'03 Legacy Idle/OD problem when A/C is on

[Timberwolf530]

I just bought a 2003 Legacy Wagon, and it has a few annoying things happening with the A/C on. First, the idle is erratic. It idles at around 800 -900 rpm at a stop in gear, but sometimes it will jump up to around 1200 – 1300 rpm. It eventually drops back down, but it’s enough to make the car jump forward if I don’t have enough pressure on the brake. The second problem is a jerk in the transmission if the car is in overdrive at speeds of approximately 35 – 45 mph. It happens when I let up on the gas pedal at these speeds and then reapply the gas. The jerk happens when I reapply. Neither one of these things occurs if the A/C is off. It idles perfect, and there is no hitch when in OD. The trans fluid was dirty (didn’t smell burnt), so I changed the spin on filter and all 10 qts of fluid with Dextron VI. I also changed plugs. Other than this and a small spark plug seal leak, which will be fixed soon, the car is perfect. Any help would be appreciated.

[Legacy777]

I believe the IAC valve is on the throttle body. It is a plunger type setup. They can sometimes get gummed/carbon'ed up. I would try removing the valve and cleaning it up and the hole up with brake cleaner.

 

If you move the plunger around when cleaning, when you first start the car up the rpms will likely race until the plunger gets adjusted properly.

 

I'd start there and see what happens.

[Timberwolf530]

I will try that tonight, but shouldn't that affect idle regardless of the A/C? When the A/C is off it idles as smooth as butter.

[Legacy777]

The motor is a stepper motor, and there may be a spot where carbon has built up, and that is the spot the ECU adjusts the IAC valve to when the AC is on.

[94Loyale]

Off the topic of the idle problem. But did you just drain the trans fluid once out of the drain plug? Cause if so, 10 quarts is way too much. Even if you let it drain all night out the plug. 5 1/2 quarts is about right on the mark when you drain and refill.

[Timberwolf530]

The manual says the tranny holds 9.8 qts, so I drained the pan, replaced the filter, and put 4 qts back in. I disconnected the return line from the tranny cooler and ran a short length of hose from the output nipple into a milk jug. Pumped out 4 more qts and added 4 more through the fill tube. By the time I got one more quart out, fluid was looking clean, so I re-connected the return line and topped it off. I drove it about 10 miles to get it hot and filled to the full mark. Recechecked the next day just to be sure, and it was right where it is supposed to be. It was right at 9.5 qts that I replaced. That's the way I have been changing tranny fluid for years. I've always figured it really doesn't make sense to change only half the fluid.

[Timberwolf530]

The motor is a stepper motor, and there may be a spot where carbon has built up, and that is the spot the ECU adjusts the IAC valve to when the AC is on.

OK, that makes sense. I have been too busy this week to get to it, but I will definitely take it off an clean it this weekend. Thanks for the explanation.

[94Loyale]

Twolf...Okay, just wanted to make sure, sorry it was off topic

[Suzam]

Twolf...Okay, just wanted to make sure, sorry it was off topic

 

 

It was a legitimate question, because I wondered too. Wouldn't be te first time someone drained the pan, read the manual and put 9+ quarts in to replace the 3-4 quarts that came out.

 

[Timberwolf530]

Update: After a month of trying to figure out what the heck was going on & cleaning carbon out of every sensor I could find, I decided to try the old turn it off and back on trick. I disconnected the battery for a couple hours, hooked it back up and it's been fine ever since. I'm glad I got it taken care of, but feel kind of dumb that I didn't try that to begin with. That's always the first thing I do to my PC, and cars today are just PCs with wheels. Anyway, I thought I would update just in case anyone in the future comes across the same problem.