Engine Sputtering on 1999 4.0 Automatic

[99’ TJ]

About a month ago I got a 1999 TJ with 66,000 miles on it. This jeep is a right hand drive and is in great condition cosmetically.
Interior was well maintained, frame is undercoated with zero rust.

The gentlemen I got it from told me there was a sputter in the engine but it was still drivable. I have driven the jeep around town cruising 45-55mph and have been on the interstate running 75mph. The jeep sputters/bucks mainly under acceleration but once you get to cruising speed it gets smoother but it's like you can feel the engine sporadically bucking.
No check engine light when I picked it up from the guy either. I did turn the key to the on position to ensure the CEL worked.

Below is a list of things the previous owner did to try and fix:

• Changed fuel pump
• New plugs, wires, and distributor
• Cut exhaust pipe before the CAT to ensure it wasn't plugged and causing a problem. This has since been patched at a muffler shop.

Since I got the jeep, I have done the following:
I read on a forum on here the NTK/NGK supplied OEM O2 sensors so I went with several other NTK/NGK sensors when replacing since I ordered direct from NGK.com.

• New upstream NTK O2 sensor (P/N 23179) - I have tried running with this unplugged as I have seen on some other threads and it does not make a noticeable difference
• New NGK ignition coil (P/N 48633 U1085)
• New NTK TPS (P/N 75426 TH0089)
• New NTK CPS (P/N 73315 EH0249)
• New NTK AIT sensor (P/N 75845 AJ0013)
• New NTK MAP sensor (P/N 74299 MA0114) check engine light came on after this (Code P0107 MAP sensor voltage too low) and it idled awful so I put original back in, cleared code and back to how it was before
• Checked spark between distributor and ignition coil
• Check spark on all spark plugs and check them for sign of oil/burnt but all looked good
• Checked fuel injector wiring with a noid light and all were good
• Cleaned IAC with throttle body cleaned and reinstalled - was not dirty at all to begin with

After doing this, the jeep is still sputtering/bucking.
I will note that after having the battery unhooked to install sensors the jeep seems to run good for the first ride then it goes back to normal again.
There is also no check engine light on. The only time the CEL came on was when I installed the NTK MAP sensor as mentioned above. As soon as I put the original back on and cleared the code it has not come back on since.

I have read several forums on this site before posting. Seems many people on here have a pile of knowledge so hopefully someone can point me in the right direction.

Looking for suggestions on what else to check or try.

• Should I take the throttle body off and make sure it is thoroughly cleaned?
• Should I replace the ECM?
• Should I check the timing?

Let me know your thoughts and I will try them out and report back.

 

[Mac57]

Checked fuel pressure? If it's bucking and sputtering mainly under acceleration could be it's temporarily starved for fuel.

 

[99’ TJ]

Checked fuel pressure? If it's bucking and sputtering mainly under acceleration could be it's temporarily starved for fuel.

I have not, called my local auto parts store Saturday to rent their fuel pressure gauge and someone already had it.
Will check back with them today. Can you tell me what PSI I should be at?
I'll report back with my readings when I get them.

 

[Mac57]

I have not, called my local auto parts store Saturday to rent their fuel pressure gauge and someone already had it.
Will check back with them today. Can you tell me what PSI I should be at?
I'll report back with my readings when I get them.

If I recall correctly you are looking for something in the 48 PSI range. Others can chime in if I'm off on this.

 

[macleanflood]

If you have a ODB diagnostic tool that will read/graph live data look at your O2 voltages, fuel trims and vacuum.

-Mac

 

[99’ TJ]

If you have a ODB diagnostic tool that will read/graph live data look at your O2 voltages, fuel trims an

If you have a ODB diagnostic tool that will read/graph live data look at your O2 voltages, fuel trims and vacuum.

-Mac

Got a guy at work bringing me one tomorrow that will read/graph live data so I will report back.

 

[Bammo68]

I installed a rebuilt motor into my RHD '99 at Christmas and swapped out the exhaust system at the same time and had a niggling issue with sputtering / loss of power that I finally resolved. The issue would seem to go away after a power reset and it wasn't super repeatable but seemed to happen once the vehicle had warmed up and was most noticeable under acceleration. Like you, I changed a few sensors thinking the TPS or IAC was the source of the issues. I then went down the O2 sensor rabbit hole thinking it was a fuelling / air issue, it wasn't.

In the end, I replaced the fuel injector connectors - mine were pretty brittle and a couple of the lock tabs had broken. I had a replacement set to put on so did that whilst I was trying to figure out the sputtering issue. Solved it instantly. Was a cheap fix and east to do with a soldering iron. I think under acceleration the lack of locking tabs was allowing at least one of the injector plugs to slightly disconnect.

May not be your issue of course but the symptoms sounded very similar to yours. Static testing of those connections wouldn't show a fault. At no stage did the engine show a CEL and there was noting obvious showing in any scan tool data.

Hope this helps...

 

[99’ TJ]

I installed a rebuilt motor into my RHD '99 at Christmas and swapped out the exhaust system at the same time and had a niggling issue with sputtering / loss of power that I finally resolved. The issue would seem to go away after a power reset and it wasn't super repeatable but seemed to happen once the vehicle had warmed up and was most noticeable under acceleration. Like you, I changed a few sensors thinking the TPS or IAC was the source of the issues. I then went down the O2 sensor rabbit hole thinking it was a fuelling / air issue, it wasn't.

In the end, I replaced the fuel injector connectors - mine were pretty brittle and a couple of the lock tabs had broken. I had a replacement set to put on so did that whilst I was trying to figure out the sputtering issue. Solved it instantly. Was a cheap fix and east to do with a soldering iron. I think under acceleration the lack of locking tabs was allowing at least one of the injector plugs to slightly disconnect.

May not be your issue of course but the symptoms sounded very similar to yours. Static testing of those connections wouldn't show a fault. At no stage did the engine show a CEL and there was noting obvious showing in any scan tool data.

Hope this helps...

I will keep this in mind. My locking tabs and connectors seemed very solid when I removed them.

Why would the static testing not show a fault in the wiring to the connector? I thought that was the purpose of testing with the noid light.

 

[Bammo68]

I will keep this in mind. My locking tabs and connectors seemed very solid when I removed them.

Why would the static testing not show a fault in the wiring to the connector? I thought that was the purpose of testing with the noid light.

Because it wasn't a static fault - it was making enough of a connection at idle and under moderate driving but when I would accelerate the connector was moving relative to the socket enough to disconnect a couple of the pins. If you are doing a noid test then I would recommend wiggling the wiring harness to see if that is triggering it. Again, my situation may be unrelated to yours, it was just interesting that the symptoms were so similar.

 

[99’ TJ]

Because it wasn't a static fault - it was making enough of a connection at idle and under moderate driving but when I would accelerate the connector was moving relative to the socket enough to disconnect a couple of the pins. If you are doing a noid test then I would recommend wiggling the wiring harness to see if that is triggering it. Again, my situation may be unrelated to yours, it was just interesting that the symptoms were so similar.

Okay, did you use any specific brand connectors? If so, can you provide a link

 

[RockyTopTJ]

About a month ago I got a 1999 TJ with 66,000 miles on it. This jeep is a right hand drive and is in great condition cosmetically.
Interior was well maintained, frame is undercoated with zero rust.

The gentlemen I got it from told me there was a sputter in the engine but it was still drivable. I have driven the jeep around town cruising 45-55mph and have been on the interstate running 75mph. The jeep sputters/bucks mainly under acceleration but once you get to cruising speed it gets smoother but it's like you can feel the engine sporadically bucking.
No check engine light when I picked it up from the guy either. I did turn the key to the on position to ensure the CEL worked.

Below is a list of things the previous owner did to try and fix:

• Changed fuel pump
• New plugs, wires, and distributor
• Cut exhaust pipe before the CAT to ensure it wasn't plugged and causing a problem. This has since been patched at a muffler shop.

Since I got the jeep, I have done the following:
I read on a forum on here the NTK/NGK supplied OEM O2 sensors so I went with several other NTK/NGK sensors when replacing since I ordered direct from NGK.com.

• New upstream NTK O2 sensor (P/N 23179) - I have tried running with this unplugged as I have seen on some other threads and it does not make a noticeable difference
• New NGK ignition coil (P/N 48633 U1085)
• New NTK TPS (P/N 75426 TH0089)
• New NTK CPS (P/N 73315 EH0249)
• New NTK AIT sensor (P/N 75845 AJ0013)
• New NTK MAP sensor (P/N 74299 MA0114) check engine light came on after this (Code P0107 MAP sensor voltage too low) and it idled awful so I put original back in, cleared code and back to how it was before
• Checked spark between distributor and ignition coil
• Check spark on all spark plugs and check them for sign of oil/burnt but all looked good
• Checked fuel injector wiring with a noid light and all were good
• Cleaned IAC with throttle body cleaned and reinstalled - was not dirty at all to begin with

After doing this, the jeep is still sputtering/bucking.
I will note that after having the battery unhooked to install sensors the jeep seems to run good for the first ride then it goes back to normal again.
There is also no check engine light on. The only time the CEL came on was when I installed the NTK MAP sensor as mentioned above. As soon as I put the original back on and cleared the code it has not come back on since.

I have read several forums on this site before posting. Seems many people on here have a pile of knowledge so hopefully someone can point me in the right direction.

Looking for suggestions on what else to check or try.

• Should I take the throttle body off and make sure it is thoroughly cleaned?
• Should I replace the ECM?
• Should I check the timing?

Let me know your thoughts and I will try them out and report back.

Change that CPS at least to a Mopar sensor. I chased problems for months and used NTK/NGK sensors and all they did was cause problems. You've already seen that issue with your MAP sensor. TJs are picky with sensors. Also I would check your distributor for any lateral play in the shaft, check the contacts, and replace the cam sensor in it with a Mopar unit.

Recently, my distributor developed so much lateral play that it broke the rotor completely off on the interstate. No check engine light either.

 

[Bammo68]

https://www.rockauto.com/en/moreinf...DGznEfnQNdYjLBj7aE2Hbwr+zoBzsYEj6tabrEIQpuQ==

 

[99’ TJ]

If you have a ODB diagnostic tool that will read/graph live data look at your O2 voltages, fuel trims and vacuum.

-Mac

I have attached all the tests I could run. All the big spikes or dips are from me revving engine up to ensure it was plotting correctly.
I do not know much about any of this, so let me know if you see any issues and what I should try if you do.

 

[99’ TJ]

I have not, called my local auto parts store Saturday to rent their fuel pressure gauge and someone already had it.
Will check back with them today. Can you tell me what PSI I should be at?
I'll report back with my readings when I get them.

43 PSI primed with key in the on position.
48 PSI running.

 

[RockyTopTJ]

43 PSI primed with key in the on position.
48 PSI running.

View attachment 599213

View attachment 599214

I believe that's right on the money

 

[99’ TJ]

Because it wasn't a static fault - it was making enough of a connection at idle and under moderate driving but when I would accelerate the connector was moving relative to the socket enough to disconnect a couple of the pins. If you are doing a noid test then I would recommend wiggling the wiring harness to see if that is triggering it. Again, my situation may be unrelated to yours, it was just interesting that the symptoms were so similar.

I ended up changing all the connectors and injectors and problem still persists.

Any suggestions on what else to try?

 

[Chris]

I ended up changing all the connectors and injectors and problem still persists.

Any suggestions on what else to try?

Here are a few more suggestions to consider as you continue to figure out the sputtering issue:

• Check Ground Connections: Ensure that all your grounds, especially for the engine and fuel injectors, are clean and tight. Poor grounding can lead to erratic performance.
• Inspect the Throttle Body: Even if you’ve cleaned it before, removing it to check for any hidden issues (like a stuck butterfly valve) might be worthwhile. Ensure that the IAC and throttle body are functioning correctly together.
• Check the Fuel Filter: If you haven’t already, consider replacing the fuel filter as a clogged one can restrict fuel flow, particularly under acceleration.
• Examine Wiring Harness: Since you mentioned changing connectors, check for any frayed or damaged wires that may not be visibly apparent. This can sometimes cause intermittent issues.
• Camshaft Position Sensor (CPS): If the CPS hasn't been switched to a Mopar part yet, it might be worth trying that route. Sometimes quality aftermarket sensors can lead to strange performance issues.
• Check for Vacuum Leaks: A vacuum leak can cause sputtering issues, especially during acceleration. A smoke test can help identify any hidden leaks.
• Review Engine Timing: If you haven’t checked the timing yet, make sure it’s set properly. A timing issue can also cause sputtering.
• Fuel Pressure Regulator: If the regulator is leaking, it can cause fluctuating fuel pressure which might lead to sputtering. Check for any signs of fuel leakage around it.
• Look for Exhaust Restrictions: While you indicated that the exhaust was checked, sometimes there can be a restriction deeper in the system, such as a partially plugged catalytic converter.

Try these suggestions one by one and see if any changes occur. Good luck, and keep us updated!

 

[99’ TJ]

I have attached all the tests I could run. All the big spikes or dips are from me revving engine up to ensure it was plotting correctly.
I do not know much about any of this, so let me know if you see any issues and what I should try if you do.

View attachment 598751

View attachment 598752

View attachment 598753

View attachment 598754

View attachment 598755

View attachment 598756

View attachment 598757

View attachment 598758

Here are a few more suggestions to consider as you continue to figure out the sputtering issue:

• Check Ground Connections: Ensure that all your grounds, especially for the engine and fuel injectors, are clean and tight. Poor grounding can lead to erratic performance.
• Inspect the Throttle Body: Even if you’ve cleaned it before, removing it to check for any hidden issues (like a stuck butterfly valve) might be worthwhile. Ensure that the IAC and throttle body are functioning correctly together.
• Check the Fuel Filter: If you haven’t already, consider replacing the fuel filter as a clogged one can restrict fuel flow, particularly under acceleration.
• Examine Wiring Harness: Since you mentioned changing connectors, check for any frayed or damaged wires that may not be visibly apparent. This can sometimes cause intermittent issues.
• Camshaft Position Sensor (CPS): If the CPS hasn't been switched to a Mopar part yet, it might be worth trying that route. Sometimes quality aftermarket sensors can lead to strange performance issues.
• Check for Vacuum Leaks: A vacuum leak can cause sputtering issues, especially during acceleration. A smoke test can help identify any hidden leaks.
• Review Engine Timing: If you haven’t checked the timing yet, make sure it’s set properly. A timing issue can also cause sputtering.
• Fuel Pressure Regulator: If the regulator is leaking, it can cause fluctuating fuel pressure which might lead to sputtering. Check for any signs of fuel leakage around it.
• Look for Exhaust Restrictions: While you indicated that the exhaust was checked, sometimes there can be a restriction deeper in the system, such as a partially plugged catalytic converter.

Try these suggestions one by one and see if any changes occur. Good luck, and keep us updated!

Thank you Chris.
I will look through these and try them out. Will report back.

 

[macleanflood]

[*]Check the Fuel Filter: If you haven’t already, consider replacing the fuel filter as a clogged one can restrict fuel flow, particularly under acceleration.

@Chris your AI is leaking out...

TJ fuel filter and regulator are both in the pump assembly.

Did you monitor fuel pressure as you drove around? I'd find a gauge I could tape to the windshield or side mirror.

-Mac