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I recently bought a 2008 Jeep Grand Cherokee with a 3.7L engine, when I went to tune it up the spark plugs were set at 0.035 which is recommend by AutoZone ( auto parts store ) but the jeep Cherokees manual says to set the spark plugs at 0.043 for a 3.7L

Any insight on the spark plug gap would be appreciated, not really sure why AutoZone is .008 off of the manuals recommendations.

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  • Welcome to Motor Vehicle Maintenance & Repair! Commented Aug 22, 2022 at 9:55
  • Did you purchase the exact type of plug the manual recommends? If you purchase aftermarket plugs with different materials or electrode design (platinum, iridium, etc.,) then the recommended gap will be different.
    – user71659
    Commented Aug 23, 2022 at 19:11

2 Answers 2

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I have no information about AutoZone in particular, but you want insight?

A plug gap of .035 will work on any vehicle that uses a spark plug. It will work, but it may not be ideal. And by "work", I mean that the car will start up and drive away without obvious problems.

A chain store parts-changer has one priority: get the customer out the door and driving down the road as quickly as possible. Spending an extra two minutes to look up the correct spark plug gap for the vehicle in the repair bay is not of any benefit to the chain store because .035 will always get the car out the door.

Can it hurt your gas mileage? Sure. Can it cause misfires on certain cars under certain load conditions? Sure it can.

If Jeep, the people who made the car, say that the correct gap is .043, then there is no other correct gap. Full stop, end of story. No excuses, no opinions and no chain store corporate policy can make any other gap correct.

Bottom line: You've been served a baloney sandwich.

You have a right to have work done according to factory specifications. If a chain store tells you that a different gap is just as good, tell them to put it in writing and accept responsibility for any harmful outcome, including poor gas mileage. Then watch how quickly they re-gap the plugs to proper specs.

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I had the same confusion with three of my cars: a '94 Ford Taurus, an '80 Olds Cutlass, and an '08 Mercedes E350. I did a lot of research on the matter. To make a long story short, the difference between those gap measurements is slight, with little real world effect. A different brand of spark plug, for example, may have slightly different characteristics (the ground wire, raw materials, etc.) and thus a slightly different gap. Car manufacturers design an engine and designate a gap specification that is approximate, knowing full well the gap will grow as the spark plugs age and engine wear varies. The gap at installation thus depends, in part, on the length of time before the next scheduled plug replacement.

Feel free to experiment with different gaps without fear of damaging the engine. Car enthusiasts often tailor a gap to fit their driving characteristics. In general, a smaller gap provides better ignitability in cold weather, at idle, and at cruising speeds; a larger gap suits heavy acceleration, wide-open throttle, high octane gasoline or more advanced base ignition timing. A universal baseline gap for a low compression, gasoline engine has always been .044, exactly what Jeep recommends in your particular case. Assuming no special modifications, .035 is perfectly acceptable; .055 equally so. You could even try .075 in mild weather and assuming your engine is very well maintained. The spark plugs would be considered worn-out once they exceed about .090.

On two of my cars, I tried different brands of sparks plugs with different pre-set gaps by the manufacturer for my particular engine. With each set of plugs, I did indeed notice slightly different performance according to what I just described. In the end, the most expensive plug by NKG was the best overall in terms of idle, acceleration, gas mileage, and plug life mileage. It had the smallest gap, very fine wire and was made out of ruthenium.

Have fun!

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