CEL P1507 Idle Air Control Valve thrown

[dcolemanca]

Recently I have had this check engine light code on my Subie Outback Legacy 1997, 109k miles. I have an OBDI-II reader so have read the code then cleared on both occasions.

 

The first time it happened I was returning to San Jose from Santa Cruz Mountains (SF Bay Area) so about 6 miles of steepish, descending winding roads. The light came on whilst descending and the engine stalled, and I had to bump start to get it going as the car wouldn't turnover - wierd. After that it ran normally all the way home. I checked the connector on the IAC valve but took no further action. The battery did not appear to be discharged when I got back.

 

The second time it happened was a similar scenario, downhill winding roads for about 6 miles, but this time there was a big loss of power, I am guessing that I had lost 2 cylinders - maybe it was running fail safe? When I read the code and cleared it the car again runs normally with full power. I plan to take the IAC valve off next weekend to clean it as I am hoping I don't need to buy a new one, I hear it costs around $330!

 

Any other suggestions as to what could be wrong, or steps to take to find the root cause?

 

It just seems wierd that it coincided both times with descending steep winding roads at around 30mph.

 

Anyone have a working IAC valve for sale by chance?

 

TIA

[frag]

If I may chance a diagnostic, I think your problem is PCV related. When you drive downhill your engine is on "compression" and generates a lot of vacuum. Maybe that is sucking oil into the IAC, flooding it and temporarily disabling it. Check or replace the PCV and see if that cures the problem. It might not be necessary to clean the IAC.

Just an idea.

Edit: thinking about it still, it seems something like that would also generate a lot of grey smoke at the tail pipe when you drive down those hills. Just a tought...

[cookie]

and given the relative cost of the items I would also try a PCV valve. Backing off would be likely to suck a lot of oil in on an engine with a bad PCV.

[dcolemanca]

Thans for the advice.

 

changed the PCV valve last night, the old one had a lot of carbon build up although seemed to operate freely, but for $4 wasn't going to quibble.

 

Not sure next time I will be coming down steep hill but when I do I will post back with the results.

[cookie]

valve for a long time. I'll try to remember to change that on my next oil change.

[dcolemanca]

UPDATE

 

I was driving home from work, no hills, after approx 3 miles CEL came on again, this time I got P1507 and P1540. The latter is to do with the VSS. The speedo is working normally.

 

I cleared both codes and they haven't come back so far.

 

Any other ideas on what could be the root cause and even better a fix?

 

I did change the PCV valve, I haven't cleaned the IAC valve yet.

[frag]

UPDATE

I did change the PCV valve, I haven't cleaned the IAC valve yet.

 

Before removing the valve to clean it, you could test the electric control module that sits on top of it. There are three contacts inside the valve connector: you should get 9 ohms between the center one and each of the other two.

[dcolemanca]

I tested the resistance from middle connector pin to the outer ones, approx 9 ohms on both.

 

I decided to tackle removing and cleaning, glad I did because I found the lower bolt was loose so I'm sure I was getting air leak.

 

The valve itself had some carbon deposit, not to bad and seemed to be operating freely. I got as much deposit off as I could with carb cleaner and reassembled.

 

Can't really tell any difference but then it was running fine when it wasn't throwing the CEL anyway.

 

I had also been getting the misfire P0303 and P0304 from time to time as well and maybe that was related to the IAC.

 

Fingers crossed that this takes care of these problems ...