1998 Subaru Outback stalls at traffic lights after driving 30 miles

There is a air bypass thingie (technical term for when you can't recall the correct name ;-) (Idle Air Control valve, maybe?) that regulates engine idle based on coolant temperature, AC on/off, ECU inputs, etc. These can get sticky with fuel deposits and such over time, and respond slowly, or not at all.

Symptoms would be as you describe; engine won't settle down to a smooth idle, or idle at all, particularly after prolonged operation at a fixed throttle position (highway).

OK idle after restart could indicate a sticking condition as well.

This is theory. 'Bout the only way I can think of to test it would be to replace the darned thing with a "known good" one; either new ($$$), or from a salvage yard. It is associated with the throttle body assembly, and probably will not be difficult to replace once you identify it; look for something on the throttle body with wiring going to it that _isn't_ the throttle position sensor.

I've never seen these symptoms on a Soobie, but my old MR2 was so bad that I finally made a metal plate to blank the air bypass off entirely, as I got tired of having to drain/refill the coolant system to service it. (Plus MR2's aren't the most user friendly things to work on.)

Maybe some one can add to this?

Good luck; let us know what you find.

ByeBye! S.

Steve Jernigan KG0MB Laboratory Manager Microelectronics Research University of Colorado (719) 262-3101

Yep, I'd say primary culprits (with no specific codes) are IAC, Engine Temp Sensor (not the coolant temp sender) maybe the MAF ($$$) possibly (least likely) intermittent Crank Angle Sensor.

I've read you must be careful what solvent is used to clean the IAC or throttle body, I'd get w'ever Subaru sells for that I think.. Unless you want to try some do-it-yourself fixes, a good Subaru dealership should be able to clean, inspect or test most of these items for you.

Oh yeah, a car this old 'could' also have agded/cracked high voltage parts, plug wires and coils. But I bet the mechanics you took it too already ruled those out.

Maybe someone else can think of something.

good luck and report back when you get it fixed!

Hmm. I just spoke to my mechanic and he says the code that came up indicates replacing the Mass Airflow Sensor ($500). Perhaps that's the "MAF ($$$)" referenced by Lucky, above? I'm worried about replacing very expensive parts that might not actually fix the problem. You know, the car just isn't worth all that much and gets lousy gas mileage anyway. (Although the heated seats are very nice.)

Is it appropriate to ask my mechanic if he'd be willing to try cleaning the "throttle body"? I'm scared of the Subaru dealership because of the trauma attached to buying this car about 5 years ago. The sales people were very creepy.

Thanks again for all the helpful suggestions, Emily

Input from hubby who drives the Subaru:

"Usually when it stalls ALL the lights come on. But sometimes just the yellow "AT Oil Temp" light flashes for a bit. What I've also noticed is a sound coming from the front, passenger side as the car's slowing down, kind of like the noise a flat tire would make, or if there was something caught in the wheel. So it COULD be brake related."

for what it's worth!

Emily

I don't always use a dealership - but in this case, they can take back an incorrectly installed electronic part like a MAF. Um, I have read of people using 'oiled'-type air filters like K&N and getting oil on the MAF sensor. They sometimes can clean the sensor with alcohol. Likely yours is bad if it's throwing a code though.

Hey, $500 is one month's car payment for some people! hah!

Usually when all the instrument panel lights come on, this is an indication something failed with the alternator.

I won't buy a new MAF. Can find plenty of used for $50 or less. I once bought one at a yard for $5 and it is still soldiering on after 4 years.

What you could be hearing is sounds from the exhaust after the engine has already stalled, but the car is still moving and driving the engine.

One way to see if it's the IAC is to look at the throttle body and see where the throttle stop is, i.e., the fixed area on the outside that contacts the movable area driven by the accelerator cable. If you can wedge a thin piece of something, like a tongue depressor or even folded up paper, between the two to bump up the idle speed to about 1000-1200 rpm, that will temporarily prevent the engine from stalling when your foot's off the gas. Just don't make it so high that the car wants to take off on its own, or wants to slam into gear from Park.

If that solves your problem, then clean or replace the IAC.

Bob

Poor fuel economy is another symptom that something is unhappy in the engine compartment. Do you know which engine is in your car? (SOHC or DOHC) Automatic SOHC cars of that vintage should deliver somewhere around 25MPG in _average_ driving (whatever that might be); the DOHC version a bit less.

In any event, about the only way to address problems of this sort is to start replacing things until you find the problem child. We call it a "witch hunt" :-) Lucky mentioned plug wires. I _always_ forget about those pesky critters. Unless you have done so in the last 20K miles, replace 'em. Toss on another MAF sensor. (Guys, '98 Legacy's MAF or MAP? Changed it somewhere in that time frame.) Put on a replacement IAC valve . . .

(Acronyms. Double OverHead Cam, Single OverHead Cam, Mass Airflow Sensor, Manifold Absolute Pressure sensor, Idle Air Control valve . . . on and on. Learn more than you ever wanted to know about it from Wikipedia: )

Emily, altho this sounds like rocket science, most of this stuff is easy to replace; pretty much all right on top of the engine. MAF takes a phillips screwdriver to undo a couple of band clamps. IAC is a couple of screws as well. Plug wires just take doin'. Access is easier if you simply remove the air intake plastic parts between the filter box and the throttle body; takes a 10mm socket wrench.

Your mechanic ought to be real happy getting paid for this kind of stuff; clean, quick, and easy. Have him pick up a complete intake system from a salvage yard, and go to it. Or give the hubby a beer and a Craftsman tool kit, and point him at the garage. Even better, roll up your sleeves and DIY (won't he be surprised?).

Our kids learned to wrench along with their drivers training (wanna drive it; gotta fix it . . .); the YL is quite adept at it, and held in awe by her boyfriend. If a 18yo college freshman (freshwoman?) can do a Legacy head gasket (pop sat in the shade with a beer and pointed), you can manage the EFI system.

If you can't find what you need at a local yard, let me know. My buddy owns a Soobie/Toyota place, and I'll pull an intake the next time I'm down for a visit, and send it along. Probably cost around $100 + shipping for everything noted above, including a throttle body. I'll send along a complete intake manifold (has a bunch of sensors on it) if necessary; Bob usually aim's 'em at the scrap aluminum bin.

ByeBye! S.

Steve Jernigan KG0MB Laboratory Manager Microelectronics Research University of Colorado (719) 262-3101