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Sparkplug Advice



Those of you who are in this forum regularly, probably got sick and tired of seeing my thread about pinging and hesitation. Guess what! Problem solved. Symtom got worse and and after talking to RKreigh last week, I resigned to the reality that I may have to remove the plenum, pull all injectors and send them out for inspection and cleaning and replce all coils and wires. Then I remembered about 7 years ago, when I tried to run Splitfire plugs, car would't even start. So, just in case, I went to a local dealer, Koon's Chevrolet, and asked for FR2LS (original plug for LT5 before Chevy went to platinum tip plugs in 93, I think). Of course they only had 4. So I drove around and eventually found a set at a local parts store, went home set the gaps at .042 and popped them in. After removing AC Delco platinum tipped plugs (about 5000 miles on them), I checked all electrodes and they looked fine. Then I noticed what appeared to be a burn stain on the shaft of one of the plugs right where metal portion joined the ceramic shaft.
Took the car out for a test drive, and what a pleasent surprise! All my prblems solved!!! LT5 ran so smooth and strong. This car never ran so smooth since I picked it up in Burmingham, Alabama 2 years ago (had original AC Delco platinum tips back then also).
Now that I had chace to think about what was happening, it all makes perfect sense.
When cold, juice leakage is not as severe and the motor is running in open loop so fuel delivery is not affected by O2 sensor reading - car runs resonably well. As the motor heats up electrical leakage becomes more severe and since the plug is recessed into the head, it starts to arc. When this happens, unburned mixture goes to the O2 sensor. Then the sensor translates this info to be mixture being too rich and reduces fuel flo to the particular bank resulting in lean mixture and pinging.
I know that platinum tipped plug sounds exotic and at triple the price copared to copper plugs, it must be good. Wrong, quality control of these plugs are terrible. Even when new, the motor somehow felt, for a lack of better words, brittle as opposed to feeling smooth and strong. LT5 with all the attention paid to it in design and assembly is a sooth running motor (remember 7200 rpm redline?). I am convinced, these pltinum tips are responsible for rough running even when they are their best.
My advise, if you are running AC DElco platinum tips, yank them out and try AC Delco FR2LS.


Site Administrator
Staff member
Sep 16, 2000
New Hampshire
1990 Corvette ZR-1
I've also had very good luck with the AC Delco Rapid Fire #5 plugs.

Hib Halverson

Technical Writer for Internet & Print Media
Jan 10, 2001
CenCoast CA
71 04 12 19
Rob said:
I've also had very good luck with the AC Delco Rapid Fire #5 plugs.

The ACD RF5 is a performance-version of the FR2LS.

The ACD RF9 is the performance-version of the FR1LS which was to be the original LT5 spark plug. Just before the cars went on sale in 1990, the recommded plug was changed to the FR2LS because of concerns GM Powertrain had about pre-delivery plug fouling.


Thanks for all the info. i am so glad to have solved the mystery, for now, I'll stay with FR2LS. Next time consensus seems to suggest that I try ACD Rapidfire #5.
Thanks guys.

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