weird "bogging" down effect

Ok the Jeep is a 97 4.0L auto 4x4 4 door country model. When I turn it on and put it in drive and take off after usually backing out of a driveway or parking spot barely using any throttle and no unusual idle or start up, it feels like it bogs down before finally running up in the RPMs. It doesn't drop in RPM from where it is after in drive but I can apply more and more throttle and it takes a few seconds before it finally kicks in and starts throttling up. Any ideas? it doesn't do it all the time only from time to time in the past but it has become more frequent recently.

does it have an extended crank at all? mine does it too, kinda sputters a little bit, after it sits overnight. however I have a bad fuel flowback valve so i have to cycle the key a few times to get fuel pressure. once its running its fine.

Ben824 said:

Ok the Jeep is a 97 4.0L auto 4x4 4 door country model. When I turn it on and put it in drive and take off after usually backing out of a driveway or parking spot barely using any throttle and no unusual idle or start up, it feels like it bogs down before finally running up in the RPMs. It doesn't drop in RPM from where it is after in drive but I can apply more and more throttle and it takes a few seconds before it finally kicks in and starts throttling up. Any ideas? it doesn't do it all the time only from time to time in the past but it has become more frequent recently.

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I have a similar problem with my 99 Cherokee Sport, 4.0 litre, AW4, but I sort of calmed it down a bit by replacing components that related to different problems. My observation on this (Using my scanner during driving of the vehicle) is that this only occurs during the Open Loop phase, whether starting out with a cold engine after the first start of the day or with a warm engine after the engine has been run. Ounce the Closed Loop phase has been achieved, the hesitation problem goes away.

This is what the 2001 FSM says to check for hesitation/sag/stumble. This list does not specify at what point hesitation, sag, or stumble is seen. The comments in brackets [ ] are mine.

CHECK PCM POWER AND GND CKT [I haven't done this, other than cleaning all grounds]
CHECKI THE FUEL PRESSURE [mine is 48 psi, steady]
CHECKI COOLANT SENSOR CALIBRATION [I have a new coolant sensor]
CHECKI THROTTLE POSITION SENSOR CALIBRATION [I have a new TPS]
CHECK MAP SENSOR CALIBRATION [I bench checked my MAP sensor OK]
CHECK THE MINIMUM IDLE AIR FLOW [I have a new orifice]
CHECK IDLE AIR CONTROL MOTOR OPERATION [I replaced the IAC]
CHECK EVAP EMISSION SYSTEM [I checked the EVAP system OK]
CHECK IAT SENSOR [I have a new IAT sensor]


As you know, in the Open Loop phase, the PCM ignores the inputs from the Oxy Sensors and relies on comparing sensor inputs to stored parameters to control fuel injection and timing. This list does not mention Crankshaft Position Sensor or Camshaft Postion Sensor inputs to the PCM. My scanner shows me that during warm-up (Open Loop) the ignition timing dances around alot, i.e., is not stable, and the short term fuel trim is really active bouncing back and forth; however, both stablize when the Closed Loop phase has been achieved. I think either the Camshaft or Crankshaft sensor, or both, are right on the ragged edge. I can't fined a simple way of checking these sensors and am reluctant to shotgun them because of the $ involved.

Hope someone comes up with a fix.

My 98 does the exact same thing. it only happens a few seconds after its started and im getting on it. Starts fine with no other issues. It lasts for just a second or two and its only once in a while, maybe a couple times a month, warm and cold engine

Maybe this can help: http://www.naxja.org/forum/showthread.php?t=1039939

O2 sensors can cause some mysterious issues.

I have replaced a few of the mentioned parts above but not all and I haven't checked any grounds for the PCM (I honestly don't know where to find that) and I haven't checked fuel pressure. It doesn't stumble or skip it just feels like a sudden heavy load was on put on the engine and it can't rev up. Since it doesn't do it enough to expect it and it only lasts a short moment I never think to look down to see if the RPMs actually drop much if at all. I almost feel like when I put my foot into it starts to rev up then falls back just slightly and then catches up. I have replaced the IAC, TPS, CPS, and coolant temp sensor before. Funny thing about the coolant temp sensor is the Jeep got to a point where it wouldn't start the first try without either giving it some gas or stopping and trying to start it a second time. One I replaced the temp sensor it never happened again. Wish I could figure this one out because it is just plain annoying when it does start to do it often.

Ben824 said:

I have replaced a few of the mentioned parts above but not all and I haven't checked any grounds for the PCM (I honestly don't know where to find that) .....

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Ben824,

Pics are of a 1999 Cherokee. Other years may differ.

Here's some pics of the main grounds in the engine compartment. G101 is/are the ground(s) for the PCM.

By the Engine Oil Dipstick Tube.



On the Coil Mount.



The other grounds are important to keep clean as well.







Under PCM.