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Old June 25th, 2020, 13:55
tjmotter's Avatar
tjmotter tjmotter is offline
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Join Date: Sep 2011
Location: Houston
Posts: 433
Shout out to MMO

'90 XJ with 200,000 miles. Left some bad gas in it last year and while it was running fine, gas mileage dropped off a cliff so I pulled the plugs to find fouling on 3 cylinders. I did a leak down test on all the cylinders (cold) and had 25% leak down on 3 but 50% leak down on the other 3 with all the air coming out of the crankcase. My working theory was that the bad gas caused the top compression rings to stick.

Tried the "old school" approach and poured a couple of ounces of Marvel Mystery Oil into each cylinder and let it sit for a couple of days. Magically I am now back to 25% leak down (cold engine) on all cylinders. I suspect it will get better once I fire it up for a bit. Pretty good considering the miles on the old girl.
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Old June 26th, 2020, 11:05
Saudade's Avatar
Saudade Saudade is offline
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Join Date: May 2005
Location: SoCal
Posts: 2,256
Re: Shout out to MMO

I use it in every oil change.
1988 Laredo 2wd Auto, AKA "Grey Ghost", 206K
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Old June 27th, 2020, 05:32
AAubinoe AAubinoe is offline
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Join Date: Jun 2020
Location: Buckeystown, MD
Posts: 7
Re: Shout out to MMO

Love that stuff, also splash a little in every small engine gas tank. Its a mystery how it works!

Also sea foam top engine spray! Most of the time when your dealing with old combustion engines its just gunk somewhere causing issues.
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