Odd Idle After 5 Speed Swap

[StoobyMcDooby]

Hello,
I have just finished my 5 speed swap on my 1991 HB. The car has 163k on it, and drove/ran perfect before converting to manual. I did all of the work myself for the swap. Anyhow, the car drives perfect since converting. However, the idle tends to stay high (around 1800 RPM) when I come to a stop, and then after 5 to 11 seconds, it calms itself down to the normal 750 RPM. Once it settles down back to normal, it stays there until I apply throttle once more.
It does NOT surge up and down. When the car is started, whether the car is cold or warmed up, it idles perfectly normal.

It is important to note, that while rolling in Neutral, no matter what speed, the idle remains constant at 1700 +- RPM, and will not settle down unless the car is completely stopped - thus indicating that the vehicle speed is affecting the idle speed.

I first used the automatic ECU that was original to the car, but have since switched to a manual ECU hoping this would resolve the issue and it has not.

Remember, the idle was PERFECT one week ago, right before I performed the swap.

Any insight would be greatly appreciated!

[StoobyMcDooby]

Anyone got any ideas?

[corn322]

Hah, my 240 does the same thing. I'm also about to put the 5 speed ECU back in to see what effect it will have.

I'm still thinking it's just a small vac leak somewhere. Have you ever adjusted the TPS? How about idle speed/timing?

This better than what it was doing last week, which was idling at ~700 at a stop then sometimes just dying like I turned the key off. Crank it back up, would idle at 700 for about 1 second, then shut back off. I could keep it alive with the gas pedal, and sometimes it would keep working.

[alms24sebring]

I think the most obvious answer would be the correct 5 speed ecu. They all have different codes but I think they all work the same more or less. Look into that. Other than that I think you need to check for some vacuum leaks. Its probly something simple and obvious as always.

[StoobyMcDooby]

I think the most obvious answer would be the correct 5 speed ecu. They all have different codes but I think they all work the same more or less. Look into that. Other than that I think you need to check for some vacuum leaks. Its probly something simple and obvious as always.

As mentioned, I am now using a manual ECU and it has not changed at all.

[Mercury_Hg]

I bet all my money on a vacuum leak. Ask me how I know.

[Hoffman5982]

I bet all my money on a vacuum leak. Ask me how I know.

Because you know everything


While it does seem obvious that it might be a vac leak, if you didn't pull the engine when doing the transmission swap, I don't see how a vac leak formed

[StoobyMcDooby]

I bet all my money on a vacuum leak. Ask me how I know.

Because you know everything


While it does seem obvious that it might be a vac leak, if you didn't pull the engine when doing the transmission swap, I don't see how a vac leak formed

Exactly. I didn't touch anything even on the motor with the exception of the starter. That's it. I did every minute of the swap myself.

I unplugged the automatic trans computer (found above the ECU), and it didn't help. When the car is idling high, I tried to close the throttle plate to make sure that it wasn't sticking open - and it was closed fully, even while idling high.

[OutToWinPAHC]

Seems weird but I cant see and issue using a manual or auto ECU, but wile you were under there ripping things off something could have been knocked loose for sure. Start with setting the TPS, check the CTS and IAC for issues, then check for vacuum leaks. Also make sure the throttle plate is closing all the way seeing that you were messing around in the pedal area, and also make sure you didn't disconnect anything near the clutch pedal aka time delay