A Spark Plug Conundrum

[sjalbert]

Several weeks ago I found that my #4 plug had virtually no gap. The gap was less then my thinnest feeler gage ( < 0.005 in). I swapped in a correctly gap plugged. Since then I've driven the jeep about 300 miles. Today I went to fill up the tank and when I started the jeep at the gas station it sounded like it was misfiring. After letting it cool down I inspected the spark plugs and the #4 plug again had virtually no gap.

I looked through the spark plug hole at the valve and didn't see any indication that the spark plug electrode had struck the valve. All four cylinders have the same Autolite spark plugs that I've used for years. My shop vac which sealed well around the spark plug hole did not appear to suck anything out.

Any idea what might be closing the gap on the plug or do I need to look forward to pulling the head?

[Bill H.]

Seth, IIRC the spark plug is not positioned above the valve but is above the piston. With a dial indicator or depth gauge check piston height on each cylinder. The top of the piston may be striking the electrode ever so slightly.

[Bill H.]

As a temporary test you can add one gasket washer to the plug and give it a go also. This won't fix the issue but it will tell you if the piston is smacking the electrode though.

Do you have any knocking or tapping?

['42 Peep's Motorpool]

Seth, IIRC the spark plug is not positioned above the valve but is above the piston.

Hi Bill,

That is true of many engines, but not our beloved L134. IIRC its more or less over part of the valves. If it was over the cylinder it would make finding TDC much easier, sadly it isn't. At this stage I would suspect a valve has to be part of the equation, so it might be worth checking the valve springs and clearance on #4.

['42 Peep's Motorpool]

I wouldn't pull the head until you can tell if there is a reason to do so. Not an expert at all, but from what I know of these engines I would start by checking that the valve clearance is correct, that the valve springs are ok and not broken, and then check to see if there is any indication of a bent valve (a leak down and vacuum test to test if it's seating fully, or abnormal noises or operation). Valves can rotate, so you may not necessarily be able to see (through the sparkplug hole) a specific point where it might have made contact with the plug if it rotated away from the hole, plus carbon might obscure evidence unless it was a pretty severe 'contact'!

What make and model sparkplug are you running? Any other symptoms you have noticed?

[sjalbert]

I’ll check for a broken valve spring. I didn’t hear anything unusual from the engine when the exhaust note changed. I’m running Motorcraft A7C’s (not the Autolite 295’s that I thought were in the Jeep).

Edit: 0822 CST
I just looked and #4 plug was missing it’s gasket. I swapped it for an Autolite 295 with a gasket.

[Bill H.]

I’ll check for a broken valve spring. I didn’t hear anything unusual from the engine when the exhaust note changed. I’m running Motorcraft A7C’s (not the Autolite 295’s that I thought were in the Jeep).

Edit: 0822 CST
I just looked and #4 plug was missing it’s gasket. I swapped it for an Autolite 295 with a gasket.

I'll wager to guess that does it.

[sjalbert]

That seems to have worked. I’m surprised that the clearance was that tight.

['42 Peep's Motorpool]

Hopefully that was the cause! Might be a good idea to double check your valve clearance too, to be on the safe side.

To have two of those missing is odd,but assuming the old one didn't stick to the head (can happen if a seeping head stud allows coolant to pool into the well), it's a good reason to inspect parts before putting them on. Always good practice to inspect parts before installation anyway, but anymore you almost have to inspect them when you buy them. Case in point, I needed a universal joint on short notice (I needed it that day) and could not order from Ron (weekend), so I went to a local big-box parts store where they sold me a very cheap made in China universal joint that had a beat up box. Upon getting home and opening it I found that aside from the low quality, it was actually missing roller bearings. Looked like someone in the store had opened it up to look at it, or even that it was a return. That experience with shoddy parts and service further reinforced my pledge to buy from Ron Fitzpatrick over any other no matter how badly I needed it, I'd rather wait for Ron's to come (quick anyway) than waste money on junk or damaged parts.

[Joe Gopan]

It's a possibility the head has been resurfaced one time too many.

[Joe Gopan]

NOS 6.48:1 Heads mike 2.140" (2-9/64").

[sjalbert]

I've never had the head off and I've been running this engine for the better part of the last eight years.

IMG_5121.jpg (11.61 KiB) Viewed 319 times

Middle Plug (Motorcraft A7C) has been in the engine since 2015 and is the one I realized was missing the crush washer.
Left Plug (Champion RJ14) is the one I installed and closed up in about 300 miles
Right Plug (Autolite 295) is what's in now (and had previously been in the engine for about 20 years)

The A7C & RJ14 clearly protrude further into the engine.

[dinof]

I run AC-45 plugs. I know these work fine.

[Joe Gopan]

I do too. As far as I am concerned they are a perfect match.
Another consideration, perhaps a piece of carbon broke loose and got mushed.

[sjalbert]

... perhaps a piece of carbon broke loose and got mushed.

That might have been the caae with the A7C (which is essentially the same plug as the AC45) because I ran it for several years without a problem. I'm not sure how the RJ14 ended up in my road spares box, but it's clearly substantially longer. With the crush washer the A7C & the 295 are the same length.