Erratic Idle

[Captain Chaos]

My 91 Shadow 2.5 T-I has an idling gremlin. At first I had a code 22 for the coolant sensor so I replaced it and the TPS at the same time hoping that either would solve the problem. I unhooked the battery to clear the code and all was well. Next day its idling at 1400-1500 RPM's in park. Shut it off and check codes, just a 12 and 55. Unhook the battery just to see and nothing, still high. While its idling it all of a sudden goes back to normal. And it does this seemingly at random. Not necessarily while hot or cold. I'm getting a second TPS just to try. I swapped the new IAC I had from my T-II Horizon, but nothing changed. This idle problem is causing it to run warm while sitting in traffic. Any suggestions?

[overlordsshadow]

Vaccum leak anywhere? All I can think of right now.

[Captain Chaos]

I don't "think" there are any, unless the intake is warped, but it seems intermittant.

[tvanlant]

May also be a speed/distance sensor problem, but that usually shows up when you check codes.

[Captain Chaos]

Its strange. I don't have any codes since I replaced the CTS. I'll adjust the screw on the TB and it will get it down around 1000-1100 RPM's then BOOM it won't idle and I can set it at 900-950, or where ever I want. Then BOOM its back to idling at 1200-1400 RPM's.

[gvare001]

May also be a speed/distance sensor problem, but that usually shows up when you check codes.

+1. It would do that to you without any codes.

[risen]

I'm with Tyler. I had my speed sensor do the same thing and it was so intermittent that it never set a code.

[Captain Chaos]

I'm with Tyler. I had my speed sensor do the same thing and it was so intermittent that it never set a code.

This is the sensor on the trans right? My speedo isn't working so I'll look into it. Thanks.

[Captain Chaos]

UPDATE: I drove the car about 20 miles yesterday and it set a code 15 for speed sensor. But there are two other threads about the sensor going bad and causing stalling, mine doesn't do that. Still a speed sensor, and that could cause my high idle?

[overlordsshadow]

Bad part = replace it and see if other problems go away. When it's electronic and there are problems never rule out that they are related.

[Captain Chaos]

Yep, got one on order.

[risen]

UPDATE: I drove the car about 20 miles yesterday and it set a code 15 for speed sensor. But there are two other threads about the sensor going bad and causing stalling, mine doesn't do that. Still a speed sensor, and that could cause my high idle?

I guess it depends on how it failed and probably the year of the car. I know that in 87 if the car thinks you're rolling above a certian speed it hold the idle up around 1500 rpm (which is annoying even with a good speed sensor). And on my car, when there was a loose connection it thought it was always rolling, so at a stoplight the idle would stay around 1500.

Other years, I don't know. Maybe if it sees excessively high speed it shuts the car down due to the speed limiter? Second part is just a guess.

[Captain Chaos]

I suppose it could be a wiring issue as it is intermittent. Guess I'll replace the sensor and rule that out.

[tvanlant]

Still a speed sensor, and that could cause my high idle?

Yes. As risen said, the computer thinks the car is rolling, so it holds the idle up. The sensor is the problem.