98 Civic DX Throttle Sticks Open

We have a 98 Honda Civic that recently developed a problem with the throttle getting stuck open. With the engine off, the throttle will return to the idle postion. However, with the engine is running, when the accelerator pedal is pressed, the engine revs increase, but then do not decrease when the pedal is let off. The same thing happens when the throttle is manually opened (not using the cable) with the engine running. The valve for the air intake doe not seem particularly dirty. I cleaned the spring assembly which I initially thought was the problem. Has anyone heard of this? Any thoughts… the car is sitting the the driveway with both of us moderately terrified of driving it?

You may have a very large vacuum leak…or your throttle body is gummed up with carbon…and holding the butterfly open more than it should. Pull your air snorkel and look into the mouth of the throttle body…use carb or T-body cleaner to clean the edges of the butterfly and all around the perimeter where it seats when closed…clean clean clean is the idea here… If this doesnt help then you need to look into a vac leak and or your AIC valve…this is the valve that gives you idle control…and if she is dirty or malfunctioning it will wreak havoc on your idle quality. But if you can correlate this to opening the throttle…then I’d start at the T-body valve cleaning…


Could also be the bores for the butterfly shaft are worn and the throttle body must be replaced.

I suggest you have the entire throttle assembly professionally disassembled and cleaned.

In the meantime, you shouldn’t drive this car until the problem is solved.

Is the throttle plate just not closing or is it just the RPMs don’t come down? Do they remain high for very long after you close the throttle or lift off the gas?

On some cars, when you close the throttle, the RMPs will remain high for a second, then slowly come down to idle speed, it may take 2-3 seconds before returning to normal. Is this the case for you?

@Keith, this isn’t a throttle-by-wire car. The 98 Civic DX has a direct cable connection. You take your foot off the gas and the RPMs are supposed to come down immediately.

I’d like to know whether the throttle plate does, in fact, close all the way. The post title suggests that the plate is sticking open. But the post itself if hopelessly ambiguous about that. It talks about the car revving rather than the throttle sticking open.

Thanks for the input. I’m planning on doing as much of the cleaning as I can to get it to a drivable state in order to drive it safely to my regular mechanic for a full cleaning. I’m not paying for it to get towed there.

When the engine is running, the throttle stays open as far as I open it. I’ll take a closer look at the plate tonight.

It seems to me that the throttle body should last more that 157k miles.

We have a '95 Civic and I just had mine apart to clean it. I’ll bet it is the same or similar to your '98. It’s really easy to get the plastic pipe off which connects to the air box at the throttle plate . . . slips off easily after you take off the clamp. The operation is super simple and you should be able to see what is causing this to stick open. Try it get a look at it . . . you’ll see how easy it is to get at. I would clean it well and lubricate the pivot points outside and inside (less on the inside so you don’t foul anything with oil). After that I would follow the throttle cable to see if it is binding anywhere. It’s a really simple set-up. Good luck! Rocketman

@SubHonSub: “It seems to me that the throttle body should last more that 157k miles.”

We Honda owners tend to be a spoiled bunch. Your car is almost 14 years old. A dirty throttle on a 13 year old car is pretty normal, especially if you haven’t kept it clean over the years. If you expect a car that old to be reliable forever, you are going to have to adjust your expectations. Mileage isn’t the only consideration. Things age with time.

I drive a 1998 Honda Civic DX, and I’ve never had a problem with a dirty throttle plate. However, I change the air filter on a regular basis and clean the throttle about once a year by spraying carb cleaner on it.

Whitey, I know how the throttle works on this car. I know its not “fly by wire”. That is why I asked the questions that I asked. This car, like all other MPI fuel injected cars with a throttle cable does have an IAC (idle air control) motor. On some cars, the IAC is programmed to open a little when the throttle is closed suddenly to keep the engine from dying. It also does something to reduce emissions though I’m not sure I understand the logic behind that.

Honda’s IAC usually closes up pretty fast compared to some other models, my Saturn will maintain engine speed for about a second before starting down and take another 2-3 seconds to return to idle.

SubHonSub, it is not necessary to remove the air dict and look at the throttle plate. The bell crank on the outside of the throttle body is directly connected to the throttle plate via the throttle shaft. If the bell crank is returning to the zero point, then I would suspect that you have carbon build up on the IAC valve stem. If this is the case, you will need to remove the IAC assembly to clean it. It probably will be easier in the long run to remove the throttle body first.

If the bell crank was loose from the throttle shaft, you should notice a delay in the increase of engine speed when you open the throttle as well as a delay in lowering when you close the throttle.

One more issue can happen, if the throttle is not returning all the way to zero when you let go of the gas, the little screw that holds the throttle plate to the shaft could have come loose. No amount of cleaning will fix this. The throttle body will have to come off to repair this.

“It also does something to reduce emissions though I’m not sure I understand the logic behind that.”

It keeps the air intake volume from changing faster than the closed-loop system can track and control the air/fuel ratio.

With carburetors and throttle body injection a film of fuel clings to the intake manifold walls. It can even puddle with low temperature and low vacuum.
If the throttle snaps shut too quickly the vacuum rises and the fuel in the manifold suddenly evaporates, creating a transient rich mixture: not good for emissions.
The reverse is also the main reason carburetors have an accelerator pump: to make up for fuel vapor that condenses onto the manifold walls when the vacuum drops.

After much investigation…turns out that the cruise control cable was holding the throttle open. For the entire time that we have owned the car, the cruise control has not worked. I disconnected the cable, and now it runs like old again. I took a drive around the block and it drives great. Has anyone ever heard of a cruise control system having that type of problem?