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Car Forum / Subaru Cars / April 2009Uncle Ben discovers the knock sensor
A month ago I had noever heard of the kinock sensor. Now I know that it is a valuable tool that everyone should be aware of. My 99 obw taught me about it through a code P0325, knock sensor ckt problem. I kicked that one upstairs to my faithful mechanic Dave, who replaced the sensor, and off I went on a 1000 mile round trip visit. To my surprise by the end of that trip I was getting 28 mpg again (75 mph hwy), just like when the car was almost new. It had degraded down to about 24 mpg in the last year. This was on 10% ethanol. I am eager to see what I now can get on 85% ethanol (E85) when the price of gasoline goes back up. Now I can see better what the knock sensor will do for a high octane fuel like E85. It advances the spark up to the knocking point and backs off a little. That explains why the mpg penalty for E85 is only 15% instead of the energy density penalty of 30%. (Last year at the peak of gas prices, E85 was 25% lower in price, giving me a miles-per- dollar savings.) It also helps explain the higher performance I get on E85. Maybe I am not the only one who didn't kinow about the knock sensor! Uncle Ben Amen brother. My soob was running a little rough and didn't seem to be getting the milage it used to so I hooked up the 'puter and pulled a P0325 like you. I removed and "tested" the sensor as it said to in the Hayes manual and the part "checked out" (open-circuit). So i put it back, cleared the code and continued to drive. While the code didn't come back for a couple of weeks, the car continued to run rough (hesitation under accelleration). Finally the MIL came back on with the same code so I actually bought a new one (80$) and put it in. Wow what difference. I'm guessing that it was failing in the other direction - always producing a bouncy voltage causing the ECU to retard the timing all the time. The other thing I read was that the ECU always listens to the knock sensor all the time to figure out, relative to MAF and RPM, when the actual *bang* is going to happening relative to the crank sensor. It then updates it's timing tables accordingly. So without a working knock sensor, your car is always driving in "limp" mode. As my MAF sensor, like the rest of the car, is 14 years old - it's output doesn't really conform to the default tables stored in the ECU and the car doesn't really run well until the ECU figures this out. cheers!  SignatureDominic Richens | knob@storm.ca "If you're not *outraged*, you're not paying attention!" >A month ago I had noever heard of the kinock sensor. Now I know that > it is a valuable tool that everyone should be aware of. [quoted text clipped - 20 lines] > > Uncle Ben >A month ago I had noever heard of the kinock sensor. Now I know that > it is a valuable tool that everyone should be aware of. [quoted text clipped - 20 lines] > > Uncle Ben Rule of thumb, if you have a knock sencor code, replace it. very few other things cause it. If you have other codes too, may want to check them out first cause they can cause a knock condition. Steve > A month ago I had noever heard of the kinock sensor. Now I know that > it is a valuable tool that everyone should be aware of. [quoted text clipped - 20 lines] > > Uncle Ben Can subies run E85? If they are not made to run it, they can't unless you convert it. Being that you just found out what the sensor is, I doubt you have done so. The amount that an engine will advance the timing to take advantage of the higher octane fuel is nowhere near what would be required to take advantage of the higher octane of E85 fuel. I'd bet that if the ECU were to max itself out as far as advanced timing, you'd only need 93 octane to stay out of the portion of the map that the ECU goes into once it recognizes knock. I've heard different numbers for the equivalent octane rating of E85, but they are all well over 100. It is very very knock resistant. Unfortunately unless a car is engineered for it, it can't be exploited. Using E85 in an engine not made for it WILL hurt the engine. The alcohol is hard on the plastics and rubbers in the fuel system. That means seals and injector parts--the important stuff. > > A month ago I had noever heard of the kinock sensor. Now I know that > > it is a valuable tool that everyone should be aware of. [quoted text clipped - 40 lines] > > - Show quoted text - Thanks for your concern, but not to worry. I did convert my car to flex-fuel with a pulse-stretcher in the electrical line to the fuel injectors. It is a little box containing a chip on a board with two cables per cylinder - one in and one out. Easy installation. Your info on damage is out of date. Since about 1996 or so, cars have been required to tolerate 10% ethanol. In fact, inspite of cover-your- a.s language in the car manuals, they tolerate much more. I have been running E85 for 10 months now on and off as prices vary. No damage apparent. None expected. The good news is that I get about 10% higher torque at low speeds and 10% higher hp at higher speeds. On cost, I break even when E85 is 15% lower. Last year it was 30% lower in my area, and I fully expect that ratio to come back as we get further into "peak oil". Some people are worried about the corn supply, but the GAO just came out with an analysis showing that diversion to ethanol was responsible for only 10-15% of the rise in corn prices last year. Most of the rise was from oil prices. Hi Uncle Ben, all! >Your info on damage is out of date. Since about 1996 or so, cars have >been required to tolerate 10% ethanol. In fact, inspite of cover-your- >a.s language in the car manuals, they tolerate much more. I think this "alcohol damages your car" rumor probably started with the race guys who run _Methanol_ fueled cars. Methanol will decidedly attack any natural rubber and most synthetic rubber parts it comes into contact with. Ethanol is a far less aggressive solvent, and unlikely to damage modern synthetic rubber compounds. Good nfo re. the knock sensor. FWIW, I will add that tired old plug wires will also cause engine roughness and loss of fuel economy. Two easy ways to test: For the brave, with the engine idling, run your finger along the wires and around the caps where they connect to the coil pack. If there's a bad wire, you'll soon know about it! (Hehheh! ;-) For the wise, open the hood in the dark with the engine idling, and watch for sparks, and a sorta glow along the wires as the plugs fire. If you see anything like this, replace 'em. Use a premium wire; Bosch, or NGK (or OEM). They're more costly, but I have found that the boots on the "generic" wires often don't fit very well. If you have over 100K on the OEM wires, I can almost guarantee they're due. ByeBye! S. Steve Jernigan KG0MB Laboratory Manager Microelectronics Research University of Colorado (719) 262-3101 > Hi Uncle Ben, all! > [quoted text clipped - 32 lines] > University of Colorado > (719) 262-3101 KG0MB: --... ...-- NQ2H > I think this "alcohol damages your car" rumor probably started with > the race guys who run _Methanol_ fueled cars. Methanol will decidedly > attack any natural rubber and most synthetic rubber parts it comes > into contact with. Ethanol is a far less aggressive solvent, and > unlikely to damage modern synthetic rubber compounds. I don't think it is a rumor. I have been told that E85 will do bad things to the seals in a fuel injection system that is not designed for it. I was told by my professor--Doc Holloway. He was the president of SAE America from 97 to 98, and knows what he is talking about. He headed up many projects through the nineties at the University of Maryland that involved converting engines to be used in International intercollegiate competitions, and was quite successful. It might be true that newer cars all use seals that are resistant to the effects of E85, but it is not a rumor that E85 is corrosive. If you are backing up your argument by testimony of a few people who have gotten away with using E85 on unmodified engines, all I can say is, wait a while, then talk to them again to see how well their fuel system is holding up. > > I think this "alcohol damages your car" rumor probably started with > > the race guys who run _Methanol_ fueled cars. Methanol will decidedly [quoted text clipped - 16 lines] > is, wait a while, then talk to them again to see how well their fuel > system is holding up. It is quite true that ethanol does attack materials that used to be used in car fuel systems. Cork is a good example; natural rubber is another. Fuel tanks used to be coated inside with something that dissolves in ethanol. But that was then. Nowadays we have abundant evidence from cars in Brazil, where they use ethanol that even has the water in it carried over in distillation, and for many years their cars have done just fine with conversion with pulse stretchers only. I'm not worried. Uncle Ben |
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