Losing Compression Past Spark Plug Threads?

[romeen]

I have low compression in cylinder #4. When I hook up an air hold line and pressurize that cylinder air escapes past the threads of the air hold fitting. Using teflon tape on the threads stops the leak. It does this a bit on the other cylinders as well but not nearly as much.

Another strange thing is that the compression on cylinder #4 is higher when the motor is cold and decreases about 25-30 psi when the motor is warm. Maybe the plug hole expanding and allowing more air to escape?

I tried some tape on the threads of the compression tester but it bumped it up by only 5 psi. I don't know if this could be because the tester has fewer threads than the air hold fitting/adapter? This pic comparing the compression tester, the air hold fitting and a spark plug shows what I mean.

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Has anyone ever encountered something like this? Other than replacing the head are there any other options (heli-coil, high temp thread sealant)?

 

[draculia]

Neither the tester, nor the plug seals with the threads, so I wouldn't worry about the threads. It's tough to see, but look into the plug recess and see if you can see what the lip around the threads looks like. That's the sealing surface. If it looks dirty then try to clean it out. Just be careful not to let stuff fall in the cylinder.

 

[BogusSVO]

the threads could be wore loose from spark plug changes over the years, or the threads have had a tap run thu them and thinned the treads out some over time.

Sav-A-Thread, spark plug thread repair kit, appx $35 at auto zone or NAPA, plus you will need a box of inserts, appox $12 (you will need the long ones)

it can be done with the head on and some common sense, But I perfer the head to be off, that way no chips fall into the cylinder

 

[romeen]

Thanks for the responses guys.

I can actually wiggle the air hold fitting a bit and the pitch of the air escaping changes so I definitely think the threads are damaged a little. But I wonder if it might leak with the compression tester or fitting but seal when using a spark plug. If I am losing compression past the plug shouldn't I see some carbon fouling or heat damage around the plug seat? I have some copper based anti-seize that I will put on the threads, I figure it can't hurt.


Edit: BogusSVO, thanks for the tip on the Sav-A-Thread. I think I will give that a try. Do you think using some heavy grease on the threads of the reamer/tap and frequently backing it out will be good enough to capture the chips?

 

[BogusSVO]

all that will help, then before installing the sav-A-thread, blow out with compressed air, then spin the engine ove a few times, that should get most all the chips out

 

[Ceddy]

If compression is leaking past the plug you can feel it while the car is running. There is a tiny vent on top of the spark plug wire boot, you can feel the air blow out and suck in quickly there.

I had this happen from a botched Threaded Insert job, ended up using a lot of red loctite to seal the Insert.

 

[JEFFREYCASTGSX]

Thanks for the responses guys.

I can actually wiggle the air hold fitting a bit and the pitch of the air escaping changes so I definitely think the threads are damaged a little. But I wonder if it might leak with the compression tester or fitting but seal when using a spark plug. If I am losing compression past the plug shouldn't I see some carbon fouling or heat damage around the plug seat? I have some copper based anti-seize that I will put on the threads, I figure it can't hurt.


Edit: BogusSVO, thanks for the tip on the Sav-A-Thread. I think I will give that a try. Do you think using some heavy grease on the threads of the reamer/tap and frequently backing it out will be good enough to capture the chips?

Whenever we tap (the 2v modular heads that spit plugs) heads we use a little bit of heavy grease and going slow at it and backing out every so often to clean the tap. I myself have never seen a single shaving fall in using that method.

 

[viprez586]

Whenever we tap (the 2v modular heads that spit plugs) heads we use a little bit of heavy grease and going slow at it and backing out every so often to clean the tap. I myself have never seen a single shaving fall in using that method.

Agreed!

What are your compression numbers?

It's odd that you have higher numbers cold. There could be some taper or out of round issues going on in that cylinder. When it heats & expands the gap presents itself.

 

[romeen]

Whenever we tap (the 2v modular heads that spit plugs) heads we use a little bit of heavy grease and going slow at it and backing out every so often to clean the tap. I myself have never seen a single shaving fall in using that method.

That's good to hear. I feel more confident in doing this now.

Agreed!

What are your compression numbers?

It's odd that you have higher numbers cold. There could be some taper or out of round issues going on in that cylinder. When it heats & expands the gap presents itself.

Yeah, it's strange and I'm not sure what to make of it. Before I give the numbers I'll say that this compression tester reads high on every car I've used it on but it is very consistent for the sake of comparison.

Warm motor:
#1=184
#2=180
#3=184
#4=105, 111

Cold motor:
#1=170
#2=163
#3=180
#4=135

I'll also add that I just finished installing EVO9 springs and retainers along with HKS 264's. Replaced valve stem seals with new viton seals. Did all of this with the head on the car. I meant to do a compression test right before the swap but forgot so I don't know if the low compression existed before this or not. Put each piston at TDC and pressurized the cylinder and could not hear any leakage past the intake or exhaust vavles of any cylinder so I think I'm ok there.

 

[romeen]

I got the Sav-A-Thread repair kit but have a question about threading in the insert. Should I use a spark plug to thread the insert in? With my luck the insert will just back out again when I go to remove the spark plug (ie-before I've had a chance to use the expander tool).

 

[habitatguy187]

Updates? It appears that my new awd is doing the same thing. Cylinder #1, #3, and #4 plug wires keep popping out. When I changed the plugs though it did look really dirty around the seat, so I'll probably try to clean that first and see what happens. I don't see it making much of a difference though.

 

[hhuricane]

I got the Sav-A-Thread repair kit but have a question about threading in the insert. Should I use a spark plug to thread the insert in? With my luck the insert will just back out again when I go to remove the spark plug (ie-before I've had a chance to use the expander tool).

No, there is a tool that you use to thread the insert in. If you bought the kit it should have included it. If you only bought the inserts you will need the kit to install it right.

 

[romeen]

Updates? It appears that my new awd is doing the same thing. Cylinder #1, #3, and #4 plug wires keep popping out. When I changed the plugs though it did look really dirty around the seat, so I'll probably try to clean that first and see what happens. I don't see it making much of a difference though.

Funny coincidence that you should bring this up since this is what I have been working on today. I installed inserts for all 4 cylinders. Pressure tested the cylinders and still have air leaking up through the spark plug wells on 1 and 4. I might have some small cracks in the plug seat right around the hole. I repeated the test but included a washer of the same size as the crush washers on the spark plugs. Still had some air escaping but not nearly as much. But this washer was not crushed and wasn't exerting as much downward force as a spark plug.

It seems to me the best thing to do is to hack up a spark plug and use that for my air connection. This should give more reliable results. I've already got one of the plugs down to only the bottom half and all of the ceramic has been removed so it's hollowed out. I still have to figure out a way to securely connect this to the air hose so there won't be any leaks.

BTW, hollowing out a plug is much more a PITA than I expected. The porcelain is super hard and takes a lot of energy to slowly drill, cut, chip away. Not sure what metal is used for the center electrode but that thing is very tough as well.

I'll post results using the modified spark plug later.

No, there is a tool that you use to thread the insert in. If you bought the kit it should have included it. If you only bought the inserts you will need the kit to install it right.

I did buy the complete kit. But unlike the Heli-coil kits mine did not come with a tool for inserting. It came with the tap and an "expanding tool" (expands the insert to lock it into place). I ended up using the tap as the insert tool and it worked fine.

 

[Benshooter]

Funny coincidence that you should bring this up since this is what I have been working on today. I installed inserts for all 4 cylinders. Pressure tested the cylinders and still have air leaking up through the spark plug wells on 1 and 4. I might have some small cracks in the plug seat right around the hole. I repeated the test but included a washer of the same size as the crush washers on the spark plugs. Still had some air escaping but not nearly as much. But this washer was not crushed and wasn't exerting as much downward force as a spark plug.

It seems to me the best thing to do is to hack up a spark plug and use that for my air connection. This should give more reliable results. I've already got one of the plugs down to only the bottom half and all of the ceramic has been removed so it's hollowed out. I still have to figure out a way to securely connect this to the air hose so there won't be any leaks.

BTW, hollowing out a plug is much more a PITA than I expected. The porcelain is super hard and takes a lot of energy to slowly drill, cut, chip away. Not sure what metal is used for the center electrode but that thing is very tough as well.

I'll post results using the modified spark plug later.

Hammer.

I did buy the complete kit. But unlike the Heli-coil kits mine did not come with a tool for inserting. It came with the tap and an "expanding tool" (expands the insert to lock it into place). I ended up using the tap as the insert tool and it worked fine.

I've helicoiled a few spark plug holes without issue. I used a little JB weld to help it seal up. This hasnt failed me yet. I would pull the head if I were you to look for cracks and to get a really good look at the spark plug sealing area.