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

Re: 4.0 light miss
 

Jeffrey DeWitt wrote:
> Gang,
>
> My good old 96 Cherokee has a slight miss, and has had it since the
> new engine was put in.

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.
http://partimages2.genpt.com/partimages/103447.jpg

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.

--
DougW

Re: 4.0 light miss
 

Jeffrey DeWitt wrote:
> Gang,
>
> My good old 96 Cherokee has a slight miss, and has had it since the
> new engine was put in.

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.
http://partimages2.genpt.com/partimages/103447.jpg

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.

--
DougW

Re: 4.0 light miss
 

Jeffrey DeWitt wrote:
> Gang,
>
> My good old 96 Cherokee has a slight miss, and has had it since the
> new engine was put in.

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.
http://partimages2.genpt.com/partimages/103447.jpg

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.

--
DougW

Re: 4.0 light miss
 

DougW wrote:
> Jeffrey DeWitt wrote:
> > Gang,
> >
> > My good old 96 Cherokee has a slight miss, and has had it since the
> > new engine was put in.
>
> 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.
> http://partimages2.genpt.com/partimages/103447.jpg
>
> 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.
>
> --
> 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

Re: 4.0 light miss
 

DougW wrote:
> Jeffrey DeWitt wrote:
> > Gang,
> >
> > My good old 96 Cherokee has a slight miss, and has had it since the
> > new engine was put in.
>
> 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.
> http://partimages2.genpt.com/partimages/103447.jpg
>
> 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.
>
> --
> 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

Re: 4.0 light miss
 

DougW wrote:
> Jeffrey DeWitt wrote:
> > Gang,
> >
> > My good old 96 Cherokee has a slight miss, and has had it since the
> > new engine was put in.
>
> 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.
> http://partimages2.genpt.com/partimages/103447.jpg
>
> 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.
>
> --
> 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

Re: 4.0 light miss
 

JeffDeWitt wrote:
> DougW wrote:
>> Jeffrey DeWitt wrote:

>>> My good old 96 Cherokee has a slight miss, and has had it since the
>>> new engine was put in.
>>
>> 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.
>> http://partimages2.genpt.com/partimages/103447.jpg
>>
>> 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.

> 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?

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.

--
DougW

Re: 4.0 light miss
 

JeffDeWitt wrote:
> DougW wrote:
>> Jeffrey DeWitt wrote:

>>> My good old 96 Cherokee has a slight miss, and has had it since the
>>> new engine was put in.
>>
>> 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.
>> http://partimages2.genpt.com/partimages/103447.jpg
>>
>> 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.

> 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?

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.

--
DougW

Re: 4.0 light miss
 

JeffDeWitt wrote:
> DougW wrote:
>> Jeffrey DeWitt wrote:

>>> My good old 96 Cherokee has a slight miss, and has had it since the
>>> new engine was put in.
>>
>> 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.
>> http://partimages2.genpt.com/partimages/103447.jpg
>>
>> 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.

> 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?

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.

--
DougW