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Odd Idle Problem on '02 Tacoma

The truck is a 2002 Tacoma V-6 w/ 5-speed manual. Recently I have noticed that as I am coming to a stop for a stop sign or traffic light the idle RPM jumps up to about 1800. The clutch is pressed in, so there is no load on the engine. The engine RPM remains elevated until the vehicle comes to a complete stop. The RPM then drops to about 800. This is so repeatable, that it almost seems like there is a wheel speed sensor telling the engine the vehicle has come to a stop (maybe there is?!).

I did not notice this problem during the colder months. The hot weather (SW Ohio) the past couple of weeks has aggravated the problem. It also seems more pronounced with the A/C operating.

Aside from the remote possibility of an engine controller problem based on wheel speed, I am thinking maybe some sort of vacuum problem. Or possibly a flakey coolant temperature sensor. If this were a carbureted engine I would think a float had a pin hole leak and had gas sloshing around in it. But obviously this thing is EFI.

Anyone hear of this sort of problem? Any ideas what to look for?

Thanks!

Don

Reply to
Don
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IAC valve?

Reply to
mrsteveo

Doesn't that engine use a throttle valve motor instead of a cable actuated valve (i.e. no IAC opening)? I'm having trouble recalling, but I think I had this problem on one several years ago, and the butterfly valve was all crapped up with gunk and sticking open for a bit after the throttle was let off after extended periods.

Reply to
qslim

Is the service engine light illuminated? Scan the computer for any set codes even the intermittant ones. I would think along the lines of the others in checking the idle air component. Also check the coolant temp sensor.

Reply to
user

First off, "Thanks!" to everyone's suggestions!

I spent a little time under the hood before a thunderstorm chased me in, so only have partial answers:

1) The engine check light is not "ON". Could there me useful codes even though the light is not "ON"? I think I know someone who has an ODBC test set, but not sure if it would read Toyota specific service codes.

2) The throttle linkage is connected to the pedal via a mechanical cable. It appears to move smoothly with no binding and sticking.

3) Started trying to figure out the IAC stuff using the manual when the rain came, so I will pick up there Thursday or Friday.

4) Once I come to a complete stop and the RPM comes down to normal I can punch the accelerator and the RPM returns to normal (~800 RPM)

5) If I coast (out of gear, foot off the accelerator) the RPM will occasionally bounce around between 800 and 1800 RPM.

That's about it for now!

Thanks again for the hints!

Don

Reply to
Don

I would check what qslm suggests and leave it at that. Since the condition is so repeatable, my guess is that it is a normal emissions control function. The idle-up will be more pronounced with the AC operating because of the AC idle-up function.

I doubt if it is a problem because vacuum problems generally cause rough idle or poor performance. You can check vacuum with a vacuum gauge to make sure there is no vacuum leak. Also check the intake tube between the air filter housing and throttle body for holes, gaps, or cracks.

Reply to
Ray O

Looks like the problem is with the IAC valve (or something attached to it). I friend of mine has a scan tool and checked for codes. We drove around for a while and nothing. He suggested disconnecting the IAC. While I got a check engine light, the idle was perfect.

Based on some searching, it appears that it is somewhat common for the IAC to go bad. Once I get a little more time, I am going to do some more testing to try to confirm it is the IAC itself. Then it will likely be a matter of springing for one and putting it in.

Thanks for the help!

Don

D>

Reply to
Don

Before you go out any buy a new IAC, try cleaning the old one with carburetor cleaner first.

Reply to
Ray O

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