Knock Sensor Question

[Poobah]

For quite some time, I have had an intermittant case of detonation on part-throttle acceleration. It only happens every once in a while so it's been very hard to track down.

I finally got a diagnostic customer snapshot of it.

The snapshot data said:

1. PD41 = 0 (The knock sensor did not detect any knocks even though my ears sure as hell did!)

2. PD40 = 0.00 (There was absolutely no spark retard resulting from knock sensor data this time. All snapshots I've taken when I wasn't hearing detonation showed between 1.5 and 9 degrees of knock sensor retard.)

3. PD08 = 23 (The spark was running about 5 to 8 degrees more advanced than any snapshots taken when I didn't hear detonation.)

Has anybody had experience with a knock sensor that acts up intermittantly?

Also, can someone give me some rough idea of the cost to replace it? If it isn't too much, I'll just change it out and see if the problem goes away.

Thanks in advance for any input.

[CadiKing]

PooBah,

I have a '99 STS.

I have noticed the same condition from time to time.

The car will appear to knock under light and heavy load.

The last time it was very embarrassing to say the least.

I was telling a friend how I loved that Caddy as my wife

was pullling away from his house...the knock sounded bad.

I have cold carbon rap which I have not been able to relieve

through WOT's.

No codes set.

I always run premium. I do WOT's every week.

What to do next?

[Scotty]

For quite some time, I have had an intermittant case of detonation on part-throttle acceleration. It only happens every once in a while so it's been very hard to track down.

I finally got a diagnostic customer snapshot of it.

The snapshot data said:

1. PD41 = 0 (The knock sensor did not detect any knocks even though my ears sure as hell did!)

2. PD40 = 0.00 (There was absolutely no spark retard resulting from knock sensor data this time. All snapshots I've taken when I wasn't hearing detonation showed between 1.5 and 9 degrees of knock sensor retard.)

3. PD08 = 23 (The spark was running about 5 to 8 degrees more advanced than any snapshots taken when I didn't hear detonation.)

Has anybody had experience with a knock sensor that acts up intermittantly?

Also, can someone give me some rough idea of the cost to replace it? If it isn't too much, I'll just change it out and see if the problem goes away.

Thanks in advance for any input.

This is a good one for bbobynski. Forgetting about the codes for a moment, this sounds like the same symptom I used to get with my 91 when I was getting an EGR problem. When the EGR doesn't flow you will get light pedal detonation. Doesn't the 94 require periodic cleaning of holes in the intake? Hopefully Guru will check in and help you out, Mike

[Poobah]

Thanks for the replies.

I didn't think to mention the following in my first post:

I give it a WOT at least 3 or 4 times a week (because it's fun).

I always use premium gas.

No codes are being set.

I replaced the EGR valve about 20,000 miles ago and cleaned out the passages and "mouse holes" in the intake and phenolic spacer shortly after that.

The car recently passed California's smog test.

The car has absolutely no drivability issues - it runs like a dream.

I've heard no detonation for, maybe, two or three months and thought the problem had gone away. All of a sudden, it's back. The only thing I can think of that's changed is the weather has warmed up a little.

From the snapshot, it seems that something in the knock sensor/spark retard circuit stops working for a moment causing the spark to run too advanced. I suspect the knock sensor is acting up because it wasn't reporting detonation and I could hear it as plain as day!

That's why I think I'll replace it (if my wallet doesn't take a major hit) and see what happens.