Update on random check engine light (code: P0325)

It sounds like your knock sensor is detecting pre-detonation in your cylinders and letting you know.

Your engine will adjust automatically, up to a point.

Your MIL/ CEL is seemingly random because it is not unusual to get more knock occurrence in hot weather or various other driving conditions that you may not immediately think about. Were you running the AC compressor, either cooling the car or defrosting the windows, which obviously would tax the engine a bit more? Going up a steep incline? Have you checked your book to see if that engine requires a premium grade gasoline? If the engine requires a premium grade, putting a lesser grade can end up costing you more dollars per mile than putting the more expensive grade in, due to the engine changing it's timing to compensate for knock conditions.

A car manual I read once summed it nicely: If you hear engine knock, you may need to use a higher grade gasoline to compensate. This can occur based on driving conditions, or even age related wear of the engine. When you return to conditions that no longer generate engine knock, you can return to a lower grade fuel. The car was designed for the low grade gasoline, but obviously there are times this may not always be ideal.

In theory, the knock sensor is only sensitive to the specific frequency of pre-ignition. But, I suppose bad motor mounts or other issues under the hood COULD be detected by it. Or the sensor itself has drifted or gone bad or it's wiring has chafed or is intermittent.

========== There was never any common conditions when it would go on or when it would go off. Didn't matter if the AC was on or off, rain or dry, highway or side street speeds, etc.

I switched back to the lower grade fuel and after running a tank through no check engine light.

PS. Never had any egine knocks either.

PSS. If it does start coming back I'll break down and replace the sensor to see if that could be faulty.

I had an old Subaru wagon with the turbo on it. It had the exact same symptoms you describe - random check engine light. Switching grades cured the problem. Should I have been running it on the low grade gasoline? No, it called for the high grade, and it only threw a check engine code when running regular, but only perhaps once every three months or so. I eventually gave up trying to save money and ran the high grade, as gas/ mileage improvement gave me a cost per mile benefit over the lower grade gasoline. I run newspapers with these cars so it's easy to generate some nice numbers on a spreadsheet and see the difference. The CEL condition I experienced in that car was never when running the AC compressor or heading up hill either. Those were just a few conditions I was proffering in a theorizing out loud sort of way. I've had engine knock in other cars under those conditions that did not generate a CEL code, so who knows.

Carl makes some very good points in his reply. My theorizing was aimed toward more ideal conditions, where only one condition could be causing your knock indication, and the knock detection system itself is working properly. In reality, every part of the system in question should be held with at least a certain degree of suspicion.

It seems this vehicle is fairly old ('97 right?) and that is why I mentioned motormounts and 'other issues'.

If I didn't do 90% driving with short hops (> 7 miles) to work I would try a mid grade vs low grade MPG comparison. Even the drive in to work can vary as some times I take side streets when traffic gets too heavy and I know I'll get almost every red light (about every 4 blocks). Of course the side streets have a stop sign on almost every intersection (bless Chicago).

BTW, to all us "lucky" midwesterners enjoying the latest snow, stay safe!

If your car wasn't engineered to take advantage of higher grade gasoline, it may not show a dollar/ mile benefit with higher grades. Was really only concerned with your knock sensor firing at random. The point can be argued about other advantages to higher grades, such as additives and cleanliness, but somebody else can do that.

Well, after my third fill-up back on the lower grade fuel, guess what came back on :-)

If my 97 sube wasn't so dang reliable I would trade her up for a new model, but gosh darn it she hasn't let me down yet!

Perhaps you have extreme carbon build-up and your 'effective' compression is higher than stock. maybe some kinda 'top cylinder treatment' and a search for a cause of 'running rich' is in order?

I dunno, just a WAG.

Carl

In all the time I've seen this thread, you haven't told us what code your system is trying to tell you when the malfunction light comes on. Have you checked the code?

If not, you are like a man who gets a letter and wonders what's inside, but never opens it to see. (I've seen this behavior here before and always wondered why.)

If you have, what was the code?

Uncle Ben

99 Legacy OB

325 is a detonation sensor code. Not uncommon on Soobs and fairly common on '99s. Put in a new knock sensor and you should be good for gold.

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Some day I may just do that. BTW, engine light off again (as expected). In my younger days I would start a pool on the next time it would come on :-O

it is an intermittent sensor - somewhat temperature dependent, ant it fails open ofter sensing a knock