Tonight I fitted NGK-BKR8EIX spark plugs as I wanted to try cooler plugs. They are iridium plugs which come gapped to about 0.7mm.

I fitted them and the car ran great on idle and cruise but as soon as I hit boost I got misfiring and backfiring, just like you do with dirty or incorrectly gapped plugs.

I refitted my PFR7's and all is fine again.

Any ideas why this may have happened. Can I get my money back. Anyone want some NGK-BKR8EIX with abotu 15 miles on them?

 

Those plugs come gapped at about 0.9mm I think, they also have less shielding than a PFR7b. They are not designated a performance plug as such, but they do work. I would say your ignition system is the cause, and the plugs just pushed it over the edge.

Paul

 

Cheers Paul for the reply.

My coils are quite new and I never get any hesitation with the PRF7's. The only performance plugs I could find that were 8's were 23 quid each!! A bit rich for me.

 

Why do you think you need such a cool plug (8)? People with 350+bhp are still using 7 grade.
I've got Blitz iridiums (made by NGK) at 7 in my STi and they're fine.
JohnD

 

I am trying a cooler plug as I think my engine may run hotter than many. My theory is that as I have a higher compression-ratio engine (wagon) the increased compression results in more heat.

The symtoms I am trying to cure are very occasional lift-off det at 6.5k+ rpm, and same thing at small throttle openings at high speed (eg, I want to hold a steady 100mph with 10% throttle then knocklink gets excited - open throttle goes quiet, close throttle, also goes quiet).

I tried something interesting on a private road the other day. Took car up to about 120mph and closed throttle completely to get car to coast and slow down. Result: the first two lights of the knockling both lit continously until I had slowed to about 100mph (ie from about 5200rpm to 4800rpm). I would normally only expect this sort of activity on boost. Sometimes if I am doing similar speeds with slight throttle opening it is worse with oranges and even a red.

 

I might just not drive so fast on motorway and get OB for silly times!

Still want to know why plugs didn't work.

17psi on PFR7's and no hesitation

5psi on the 8's and misfiring bigtime!

 

It could be lighting the KL as you are slowing because the release bearing in the gearbox is spinning. Mine used to do that and i initially thought it was lift-off det. I never worried about it after i knew the cause and it was always lighting from high revs falling away.

Hope that helps
Stuart

 

Yes, this could be very helpful. How did you know that it was the release bearing?

..having thought about it though, I get KL activity with the clutch engaged,....and I am pretty sure that a mapper heard the lift-off det with his det-cans..

 

I had same prob after fitting iridium plugs.....missfire on boost .
Put old plugs back in was fine.
I cured it by re-gapping iridium plugs to 0.6mm.....runs a treat now...well better than it did with the standard plugs in it.
Im running between 17-20psi.

 

thanks for the tip karlos

 

Let me know if it cures your problem .

 

STI sells the BKR7EIX rebadget at 0,6mm gap so it's obvious the 8's need at least this gap or less to work!

 

Well I got the offending spark plugs out of my tool box today to have a look at the gap, and measured it to find it was set at about 0.8.

I also found that the insulation on two of the spark plugs was cracked pretty badly, one with a huge chip. I think this might be part of the problem (!).

I have contacted the sparkplug company to see if they can replace them.