Knock Sensor Replacement

[Lemsteraak]

Hi Guys,

Was wondering how big of a job it is to replace a knock sensor. I'm Getting a little transition knock when the engine is under slight load when the temperature around here gets over 95 degrees. I'm running 93 octane, have a really good radiator and otherwise the engine runs great at 190,000 miles. This engine doesn't have an EGR valve which helps elevate the combustion temp.

Best car I've ever owned

Rich
'93 MTX SHO
Tampa FL

 

[St Louis SHO]

Carbon buildup can cause this, and at 190,*** i'd put my money on that, and not a bad sensor.

 

[frosho]

Perhaps it's time to try some Auto-Rx?

 

[Mr Anonymous]

Never seen a knock sensor go bad in these cars.

The computer can only pull out timing so fast when it detects knock, often not fast enough and it's also limited to retarding a max of 10 degrees from the borderline knock table IIRC.

As mentioned carbon buildup is a common cause. Another common cause is a slightly lean mixture. Try cleaning your MAF sensor and resetting the keep-alive memory of the computer and see if that changes anything.

 

[lowc]

x3 on the carbon build up imo the best way to decarb is to pull off the intake get a few cans of carb clean and a old tooth brush and scrub away then blow it out with compressed air if available if not just remeber to let it air out before you start it back up bad things will happen if you start it with it full of carb clean

 

[rubydist]

The computer can only pull out timing so fast when it detects knock, often not fast enough and it's also limited to retarding a max of 10 degrees from the borderline knock table IIRC.

The computer can pull timing out plenty fast - typically 2 degrees per knock event detected, so it can pull out the full 10* in less than 2 revolutions of the crank, but as you say the 10* limit is not enough if you have a problem like carbon buildup.

The good news is that a little slight knock at transition under light load is nothing to be worried about, as it will not create combustion pressures high enough to cause any damage. Its knock under full or nearly full power that one needs to be worried about.

 

[St Louis SHO]

The computer can pull timing out plenty fast - typically 2 degrees per knock event detected, so it can pull out the full 10* in less than 2 revolutions of the crank, but as you say the 10* limit is not enough if you have a problem like carbon buildup.

The good news is that a little slight knock at transition under light load is nothing to be worried about, as it will not create combustion pressures high enough to cause any damage. Its knock under full or nearly full power that one needs to be worried about.

This over 100k miles is what leads to rod bearing failures....

 

[Lemsteraak]

Thanks Guys,

Really good information, you guys know your stuff.

I agree that carbon is probably the culprit, it absorbs fuel and leans the mixture, causing high combustion temp and knock. I just don't know where to look.

Didn't occur to me to clean the intake, I figure because there isn't an EGR valve there isn't exhaust running through and thus no carbon. I'll get the gaskets disassemble and clean it and the MAF sensor.

I do run a can of Sea Foam through the intake about every 5K to break up carbon in the head. I was really impressed the first time. I failed the new enhanced CA smog test reading high NOx. I ran a can of Sea Foam through and tested it again to get a base line number before I started work. It passed easily, as far as I can tell the problem was carbon in the head leaning out the mixture.

Rich
'93 MTX SHO
Tampa FL

 

[DavesNOThereMAN]

Thanks Guys,

Really good information, you guys know your stuff.

I agree that carbon is probably the culprit, it absorbs fuel and leans the mixture, causing high combustion temp and knock. I just don't know where to look.

Didn't occur to me to clean the intake, I figure because there isn't an EGR valve there isn't exhaust running through and thus no carbon. I'll get the gaskets disassemble and clean it and the MAF sensor.

I do run a can of Sea Foam through the intake about every 5K to break up carbon in the head. I was really impressed the first time. I failed the new enhanced CA smog test reading high NOx. I ran a can of Sea Foam through and tested it again to get a base line number before I started work. It passed easily, as far as I can tell the problem was carbon in the head leaning out the mixture.

Rich
'93 MTX SHO
Tampa FL

Even though there is no EGR valve the PCV system still dumps goopy oily waste into the intake.. I recently removed my intake as well and it was covered in thick black soot. Tanked it and cleaned it all out and is pristine now. Install a catch can and you will see how much nasty oil and crap gets caught up and your intake will stay cleaner longer.

 

[sho_sc]

Also, from experience. The solenoid that controlls the secondaries "shares" the V+ with the MAF. When it gets old, it can add all kinds of resistence into the circuit causing the MAF to report a lower voltage (temp. lean condition). This condition happens normally right before the secondaries open (3500-3800 RPM). You can check it by unplugging it to see if the knock goes away.

 

[turbo79]

Also, from experience. The solenoid that controlls the secondaries "shares" the V+ with the MAF. When it gets old, it can add all kinds of resistence into the circuit causing the MAF to report a lower voltage (temp. lean condition). This condition happens normally right before the secondaries open (3500-3800 RPM). You can check it by unplugging it to see if the knock goes away.

Where is the solenoid you mention located? (And what the "V+" is?)

And thank you!

 

[sho_sc]

Where is the solenoid you mention located? (And what the "V+" is?)

And thank you!

Behind the engine, about mid-way, attached (well should be) to the intake, it has 2 vacuum lines; which one has a vacuum canster.

postive voltage coming from the EEC.