Trouble with engines revs rolling to a stop

[jefferson]

My engine revs at 2000 rpm when I have the clutch in rolling to a stop. As soon as the car stops the revs drop and then slowly return to idle. This really annoys me. I know it can be normal as there are times when the car will return to a normal idle speed while rolling up to a stop. Is this an IAC issue or something else?

Jeff

[Legacy777]

Any more info you can provide? When did it start doing this? Did you do anything prior to it occuring?

[jefferson]

It has pretty much done it since I got the car and I am just getting sick and tired of it. It seems to have a mind of it's own as far as it doesn't always do it and there is nothing I can put my finger on that is different. One day you drive it and it is fine and then it revs for the next week.

I was scoping things out for doing the neon coil conversion and I found my plug wires weren't tight into the coil. The car smoothed out and idled as it should when rolling up to a stop. The next day it was right back to revving. The neon coil didn't have any effect either. I attacked my grounds the other day with no real positive result. I just loosened and then tightened the ecu ground a few times, I didn't actually take it off and clean it.

I am sure that the iac does need cleaning, but that doesnt explain why it revs until the second the car stops and then it is almost like a solenoid clicks and the revs drop about 250 to 500 and then slowly drops to idle.

Any and all ideas appreciated.

Jeff

[Aerotech]

Any Check Engine light?

A bad speed sensor (VSS) input will do this sometimes.

[jefferson]

There are not codes. Is that speed sensor located on the transmission?

I am going to clean the iac and see what that does. What I can't understand it why the iac is being told it needs to rev the engine while the car is moving. I will also have to check out that speed sensor. Thanks.

Jeff

[ericem]

There are not codes. Is that speed sensor located on the transmission?

I am going to clean the iac and see what that does. What I can't understand it why the iac is being told it needs to rev the engine while the car is moving. I will also have to check out that speed sensor. Thanks.

Jeff

could just be sticking. Spray some throttle body cleaner down the IAC tube.

[jefferson]

I've done that several times with no real change in anything. If it was sticking I don't see why the rpms drop the instant the car stops. You would think the rpms would stay where they are regardless of the vehicles motion with the iac sticking. I am going to check with my suby tech friend and see if he has any ideas also.

Jeff

[slunk33]

could just be sticking. Spray some throttle body cleaner down the IAC tube.

Sorry for my ignorance, but what is the IAC?

[jefferson]

The iac is the idle air control valve.

I have done some more research and it seems the manual transmissions have no speed sensor. There is some sort of neutral switch on the tranny. I have played around a little with putting the car in neutral as I roll to a stop and the car doesn't seem to rev so high. Not enough time to tell for sure if it makes a difference.

I am going to check and clean connections on the iac and the neutral switch just for grins.

What I really don't understand is how the iac knows the car has come to a stop with no sort of speed sensor. Maybe I have some sort of hybrid that does have one. I am kind of stumped.

Something I just thought of though is that the cruise control has to get a speed signal from somewhere. I will have to look into that and I may disconnect the throttle cable from the cruise control and see what and any difference that makes.

Jeff

[slunk33]

I wonder if there is a speed sensor behind the speedometer, I thought I read that someplace.

Is there an IAC on a 4eat?

Thanks!

[jefferson]

Yes there is one on the auto cars also.

[slunk33]

Where is the IAC located? I may as well clean mine, couldn't hurt.

[Legacy777]

There is most definitely a speed sensor for the manual transmission cars. The main speed sensor for both the AT & MT cars is located in the dash (combination meter). That is why the speedo is cable driven. The AT's have a second speed sensor located in the rear of the transmission to detect rear driveline speed so the TCU can adjust the AWD.

Here are some pictures that show various parts on the engine.

http://www.main.experiencetherave.com/s ... injectors/

Here's the IAC valve
http://www.main.experiencetherave.com/s ... P_2470.JPG

[slunk33]

Cool, thanks Josh for the images.

[jefferson]

Well put it down to bad info in other threads. I kind of figured there would have to be, but all I could find was references to the one for the auto trans. Said there wasn't one on the manual although they may have been talking about sensors on the transmission.

I still haven't been able to deternine if sticking the tranny in neutral has a positive effect. It was idling fine this morning every time I rolled to a stop.

Jeff

[kbeefy]

I've noticed similar behavior on my '92 (turbo MT).

When cold if I clutch it the rpm's jump to almost 3K.
before being completely warmed up, rpm's will rise to about 2100 for a second or two when clutch is disengaged. Could be the same when warm, I don't recall.

[jefferson]

When mine is cold the revs go up real bad during a shift and does it until warmed up. Another very annoying trait.

I did a little more experimentation last night and during an extended coast down a hill with the clutch in, the revs would go up when you put it in gear and go down when it went into neutral. It was only around 200 to 250 rpm so it wasn't a huge difference, but it was noticeable. It wasn't doing it's 2000 rpm revving last night so I need to do some more investigation on it.

Jeff