1994 YJ Help! Is the computer the problem
 

My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It
has since been to the my local mechanic and the dealer. The problem
starts after 14 to 20 miles of driving. The engine starts running a
little rough just barely noticeable until you push in the clutch. Then
the idle drops off very quickly to about 200 to 300 rpm. When you let
out the clutch and give it the gas its like driving into a very strong
head wind. If you push on most of the time the engine will surge a
couple of times and start running ok again. I have had the problem
show up both when the engine was cold but mostly it happens after
driving it a while. Once the problem occurs if you rev the engine from
idle it will backfire out the throttle body. The timing appears to be
advanced. There are no codes stored except the usual 12, 33 (I dont
have AC). I have listened to the injectors with a mechanics
stethoscope and they are all just clicking away.

My mechanic thinks it's the computer, but from what I have read here
computers dont go bad very often. I brought it to the dealer for a
diagnostic they found a massive vacuum leak at the throttle body and
replaced. They also suggested the cat converter may be bad.

The dealer said the compression was good and my mechanic said he
checked the grounds.

I have replaced
Rotor
Distrib Cap
Plug Wires
Coil
Plugs
Radiator (it needed it lots of fins were gone)
Coolant Temperature Sensor
Vehicle Speed Sensor
Throttle body
Cat Converter
Oil
Oil Filter
I have also added a bottle of fuel injector cleaner in the gas.

When its not acting up it runs like a dream, all kinds of power,
well at least for a 4.2L. I really dont want to replace this one I just
got it setup the way I wanted it.

The computer is next.

Does anyone have any ideas.

Re: 1994 YJ Help! Is the computer the problem
 

Its not a 4.2L it a 2.5L I-4 Sorry...

Re: 1994 YJ Help! Is the computer the problem
 

Its not a 4.2L it a 2.5L I-4 Sorry...

Re: 1994 YJ Help! Is the computer the problem
 

Its not a 4.2L it a 2.5L I-4 Sorry...

Re: 1994 YJ Help! Is the computer the problem
 

You know, the ignition control module is a rather cheap swap out, looking at
the list of stuff you have done. I had an 87 YJ 6 cyl that had mysterious
problems with cutting off and not starting when warm and such. Putting in a
new ICM made it all go away. In 2000 it was about $65US in the NAPA parts
store. The odd part was where it was located - in the wheel well on the
other side of the sheet metal from the engine (on the tire side). Just
unbolt it from the sheet metal and unplug it - that is it and you can do it
yourself.

Stay tuned for other thoughts here, but this is what worked for me back
then. Hope this helps,
Tomes

"aj" <ajleduc@msn.com> wrote in message
news:1135339317.565507.289380@g49g2000cwa.googlegr oups.com...
> My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It
> has since been to the my local mechanic and the dealer. The problem
> starts after 14 to 20 miles of driving. The engine starts running a
> little rough just barely noticeable until you push in the clutch. Then
> the idle drops off very quickly to about 200 to 300 rpm. When you let
> out the clutch and give it the gas its like driving into a very strong
> head wind. If you push on most of the time the engine will surge a
> couple of times and start running ok again. I have had the problem
> show up both when the engine was cold but mostly it happens after
> driving it a while. Once the problem occurs if you rev the engine from
> idle it will backfire out the throttle body. The timing appears to be
> advanced. There are no codes stored except the usual 12, 33 (I dont
> have AC). I have listened to the injectors with a mechanics
> stethoscope and they are all just clicking away. Its a 4 2.5L.
>
> My mechanic thinks it's the computer, but from what I have read here
> computers dont go bad very often. I brought it to the dealer for a
> diagnostic they found a massive vacuum leak at the throttle body and
> replaced. They also suggested the cat converter may be bad.
>
> The dealer said the compression was good and my mechanic said he
> checked the grounds.
>
> I have replaced
> Rotor
> Distrib Cap
> Plug Wires
> Coil
> Plugs
> Radiator (it needed it lots of fins were gone)
> Coolant Temperature Sensor
> Vehicle Speed Sensor
> Throttle body
> Cat Converter
> Oil
> Oil Filter
> I have also added a bottle of fuel injector cleaner in the gas.
>
> When its not acting up it runs like a dream, all kinds of power,
> well at least for a 4.2L. I really dont want to replace this one I just
> got it setup the way I wanted it.
>
> The computer is next.
>
> Does anyone have any ideas.
>

Re: 1994 YJ Help! Is the computer the problem
 

You know, the ignition control module is a rather cheap swap out, looking at
the list of stuff you have done. I had an 87 YJ 6 cyl that had mysterious
problems with cutting off and not starting when warm and such. Putting in a
new ICM made it all go away. In 2000 it was about $65US in the NAPA parts
store. The odd part was where it was located - in the wheel well on the
other side of the sheet metal from the engine (on the tire side). Just
unbolt it from the sheet metal and unplug it - that is it and you can do it
yourself.

Stay tuned for other thoughts here, but this is what worked for me back
then. Hope this helps,
Tomes

"aj" <ajleduc@msn.com> wrote in message
news:1135339317.565507.289380@g49g2000cwa.googlegr oups.com...
> My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It
> has since been to the my local mechanic and the dealer. The problem
> starts after 14 to 20 miles of driving. The engine starts running a
> little rough just barely noticeable until you push in the clutch. Then
> the idle drops off very quickly to about 200 to 300 rpm. When you let
> out the clutch and give it the gas its like driving into a very strong
> head wind. If you push on most of the time the engine will surge a
> couple of times and start running ok again. I have had the problem
> show up both when the engine was cold but mostly it happens after
> driving it a while. Once the problem occurs if you rev the engine from
> idle it will backfire out the throttle body. The timing appears to be
> advanced. There are no codes stored except the usual 12, 33 (I dont
> have AC). I have listened to the injectors with a mechanics
> stethoscope and they are all just clicking away. Its a 4 2.5L.
>
> My mechanic thinks it's the computer, but from what I have read here
> computers dont go bad very often. I brought it to the dealer for a
> diagnostic they found a massive vacuum leak at the throttle body and
> replaced. They also suggested the cat converter may be bad.
>
> The dealer said the compression was good and my mechanic said he
> checked the grounds.
>
> I have replaced
> Rotor
> Distrib Cap
> Plug Wires
> Coil
> Plugs
> Radiator (it needed it lots of fins were gone)
> Coolant Temperature Sensor
> Vehicle Speed Sensor
> Throttle body
> Cat Converter
> Oil
> Oil Filter
> I have also added a bottle of fuel injector cleaner in the gas.
>
> When its not acting up it runs like a dream, all kinds of power,
> well at least for a 4.2L. I really dont want to replace this one I just
> got it setup the way I wanted it.
>
> The computer is next.
>
> Does anyone have any ideas.
>

Re: 1994 YJ Help! Is the computer the problem
 

You know, the ignition control module is a rather cheap swap out, looking at
the list of stuff you have done. I had an 87 YJ 6 cyl that had mysterious
problems with cutting off and not starting when warm and such. Putting in a
new ICM made it all go away. In 2000 it was about $65US in the NAPA parts
store. The odd part was where it was located - in the wheel well on the
other side of the sheet metal from the engine (on the tire side). Just
unbolt it from the sheet metal and unplug it - that is it and you can do it
yourself.

Stay tuned for other thoughts here, but this is what worked for me back
then. Hope this helps,
Tomes

"aj" <ajleduc@msn.com> wrote in message
news:1135339317.565507.289380@g49g2000cwa.googlegr oups.com...
> My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It
> has since been to the my local mechanic and the dealer. The problem
> starts after 14 to 20 miles of driving. The engine starts running a
> little rough just barely noticeable until you push in the clutch. Then
> the idle drops off very quickly to about 200 to 300 rpm. When you let
> out the clutch and give it the gas its like driving into a very strong
> head wind. If you push on most of the time the engine will surge a
> couple of times and start running ok again. I have had the problem
> show up both when the engine was cold but mostly it happens after
> driving it a while. Once the problem occurs if you rev the engine from
> idle it will backfire out the throttle body. The timing appears to be
> advanced. There are no codes stored except the usual 12, 33 (I dont
> have AC). I have listened to the injectors with a mechanics
> stethoscope and they are all just clicking away. Its a 4 2.5L.
>
> My mechanic thinks it's the computer, but from what I have read here
> computers dont go bad very often. I brought it to the dealer for a
> diagnostic they found a massive vacuum leak at the throttle body and
> replaced. They also suggested the cat converter may be bad.
>
> The dealer said the compression was good and my mechanic said he
> checked the grounds.
>
> I have replaced
> Rotor
> Distrib Cap
> Plug Wires
> Coil
> Plugs
> Radiator (it needed it lots of fins were gone)
> Coolant Temperature Sensor
> Vehicle Speed Sensor
> Throttle body
> Cat Converter
> Oil
> Oil Filter
> I have also added a bottle of fuel injector cleaner in the gas.
>
> When its not acting up it runs like a dream, all kinds of power,
> well at least for a 4.2L. I really dont want to replace this one I just
> got it setup the way I wanted it.
>
> The computer is next.
>
> Does anyone have any ideas.
>

Re: 1994 YJ Help! Is the computer the problem
 

The 4.2 is a carburetor engine so you are going to get all kinds of
wrong advise....

You later state it is a 2.5.....

I would be suspecting the electrical connections on the throttle body.
When they get corroded, the gas pedal response can act like you
describe.

The pins are very small computer signal pins and can look perfectly
clean but be corroded enough to interfere with the signal.

I recommend a good clean of every sensor connection you can get your
hands on with a spray contact cleaner. I then use dielectric grease on
the plug's seal skirts. The best spray is sold at electronic shops, but
in a pinch, WD40 will work as an electrical cleaner.

I would try the clean before tossing any expensive parts at it.

A massive vacuum leak at the TB is just plain -------- I am sorry to
tell you. A vacuum leak at the TB causes 'high' idle, not low idle,
they lied to you.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Aug./05 http://www.imagestation.com/album/in...?id=2120343242
(More Off Road album links at bottom of the view page)

aj wrote:
>
> My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It
> has since been to the my local mechanic and the dealer. The problem
> starts after 14 to 20 miles of driving. The engine starts running a
> little rough just barely noticeable until you push in the clutch. Then
> the idle drops off very quickly to about 200 to 300 rpm. When you let
> out the clutch and give it the gas its like driving into a very strong
> head wind. If you push on most of the time the engine will surge a
> couple of times and start running ok again. I have had the problem
> show up both when the engine was cold but mostly it happens after
> driving it a while. Once the problem occurs if you rev the engine from
> idle it will backfire out the throttle body. The timing appears to be
> advanced. There are no codes stored except the usual 12, 33 (I dont
> have AC). I have listened to the injectors with a mechanics
> stethoscope and they are all just clicking away.
>
> My mechanic thinks it's the computer, but from what I have read here
> computers dont go bad very often. I brought it to the dealer for a
> diagnostic they found a massive vacuum leak at the throttle body and
> replaced. They also suggested the cat converter may be bad.
>
> The dealer said the compression was good and my mechanic said he
> checked the grounds.
>
> I have replaced
> Rotor
> Distrib Cap
> Plug Wires
> Coil
> Plugs
> Radiator (it needed it lots of fins were gone)
> Coolant Temperature Sensor
> Vehicle Speed Sensor
> Throttle body
> Cat Converter
> Oil
> Oil Filter
> I have also added a bottle of fuel injector cleaner in the gas.
>
> When its not acting up it runs like a dream, all kinds of power,
> well at least for a 4.2L. I really dont want to replace this one I just
> got it setup the way I wanted it.
>
> The computer is next.
>
> Does anyone have any ideas.

Re: 1994 YJ Help! Is the computer the problem
 

The 4.2 is a carburetor engine so you are going to get all kinds of
wrong advise....

You later state it is a 2.5.....

I would be suspecting the electrical connections on the throttle body.
When they get corroded, the gas pedal response can act like you
describe.

The pins are very small computer signal pins and can look perfectly
clean but be corroded enough to interfere with the signal.

I recommend a good clean of every sensor connection you can get your
hands on with a spray contact cleaner. I then use dielectric grease on
the plug's seal skirts. The best spray is sold at electronic shops, but
in a pinch, WD40 will work as an electrical cleaner.

I would try the clean before tossing any expensive parts at it.

A massive vacuum leak at the TB is just plain -------- I am sorry to
tell you. A vacuum leak at the TB causes 'high' idle, not low idle,
they lied to you.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Aug./05 http://www.imagestation.com/album/in...?id=2120343242
(More Off Road album links at bottom of the view page)

aj wrote:
>
> My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It
> has since been to the my local mechanic and the dealer. The problem
> starts after 14 to 20 miles of driving. The engine starts running a
> little rough just barely noticeable until you push in the clutch. Then
> the idle drops off very quickly to about 200 to 300 rpm. When you let
> out the clutch and give it the gas its like driving into a very strong
> head wind. If you push on most of the time the engine will surge a
> couple of times and start running ok again. I have had the problem
> show up both when the engine was cold but mostly it happens after
> driving it a while. Once the problem occurs if you rev the engine from
> idle it will backfire out the throttle body. The timing appears to be
> advanced. There are no codes stored except the usual 12, 33 (I dont
> have AC). I have listened to the injectors with a mechanics
> stethoscope and they are all just clicking away.
>
> My mechanic thinks it's the computer, but from what I have read here
> computers dont go bad very often. I brought it to the dealer for a
> diagnostic they found a massive vacuum leak at the throttle body and
> replaced. They also suggested the cat converter may be bad.
>
> The dealer said the compression was good and my mechanic said he
> checked the grounds.
>
> I have replaced
> Rotor
> Distrib Cap
> Plug Wires
> Coil
> Plugs
> Radiator (it needed it lots of fins were gone)
> Coolant Temperature Sensor
> Vehicle Speed Sensor
> Throttle body
> Cat Converter
> Oil
> Oil Filter
> I have also added a bottle of fuel injector cleaner in the gas.
>
> When its not acting up it runs like a dream, all kinds of power,
> well at least for a 4.2L. I really dont want to replace this one I just
> got it setup the way I wanted it.
>
> The computer is next.
>
> Does anyone have any ideas.

Re: 1994 YJ Help! Is the computer the problem
 

The 4.2 is a carburetor engine so you are going to get all kinds of
wrong advise....

You later state it is a 2.5.....

I would be suspecting the electrical connections on the throttle body.
When they get corroded, the gas pedal response can act like you
describe.

The pins are very small computer signal pins and can look perfectly
clean but be corroded enough to interfere with the signal.

I recommend a good clean of every sensor connection you can get your
hands on with a spray contact cleaner. I then use dielectric grease on
the plug's seal skirts. The best spray is sold at electronic shops, but
in a pinch, WD40 will work as an electrical cleaner.

I would try the clean before tossing any expensive parts at it.

A massive vacuum leak at the TB is just plain -------- I am sorry to
tell you. A vacuum leak at the TB causes 'high' idle, not low idle,
they lied to you.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Aug./05 http://www.imagestation.com/album/in...?id=2120343242
(More Off Road album links at bottom of the view page)

aj wrote:
>
> My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It
> has since been to the my local mechanic and the dealer. The problem
> starts after 14 to 20 miles of driving. The engine starts running a
> little rough just barely noticeable until you push in the clutch. Then
> the idle drops off very quickly to about 200 to 300 rpm. When you let
> out the clutch and give it the gas its like driving into a very strong
> head wind. If you push on most of the time the engine will surge a
> couple of times and start running ok again. I have had the problem
> show up both when the engine was cold but mostly it happens after
> driving it a while. Once the problem occurs if you rev the engine from
> idle it will backfire out the throttle body. The timing appears to be
> advanced. There are no codes stored except the usual 12, 33 (I dont
> have AC). I have listened to the injectors with a mechanics
> stethoscope and they are all just clicking away.
>
> My mechanic thinks it's the computer, but from what I have read here
> computers dont go bad very often. I brought it to the dealer for a
> diagnostic they found a massive vacuum leak at the throttle body and
> replaced. They also suggested the cat converter may be bad.
>
> The dealer said the compression was good and my mechanic said he
> checked the grounds.
>
> I have replaced
> Rotor
> Distrib Cap
> Plug Wires
> Coil
> Plugs
> Radiator (it needed it lots of fins were gone)
> Coolant Temperature Sensor
> Vehicle Speed Sensor
> Throttle body
> Cat Converter
> Oil
> Oil Filter
> I have also added a bottle of fuel injector cleaner in the gas.
>
> When its not acting up it runs like a dream, all kinds of power,
> well at least for a 4.2L. I really dont want to replace this one I just
> got it setup the way I wanted it.
>
> The computer is next.
>
> Does anyone have any ideas.