1999 Forester idling very low after warming up

[sewardflynn]

Hello! I am brand new to the forum and admittedly not too knowledgeable about cars. We have 99 Forester with about 120,000 miles. It had check engine light on for a long time - our mechanic stated it was an evaporative emissions code, not necessary to fix and very pricey, so we drove it like that for a couple of years. Exhaust system also required work for a long time. We finally did these repairs just in the last couple of days - we found a different, affordable mechanic, who seems very trustworthy. We found them after we bought an Outback from them a year ago. The car has always gone through a lot of oil, too, and we were not always super good about keeping it up. Sometimes we would check it and it wouldn't register on the dipstick at all. It doesn't have any leaks though, just eats oil.

A couple days before taking it in to the mechanic to be fixed, it did something new. It started idling really low. It would start up well, even in the subzero temperatures we are having here in Minnesota, and then after warming up and driving it for a while, it would start to idle low, around 500 rpms, and feel like it was going to die. The air blowing out of the heater would also really slow down, and lights would dim a bit until it got more gas. We mentioned this when we brought in. Mechanic says they checked the alternator and the battery and both were OK. They replaced the MAF sensor and the front O2 sensor. They didn't experience the symptom while it was in their care and sent it home with us. However, it is definitely still occurring. Having just invested all this money in a car we thought was on its last leg is kind of a bummer. Any thoughts? 

[lmdew]

The IAC controls the idle speed.  It's controlled by the ECU.  Try turning on the AC, that should bump the idle some.  

 

It could be many things, a good scan will tell you a lot.  Is the CEL on?  If so get the codes and post back.

[sewardflynn]

Thanks for your reply. No, it is not on. It was, and the codes were P0440 and P0170. Our mechanic said they fixed that by replacing the MAF sensor and the front O2 sensor. When It is idling low, it kind of revs itself up and down on its own every 3-4 seconds or so. I have been dealing with it by just giving it some gas while it is in neutral while I am at a stoplight, etc. Would the low idle itself be causing the lights to dim and the fan to slow down? Or does it sound like there is another problem causing all of it? I should take it back in but the mechanic is an hour away..

[BB's93LegacyL]

I'm interested in the answer to this problem as I have been having the same thing happen with my '93 Legacy wagon with 139K miles.  Warm idle looks to be about 500 rpm and the engine seems to be running a little rough, like while waiting at a stoplight. I also notice the slight surging of the revs maybe to 700rpm. I thought with the extreme cold in the minus teens and 20's I maybe had moisture in my gas or something simple like that related to fuel delivery, unique to severely cold weather.

Re: turning on the AC - if you have the heater control set to defrost, that would also cycle the AC compressor wouldn't it, as the defrost setting uses the AC compressor to dry the air inside the car?

[Fairtax4me]

The IAC valve on the 99 Foresters is known to fail and cause idle problems. I found one on eBay when a friend needed one a few years ago. $75 vs the $200+ that the local parts stores wanted. Couldn't find one in the local junkyards.

 

For the oil consumption, replace the PCV valve. Get one from a dealer. A little extra money but it makes a huge difference.

 

 

 

To BB, the IAC on yours is easily cleaned and that often makes the difference. Buy a can of throttle body + air intake cleaner. Remove the hose on the IAC and spray it down really well a few times. Make sure the valve inside flips back and forth easily. Also clean the throttle plate and bore inside the throttle body.

You should also make sure there are no vacuum leaks. And if it hasn't had basic tune-up stuff recently; plugs, wires, air and fuel filters, PCV valve; that always helps.

[BB's93LegacyL]

Thanks, Fairtax. As the second owner, I have good reason (service records) to believe that this 21 year-old car with 139K miles still has original plugs, wires, PCV valve, and fuel filter. I put in a K&N air filter awhile back, and it's still clean. I will follow your suggestions. I appreciate you taking your time to help.

[Fairtax4me]

Take out the K&N. The oil in the filter will coat the element in the MAF sensor and cause drivability and power problems.

 

Plain paper filters work best in these. The stock filter is plenty large enough to deliver all the air the engine needs. If you want some extra sound, pull the water separator out of the fender.

[BB's93LegacyL]

Thanks again!

[Rooster2]

Be sure to keep the oil level up near full. Running out of oil is a sure fire way to ruin the motor.

[mdjdc]

Just for information sake, I have been running K&N filters in my 2 forresters and outback or several years now.  I have not had any problems with the filters or driveabilty due to MAF issues.  I think the problem arises when people clean the filters and then over oil the filters.  If the filter is properly maintained, there should be no problems