Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Dashpot on early 90's Fuel Injected Corolla? How does it work?

PleaselLook at the diagram in the link below. See that part 22202? It’s called out in the expoded diagram below/right. Right under the throttle spring? On my Ford truck, there’s a similar part and it is called a “dashpot” on the Truck. I presume that is what it is called on the Corolla too. But, since the Corolla has electronic fuel injection w/a IAC controlled by the ECM, I’m wondering why a dashpot is there on a early 90’s Corolla.

This gadget appears to be there for a reason, but I don’t understand what. It provides a stop for the throttle closing position. If it moved, it would raise or lower the rpm. But it doesn’t appear to be connected to any electrical wires. So the ECM couldn’t make it move. So does it in fact move? Just curious, what does it do and how does it work? Anybody ever had to replace one?

Mainly, a dashpot prevents the throttle from slamming shut. The engine could stall when the throttle closes suddenly when the driver stops the car.

Some of them also had springs and a vacuum line. They would help to prevent the engine from stalling at idle. If vacuum dropped, the spring would try to open the throttle a little. Some would actually work.

Replace one? Darned if I ever cared to even find out how to check one. I guess if it wouldn’t hold vacuum or resist sudden throttle closure you would change it. I don’t think it was a big money maker. Not when carburetor swapping was the best way to go.

Oh, it is sort of like one of those spring-type door stops. Provides a lititle cushioning when the driver completely takes his/her foot off the gas to slow down suddenly. I can see that. And there’s no need for any electronic control. It does feel kind of springy-like when I touch the end of it. Thanks for the explanation.

I guess I just talked without explaining it. You really explained it.

I could understand the need for the one on my Ford truck better. I mean before your comment. The one on the truck, it is electric controlled. It has a wire going to it. When the engine is on, it become energized and pokes out a few mm, and holds the throttle open a bit. When the engine is off, it denergizes, pulls back, allowing the throttle plate to close completely. Stops all airflow through the carb and prevents unwanted dieseling.

The dashpot also reduces emissions by keeping the vacuum and airflow from changing more rapidly than the fuel system can track and maintain a decent fuel/air ratio.

Even my 1975 carbureted Civic had a dashpot.
Carburetor have an accelerator pump to prevent a momentary lean condition when the throttle is opened.
Conversely, closing the throttle would cause a momentary rich condition.
Not a problem before tight emission regulations.
Slowing the throttle closing reduces the rich transient.

I believe that controls the idle while the A/C is operating.

@casper, the IAC does that in my '88 Toyota. I have a dashpot, and it is just an air/spring device that prevents the throttle from slamming shut, and bleeds the air cushion in a few seconds. Really noticable in traffic. I’ll push the throttle a little to pull up, and notice the RPMs drop to 1,100 just as I let off, then settle down to 750 RPM, proper idle speed, after a few seconds. There are no wires or active vacuum lines on mine.

After futher review, pg. FI-62 in the FSM identifies this as a Dash Pot and shows the adjustment procedure.

@GeargeSanJose If you need the inspect and adjust procedure, I’ll be happy to post it for you.

Thanks for offering @casper, but I was just curious is all. My early 90’s Corolla’s dashpot is still working like new. But it’s interesting how useful that gadget is. And it is really sort of like a mini-air shock, with a designed in time constant. I was thinking it was just a flexible bumper to reduce wear and tear on the throttle plate/house contact area, but from the posts above it’s for far more reasons that that. A completely passive device. Something so seemingly innocuous. Yet so important. I wonder if Toyota has a team of mechanical design engineers holed up in an office in Tokyo working 12 hours a day that specialize only in dashpots? lol

If you think that’s interesting, some of the older Toyota designs have a small device on the fuel rail where the pressurized fuel comes in. It is called a fuel dampener, and is basically a shock absorber for the fuel rails to smooth out the fuel pump pulses. However, these can go bad after many years and amplify the pulses instead of dampen them, causing the engine to idle erratically. I discovered this in a Toyota thread just reading the various posts.I thought at one time that may have been a problem I was having, but it turned out to be dirty injectors that a second helping of Techron took care of.

My 75 Corolla had a kind of dial on the distributor to adjust the timing +/- 3 degrees to compensate for different fuel octanes and/or quality. Or so the owners manual stated.

Retard (-) for a lower octane maybe. Interesting. Could be quite useful when driving a car in a 3rd world country where the fuel octane varies by where you buy it, place to place. Good post.