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Knock Sensor Threads Leaking


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I have an 04 Response Lxi with Monsoon 340. 

Ever since I bought the boat a few years ago the threads in the block where the Knock sensor goes were rusted really bad and were cross threaded.  The knock sensor was just kind of forced in there.  Well this spring I could not get it to stay in so I finally went and bought the correct size thread chaser and cleaned out the threads which worked very well, except I noticed when I ran the boat even at idle there is a small drip leaking out through the threads.   The higher you run the RPMS the faster it drips.  (I also put in a brand new sensor and torqued to 15 ft-lbs which is the spec).  I know you are not supposed to use teflon but is there anything else that can be  used like "Perfect Seal" or pipe dope or anything to stop the leak that wont affect the sensor?  I dont want to overtighten as I am afraid my threads are probably weak  already.  

The manual states not to use any sealant so I am not sure if I should just let it drip or what to do.  

Edited by MTBoy
Clarify
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Maximum pressure at the knock sensor is 7 PSI, which I measured once on the drain plug side. Can you find a compatible sensor with slightly larger threads and tap the port to match?

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For the old single wire sensors you need to find a conductive sealant. 

Other that that, if it was mine I would get an adapter/extender fitting that you could leave in the block and flux/solder/braze the heck out of the threads on the adapter to block connection.  Drilling/tapping cast iron sucks especially with limited access..

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old jeep, 

Is that an electrically conductive sealant?   I have alot of that Mercury Perfect Seal but not sure if that is considered conductive.  

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I called Indmar today.  First of all I was impressed that they picked up the phone and talked to me.  PCM never even returned calls when I had a Nautique. 

But the first thing he told me was teflon.  I told him the manual says not to do that.  He said, that they come with a coating on it and go ahead with pipe thread sealant.  

Not sure about that.  I think I will still try the copper antisieze first.  

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55 minutes ago, MTBoy said:

I called Indmar today.  First of all I was impressed that they picked up the phone and talked to me.  PCM never even returned calls when I had a Nautique. 

But the first thing he told me was teflon.  I told him the manual says not to do that.  He said, that they come with a coating on it and go ahead with pipe thread sealant.  

Not sure about that.  I think I will still try the copper antisieze first.  

Antisieze is not a sealant.  

  • Like 1
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1 hour ago, MTBoy said:

Not sure about that.

It's a tapered pipe thread; tighten it enough and metal will touch metal.  Try the Teflon tape and a new brass nipple so you don't have to do this again.

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Not quite following you justgary, if you use a brass nipple and teflon dont you have the same issue where haven’t established a ground?

 

i know antiseize is not a sealant but i think it would act like one.   Right now i have marine perfect seal on it and it works but just not sure if that is ok like Indmar told me it was.  

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MT boy,

For the first time in my 2005 Response my knock sensor is leaking, bought a new one, etc.  but I can't get it to stop leaking.

Can you tell me what you ended up using to stop your leak and I will follow your example.

Thanks,  Carol

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Another recent thread prompted me to remember the old can of aviation shellac.  I would be tempted to try that on the threads.  If you start with clean threads on the block, I suspect you will get adequate conduction since the threads are tapered and will squeeze the goop out as you tighten the fitting.

I'm not sorry I don't have direct experience with this problem; I converted to closed cooling so I don't have to bother except to change the coolant every three to five years.

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@cjespinoza

right now i have perfect seal on the threads and it doesn’t leak, running with the bucket method in my driveway.  I haven’t had the boat on the lake yet at high rpms.  
May try one of the other things listed on here except for the newer boat that gary keeps rubbing in😂. Can’t afford that.

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9 minutes ago, MTBoy said:

May try one of the other things listed on here except for the newer boat that gary keeps rubbing in😂. Can’t afford that.

It wasn't me; my boat is older than yours!

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The latest advice I received from SKIDIM Discount marine was to use Permatex High Temp Thread Sealant.  So, that's what I am going to try.

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The anti-seeze works well after you pull the plug to drain the block for winterization. Pull the plug, wait a few days for it to dry out.

Then thread with anti-seeze and put the plug back in hand tightened. This helps prevent the threads from corroding over the winter. 

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  • 4 months later...

I've taken my knock sensor out every year for over 20 years; cleared rust in the hole with a screwdriver, chased/ cleaned the ( 1/2"-20UNF ) straight threads with a tap, applied a couple wraps of teflon to the knock sensor threads, and hand tightened the knock sensor. Thank goodness I've never noticed any pre-ignition or any engine issues.

The length of my knock sensor male tapered thread ( 1/8"-18NPT )  has become shorter every year from rusting inside the block. It's at the point where I need to replace it. I found a 5.7L Indmar replacement at Bakes Marine for $119.99, but I also found a GM ACDelco OEM knock sensor on Amazon for $28.11. The top part number on the Amazon Part ( 1045612 ) matches the number on my sensor. However, the bottom numbers do not match. MIne is 5020-21, the Amazon GM part number appears to be 0070-??.  Does anyone have any advice on the GM replacement knock sensor that I've found?

I was also thinking about installing a brass male to female 1/8" NPT adapter into my block if I were to use my current knock sensor. My other option is the new sensor with longer male threads directly to the blaock threads.

The other benefit of the brass fitting is its softness to seal to the block threads and the knock sensor threads. Any thoughts on the brass adapter?  

 

2001 Malibu 5.7L knock sensor.jpg

Bakes 5.7L indmar knock sensor.JPG

OEM GM knock sensor.JPG

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The bottom number is most likely a date or batch code, and probably safe to ignore.

Of course, you could just freshen up your old threads with a 1/8NPT die so the taper moves up the shaft a bit.  Probably wouldn't take much.

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I bought the same sensor you found on Amazon (ACDelco 213-96) and installed back in 2018, and have had no issues.  Also I occasionally use a 1/4-18 NPT tap to clean up the threads in the block, as justgary suggests above.

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@Vach6 @justgary @butter71 I'm confused as there's mention of 1/2"-20UNF and 1/8"-18NPT and 1/8NPT and 1/4-18 NPT. I've grown up fully metric so imperial is like a foreign language! Surely these are all quite different? My knock sensor only screws in 3 or 4 turns and I've noticed the engine block threads look quite full of rust. What's the correct tap?   

 

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15 minutes ago, uk_exile said:

@Vach6 @justgary @butter71 I'm confused as there's mention of 1/2"-20UNF and 1/8"-18NPT and 1/8NPT and 1/4-18 NPT. I've grown up fully metric so imperial is like a foreign language! Surely these are all quite different? My knock sensor only screws in 3 or 4 turns and I've noticed the engine block threads look quite full of rust. What's the correct tap?   

 

Pretty sure 1/4-18 NPT is the correct thread size for these knock sensors.  That's definitely the tap size I use.  I'm guessing the other are typos, etc...

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P.S.  I've found 3 or 4 turns is pretty normal.  NPT is a tapered pipe thread so it tightens and hopefully seals once you get it threaded to a certain point, due to the taper.

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