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GM 3.4 DOHC - Knock sensor circuit engine code


dkas

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So how bad is this and how expensive is this going to be to fit? Engine light come on, scanner says it is a knock sensor circuit that is bad. Where is it located? I have a 1994 Cutlass convertible that idles perfect, and misfires over 50 mph so badly it is not drivable.

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complete PITA to get to if it is the sensor. it's right between the alternator and 7X crank sensor.

 

if you're lucky, there is a cut wire somewhere or the knock filter on the MEMCAL is dead.

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I'd be more inclined to believe that a severe misfire is causing the knock sensor to be very active, leading to the code. At 50 MPH, the car should be in 4th, and the TCC locked--making that misfire very unpleasant.

 

Can a bad knock sensor even cause a misfire?
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Can a bad knock sensor even cause a misfire?

 

cause? no.

slight misfire cause knock sensor to go nuts and feel like a bad misfire? yes. this happened with Aaron's 94GP last year at the meet. changed the spark plugs(which had opened up to like .120") and it disappeared. was pulling ~9* at higher throttle and felt REALLY choppy.

 

I'd be more inclined to believe that a severe misfire is causing the knock sensor to be very active, leading to the code. At 50 MPH, the car should be in 4th, and the TCC locked--making that misfire very unpleasant.

 

VERY possible.

 

 

 

dkas, did you get the exact code number?

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You just reminded me of something there...once the timing gets pulled, combustion temps rise, contributing to an even greater likelihood of detonation---leading to more knock. It sort of feeds on itself.

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can be an issue, especially during sustained events.... the occasional knock event that causes ~2* to be pulled instantly and filtered out within a second? not a problem.

 

driving 2 miles with 10* retarded timing? glowing manifolds, downpipe and cat, not to mention what happens to head temperatures.

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I've got a pair of 3.4s that have such horrible piston slap that the knock sensor is pulling timing, with an expected reduction in fuel economy and power.

 

Tens of thousands of miles on knocking pistons--doesn't seem to be doing any actual damage, except to my wallet when I fill the tank.

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About a year and half ago I put new wires and plugs in, so I would be surprised if it is a spark plug. Also, we did find a broken wire on the connector for the knock sensor, fixing that did not help. The car is still misfiring over 50mph. It idles like a champ however! Mechanic does not think it is a time belt issue because he feels it would misfire at idle as well. Not sure where to go next to find the problem. Couple questions 1) Do you think a hole in the exhaust would cause this? The exhaust tone has changed and seems a bit louder but not a lot. 2) should my next move be to replace the ECM? Thanks for all the info so far. Oh, I brought the car home tonight because the mechanic is booked solid for the rest of the week. it goes back next Wednesday. when I got home it seemed really, really hot, but the heat gauge did not seem to be picking up and reading how hot it seemed to be. It smelled awful. I have been advised not to drive it.

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May have found the problem. The exhaust clamp has broken off at the rear where the single pipe splits to go to the dual exhaust. If it is bouncing around, could it be setting off the knock sensor? maybe just wishful thinking that it would be a quick and cheap fix! lol

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it's a possibility, though the wire on the knock sensor connector MAY be the real problem. mine was partially broken for ~4 years before it finally went completely. had to push/twist every couple of months for it to keep good contact. you won't have the same experience with a 91-95 LQ1 though, since the only knock sensor is obviously mounted towards the firewall.

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no, it is hard to get to in my 1994. we did find the wire was broken so we fix it. new connector and all. problem is still there. it made no difference at all...still misfiring.

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