WHAT ARE THE SYMPTOMS OF A BAD KNOCK SENSOR ?

[BOUDREAUX]

Can the knock sensor (KS) function erraticly? What symptoms does one usually see/smell/feel when KS is bad?

Boudreaux

[Q45tech]

A failed KS test at startup reduces the ignition advance under cruise and steady state conditions. Called low load

Sluggish acceleration especially on highway cruise at less than WOT....mild acceleration* bucking/surging, a minor decrease in highway [1-2] MPG.

Different year Q behave differently due to differences in CONDITION, software, gasoline, temperature, and location.

At a steady state rpm [say 60 mph] the advance can be as high as 40 degrees dropping to 22-26 degrees under acceleration.

Only takes a few minutes to measure KS resistance once you have test equipment and learn the steps.

Generally KS last around 10 years but this varies with local ambient temperatures and how frequently you crank and shutoff to create extreme conditions [hot soak] under the plenum.

The harness is always changed with BOTH KS SENSORS due to high labor cost and extra damage [to hoses] that removing plenum creates.

[BOUDREAUX]

So does the knock sensor mostly effect engine performance at higher sppeds or can you have issues around town and when accelerating from a stop.

Boudreaux

[Q45tech]

When you suddenly depress throttle the cruise advance drops from 40 to 15 then quickly rises to 20-24 depending on rpm and whether a downshift occurs.

Speed has little to do with it just start rpm and resulting rpm.

A 32 valve engine is marginal on torque until 2,500-3,000 rpm why downshifts occur.

The worst case might be 25-45 mph and again around 55 mph pulling a slight incline. Turning AC on makes it more noticeable.

[qsiguy]

My '94 dyno'd at only 151 hp with a bad knock sensor and the OEM ECU. That's about a 70-80 HP drop in power.

[Q451990]

So does the knock sensor mostly effect engine performance at higher sppeds or can you have issues around town and when accelerating from a stop.

I typically notice it at a lighter cruise. Think of when an average car will experience spark knock the most... say going up a slow incline at 40MPH with the transmission in 4th gear - essentially those times that the engine is lugging. It's worse in hot ambient temperatures too.

That's when you're going to experience the most problems with bad knock sensors too. The reason being - the ECU knows it's bad, so it puts everything into failsafe mode... using all of it's safety margin with the timing advance, etc. AS-IF the engine were knocking all the time - because it knows it can't really tell what's going on without it's "ears" (the knock sensors).

The difference between driving with bad vs. working is very noticeable to me. For some drivers, it's probably more of a "something doesn't feel quite right" type of thing.

Heath

[sdkhalsa]

Major symptom was lack of acceleration. When I connected my consult software(datascan) I noticed a sharp retarding of ignition timing, which was pretty much my notice of a bad knock sensor. On ohming the sensors, I found that one was open most of the time. To temporarily solve the problem I put a 550K resistor in parallel with the bad sensor, and this worked fine. Just now getting ready to replace the sensors as I need to go in anyway to take care of an injector problem.

[Q45tech]

The resistor trick is an emergency solution not a long term solution especially in Summer heat!!!!!!!