WRX knock problem

[evelrod]

This is just an FYI, really. But....

We just, after several days of fiddling, found out why my crew chief's somewhat modded WRX would just 'lay down' at several different RPM's, always the same. Apparently the knock sensor would retard the timing when it "heard" the rear alternator bearing that was going bad 'vibrate' at certain rpm's. New bearing and now the thing runs like stink. It's making good power at 18psi boost with no flat spots now. I'm too far out of the loop on some of the newer technology, I guess...Whoda thunk it?

Rod

 

[MintJulep]

weird.

How did you manage to figure it out?

 

[patprimmer]

Maybe the old technique of:-

It used to run alright. What changed, how long has it been making that noise. Then noise is vibration, knock is vibration, I wonder?

Using the old knock sensors on the sides of the head heard the bearing and replaced it and then it was noticed the problem coincidently went away.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

I had a very used non-turbo 1985 volvo station wagon with a noisy exhaust heat shield. One day I took off the shield to get rid of the noise. It felt noticeably stronger and more responsive immediately

 

[patprimmer]

It must have been a VERY heavy heat shield as I suspect a 1085 Volvo has no knock sensor to be tricked by a noise.

Also I expect that an exhaust rattle is a very different frequency to knock, where a noisy bearing might be in the same frequency as knock.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[kenre]

Ive diagnosed many a noise by just removing the drive belts from an engine....

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[patprimmer]

I think the point is the knock sensor was picking up vibrations from the worn alternator bearing and falsely sensing knock and retarding the engine as a consequence, thus killing power output.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

"It must have been a VERY heavy heat shield as I suspect a 1085 Volvo has no knock sensor to be tricked by a noise."

The 1085s didn't have them. That 1985 did. Most US models of 240 got them in 1983 or so.

The factory recommended valve clearance for 240s got tightened up at some point, reportedly to avoid fooling the knock sensor with heavier valve clickety clack.

 

[evelrod]

We hooked up the laptop and ran the engine in the range where the ECU was affecting timing and stuck our heads under the bonnet with a long screwdriver against our ear. Actually, you could hear it several feet away, it was BAAAAAD!

Rod

 

[carnage1]

hmm, just picturing that and wondering which end of the screwdriver you listen to ? never heard that trick before

 

[patprimmer]

You need a full steel shank screwdriver for best results (the type designed to be hit with a hammer without damaging the plastic handle.

Obviously you place the blunt end against your ear and touch the sharp end on the susso part. It is quite an efficient stethoscope and makes one realise just how much engine noise can be transmitted into the passenger compartment via control rods, cables and levers.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

Pat, I only did the screwdriver bit 'cause I wasn't in my shop. My wife is a nurse and I have quite a selection of stethoscopes...one with an 18 inch brazing rod as a probe. I hear you can even "buy" them at the auto parts emporiums, these days!

Rod

 

[Tmoose]

Some of the folks on the vibration analysis groups call them "yellow handled analyzers"

 

[MechanicalChef]

Interesting approach. Back in school, my NVH professor's method for diagnosing a noisy component was to go around and hit parts with a ball-peen hammer. Worked well on tractors. Probably not so much on body panels.

 

[RossABQ]

I had an '87 VW GTi that wouldn't run right above 2,000 RPM unless the A/C was on. Knock sensor tested good, but I always wondered if the ECU was ignoring the knock sensor when the A/C pump was on to avoid just this kind of problem? I could find no electrical problems, no codes, etc.

 

[patprimmer]

Maybe the knock was coming from the compressor itself.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RossABQ]

What I meant is, the car ran perfectly WITH the A/C running. If the compressor was knocking it would have run poorly with it on.

 

[tommurphy73]

Interesting. Good to see that old fashioned diagnostic methods like the screwdriver are still valid in this electronic age.

Web design company

 

[modesign]

Years ago when I had a Shelby Daytona in the US. It had the same lack of boost problems. Seemed to be worse in winter. The engine was a 4 cylinder. Spoke to Shelby who immediately said that it was piston slap (deliberately wide clearances) being picked up by the knock sensor and that I should change to a particular full synthetic oil. Problem solved.

 

[3star2nr]

yeah its amazing how sensitive knock sensors are, and how quickly modern ECUs respond to them. just tapping the block with a screw driver is enough for the knock sensor to "hear"

Thats an interesting case though