2000 Legacy, overheats when a/c is on.

[rysbrnr]

Hi everyone,

new to the Subie world. Just heard about the good rep so I bought one for a daily driver. It's a 2000 Legacy L with 180k miles! The car is very clean and looks very new inside. The problem is, when I turn the a/c on it overheats and does so within minutes. When you turn the a/c off, everything is okay. I tried the a/c on the highway, same thing, the temp gauges still rises. Also, today the CEL came on and according to Auto Zone it was the knock senor....I had them reset it and it came on again later in the day.

 

Does anyone know where to start with the "overheating while A/C is on" problem. And what about the knock sensor? is the common problem where I could just replace the sensor with a new one? Any help or ideas are greatly appreciated.

 

rysbrnr

[gbrand]

perhaps cooling fan(s) not working. On my 91 there are 2 fans, one to cool controlled by thermostat and the second comes constant when the A/C is on. Maybe someone with a newer model can comment on how your car is setup.

 

 

Knock sensor fairly common issue, do a search in this forum and you should get lots of info

[Suzam]

If you have observed the fans operating correctly (1 always on when AC is on) and as temp rises both should run, the highway speeds would more than keep the coolant at operating temperature. So I'd rule out fans if they are working.

 

I'd think maybe low level or sluggish coolant movement?

One of the following could cause the symtoms.

 

Water pump needs replacing?

Clogged cooling system or radiator?

Air to bleed out of the system?

Timing belt could need replacement?

 

Any history on the services done prior to your ownership?

[rysbrnr]

thanks for the input guys. Tonight I just went out for another cruise to see if would still do it. It doesn't now?!?! I let the car idle while the a/c was cranking away, then I drove around town at lower speeds (non-highway speed) and the temp needle didn't move above mid-point. What gives? Both the fans work and come on as usual when the a/c is turned on. What I did notice was that the non-a/c fan was running a lot more than on my other cars... maybe a sign that the cooling system isn't working too efficiently? I just bought this car today and during all the test drives in town and highway, it didn't overheat. This is a strange intermittant problem. When it overheated today, it was around noon and a lil warmer than my test drive tonight. I'm going to try flushing the coolant tommorrow and we'll see how it goes.

Anyone got any ideas on the knock sensor CEL? Is there an easy way to pull the CEL codes yourself without using a scan tool? How do you clear the codes?

[gbrand]

As 2000 MY is OBD2, you will most likely need a scanner to pull the codes. many autoparts stores(autozone, etc.) will pull the codes for you free. make sure you get the actual code number(s0, not just the parts monkey's description of what he thinks is wrong and you need to buy parts for.

 

A careful evaluation of the cooling system should reveal the problem, hoepfully not the dreaded head gasket.

[1 Lucky Texan]

thanks for the input guys. Tonight I just went out for another cruise to see if would still do it. It doesn't now?!?! I let the car idle while the a/c was cranking away, then I drove around town at lower speeds (non-highway speed) and the temp needle didn't move above mid-point. What gives? Both the fans work and come on as usual when the a/c is turned on. What I did notice was that the non-a/c fan was running a lot more than on my other cars... maybe a sign that the cooling system isn't working too efficiently? I just bought this car today and during all the test drives in town and highway, it didn't overheat. This is a strange intermittant problem. When it overheated today, it was around noon and a lil warmer than my test drive tonight. I'm going to try flushing the coolant tommorrow and we'll see how it goes.

Anyone got any ideas on the knock sensor CEL? Is there an easy way to pull the CEL codes yourself without using a scan tool? How do you clear the codes?

 

If you are new to Soobs PLEASE search here for coolant flushing/filling posts - it can be a little trickier to refill soobs confidently. I think Setright has a good procedure outlined somwhere. basically, you have to fill in 1-2 certain ways and burp the system multiple times to assure no entrapped air.

 

Carl

[rysbrnr]

I started looking for that post by Setright about coolant flushing procedure. No luck finding it so far. Just be spending some time searching I found that the "phase 2" 2.5 SOHC in my Legacy has less of a chance of having the dreaded head gasket failure. Glad to know that now! However it's been said that my SOHC 2.5L may have chances of developing an external coolant leak... so far no green fluids on my driveway (knock on wood). Above the knock sensor problem, the intermittant overheat issue still worries me. I don't want my pregnant wife driving around in it and having it overheat on her! Back to searching and reading I guess...

[1 Lucky Texan]

I started looking for that post by Setright about coolant flushing procedure. No luck finding it so far. Just be spending some time searching I found that the "phase 2" 2.5 SOHC in my Legacy has less of a chance of having the dreaded head gasket failure. Glad to know that now! However it's been said that my SOHC 2.5L may have chances of developing an external coolant leak... so far no green fluids on my driveway (knock on wood). Above the knock sensor problem, the intermittant overheat issue still worries me. I don't want my pregnant wife driving around in it and having it overheat on her! Back to searching and reading I guess...

 

here's one;

http://www.ultimatesubaru.org/forum/showthread.php?t=37017&highlight=coolant

[Johngenx]

Also, today the CEL came on and according to Auto Zone it was the knock senor....I had them reset it and it came on again later in the day.

 

That would be because you didn't repair the faulty knock sensor. The CEL is not there to annoy you, but it's actually connected to things. If you don't remedy the problem that triggered the code, the light will not stay out. Go buy a scanner and you could reset the light every day if you wanted, but it will keep coming back on until the fault is repaired.

[rysbrnr]

That would be because you didn't repair the faulty knock sensor. The CEL is not there to annoy you, but it's actually connected to things. If you don't remedy the problem that triggered the code, the light will not stay out. Go buy a scanner and you could reset the light every day if you wanted, but it will keep coming back on until the fault is repaired.

 

The CEL doesn't always mean you need to replace a sensor to remedy the light. Knock can be induced by several things. It could be low octane, too much ignition timing, hot spots in the combustion chamber, high intake air temps, etc. Throwing a new sensor at the CEL isn't garaunteed to fix it. For example, today I drove for 30 minutes and the CEL never came on at all... and didn't reset the CEL previously.

 

I haven't taken any action towards my CEL and overheating problem yet because I'm still "collecting data". Being that this a "new to me" car, I need to drive it enough to get some type of owner history on it. I need to know WHEN it overheats, WHAT the conditions are when it does, etc. Right now the problems I'm having are intermittant which makes things harder to troubleshoot. Once I can get it where I can predict reliably WHEN the problem will occur, I'll have a more accurate diagnosis.

 

A question for Subie guru's.

 

What makes the CEL for knock sensor come on?

- Anytime it hears ANY pinging in the engine?

- If the knock sensor is unplugged or when the ECU doesn't "see" the knock sensor hooked up?

Does Subaru's ECU knock sensing circuit detect knock according the frequency of the sound picked up the microphone of the knock sensor?

Is the output of the subaru knock sensor a voltage 0-5v?

 

 

It's normal that a car knocks a lil from time to time and the knock sensor is just there to tell the ECU to retard timing to protect the engine. I don't want to just throw a new sensor on only to find that something else causes the knock CEL. Then again I don't know anything about the reliability of the Subaru knock sensor... which why I'm here on this site researching the car I just bought.

 

On my turbo DSM, I have a datalogger that reads ALL of the what the ECU sees and puts it in a graph. I can see exactly how many counts of knock occurs, at any given RPM. I can see exactly how much timing advance the ECU is giving the the engine along with o2 sensor voltages at any given RPM versus % of throttle (load). Johngenx, I have kinda have an idea on how cars, ECU's, CEL's, and sensors etc work. Thanks for your input though:)

[1 Lucky Texan]

The CEL doesn't always mean you need to replace a sensor to remedy the light. Knock can be induced by several things. It could be low octane, too much ignition timing, hot spots in the combustion chamber, high intake air temps, etc. Throwing a new sensor at the CEL isn't garaunteed to fix it. For example, today I drove for 30 minutes and the CEL never came on at all... and didn't reset the CEL previously.

 

I haven't taken any action towards my CEL and overheating problem yet because I'm still "collecting data". Being that this a "new to me" car, I need to drive it enough to get some type of owner history on it. I need to know WHEN it overheats, WHAT the conditions are when it does, etc. Right now the problems I'm having are intermittant which makes things harder to troubleshoot. Once I can get it where I can predict reliably WHEN the problem will occur, I'll have a more accurate diagnosis.

 

A question for Subie guru's.

 

What makes the CEL for knock sensor come on?

- Anytime it hears ANY pinging in the engine?

- If the knock sensor is unplugged or when the ECU doesn't "see" the knock sensor hooked up?

Does Subaru's ECU knock sensing circuit detect knock according the frequency of the sound picked up the microphone of the knock sensor?

Is the output of the subaru knock sensor a voltage 0-5v?

 

 

It's normal that a car knocks a lil from time to time and the knock sensor is just there to tell the ECU to retard timing to protect the engine. I don't want to just throw a new sensor on only to find that something else causes the knock CEL. Then again I don't know anything about the reliability of the Subaru knock sensor... which why I'm here on this site researching the car I just bought.

 

On my turbo DSM, I have a datalogger that reads ALL of the what the ECU sees and puts it in a graph. I can see exactly how many counts of knock occurs, at any given RPM. I can see exactly how much timing advance the ECU is giving the the engine along with o2 sensor voltages at any given RPM versus % of throttle (load). Johngenx, I have kinda have an idea on how cars, ECU's, CEL's, and sensors etc work. Thanks for your input though:)

 

Maybe someone remembers the specifics, but I have a vague memory of there being a TSB or recall for knock sensors. I just can't remember the year/model. And I think it was because they were too sensitive. Do the plugs look good? if you had a carbon buildup a seafoam or other treatment may clear som out.

 

Carl

[Johngenx]

Yeah, the reason I jumped to the knock sensor diagnosis is that it's pretty common failure on the early SOHC 2.5L engines. Our 01 OBW and 01 Forester have both had their knock sensors replaced thanks to a CEL and indicator code. My father's OBW has also experienced the same problem.

 

Normally, I wouldn't advocate throwing parts at a problem, and I'd probably use a known-good knock sensor to test it, but since this is a common fault, it's a good place to start.

[jib]

Longshot - Verify the fans are blowing in the right direction. They run when the A/C is on and being reversed would essentially stop the flow of air through the radiator when driving. I've seen this on other cars. I don't know how easy it is to swap the blower direction on a Subie though. I've never been in there.

 

Jack