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spark plug replacement


New Member
February 15, 2013
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City, State
Year, Model & Trim Level
1991 Ford Explorer
i'm curious about spark plug replacement - and how often it should be done for older vehicles. Specifically my Explorer is 22 with almost all original parts. I'm only the 2nd owner and it's been in good care since 2006.

in general spark plug replace is at 20K. i'm only at 17k and i've got a shuddering between 1500 - 2500 RPMs in 1st, 2nd and 3rd gear. i realize that spark plugs might not be in the issue here, but i thought i'd ask around and see what the forum's experience with older Explorer models has been.


Elite Explorer
November 6, 2000
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Year, Model & Trim Level
2018 EX Sport Concept
The original spark plugs from the factory on the 91-94 Explorers were Motorcraft "Platinum Plus" double platinum plugs, with suggested replacement at ~60,000 miles.

As the original Platinum Plus plugs are no longer available, the newer Motorcraft plugs spec'd for the 4.0L OHV V6 are just single Platnum plugs, but have the added benefit of a newer "fine wire" electrode design, carried over from Iridium spark plugs. They are very good, and still have the ~60,000 mile replacement interval.

Of course, when it's time to replace the spark plugs varies by the vehicle, the shape the engine is in, the type of use it sees, and what plug you use, among other things. If you use cheap copper plugs, you might need to replace them every 10-20K. Cheap platinum plugs might need to be replaced every 30-40K, or even more often. If you use the correct Motorcraft plugs, generally you can get 60K out of them, or replace them after just 50K if you're really picky about performance.

Shuddering might be the plugs, the previous owner might have thrown cheap plugs in there, and if they are worn out, they might cause it. Might want to pull one of the easier plugs to get to on the driver's side and see what's up. Could also be an air intake issue, anything from a dirty MAF sensor to a bad IAC idle sensor, dirty/clogged air filter, or something transmission related.