Hey, my dad's car has been acting up.. he gets the error code for the knock sensor (I forget the code right now). It's a 2.8 MPFI, but I assume that a knock sensor is the same no matter what engine it's in...

We ran the diagnostic out of the GM book- the knock sensor & wiring turn out fine. The SES light will go away after we're done, and come back on after a drive or two. So we test it again, same thing. The last time we worked on it, I replaced the connector; thinking it was loose, and not making contact. For giggles, we tested the knock sensor & wiring again- perfect. So, what the heck's going on?

The catalytic convertor had recently broken up, and was replaced. My dad thinks that maybe pieces of the cat are in the exhaust, and that maybe it's causing the engine to knock, and set a code. But if that were the case, wouldn't the engine run really badly, but NOT set the knock sensor error code?

Should we just replace the sensor? I find it odd that every time, the sensor tests out fine. If not, I guess I'll have to trace all the wiring back and look for an intermittant... (sigh)

Thanks!


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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If the Knock sensor is checking out fine, than I would look towards a tuning issue that may be causing the knock. Timing or fuel delivery possibly...or have you tried to use a higher octane gas to see if that helps at all? If there were pieces of the cat in the exhaust they would more than likely get blown out I think. DOes it seem to run fine as far as not cutting out at all?

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The car barely runs, actually! The GM book says the knock sensor will retard the timing up to 15 degrees- that's a helluva lot!

If it was a tuning issue though, why would the SES light come on, and the computer set the error code for the knock sensor? That's what has me baffled. I think you're right though about any pieces of the catalyst; it probably has nothing to do with this problem. I just can't imagine the knock sensor setting a code if it's fine... see my confusion? It's a distributorless car (DIS), so there's no timing, but my dad -does- use 87 octane. Maybe we'll give it some octane boost; I don't think we'd make it to a gas station.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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Your best bet at this point is to bypass the knock sensor so you can rule out whether knock or some other engine vibration is causing the excessive knock your ECM thinks is going on, or whether something in the ESC circuit or the ECM is fried. Just put a 3.9k phm resistor in place of the KS, connected to ground. This way you KNOW that you should not be getting any knock, and you know what kind of problem you're chasing.
...ed

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Ed Maher - Moderator @ The Carb Board
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[quote] Posted by TOMP If it was a tuning issue though, why would the SES light come on, and the computer set the error code for the knock sensor? [quote]

I agree with Ed about bypassing the knock sensor for sure to see if that is the cause.
The reason the knock sensor is setting the code may not be because it is faulty. It also sets the code when it is constantly receiving knock to let you know there is a problem. If it is barely running, then I think that it is something other than the knock sensor though. Timing and/or fuel delivery would be my first guess. Try bypassing it first since that is really easy to do, and then go from there if that is not the problem.

 

Thanks! 3.9k ohm, eh? I must have one of those in the basement somewhere! Wow, that's weird that the computer actually sets the code if the engine knocks too much- I wonder why the diag chart didn't mention that! I'll let you guys know how it goes! Thanks again!


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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Again, Thanks! I couldn't find a 3.9k ohm resistor, so I adjusted a 5k pot for 3.9k, and put it in place of the knock sensor. I started the car, and still got the SES light. So I put my meter on in place of the resistor, and did a scientific (heheh) "wiggle the wire" test on the knock sensor wire, and the voltage didn't change. So... the GM diag chart tells me to check the Memcal (chip) to see if it's fully seated. (The ECM was replaced a year ago.)

Man, what a pain that ECM was to get at! I'll never take my Firebird's for granted anymore! I had to pull the glovebox out, to get to the ECM-to-ECM bracket bolts, and then I couldn't pry the ECM out because the air conditioning duct was in the way. I wound up removing the dash-to-car bolts, pulling the dash away from the car, and then pulling the ECM out.

So it looks like we need to find a new MEMCAL; problem is, the car had that service bulletin for "MAF to SD" done on it. If we can't find that chip, we'll have to go backwards and hook the MAF back up. Oh well.. fun stuff! So thanks again for the knock sensor bypass info!


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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