm) and start running like complete shit. This means serious power loss throughout the rpm band and a very good possibility of detonation.
In normal operation these units turn on and off rapidly (via pulse width modulation usually), controlled by the PCM. The speed of opening and closing determines the amount of additional air allowed into the throttle body and this is how your PCM controls your idle speed. The little test with the battery maintains this (small) valve at it's maximum opening and will get you nothing like full throttle. Don't believe it? Have a look at the IAC valve and compare it to the throttle body. The entire IAC will usually fit right into your TB. It's opening is minuscule in comparison. SO, while I can't say what your problem was, I can be pretty certain it wasn't this particular part alone.
posted by IronLizard at 5:06 PM on July 23, 2007

IronLizard, GM IAC valves have 4 leads, and use a stepper motor to control the pintle position. You can't step this with a 9V battery (but if you had 2 batteries, and could switch them on and off at 128 Hz or so ...), but a scan tool like a Tech 2 will have a built in device override that will let you command any idle speed you want for testing purposes. I have personally done tests on GM 4 cylinder engines where I commanded a wide-open IAC, and in neutral you can get up to almost 3000 RPM, but in drive you won't get much higher than 1200 or so. This is still small compared to the total airflow through the throttle body. Also, the engine doesn't run bad, the airflow is compensated for (on GM engines, at least).
posted by rfs at 8:42 PM on July 23, 2007

I'm a bit dated, I see. If it's a 4-wire stepper you'd have to do it with an h-bridge and controller of some sort (not having an appropriate scan tool, of course). Hmmm, easier just to unbolt it and vary the size of the opening with your finger :)
posted by IronLizard at 9:11 PM on July 23, 2007

I should clarify my origional post. I was not driving the truck when this occured. The information I have is what my very upset family member told me over the phone. I do have the old IAC valvce that was reolaced. It has a carbon buildup.
posted by Raybun at 4:47 AM on July 24, 2007

I was driving my 1990 camry when the throttle got stuck full open - luckily, it was late at night on the freeway, and I was able to put it in neutral, cut the ignition, and coast to the side of the road safely. It turned out that a small flange had fallen in the engine and was blocking the cable return. this happened shortly after a transmission rebuild, and interestingly enough, that flange was NOT from any part on my car - but the transmission shop disavowed all knowledge of how this could have happened.
posted by sluggo at 7:00 AM on July 24, 2007