4.0 light miss

Gang,

My good old 96 Cherokee has a slight miss, and has had it since the new engine was put in.

Driving down the road at a steady speed or under hard acceleration it's unnoticeable, it's most noticeable under light to moderate acceleration.

When the new engine was put in it got new plugs, wires, filters, distributor cap, rotor and front 02 sensor.

I assumed the miss was due to the 7+ month old gas in the tank, so I started using Techron fuel system cleaner.

10,000 miles and a gallon or so of cleaner later the problem is still there. Sometimes it almost goes away with the cleaner in it but not entirely. Then something interesting happened last week.

I put the new downstream O2 sensor in, and of course had the battery disconnected so the computer would reset. When driving it later much to my surprise for the first 20 min or the engine ran absolutely beautifully, then it stated acting up again.

The O2 sensor operation was successful and the "Check Engine" light is staying out, but something is off enough to keep the engine from running quite right.

So it seems as though when it's in learning mode (using default settings?) it works great, but as it picks up the information from the Jeeps sensors it gets out of whack.

Have I got a sensor going (gone) bad but not quite enough to trigger the light or..?

Any suggestions?

Thanks!

Jeff DeWitt

Reply to
Jeffrey DeWitt
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I had a slight miss on my 93 4.0 for quite a while and it turned out to be the coil assembly. Coil and resistance wire were a recall for several years and if you don't have the correct one it seems to throw the timing off. Dunno exacly how, but it did solve my problem.

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The ZJ can now run 91 with no problem.. keep in mind my 4.0 has a blower and prior to this I had to run 93 or suffer highway ping.

And also the fuel filter, but usually that is ping only at full throttle.

Reply to
DougW

I've got no problem replacing the coil if that will fix it, but if it is the coil why will the engine sometimes run better with injector cleaner in it, and why did it run so well for a few min. while the computer was resetting?

Jeff DeWitt

Reply to
JeffDeWitt

That was my thought as well. Close as I can tell, the new coil has a longer connecting wire and uses a resistance balast, that might account for a "tiny" bit more delay.

Adding injector cleaner, like running high octane fuel or running in startup where the engine is made rich, are all the same things. The computer only adjusts timing so much, it assumes the rest of the system is working properly.

All I can say is that doing that got rid of my pinging. Worst off you have a spare coil.

Reply to
DougW

You might want to check the fuel pressure on the rail as well - marginal pressure or low pump capacity can cause what you see as well.

Reply to
Will Honea

I might go ahead and put a new coil on anyway, it's easy and it IS the orginal coil so it shouldn't hurt to put in a new one, and as Doug points out it's not bad to have a spare.

I've also w> >

Reply to
JeffDeWitt

I might go ahead and put a new coil on anyway, it's easy and it IS the orginal coil so it shouldn't hurt to put in a new one, and as Doug points out it's not bad to have a spare.

I've also w> >

Reply to
JeffDeWitt

Double check the part but I used the NAPA part ECHIC409. It's the same one that was used for a recall Jeep had a long time back. Comes with a coil and adapter harness. You have to use the harness with this coil as it contains a balast resistor.

Other than that it depends on if you have blades or pins. My factory connector is pins, the later model is blade.

Fuel pump pressure is easy to check if you have an old R-12 gauge or stop by autozone (or sears) and buy one. Just hook it up and duct tape it to the windshield.

My pump still works. Generally if the filter is clogged or the pump is shot, you will get real bad ping under load and at wide-open-throttle. Both times I've gotten ping at WOT it's been due to a clogged filter.

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Reply to
DougW

Hmm, picked up a new coil on the way home from work yesterday, nothing in the box but the coil, and it was a Borg-Warner brand coil.

Since when did Borg-Warner start making ignition parts?

I believe mine has the blade pins.

Jeff DeWitt

DougW wrote:

Reply to
JeffDeWitt

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