spark plug gap closed on two plugs, what causes this? Options

[siverson]

hi - quick question from the garage. cdi box died on my six. got replacement and car starts right up (good). but runs like crapola right away. pulled the plugs and on two the electrode (term?) is no touching the plug.

is this why my cdi boxed died?

or is this a result of it dying?

or?

plugs were 1k miles old. cdi box was 46 years old.

[siverson]

914

[siverson]

914

[stugray]

Cyl touching? (Does it have High CR pistons? does that bank run a lot hotter then the others?)

Dropped during installation?

Running SOOO hot it is melting the electrode? (highly unlikely in an aircooled engine or likely toast by now?)

[nine9three]

The only thing I know that would cause this would be the wrong plug (thread body too long). Your pistons are making contact with the plugs. Bad deal.

[veekry9]

(Pelican Pix)
They can look as bad as this before they leave the engine.
https://www.google.ca/search?q=porsche+911+...rP5n8IZN9mSM%3A

What you typed:
"...pulled the plugs and on two the electrode (term?) is no touching the plug."
What you meant:
"...pulled the plugs and on two,the cathodes are now touching the electrodes."(meaning,the gaps are closed)

The closed gap will indeed ruin the output transistors of the cdi amplifier.
Carbon buildup on the pistons are usually the culprits,running way rich and or lousy valve guides or poor ring sealing.
A short term fix is a shim for the plugs,to recess them up,away from the pistons.
For sure use copper formula antiseize on the plug threads.
https://www.google.ca/search?q=copper+anti+...RjOD2EQ_AUIBigB
A look inside to check the condition of the cylinders,with your recording borescope,will give a good reading of the overall condition.
https://www.google.ca/search?q=recording+cc...1JWDlQrLkBnM%3A
Roll the engine over to look at each valve in turn,(12),inspecting the seats and valve head.A comprehensive leakdown test is next.
A black,wet looking carbon deposit on pistons,chamber and valves means it's time for some headwork,at minimum.
A single plug six is sensitive,and must be maintained,otherwise..(Kaboom!).
BTW,a solid state ignition makes a hotter spark,the signal edges much sharper than what mechanical points can produce.
http://www.123ignition.nl/brand/porsche.html
There,spent some of your money in a most cost effective way,avoiding the inevitable explosion,and rod through block tears.
(IMG:style_emoticons/default/biggrin.gif)
/
https://www.google.ca/search?q=spark+plug+s...8dHcfDRqr2aM%3A
Bill Jenkins found hp by indexing the plugs in his prostock 'sbc'engines,so too should the gap be pointed into the center of the cylinder of the 911 'hemi' engine.
/

[brant]

You missed a shift or most likely over revved it on engine braking at down shift

(Engine braking on track is a really easy way to exceed red line and it's quite educational to watch you tach shift light a few laps each weekend to curb bad habits)

Change the plugs and you will be fine
I've put 50 race hours on a motor after the same thing happened to me.

[r_towle]

I'm very unclear on how a mis shift or over revving could make a piston hit a spark plug....
I would suggest you may be using the wrong plugs...too much thread

[brant]

on a -6 with high compression and high lift cams the tolerances are very close

a missed shift or too much rpm causes things to kiss
"hopefully softly"

I've broken a rocker before from a missed shift...
I've even kissed a valve softly enough not to hurt the rocker, but enough to loosen the adjustment lock nut...
I've seen valves bend slightly... enough to have to be replaced...
others have done much worse...

not sure if its valve float? but an over rev allows things to touch with all of the inertia at high rpm.... but it can happen
(I run 9k valve train for this reason)

valves can touch
plugs can touch
and much worse also.

[Luke M]

I would check to make sure those are the proper plugs first.
Like stated before they could be too long so the piston hits them.
Do you have a bore scope or can you get a hold of one ? Check to see if the top of the piston has a nick/mark in it from striking the plug.

On an over rev/missed shift, I've seen the rocker arms snap but not the plugs being struck. I guess anything can happen at that point. Hope it's an easy fix for you.

Is this from the orange or green 6 engine ?

[nine9three]

Carbon buildup on the pistons are usually the culprits,running way rich and or lousy valve guides or poor ring sealing.
A short term fix is a shim for the plugs,to recess them up,away from the pistons.
For sure use copper formula antiseize on the plug threads.

After reviewing the photos again, I believe this is your problem. The fact that some are touching and others are not leads me to this result. The plugs are certainly carbon fouled. I also agree that shimming the plugs would be a temporary fix. Getting to the source of the over rich situation is the real issue. Fuel, valve adjustment, timing etc. Good Luck!

[Frankvw]

Mmmmm interesting topic.
but to be honest...I am really lost about:
" missed shift or too much rpm causes things to kiss"
That is new to me and never heard/read this before.
the sparkplug is fixed location and the piston cannot go more up then tdc, right ?? So...also not when revving or shifting...
is there something wrong with my logic in this ??

[brant]

Mmmmm interesting topic.
but to be honest...I am really lost about:
" missed shift or too much rpm causes things to kiss"
That is new to me and never heard/read this before.
the sparkplug is fixed location and the piston cannot go more up then tdc, right ?? So...also not when revving or shifting...
is there something wrong with my logic in this ??

it happens
its happened to me and many others

[siverson]

Interesting, thanks for the feedback.

I don't think the plug length is the issue. The plugs were installed by the shop that built the engine and they use them all the time.

I also had several other new plugs in the garage are various brands for Porsche six cylinder engines, and they were all the same length.

So, just to test, I installed a new set of FR5DTC plugs. They aren't the best for this 2.0 engine (they are correct for the 964 3.6) but everything runs and sounds fine. Vrooom. I've ordered a set of WR5DC (what CDI box mfgr/PartsKlassik recommends) that I'll replace these with.

The tach seems more bouncy than normal with the CDI box, but I'll take a look at the points (or maybe replace with Pertronix).

I'll have to hook up my Air/Fuel ratio meter to see whats up. A boroscope would be nice, but I don't have one... Hmmm...

A piston hitting a spark plug on high rev doesn't really make sense to me either, but I also have WAY less experience with this stuff than brant.

-Steve

[siverson]

New plugs.

-Steve

[siverson]

In there now.

-Steve

[brant]

Interesting, thanks for the feedback.

I don't think the plug length is the issue. The plugs were installed by the shop that built the engine and they use them all the time.

I also had several other new plugs in the garage are various brands for Porsche six cylinder engines, and they were all the same length.

So, just to test, I installed a new set of FR5DTC plugs. They aren't the best for this 2.0 engine (they are correct for the 964 3.6) but everything runs and sounds fine. Vrooom. I've ordered a set of WR5DC (what CDI box mfgr/PartsKlassik recommends) that I'll replace these with.

The tach seems more bouncy than normal with the CDI box, but I'll take a look at the points (or maybe replace with Pertronix).

I'll have to hook up my Air/Fuel ratio meter to see whats up. A boroscope would be nice, but I don't have one... Hmmm...

A piston hitting a spark plug on high rev doesn't really make sense to me either, but I also have WAY less experience with this stuff than brant.

-Steve

I don't know why... I'm hoping jeff or someone chimes in on that
but when I had it happen it was a 2.0 motor also
and I know the factory changed the plug location and combustion shape on the 2.2 heads....

I wonder if the plug location on the 2.0 head is closer/more susceptible to this?

[stugray]

Engine parts DO flex.
The greatest stress in an engine is the near instantaneous reversal of direction of the piston at TDC.
That is why over-revs can take out wrist pins in addition to valvetrain components.

So it IS conceivable that an over-rev could flex the crank such that the piston is allowed to travel in the cyl a little further than normal.

Excess heat can also cause the piston and the rod to expand more than normal and protrude further into the comb chamber.
And yes you can buy plug washers (shims) that are specifically for setting the plugs "clocking".
They would pull the plug out a little.

[r_towle]

on a -6 with high compression and high lift cams the tolerances are very close

a missed shift or too much rpm causes things to kiss
"hopefully softly"

I've broken a rocker before from a missed shift...
I've even kissed a valve softly enough not to hurt the rocker, but enough to loosen the adjustment lock nut...
I've seen valves bend slightly... enough to have to be replaced...
others have done much worse...

not sure if its valve float? but an over rev allows things to touch with all of the inertia at high rpm.... but it can happen
(I run 9k valve train for this reason)

valves can touch
plugs can touch
and much worse also.

Hitting valves, no question.
An over revving situation making a piston hit a plug....I can't see it unless it flexes the crank, or stretches the rod....

Carbon build up...that makes sense.