Questions and Answers

Subaru Outback DTC P1507 Q. Hi, I have a 1997 Subaru Outback, 90,000 miles, 2.5 liter DOHC engine and a manual transmission. I intermittently get DTC P1507 - "Idle Air System Malfunction (fail-safe)" under certain driving conditions. Here's what happens: If I'm stuck in stop and go traffic or just changing speed a lot, in mountainous terrain, etc, it will suddenly start to idle very rough and lose power and pop the code.

If I turn it off and back on, it's fine. Seems like maybe I have to let it out of gear and idle to let it start doing this. Sometimes this also happens after a long time cruising at high speed, but usually I'll go to get off at an exit, let the car idle out of gear as I slow down, and then realize it has stalled completely and it pops the same code. But once again if I restart it, it's fine.

I also notice a general lack of good driveability at lower speeds. It tends to have some hesitation in the lower gears when hitting the gas between 2,000 and 2,500 rpm's and feels a little jerky when starting in first. And one more thing: The engine doesn't seem to slow the car down as much as it should when heading down steep inclines.

I did an experiment one morning when leaving my home, which is on a steep road, and completely blocked off the hose for the IAC valve intake with a plastic bag. This totally screwed up the idle, but it helped the engine keep the car slowed down as I went down the hill, more like what I'd expect.

I recently replaced the plugs and wires with OEM replacements. Installed a K&N air filter, tried a new IAC, replaced the neutral lockout switch on the transmission, tested the TPS in my garage, (might malfunction at high temp?), all to no avail. I'm thinking something that tells the computer how to control the IAC is malfunctioning, but what?

Thanks for any help!
Pat

A. This is as easy a DTC to troubleshoot as there is.

Added 10/08/2005:

Hi Vince, I emailed you a while ago concerning my Subaru Outback with DTC P1507 issues Subaru Outback DTC P1507.Thanks for the response, but none of that information helped.

However, I finally found the source of the problem. Turns out the previous owner put a 'chip' on the car computer and didn't tell me. I took the chip out and haven't had a CEL since.

Just thought you might like to know.
Pat

I'm glad you found the problem. I, too, was unaware of any modifications to the PCM. This is one of the reasons I don't recommend installing "performance" chips since they do tend to mess up engine systems. Under normal conditions the information I gave you would have been valid.