Hesitation and misfire under load

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Krischick33

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May 10, 2020
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97 2.5 5 speed. Still chasing a few issues.
  1. Rough idle - worst when cold
  2. Hesitation or skip under load. This happens between 2000 and 2800 rpms.
To date I have done the following:
  • Exhaust/Intake manifold
  • Flushed/cleaned fuel injectors
  • Replaced both O2 sensors - NTK
  • CPS, MAP and TPS sensors (mopar)
  • New plugs, wires, cap, rotor and ignition coil.
  • Seafoam
The only thing I haven't done yet is Deal with the fuel pump. I think the pump is ok but I'm not holding fuel pressure.

Currently doing the key turn to build pressure before starting.

It seems to run strongly if push through the gears aggressively. Just the rough idle and hesitation are driving me nuts.

Currently no codes but I've been disconnecting the battery while working.

Any suggestions?

BTW, I have a scanner Ancel BD310 on order.
 
Krischick33 said:
97 2.5 5 speed. Still chasing a few issues. 1. Rough idle - worst when cold. 2. Hesitation or skip under load. This happens between 2000 and 2800 rpms.
To date I have done the following:
Exhaust/Intake manifold
Flushed/cleaned fuel injectors
Replaced both O2 sensors - NTK
CPS, MAP and TPS sensors (mopar)
New plugs, wires, cap, rotor and ignition coil.
Seafoam
The only thing I haven't done yet is Deal with the fuel pump. I think the pump is ok but I'm not holding fuel pressure. Currently doing the key turn to build pressure before starting.
It seems to run strongly if push through the gears aggressively. Just the rough idle and hesitation are driving me nuts.
Currently no codes but I've been disconnecting the battery while working.
Any suggestions?
BTW, I have a scanner Ancel BD310 on order.
Click to expand...
Bump
 
How does it lug at 1500rpm under load? Mines been getting rougher for years and has those lean hiccups you mention. The heads coming off my 2001 2.5 tomorrow. After chasing this issue for a long time and throwing all sorts of sensors/parts, it came down to a mechanical issue. New valve springs, valve seals, lifters didn't help either. Turns out, the valves on #1 and #3 are not sealing on the seats. Only a proper leak-down test (by the dealer) revealed the issue. I should have it together in a few weeks after the machine shop does their thing. We'll know them.
 
I can only speak in generalities, but a skip only under load is often caused by poor fuel, bad compression, weak spark or ignition timing. The fuel is easy to check by switching to a high octane (less likely to pre-ignition) and ethanol free. This is unlikely to be th problem, but the easiest to check for.

There is also a slight possibility of a fuel pressure or supply issue, but the chances are very slim. Most likely to be a spark/timing thing. What was the very last thing you had done to the Jeep before this started? Often looking there is a good start. Fat fingers could have knocked wires loose or off enough to cause another issue to start.

Check that you have all good and clean battery cables and ground cables. Pull spark plugs and look for carbon or signs of preignition. Check for good voltage across the coil/s. But the very first thing is to check for any codes being thrown that can direct you to the system that is faulting to the computer. Often a multimeter is your best friend in this. There are a bunch of sensors that have to work together in a modern(ish) engine to make it run at it's best. Hope this gives you a start.
 
RaymondT said:
How does it lug at 1500rpm under load? Mines been getting rougher for years and has those lean hiccups you mention. The heads coming off my 2001 2.5 tomorrow. After chasing this issue for a long time and throwing all sorts of sensors/parts, it came down to a mechanical issue. New valve springs, valve seals, lifters didn't help either. Turns out, the valves on #1 and #3 are not sealing on the seats. Only a proper leak-down test (by the dealer) revealed the issue. I should have it together in a few weeks after the machine shop does their thing. We'll know them.
Click to expand...

Good advice here. Go back to basics with a compression test as a beginning. Easy to do as is the leak down test if indicated. How many miles on the engine? In the old days the compression gauge and vacuum gauge were always in easy reach.
 
Thanks for the responses. I just changed the fuel pump due to a bad pressure regulator. Was hoping this would stop the rough idle ans hesitation. No luck.
Answers to questions. 107 K miles and currently no CEL. Due to covid I haven't been able to register the jeep yet so just driving around the neighborhood for now.
I'm thinking its related to a timing issue. About 2 out of 10 times when I start it does something weird. Difficult to describe but while cranking it seems like the battery is low (its not). Difficult to turnover and just seems like something is out of wack. Its coughs. I need to keep trying and eventually it starts.
I changed the CPS most recently.
What else controls timing?
 
Checked all grounds. Removed, cleaned and reconnected. Still rough idle and hesitation/skip while between 2000 and 2700 rpm. I received my BT OBD reader today and downloaded Torque pro lite. I'd like to record some readings. Can anyone recommend what would be the most important to track.
 
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