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HELP: Mystery Idle Problem

Spencer Timkar

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I have a '95 Del Sol with a D16Y8 engine swap. Previous owner replaced a lot of sensors but didn't have a lot of attention to detail and there are a lot of weird and missing things. The TPS sensor and the Idle Air Control valve both were replaced recently and seem to be in good condition.

On cold start the RPM sits around 2000, once it heats up it goes down to a more reasonable level, but oftentimes it sits too low (sometimes below the 500 marker). The engine gives a code for the IACV and sometimes a code for the TPS sensor. And generally the engine seems to run rich though I can't confirm, but I smell gas pretty much every time I get out of the car. And the shift lock is always engaged, meaning I have to use a key to get it out of park every time I want to drive it.

When I got the car there was a problem where the rpms would surge/buck around 1500 RPM pretty bad, but this was fixed when I just shoved the TPS sensor to the other end of its range. Recently after having read up on some of the issues I decided to try and calibrate the TPS sensor more accurately. I got out the multimeter and set it so it was at a nice .5V when closed. This fixed the shift lock issue but the surging/bucking came back. I would put it up .05V and retest until it went away. The bucking stayed until .7V but then the shift lock problem came right back. I know that its supposed to be set to .5 when closed but this seems to create a problem.

I checked out the IACV and it looked good and there was little to no carbon built up inside. I cleaned it out the best I could and the problem persisted. Then I decided to try and clean the hole into the intake manifold that the IACV connects to and there was a huge amount of carbon built up. I pretty much used a screwdriver and wrapped it in paper towels and tried to scrape out as much carbon as I could. By the time I was done, nothing was coming out on the towel. I bought some carburetor cleaner and was thinking about taking the whole intake system off and cleaning it out thoroughly but I haven't had the time yet. After all that the engine code for the IACV still persists.

I'm thinking that the problem with the IACV is really what is causing the bucking but setting the TPS to a high voltage is a bandaid fix? I'm really not sure. I also think it's possible there could be a vacuum leak partially causing the high idle, but it seems like 2000 is way too high to be caused by a vacuum leak?

Any knowledge would be helpful. Thanks for reading!

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