I have a 1996 Honda Accord LX that is equipped with a V6 engine and ABS.

A recent problem that occurs is that the engine idle will go very high when it is shifted to reverse gear. I have to press the brake pedal as hard as possible to get it to stop when backing out of the driveway. The car will return to a normal idle when I shift into drive and then it runs otherwise fine. I should mention that this problem first occurred a few months ago when my daughter slid into a tree when the roads were ice covered, but only going about 15 MPH through the neighborhood. Seems to be some type of vacuum-line related problem, but there are no obvious leaks. What air valve could possibly be triggered by the reverse gear? What could be causing the high-idle problem?

 

If you open the hood and have someone watch the throttle cables while you do this, do you see the cables getting pulled somehow?
Sounds a bit like a short in a wire that's touching the reverse otherwise.

 

I've looked at the throttle cable when the high idle occurs, but it does not noticeably move at all. It seems that more air is being sucked in somewhere when the gear lever is shifted to reverse.

 

Thermovalve or FITV? On the inside here is a wax disc back it off a couple turns.

 

I actually have already adjusted the FITV (fast idle thermo valve) by turning it down just to get the high idle from getting too high. Car will start with a minimum idle, but idle still gets significantly high when shifting into reverse. Engine RPM will actually continue to increase if I leave it in reverse for too long. What valve or solenoid could be triggered by shifting into reverse gear?

 

Is this hot cold related?

You are shifting into gear creating load on the engine. If there is enough excessive idle it will make it run away.

Most idle problems like this are handled by a relationship including 3 things. 1 The idle set allen screw and lock nut on the throttle linkage plate. 2 The slot screw idle port adjustment at the top of the TB. 3 The wax disc on the inside of the thermovalve. It is a balance.

Are you able to place a reversed vacuum cap in the idle port on the inside of the throttle body? Or have someone plug it with their thumb while you start the vehicle and shift into reverse? What happens?

 

The high-idle problem occurs when the engine is both hot and cold. Happens in the morning when I back out of the driveway, and will happen if I go somewhere (for pick-up) and back out of a parking spot.
Shifting into reverse is not hard on the engine since idle RPM's have not increased yet. However, shifting from reverse to drive does create a big load on the engine. RPM's are so high that car wants to take off unless I hold brake pedal to the floor. The RPM's will drop back to normal as the car is driven in the "drive" gear.

I will try your suggestion of placing a reversed vacuum cap in the idle port to see what happens. I'm planning to work on it this weekend.

 

But it is not hot and cold related?

If it goes to normal driving in D, its not a vacuum leak.

Dude you should make a video.

It needs air to increase the RPM. If you plug the idle port it will not be able to suck air from there. There can only be so many places or reasons. Look for a vacuum leak anywhere behind the throttle plates, like the PCV hose. FPR system.etc.

 

The high-idle problem I'm having is not related to the temperature of the engine. The problem occurs when the engine is either hot or cold. The engine RPM's will get very high right after the car is shifted to Reverse gear. It seems that the Reverse gear is triggering something that allows more air into the intake manifold. Since the idle returns to normal when the car is shifted to Drive, I'm thinking that a solenoid must be switching (when in Reverse) and creating the imbalance. I've replaced the PCV valve and grommet, but high-idle still occurs.

 

Put it in gear load goes up.

Look at the idle port. On the inside of the throttle body. I am suspecting there could be a failure there. For diagnosis a thumb or cap needs to block it off. Tell me if that changes the condition and what is the result? I am expecting a change, lowering the RPM and limiting the air passage,. Keeping it from surging when placed in reverse.

It should isolate the suspected area.

 

As an update, I was able to solve the high-idle problem with my 96 Honda Accord. After closing off some vacuum line ports, I found out that cruise control module was causing the idle to surge. Turns out the cable between cruise-control diaphragm and throttle body was being pinched by power-steering pipe. For some strange reason, cable was routed under the power-steering pipe instead of over it. I just disconnected the cable from cruise diaphragm and re-routed it so that it was free of interference, then reconnected cable end. Idle is now back to normal when shifting to Reverse gear.

 

Dang, nice diagnosing.