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97 4.0 driveability issue

hysiderafter

hysiderafter

NAXJA Member #1296
Location
Port Orchard, Washington
For the last couple weeks my 4.0 has been doing something strange. After being driven long enough to come to operating temperature, if I park it for a while (10-15 minutes) and then start it and take off again, it stumbles while trying to accelerate and acts like it's going to die though it never actually does. If I back off on the throttle for a few seconds it then clears up and runs normally. If I start it cold and take off it will run all day with no issues around town or on the highway. Plugs new about 5K miles ago. I replaced the distributor cap, rotor and plug wires yesterday as the cap and rotor were somewhat pitted. That didn't change anything. I had planned on changing the crank position sensor anyway since it has 180K on it so I did that today. Again, no change whatsoever (but I have a spare CKP now just in case). No DTC's are coming up at all. Any ideas on what might be causing this or how to troubleshoot it with no DTC's showing?
Thanks, Steve
 
hysiderafter said:
For the last couple weeks my 4.0 has been doing something strange. After being driven long enough to come to operating temperature, if I park it for a while (10-15 minutes) and then start it and take off again, it stumbles while trying to accelerate and acts like it's going to die though it never actually does. If I back off on the throttle for a few seconds it then clears up and runs normally. If I start it cold and take off it will run all day with no issues around town or on the highway. Plugs new about 5K miles ago. I replaced the distributor cap, rotor and plug wires yesterday as the cap and rotor were somewhat pitted. That didn't change anything. I had planned on changing the crank position sensor anyway since it has 180K on it so I did that today. Again, no change whatsoever (but I have a spare CKP now just in case). No DTC's are coming up at all. Any ideas on what might be causing this or how to troubleshoot it with no DTC's showing?
Thanks, Steve
Click to expand...

I would throw a meter on your Throttle position sensor and see if that is nice and smooth, at least for a start.
 
The o2 sensor caused that issue on my 98. Exactly as you described.
 
Could be the "heatsoak" issue. Especially common on the #3 fuel injector. Can cause a stumble start upon a hot restart. Search for more information on it.
 
Talyn said:
Wrong year for the #3 injector issue.
Click to expand...

Agree, that would be 99-01 for the XJs with heat soak. However, does sound a little like the OP has one or more injectors that are leaking and pooling fuel in the intake/cylinders.

OP, grab a fuel pressure gauge and see how long the system holds pressure after you shut it off hot.
 
I checked fuel pressure last weekend. Running = 45psi, after shutting off drops to 40 and after 30 minutes down to 30 psi. Haven't checked TPS but I changed O2 sensor today. I figured with 180K on it I wouldn't feel bad even if it didn't turn out to be the problem. I didn't have time during lunch to test drive though and see if it fixed anything.
Thanks for the replies, Steve
 
hysiderafter said:
I checked fuel pressure last weekend. Running = 45psi, after shutting off drops to 40 and after 30 minutes down to 30 psi. Haven't checked TPS but I changed O2 sensor today. I figured with 180K on it I wouldn't feel bad even if it didn't turn out to be the problem. I didn't have time during lunch to test drive though and see if it fixed anything.
Thanks for the replies, Steve
Click to expand...

It is also a good idea to zero out the ECU to factory settings every once in a while, warm it up, disconnect the battery and let it set for a hour or so then reconnect it. Then start it up and take it out for 45 min drive on open roads, not traffic where it will relearn it's parameters. I usually jump on interstate 80 and drive a few exits out and back. It always runs much better for a few months after that, I figure the ECU must be picking up bad habits.
 
Cookie to Ronbo, well done, Sir!
 
hysiderafter said:
Since changing the O2 sensor the problem has gone away. Hasn't stumbled once in the last 5 days. Thanks for the tip. Steve
Click to expand...

It never ceases to amaze me how many bad o2 sensors, in all makes of vehicles, never show a code.
 
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