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4.0 pings with 87 octane

Terrific

NAXJA Forum User
Location
Brooksville, FL
I have a 97 with the 4.0. When I run 87 octane it pings badly when under load. With 93 octane it runs perfectly, no pinging at all.

Anyone have an idea what the problem could be? As far as I know the computer is untouched. The intake and exhaust is stock. I have run fuel injector cleaner through it every 3000 miles for the last 2 years.

It had this problem when I bought it, but I just ran 93 octane. Now I'd like to run 87 to save that 20 cents per gallon.

Could it be timing? Can the computer be modified to adjust the timing, or is the problem somewhere else?
 
x2 on carbon buildup. Your 4.0 should NOT ping with 87 octane gasoline.
Mechanical or chemical removal of the carbon may be the answer. Base timing is not adjustable......
 
They shouldn't ping on 87...but with the wide quench these motors come with stock they build up quite the coating of carbon on the piston tops effectively raising the compression ratio as the miles go by.
 
Slo-Sho said:
They shouldn't ping on 87...but with the wide quench these motors come with stock they build up quite the coating of carbon on the piston tops effectively raising the compression ratio as the miles go by.

yup, also the carbon deposits will trap heat and cause pre-ignition
 
Slo-Sho said:
They shouldn't ping on 87...but with the wide quench these motors come with stock they build up quite the coating of carbon on the piston tops effectively raising the compression ratio as the miles go by.
Has anyone ever documented the increased compression amount?

There is another active thread about ethanol where some guy is using 60/40 blend of E10/E85 and getting a 2 mpg mileage increase on an old 4.0. I can't help but wonder if his compression hasn't changed due to carbon on the piston tops.

I just started pulling a head on an 89 (head gasket leak between #4 & #5) and found 1/4" layers of solid hard carbon :eek: laying on the top of the head in the valve area between the valves and springs . How knows what is waiting underneath?

Let's see, wrong spark plugs can cause pinging too, or poor engine cooling, running too hot.
 
my 96 does the same thing . it just started a couple of months ago. everyone sayes its carbon build up. i had it decorbonized at the dealer ran two cans of seafoam through the intake and it still pings like hell during load on 87 octane . on 93 it runs great . could an 02 sensor or something else be going bad and cause this ? JIM
 
It isn't carbon, in my case, either. I've gone that route with Justice Brothers fuel injector cleaner and carbon blaster every 3000 miles for the last 2+ years in the tank, and twice through the throttle body. I used Sea Foam once as well. Exhaust was clear as can be. No black smoke in the least.

The vehicle has less than 75000 miles on it.

The engine temp tops out at about 200. It pings, under load, cold or hot. Plugs were fine and a new set of Bosch Platinum 4's didn't help either. Plug wires are new MSD 8.5 mm superconductors.
 
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Late models don't have an EGR

I would pull the platinum plugs. The 4.0 ignition system doesn't have the spark to run right on Platinum plugs. I run NGKs and have been very happy.

You can try to decarbonize with water as well as the Seafoam/Justice Brothers/ect..

Get a plant mister and spray into the intake while running. It works, but i get nervous doing it :looney:
 
Jim Mesthene said:
That's a classic symptom of an EGR system that is plugged or disconnected.

Perhaps on other vehicles - the RENIX era runs fine without the EGR connected, as 5-90, me, and others have tested. Mine is currently blocked off. Considering the low compression ratio of the Jeep 4.0L, it shouldn't ping at all. The HO motors didn't even have EGR's.
 
crank position sensor sending too much of an advanced signal, or do you think the fuel isn't sync'd up with the spark(cam position sensor-->distributor needs indexing)? It could be a o2 sensor that thinks the engine is running rich and is therefore telling the ecu to lean out the mixture.
 
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gradon said:
crank position sensor sending too much of an advanced signal, or do you think the fuel isn't sync'd up with the spark(cam position sensor-->distributor needs indexing)? It could be a o2 sensor that thinks the engine is running rich and is therefore telling the ecu to lean out the mixture.

How do you index the distributor?
 
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