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Bosch Platinum Plugs

XJDanny

NAXJA Forum User
Just purchased 6 Bosch Platinum plugs for my '90 4.0l. I have used them before, and currently have them in, but its time to change them. The plugs I got are Bosch 4202. I have always gapped them to .035, the factory spec, but now I'm hearing that one does not need to gap platinum plugs. The gap on these Bosch plugs is larger than factory spec. Do I gap to spec or leave them alone. Any comments?

XJ Danny
'90 Laredo
208K mi
 
Gap them to factory specifications. I did that with my Bosch platinum plugs (single side electrode type) with good results- also a 1990 4.0 engine. If you have a super high voltage aftermarket ignition system, they use a different (wider) gap. Standard ignition system = standard gap.....works for me. 4xBob
 
i think that you don't have to re-gap them.
 
So how does it run with the Bosch platinum plugs? I have read on other boards that many members who run Bosch platinums are not at all satisfied with their performance.
 
I just bought a 92 XJ. It was getting really bad MPG, like 8-10. I pulled a plug and found it to be a Bosch Platinum... half the ceramic nose was missing. I pulled number two, same condition. Three? same thing. Four? Ditto. Five? 3/4 missing. Six? same thing.

Put in Champ truck plugs... MPG @ 19! My partner who was a mechanic at a local Jeep dealership says that any aftermarket plug like the Bosch, SplitFire etc is a waste of money.

After having a lengthy (1 hour) discussion with a SplitFire "application engineer," I concluded that my mechanic was right. This discussion was a result of emission testing data that showed the SplitFire fires hotter, causing leaner combustion and raising the NOx over the Champ plugs. The AE confirmed this as a design goal which helps them advertize improved MPG and therefor perfomance.

You can gain the same effect with adding the adjustabe MAP unit for the years that there is one.
 
I happened to have some Bosch plugs on the shelf, put them in my 88 XJ, along with some new cables. Ran like crap. Even days later after the computer had enough starts to relearn a little.
Got curious and measured the resistance in the Bosch plugs and the recommended Champion, was a rather large difference, To make a long story short, the Champs worked better, it worked even better when I went back to the OEM coil to distributor cable. The spark plug cables and the small change in resistance, didn´t seem to make much difference, but a resistance change in the coil wire and the plug, was noticeable.
May just be a quirk of my XJ, but have noticed on many others, the Champs seemed to work just a little better.
Have had pretty good luck with A/C and NGK, but haven´t done a resistance comparision on them.
Could be talking through my exhaust opening, but I believe the Renix system is somewhat sensitive to resistance values in the high voltage circuit.
 
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