I have to run on 89+ octane! How come??

If I run 87 octane gas, I get pinging with the AC on or going up a hill. Anything that loads the engine makes it ping on 87. 89 or 91 it runs fine, regardless of the brand of gas. We're talking about the 4.0 6 cylinder on a 97 Cherokee with 160k miles on it. I've been told the timing could be off but that when the vehicle is smogged in California, they check that and it has to be on. (and I just had it smogged) What other issues could be the cause of my pinging problem? I'd like to be able to run the cheaper gas since this thing only gets 14-15 mpg in mixed city/highway driving anyway.

There is a technical service bulletin out for this extact thing. Engine pinging when under load, or "spark knock." You need your ECU reflashed and your timing retarded. Should be about $100 at the dealer.

Effects '96 and '97 4.0 Cherokees

Reference number is 18-24-98


I have the same problem but haven't done the reflash yet. I put in a set of spark plugs that run cooler and this took care of probably 90% of the pinging.

Two questions:

1. Is there a DIY for an ECU flash?

2. What plugs run cooler?

javajeep said:

Two questions:

1. Is there a DIY for an ECU flash?

2. What plugs run cooler?

Click to expand...

You cannot flash the ECU yourself.

The spark knock may not be plug related. Thus, even if you change to cooler plugs, you may still get spark knock.

Before you go paying the dealer to do a reflash, are your plugs/wires/cap/rotor and fuel filter all known to be good?

anony91xj said:

Before you go paying the dealer to do a reflash, are your plugs/wires/cap/rotor and fuel filter all known to be good?

Click to expand...

New plugs, wires, cap & rotor. Fuel filter??? I don't know. It's in the gas tank right? I don't really want to pull that thing out to check if I don't have to.

I may be stating the obvious but in my '99 Jeep XJ's Instruction Manual it states that 87 Octane is the recommended fuel.

I noticed quite a bit of difference after running 85 in my 98 (when i was a bit more stressed for money), then when I got my '99 I swore to 87 or higher after reading the manual and dig the performance and .5 mpg it saves me.

Maybe that has to do with the sounds you are hearing. If not I bet it is the hardware.

(this is from a newbie so take it as you will):flame:

One thing that can cause pinging, even when the timing is acceptable, is carbon deposits in the upper combustion chamber. The best cleaner I have found to date is GM Cleenz upper cylinder cleaner. You pour some down the throttle body, ending with a good shot that stalls the engine (or shut it off then). Wait a while, then start it up again. This process makes a lot of white (grey?) smoke, so think about where you do it!