Greetings:
Background: I have a '96 Grand Cherokee 4.0 that is in great shape now with 170K miles. I bought it 1.5 years ago with 133K on the clock. I had a frustrating problem with spark knock under load on hills and highway grades when I used the owner's manual recommended 87 octane fuel. This engine does not have a knock sensor. It does not burn oil and I always try to use top-tier fuels. Driving mix is 70/30 highway/city so carbon buildup is unlikely. A piston soak with Mopar combustion chamber cleaner didn't help. Using 89 octane eliminated most of the pinging but I wanted to solve it since this engine is supposed to run on 87. This is mostly on principal - running 89 or 93 won't break me.
I tried a new cap/rotor/wires (Blue Streak Standard Ignition brand) and a set of OEM type Champion RC12LYC plugs as it was time for a tune-up anyhow. I also tried a set of Bosch Copper Plus plugs. None of this helped. No OBDII codes set and the exhaust after the manifold and O2 sensors are recent. No vacuum leaks were found either.
After reading on the 'net that some Dodge 5.2/5.9 truck owners solved pinging problems by running a colder plug (Champion RC9YC) and that the late '80's-early '90's Jeep 4.0 used that plug, I thought that might work. A lot of the Jeep 4.6 stroker motors are using this plug too. I popped in a set of the RC9YC Champions and my pinging is gone and the Jeep still runs great otherwise. After 200 miles I pulled one plug to make sure it wasn't too cold of a heat range and it looked great - no fouling and a nice color.
In closing I think the problem is a combination of the stock heat range being too high and the original RC12LYC plug design has an extremely long nose that probably exacerbates the problem. If your 4.0 pings I'd give this a try as it is a cheap experiment even if it does not solve your problem.
Andrew S.
Background: I have a '96 Grand Cherokee 4.0 that is in great shape now with 170K miles. I bought it 1.5 years ago with 133K on the clock. I had a frustrating problem with spark knock under load on hills and highway grades when I used the owner's manual recommended 87 octane fuel. This engine does not have a knock sensor. It does not burn oil and I always try to use top-tier fuels. Driving mix is 70/30 highway/city so carbon buildup is unlikely. A piston soak with Mopar combustion chamber cleaner didn't help. Using 89 octane eliminated most of the pinging but I wanted to solve it since this engine is supposed to run on 87. This is mostly on principal - running 89 or 93 won't break me.
I tried a new cap/rotor/wires (Blue Streak Standard Ignition brand) and a set of OEM type Champion RC12LYC plugs as it was time for a tune-up anyhow. I also tried a set of Bosch Copper Plus plugs. None of this helped. No OBDII codes set and the exhaust after the manifold and O2 sensors are recent. No vacuum leaks were found either.
After reading on the 'net that some Dodge 5.2/5.9 truck owners solved pinging problems by running a colder plug (Champion RC9YC) and that the late '80's-early '90's Jeep 4.0 used that plug, I thought that might work. A lot of the Jeep 4.6 stroker motors are using this plug too. I popped in a set of the RC9YC Champions and my pinging is gone and the Jeep still runs great otherwise. After 200 miles I pulled one plug to make sure it wasn't too cold of a heat range and it looked great - no fouling and a nice color.
In closing I think the problem is a combination of the stock heat range being too high and the original RC12LYC plug design has an extremely long nose that probably exacerbates the problem. If your 4.0 pings I'd give this a try as it is a cheap experiment even if it does not solve your problem.
Andrew S.
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