Throttle stuck on highway

[454cid]

This afternoon, going down the highway, in my 96 850, I let off the gas, and the car kept going as if I had cruise on. I didn't recall setting cruise, but I tapped the brake a couple of times, and the car only slowed when actively braking. Coming to a light, I got on the brakes heavy, and put the car in park. The engine immediately went to about 5000 rpm. I popped the hood, and looked at the throttle body, and didn't really see anything obviously wrong. Working it by hand, I did feel a catch/release, and it settled to normal idle. I then proceeded on my way to the store, and then home. I've messed with it several times since, and I feel and see nothing unusual, from the throttle body side, or the pedal side.

I've read, that carbon build up can make for a sticky throttle, but in my experience that's a slow build up that becomes more a more noticeable, not a sudden stuck throttle situation that appears out of no where.

Any ideas?

[abscate]

Take a good look at the linkage to make sure it isn’t binding.

Then pop off the snorkel off and clean that TB!

Danger Will Robinson !!

[BlackBart]

Was it sloppy / icy and the linkage froze? I had a carburetor freeze open in a blizzard once on an Interstate - scary.

[erikv11]

My 850 NA throttle plate froze open on the highway three or four separate times. Not dirty, just got moisture in there that froze.

Is yours an NA, if so is the airbox flap working; do you live in a cold climate?

e.g. viewtopic.php?t=43312

[454cid]

Take a good look at the linkage to make sure it isn’t binding.

Then pop off the snorkel off and clean that TB!

Danger Will Robinson !!

Binding, I checked for. It seemed very smooth, when checked in the driveway.

I will see about cleaning it out.

[454cid]

Was it sloppy / icy and the linkage froze? I had a carburetor freeze open in a blizzard once on an Interstate - scary.

That occurred to me, but I didn't see any trace of snow/ice under the hood. Dry as a bone, looking.

[454cid]

My 850 NA throttle plate froze open on the highway three or four separate times. Not dirty, just got moisture in there that froze.

Is yours an NA, if so is the airbox flap working; do you live in a cold climate?

e.g. viewtopic.php?t=43312

THAT, I completely forgot about. I removed that system from my car, since it was stuck in the heated air mode. The actuator on my car is not able to be replaced, so I swapped in a lower airbox from a Turbo. I didn't have any issue with it last winter, and I don't think it was particularly humid today, as it was in the 20's, I think. Still that may be what happened.

I tossed my non-filter HVAC frame, but I think I still have the original lower airbox. Maybe I'll swap that back in tomorrow, if the weather is tolerable.

[abscate]

I’ve got a spare heated box if needed

[454cid]

I’ve got a spare heated box if needed

I just re-installed my original. It pulls heated air 100% of the time, but for this time of year I think that will be fine. It was very easy to do with the engine idling.... the air filter sticks to the upper housing and the engine doesn't know anything is amiss!

I did test the air flap. I had forgotten that vacuum was involved. The diaphragm does move, but not correctly. The post/rod/plunger/whatever seems to be two concentric pieces. With vacuum applied, the outer moves, but the center portion keeps the flap closed to cold air, and actually further closes the slight gap that is present at rest. I remember from working with it before, that none of it would come apart.

I could not bring myself to clean the throttlebody.... too cold.

[scot850]

I'm probably wrong, but that thermostat in the airbox used to be replaceable. Then I think the thermostat went NLA, but I believe someone figured out one from a 240 would work?

I should add that it may be worth trying spraying the throttle cable return spring with WD-40. They can get sticky and in cold weather it will amplify the issue.

Neil.