My '99 Outback is plagued lately by an idle malfunction. Have been fighting this for weeks. Code scanner yeilds DTC of P1507, “idle control malfuncion”. No other codes. My repair manual says check for vacuum leak, loose intake, bad vacuum hoses, replace IAC. I have replaced the IAC, all vacuum tubing except large line to brake vacuum booster (when I disconnected it there was an audible rush of air leading me to believe all was intact there), & PCV. Also checked the intake bolts for loosness (found all to be tight). There seems to be a direct coorelation between lugging the engine down & the check engine light coming on. After all the above measures (the vacuum hoses were the last done) I actually got away with driving about 30 miles before I was late with a down-shift, lugged the engine, & the light is back. Any additional insight apprecieated.
Is there a driveability problem or is it just the CEL nagging you? Does the engine idle properly?
“… line to brake vacuum booster (when I disconnected it there was an audible rush of air leading me to believe all was intact there)…”
This does not rule out a leak in this line. I’d replace it.
Idle control problems can be difficult to solve. It’s most likely air getting into the engine via a path which it shouldn’t (for that operating condition anyway). It sounds like you are on the right track. I had an idle speed problem on my Corolla last year and the folks here guided me through how to get things working again. It was a fairly involved process, but I got it working ok eventually. The link below for that discussion might be helpful.
“I was late with a down-shift, lugged the engine, & the light is back.”
Well, don’t do that.
Maybe a problem with the throttle body/throttle position sensor (TPS).
“I had this code and went crazy with it for weeks. replaced the TPS, IACV, checked over and over vacuum leaks, replaced the neutral safety switch. In the end I brought it to the local independent subaru specialist who replaced the throttle body and adjusted the TPS which took care of it.”
Since you say it happens when you lug the engine (low throttle position) I would start with adjusting the TPS and seeing if that does it.