Wrong Spark Plug Gap Results

[barbarian]

2006-Merc 250xs
ser# 1e000996

Motor been running perfect IMO, but it was time for some PM. While checking everything, the shop said plugs looked good but gapped wrong so they adjusted.

My question is what is performance issues that would result in the gap being too much and/or too little? The skeptic in me isn't sure 1) they did anything, 2) went from right to wrong.

[krindgen]

As a rule, a properly gapped spark plug will burn hot without being too wide at high rpm to cause a misfire. Ironically, the car manufacturer's recommended spark plug gap is not optimal! The recommended spark plug gap is designed to be adequate for cold starting and smooth driving on a car that is in need of an engine tune up. If you drive your car normally and tune the engine regularly, you can increase the spark plug gap by about 0.010" for better performance and better fuel economy. However, if you drive at full throttle most of the time, you should reduce the gap by about 0.010" for better performance. The spark plug itself, and the residue that forms on it, would indicate whether the gap is too big or too small. A light brownish discoloration of the tip of to porcelain insulator indicates the proper operation of the spark plugs with the gap being ideal or close to ideal for the most recent engine speeds. Thus, to check the spark plug gap at high engine speeds, you'd need to run at full throttle and immediately turn the ignition off without allowing the engine to idle. But ultimately, you'd need to run your car on a dynamometer to find the best spark plug gap, and the right ignition timing for your engine.


I was curious. In your case the delta in performance may be imperceptible at the wheel.

[SilverFox579]

From the FAQ at the top of the forum....

Spark Plugs:
-QL77CC (Champion) for 3 Liter EFI's (XB models or 2001 and EARLIER: .035" / All 2002-> Down-draft intake models: .040")
-IZFR5G (NGK)@ .032-.033" for Standard Opti's 135-225HP, 2001 200XS, and 200 ProXS PRIOR TO Serial Number#1B912786***
-IZFR5J (NGK) @.032-.033 for 75-115 Opti's
-IZFR6J (NGK)for 150/175/200 ProXS (200 ProXS Serial #1B912786-up) & 225-250 ProXS and Racing XS Motors
-2006 & prior ProXS: .032-.033"
-2007 & up: .042-.044" gap for best idle performance. (150/175/225/250 ProXS/XS)
- 2007 & up 225-250 XS/ProXS can use NGK- IZFR6J (gapped to .042-.044") OR IZFR6J-11 (pre-gapped, currently a "factory only" plug)
-2007 & up 225 ProXS: .042-.044" (IZFR6J)
-175 ProXS: .042-.044" (IZFR6J)
*** 200 ProXS Models manufactured PRIOR TO 2/1/12 utilize IZFR5G(NGK) Plugs.
*** 200 ProXS Models manufactured AFTER 2/1/12 (Serial Number 1B912786 and later) will utilize NGK- IZFR6J (gapped at .042-.044") due to production changes in the engines.

-IZFR7M (NGK) for 300XS: .044" Gap

Keep in mind the service interval for Optimax plugs is replace every 365days or 100hrs whichever comes first. At minimum I'd ask the shop what plug gap they used.