'94 3.9L 14 CUX system. Recent installation of Bosch Platinum +2 plugs. Purchased on clearance from Rock Auto after they sent me a nice email about sale parts for my vehicles. Previous plugs were NGK fine wire platinum.

Both types perform well. Plug color on the used NGKs was perfect rusty color. First tank with the +2 gained approximately 12 - 15% fuel economy, and it just felt a bit more peppy. So, what changed?

The gap is wider (0.063 inch, 1.6 mm) on the +2. Compared to spec 0.035 inch on the NGKs. That is nearly double! Also changed from summer to winter in eastern Washington State. I use Shell mid-grade gas with a bottle of octane booster. Only drive it on weekends.

I always figured a spark is a spark, but now I find out racers often widen the gaps to get a bit of a boost. My mechanic said the coil will be stressed a bit more due to the wider gap, since voltage builds up more before the spark jumps the gap. I am OK with that, and carry a spare coil in my trail bag. For 12 - 15% savings on fuel, it is worth it to me. Any other members experiment with plugs? In the mean time the truck is running great.

 

I was just thinking about this. How would this play out with a GEMS engine?

 

I’ll be interested to hear how this works out. That’s a lot bigger gap and I agree that it might be a bit hard on the coils.

 

My last load of Denso 5305 came out of the box with like a 1.2mm gap and ive just been running that.
No idea about fuel economy on it though. I also am running a fresh coil pack so im not overly worried about it.

 

My plan is to go to 0.043 (1.1 mm) to 0.055 (1.4 mm) on my GEMs truck, but I'm not in a hurry since it runs great and it's my wife's winter snow day car. The high voltage builds up a bit higher, so any weakness could appear, such as dirt or moisture that would cause tracking, or poor insulation that could flash over to ground. The high tension wire supports need to be well maintained.

 

There are upgraded coils available, but no idea if its worth the cash or not...

https://performancedistributors.com/...4-0l-1996-1998

 

I'm going to get those. It makes sense to the electrical engineer in me.

If it saves $5 - 10 per tank, they pay for themselves in short order.

 

John, keep us posted on this. If your fuel economy really does improve that much it would be worthwhile.

 

JohnZo, let me know how those work out for you. The set of four is only $109? I'm pretty sure my coils are the originals and probably need to be replaced. I've had a flashing check engine light twice recently a few minutes after startup when the temp is in the single to low double digits. The code is (P1314) and a flashing CE means it's actively misfiring. It's weird because I don't feel a misfire. No missing, stumbling, or loss of power. It only flashes for around 10 seconds or so and then the CE light illuminates. When started the next day the CE light goes out and it won't happen again until startup in super low temps.

My old 2000 BMW 540 used to experience similar behavior. The coil overs would crack and arc to the block, but mostly only when the temps plummeted. That was 20 years ago and the cost of each coil over back then was more than this entire set!

 

Gapping spark plugs 0.045 to 0.055 is something Rover guys have been doing for decades with the cheap Autolite copper plugs. Runs better, but you have to change them more often.