Root Cause of Misfire / Fouled Spark Plug


I had a misfire on A bank (1990 5.3 V12, 54kmi) that was most noticeable at idle and clearly shown via paper test on exhaust. To diagnose, I pulled spark plugs 1&2 from each bank to compare (no AC compressor in the way).

On the left is A bank cyl 1 spark plug and right is B bank cyl 1. A bank cyl 1 showed fouled and was dry with little oil on the threads. Cylinder 2 plugs for both banks looked identical to B bank cylinder 1. I swapped the plugs in the photos and the misfire moved to B bank. I replaced the fouled plug and the car is extremely smooth, could not tell it started up when I first tried.

Now, I am waiting to drive the car more and pull the A bank cylinder 1 plug to check its condition.These plugs were replaced about a year ago and have maybe 500miles max on them so I have a hard time buying the plug was bad from the start.

Are there any checks or ideas as to what might have caused this?

More info:

  1. NGK BR7EFS plugs gapped 25thou
  2. new HT leads (from Moss motors highly recommend you don’t waste money on these…)
  3. new distributor cap, rotor, and ignition coils
  4. injectors and efi harness rebuilt, verified all injectors clicking
  5. Compression (cold and dry)
    A → 1: 180 2: 175 3: 180 4: 150 5: 170 6: 160
    B → 1: 180 2: 160 3: 155 4: 140 (wet → 180) 5: 170 6: 140 (wet → 155)


Okerman’s Razer: “newish” plug your car did not like. Trouble moved… I can add little. Invest $4 in a new plug + enjoy! Post script- you got lucky- rock on!

I would pull all plugs. Something is making A bank over fuel? You transferred the misfire by transferring a fouled plug.
New plug fixed it, until that plug gets fouled again.

I suspect low mileage and low temperature running is what is causing this. A good high load run would probably clean up the fouling.
Find the longest, steepest hill you can and floor the accelerator while going uphill, which will raise cylinder temperatures enough to burn off deposits.

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Similar to what we call ‘Italian tune-ups’.

Personally I wouldn’t give the matter much more thought until you’ve driven quite bit more with the new plug and positively confirm that this is a repeatable fault.

To be honest I’d be more concerned as to why compression is low on several cylinders. Spec is +/- 200

As for Italian Tune-ups, I’m an advocate. My V12 sees a lot of short trip, city driving. An ITU once a month is required. More often when conditions and opportunity permit…just for good measure :slight_smile: .

In other words, “Give 'er the boot” and don’t be bashful about it !


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Brand new parts fail sometimes…I remember on a different ignition going through three condensers in one day…last one lasted three years

Nothing like seeing an old Jaguar XJS cruising in it traffic lane, just hanging with traffic, then seeing that cat unexpectedly pounce out and around traffic and you see the taillights get smaller and smaller as it leaves traffic behind.

Well … nothing other than being the driver of that old XJS!

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As Doug said, the compression readings are a bit low on a few cylinders. 180 is fine on a cold engine. But 140-160 are a bit low compared to the 180s. How long have you had the car? Is it a high mileage engine? I would not risk an Italian Tune up until compression readings were more in line with each other. Have you switched to Synthetic Oil? Hopefully the rings just need a bit of cleaning which will take time. I’m a proponent of adding some MMO to the oil a few months before an oil change. I think it helps, but I understand some people would never pour anything but oil into the crankcase. My cold compression is 180-195 and warm compression is 215-225.

Based on what you’ve shown, I still feel the A bank is over fueling. Not familiar with Marelli engines and how that could happen. But yeah, drive a while with the new spark plugs and see if the problem comes back. But I doubt the spark plugs were bad and caused the fouling.

Cheers all for the comments. Once able, I’ll do some driving and give it some gas, most of the driving since the plugs have been replaced has been shorter drives and taking it easy but more often just short on/off cycles.

To some of the questions, I think I need to redo the compression test. I did mine without a tender on the battery, cold, throttle closed, and only 3-4 cycles of the starter as I did this alone and couldn’t see when the gauge leveled out.

I’m not sure about A bank over fueling as spark plug for cylinder 2 was identical to both B bank plugs pulled.

Engine has 54k miles, was sitting for the past 5-10 years until I started working on it 2 years ago. When I did the compression test the oil was synthetic but Ive since switched over to conventional.

Very similar to mine when i bought it. 48K miles and sitting 5+ years.

I’m a big believer in synthetic oil, it is superior to conventional in so many ways.
It also slowly cleans old varnish leftover from conventional oils.

I also ran lots of fuel cleaners over the years and like i said MMO in the crankcase a few times.

Car has 64000 miles now. I’ve removed pan and valve covers. Everything is super clean.
With intake manifold off, i peeked down at a few intake valves that i could see, also super clean.

Hopefully your engine just needs some TLC like mine did. These cars do NOT like to sit.