Unintentional acceleration.

[theroughlife]

First of all, I'm new to this forum and also fairly green when it comes to engine repairs. But I am very competent and willing to learn.

So, recently my 84 tercel got stuck accelerating when I'm not pushing down the pedal. It revs high even in park or neutral. When I push the pedal it can still get faster but it never idles. I have the air filter off and have access to the throttle body but I have no idea what I'm looking for. Can anyone help??

[Gottolovem]

Welcome to the forum
lets start with don't drive it if the throttle sticks very dangerous
The first thig I would do is lube everything up and look to see if their is a kink in your cable. Disconnect the cable at the carburetor see if it's the cable or carb that's getting hung up. Use a "dry" spray to loosen things up like graphite or silicone! Wd-40 will gather dust and dirt over time and make it worse.
If you find it's stiff at the carb you may have bigger trouble but use the spray anyway. Also take a look at the return spring on the carb it may be crap.
Good luck

[xirdneh]

make sure the cable is not too tight at the carb. there are two 12mm nuts to loosen and adjust on the cable

[Petros]

welcome to the forum!

You come to the right place. First of all, I know you guys are trying to be helpful, but we really need more information before we can give him the correct advise.

I would like to get a clarification: is the "unintentional acceleration" due to the high idle speed? or the the car surge ahead from a steady state speed condition before you step down on the pedal? these are two different issues. With a high idle speed it will just put along at some speed like 10 mph without touching the throttle, the words "unintentional acceleration" implies to me that when driving at a stead state it will suddenly increse speed without throttle input, but your description sounds like it is just a very fast idle speed.

If it is a fast idle, there are a number issues that can cause that. It would help to know if it was acting normal and than it suddenly startered idling higher, or it was servcied, or there was some other change or maintaince on the engine recently, and it started doing a high idle after the change was made.

Usually the mechancial items such as the throttle linkage, will not suddenly change by itself, so I would not change the throttle cable adjustment, nor the adjust the idle speed setting, if there was no one messing with these items recently. The causes is most likely a vacuum leak in one of the many vac hoses on the system, or a failure of the one of the vac operated diaphrams or other vac devices.

It is it idling high, like in the 1500-2000 rpm range, it is like a bad vac leak caused by one of the hoses comng off. the first one I would suspect is the larger one that comes from the bottom of the carburator area (on the side of the carb nearest the valve cover), that plugs into the under side of the air filter assembly (on to a while plastic valve body). it that one is off it will act this way, you have to lift the front edge of the air filter assembly to see this (a flash light helps). You have to loosen the two 10 mm bolts on the assembly that screws to the top of the valve cover, and the center wing nut on the airfilter assembly to lift the front end to inspect if this larger vac line is connected to the underside of the air cleaner assembly.

Another sourse of large vacuum leak could the the large vac line that goes from the aft side of the intake manifold up to the fire wall over to the large black pie-pan-like chamber on the brake master cylinder (this is the power brake booster). If the power brakes are not working well, it also might be possible that the internal diafram is ruptured and the power booster needs to be replaced. These are vairly reliable however and I would not suspect that unless it is obvious the brakes appear weak or hard to depress.

Next I would carefully compare all of the vac lines, one at a time, with the vacuum diagram for your engine (varies slightly from fed to calif, to Canadian versions). You can find the vac diagram in the on-line service manual elsewhere on this forum, or in one of the threads on this topic in the forum. Trace each of the vac lines (I use my finger to follow each one to stay consistent), from one end to the other, looking for correct routing and that all are connected at each end, and connected to the correct place. do not assume that who ever worked on your car last got them all back in the correct place (even if they were a regular mechanic, on cars this old many are not familiar with them but do not want to admit it).

Another way to look for a vacuum leak is to spray carb cleaner, or even raw propane gas, around in short spurts, at various places around the carb and all the vac lines and vac operated equipment, and all the gasket lines under the carb, and the intake manifold. If you spary where the leak is located the engine speed will change, either speed up, or slow down, momentarily and than return to normal.

If all of this checks out, than there are several items that could mechanically cause the throttle to not return back to normal idle speed. though these rare ever go out of adjsutment on their own, but only after there was a change in the system, like a rebuilt carb, replaced throttle cable, etc. These are the idle set screws, the throttle cable, and the automatic trans step down linkage. the idle sets scew is on the carb next to where the cable attaches to the throttle crank. The fast idle screw is on the throttle crank itself (smaller screw than idle screws, moves when the throttle linkage is operated), and the device on the other side of the carb from the throttle cable, on the throttle shaft that has a screw adjustment (it delays the close of the throttle by a few seconds to prevent it from slamming shut to reduced emissions, if out of adjustment it might hold the throttle open causing a fast idle).

the auto trans step down linkage is on the same side of the carb as the throttle cable, it goes down from the carb crank, though a linkage and into a crank on the side of the trans. this allow the auto trans to step down to the next lowest gear when either accelerating to pass, or when going up a long steep hill and you have the throttle held all most all the way open. the is a simple threaded rod and lock nut adjustment on it, this too should not go out of adjustment unless the trans was recently replaced or something bent or damaged this linkage.

check this stuff out and get back with us.

good luck