99 4.0 engine loses power momentarily around 2000 RPM

[Bammo68]

So unsure what Karma God I upset but after resolving a bunch of annoying issues and getting my new rebuilt 99 engine running smoothly at long last I have a new issue.

Under mild acceleration the engine seems to hiccup somewhere in the range of 1700-2000 RPM. A momentary loss of power and then it continues - seems to be fairly repeatable and its a new issue. Has run for about 300 miles since the new engine was installed. Mechanically the engine seems fine and timing is spot on. No CEL and no codes.

Things that I have *not* recently replaced with new in the engine include:

1. Crank position sensor
2. MAP and IAC sensors though I cleaned out the IAC sensor
3. IAT Sensor (I have a bunch of spares)
4. Bottom half of the distributor (new cap, rotor etc)
5. Fuel injectors and rest of fuel system except for the wiring harness to the injectors
6. ECU

After several hours searching the interwebz and watching endless YT videos I have come to the conclusion that this is not an uncommon issue and no one seems to have a solid understanding as to what causes it. I see lots of people pointing the finger at the CPS, Cam position sensor (mine is new), TPS (mine is new), IAC, distributor and the MAP sensor. Oh and new NTK O2 sensors. Most of the YT videos don't have a resolution or if they do, they don't go the extra step to identify what the underlying cause of the fault was. They just load the parts cannon and declare it "fixed".

I'm running out of ideas and was going to partially load the parts cannon and do a new CPS & MAP sensor as my first focus area. Any thoughts? I didn't think the IAT sensor would be at fault here. Could it be the new TPS sensor I recently installed?

 

[Chris]

So unsure what Karma God I upset but after resolving a bunch of annoying issues and getting my new rebuilt 99 engine running smoothly at long last I have a new issue.

Under mild acceleration the engine seems to hiccup somewhere in the range of 1700-2000 RPM. A momentary loss of power and then it continues - seems to be fairly repeatable and its a new issue. Has run for about 300 miles since the new engine was installed. Mechanically the engine seems fine and timing is spot on. No CEL and no codes.

Things that I have *not* recently replaced with new in the engine include:

1. Crank position sensor
2. MAP and IAC sensors though I cleaned out the IAC sensor
3. IAT Sensor (I have a bunch of spares)
4. Bottom half of the distributor (new cap, rotor etc)
5. Fuel injectors and rest of fuel system except for the wiring harness to the injectors
6. ECU

After several hours searching the interwebz and watching endless YT videos I have come to the conclusion that this is not an uncommon issue and no one seems to have a solid understanding as to what causes it. I see lots of people pointing the finger at the CPS, Cam position sensor (mine is new), TPS (mine is new), IAC, distributor and the MAP sensor. Oh and new NTK O2 sensors. Most of the YT videos don't have a resolution or if they do, they don't go the extra step to identify what the underlying cause of the fault was. They just load the parts cannon and declare it "fixed".

I'm running out of ideas and was going to partially load the parts cannon and do a new CPS & MAP sensor as my first focus area. Any thoughts? I didn't think the IAT sensor would be at fault here. Could it be the new TPS sensor I recently installed?

Even though it's new, it’s worth checking the TPS calibration. Sometimes, new sensors can be faulty or mis-calibrated. This is especially the case with cheap, aftermarket sensors.. Use a multimeter to verify that the voltage readings are smooth and proportional throughout the throttle range.

A vacuum leak can cause erratic idle and throttle response. Double-check all vacuum lines and the intake manifold to ensure there are no cracks or loose connections. It might be helpful to spray some starting fluid around suspected areas to check for a rise in RPM, indicating a leak.

Given that the wiring harness to the fuel injectors hasn't been replaced, inspect the harness thoroughly. Look for damaged wires or poor connections that might be causing intermittent issues. You'd be surprised how often this ends up being the issue, especially on these older TJs. Breaks in the wiring can be hard to find but it's a serious and well documented issue.

A partially clogged catalytic converter could also trigger performance issues. If you suspect this might be the case, you may want to check back pressure or remove the cat temporarily to see if performance improves.

In addition, reinstalling the CPS or making sure it is properly aligned could be worth investigating again.

 

[macleanflood]

I'd get to reading and graphing live ODB data. An expensive scanner or a $30 Bluetooth dongle and Torque Lite will do it.

I'd be monitoring O2 voltages, vacuum, fuel trims for starters.

Get some data before firing the canon.

-Mac

 

[AaronPaul]

So unsure what Karma God I upset but after resolving a bunch of annoying issues and getting my new rebuilt 99 engine running smoothly at long last I have a new issue.

Under mild acceleration the engine seems to hiccup somewhere in the range of 1700-2000 RPM. A momentary loss of power and then it continues - seems to be fairly repeatable and its a new issue. Has run for about 300 miles since the new engine was installed. Mechanically the engine seems fine and timing is spot on. No CEL and no codes.

Things that I have *not* recently replaced with new in the engine include:

1. Crank position sensor
2. MAP and IAC sensors though I cleaned out the IAC sensor
3. IAT Sensor (I have a bunch of spares)
4. Bottom half of the distributor (new cap, rotor etc)
5. Fuel injectors and rest of fuel system except for the wiring harness to the injectors
6. ECU

After several hours searching the interwebz and watching endless YT videos I have come to the conclusion that this is not an uncommon issue and no one seems to have a solid understanding as to what causes it. I see lots of people pointing the finger at the CPS, Cam position sensor (mine is new), TPS (mine is new), IAC, distributor and the MAP sensor. Oh and new NTK O2 sensors. Most of the YT videos don't have a resolution or if they do, they don't go the extra step to identify what the underlying cause of the fault was. They just load the parts cannon and declare it "fixed".

I'm running out of ideas and was going to partially load the parts cannon and do a new CPS & MAP sensor as my first focus area. Any thoughts? I didn't think the IAT sensor would be at fault here. Could it be the new TPS sensor I recently installed?

Personally, I would check these 2 things first if it was me:

Crankshaft Position Sensor (CPS)​

Even though it's not always a frequent culprit, the CPS is a good place to start. It controls ignition timing, and if it's malfunctioning or out of alignment, it can cause intermittent power loss or hesitation. This is especially true under load, like during moderate acceleration. It’s also worth verifying that the wiring and connector are intact.

​

Throttle Position Sensor (TPS)​

Though you’ve already replaced it, a new TPS can sometimes be out of specification, or the sensor's voltage might be improperly calibrated. This can lead to irregular throttle responses or engine hiccups. Double-check the TPS voltage and reset it as needed.

Given the symptoms and troubleshooting you've done, I would focus on checking the CPS, TPS, and fuel delivery first. If the issue persists, the vacuum leaks or wiring issues could be more subtle culprits.

 

[bohuggabee]

So unsure what Karma God I upset but after resolving a bunch of annoying issues and getting my new rebuilt 99 engine running smoothly at long last I have a new issue.

Under mild acceleration the engine seems to hiccup somewhere in the range of 1700-2000 RPM. A momentary loss of power and then it continues - seems to be fairly repeatable and its a new issue. Has run for about 300 miles since the new engine was installed. Mechanically the engine seems fine and timing is spot on. No CEL and no codes.

Things that I have *not* recently replaced with new in the engine include:

1. Crank position sensor
2. MAP and IAC sensors though I cleaned out the IAC sensor
3. IAT Sensor (I have a bunch of spares)
4. Bottom half of the distributor (new cap, rotor etc)
5. Fuel injectors and rest of fuel system except for the wiring harness to the injectors
6. ECU

After several hours searching the interwebz and watching endless YT videos I have come to the conclusion that this is not an uncommon issue and no one seems to have a solid understanding as to what causes it. I see lots of people pointing the finger at the CPS, Cam position sensor (mine is new), TPS (mine is new), IAC, distributor and the MAP sensor. Oh and new NTK O2 sensors. Most of the YT videos don't have a resolution or if they do, they don't go the extra step to identify what the underlying cause of the fault was. They just load the parts cannon and declare it "fixed".

I'm running out of ideas and was going to partially load the parts cannon and do a new CPS & MAP sensor as my first focus area. Any thoughts? I didn't think the IAT sensor would be at fault here. Could it be the new TPS sensor I recently installed?

I rarely push my 4.0 to 2,000 rpm. But you said mild acceleration so this is weird. If its rebuilt. It needs to be broken in. Make sure you have heavy oil. But this is the trick. Get a high mile rated oil filter. Try the 20,000 mile rated k&m filters. Keeps oil pressure freer . Expensive filter. But for these engines are a godsend. You need a nice thick oil though. The engine has plenty of torque. If its running rough. Shaking or knocking. Try 4 hole injectors. They heat soak, need good seals but my engine runs like butter. Ive had 4 hole bosch for 6 years now. Better fuel economy. Smoother engine. Longevity. Just dont sit out in a hot day...

1700-2000 rpm is strange because the engine should be running in cycle enough for enough torque for any hill, and enough fuel. So most likely its the fuel line dropping pressure.

Im personally considering a dual fuel pump mod. One external and one internal. Where i can control choke and bypass the external when at speed.

But i dont have issues at speed. Its during less speed i can have throttle loss and chokes from fuel pressure loss. So at speed it could be your clutch failing. A new engine. That high rpm... you might be shocking the clutch plates. Check the clutch fluid first

 

[Bammo68]

OK bit of an update just to make things more confusing. Every other niggle with the TJ is now sorted - new engine is purring. This minor hesitation persists, but its very repeatable now and the conditions under which it occurs are a bit different from when I first described it. Now it only seems to occur when the car is cold and it happens about a minute or so after I start driving - almost always as I am turning right at the end of my street. It seems to just hesitate and revs drop right down - never quite stalling but no power.

Whole thing lasts between 1 and 3 seconds and then completely goes away not to re-occur until the next time I do a cold start. It seems to happen at low revs only - the 2000RPM figure I mentioned earlier no longer seems to apply and it does it as I start to accelerate after coming to a stop at the intersection. Does it with both an empty tank and a full tank so I don't think its the fuel pickup being blocked.

It's a weird one - from a practical perspective its not causing me too much grief so I am reluctant to invest a ton of time into resolving it but the OCD part of me is freaking out!

 

[macleanflood]

My previous advice stands about monitoring and logging live ODB data.

I wonder if you have a defect in your throttle body or intake. Maybe a crack or worn butterfly valve shaft. Maybe a smoke test might reveal things.

Good luck and nice progress!

-Mac

 

[ColoJeep]

My 99 had a similar issue years ago , momentary power loss going uphill , I thought it was a weak fuel pump , changed the pump and it still would happen.
No Codes , I was looking over everything and found a frayed wire from the upsteam O2 sensor , it would occasionally move and touch/ rub on the body going uphill . Fixed the wires and the problem went away ! No issues since 2012.

 

[Bammo68]

6 months later and....still the same issue. It only ever does it now when the engine is cold and its always as I turn right at the end of the street. It hiccups, I sometimes get a pop like a mild backfire and then it runs normally with absolutely no other symptoms. Way too consistent to be an intermittent issue but its got me very confused!

 

[SkylinesSuck]

I hate to always suggest the same thing, but it very much sounds like a short in the wiring harness to me. Start wiggling stuff around while the engine is running and see if it stumbles. Pay particular attention to where the harness snakes around the back of the cylinder head.

 

[GoldenGorilla]

The fact that it seems to tie to a particular action has me thinking wiring harness issues too but does not rule out a couple other things. Especially since cold start is in play. A scan of live system data while running and having this issue will also be valuable. Though I'll note my TJ stumbles in a specific rpm range only when I have VCM Scanner running live on my laptop and I have NEVER found it in a scan. It doesn't ever do this with my bluetooth ODBII and the Torque app scanning live.

There are some time dependent things worth checking as well. I only have the '04 and '06 years PCM settings as refence, but there are some warmup timings I've noticed in each config. I believe they are related to bringing the cat up to temp and the engine coolant up to temp on cold start. The 99 PCM settings may be completely different as things are with these TJs.

The first warmup cycle on my 04 is 208sec followed by an additional 30sec. Point being, the PCM is doing something a little different in these time periods. If you start a stopwatch from start, before driving your route to the repeatable issue, it might give us a little more info on specific parts to help you focus on. You might also try letting it warm up in the driveway for 5 minutes to get past these warmup timers before setting out and see if it shows the same stumble behavior.

I'm sure we can get to the bottom without deploying the old parts cannon.

 

[ColoJeep]

The fact that it seems to tie to a particular action has me thinking wiring harness issues too but does not rule out a couple other things. Especially since cold start is in play. A scan of live system data while running and having this issue will also be valuable. Though I'll note my TJ stumbles in a specific rpm range only when I have VCM Scanner running live on my laptop and I have NEVER found it in a scan. It doesn't ever do this with my bluetooth ODBII and the Torque app scanning live.

There are some time dependent things worth checking as well. I only have the '04 and '06 years PCM settings as refence, but there are some warmup timings I've noticed in each config. I believe they are related to bringing the cat up to temp and the engine coolant up to temp on cold start. The 99 PCM settings may be completely different as things are with these TJs.

The first warmup cycle on my 04 is 208sec followed by an additional 30sec. Point being, the PCM is doing something a little different in these time periods. If you start a stopwatch from start, before driving your route to the repeatable issue, it might give us a little more info on specific parts to help you focus on. You might also try letting it warm up in the driveway for 5 minutes to get past these warmup timers before setting out and see if it shows the same stumble behavior.

I'm sure we can get to the bottom without deploying the old parts cannon.

Years ago I figured out a similar issue on my little brothers 87 K-5 Blazer . It would stumble and try to stop running for 2 - 3 seconds when the TBI went from open loop to closed loop warming up . It ran perfect after the 2-3 seconds and before in open loop . It was the PCM. Replaced it and reused the E-prom .

 

[Bammo68]

You might also try letting it warm up in the driveway for 5 minutes to get past these warmup timers before setting out and see if it shows the same stumble behavior.

That's a really interesting idea! will try that tomorrow...

 

[GoldenGorilla]

I looked up those warm up spark tables on my TJ and they have quite a bit less advance than normal running. Generally -3° to -8° off normal. There might be something parallel on fuel at warm up to run a bit rich to prime and fire up the cats. A common strategy.

It makes sense as I've always had a short period on warmup where it seemed like it would bog a little on the upshift from 1-2 or 2-3. I never really thought of it as cutting out but you might.

 

[Bammo68]

I never really thought of it as cutting out but you might.

Oh it definitely cuts - like total power loss for just a second or so, usually accompanied with a "blat" out of the exhaust like a mild backfire. Its just resoundingly odd to me that it *only* does it once, almost always at the exact same spot at the end of my street and then never so much as hints at doing it again until the next time I drive it when totally cold.

 

[SkylinesSuck]

Its just resoundingly odd to me that it *only* does it once, almost always at the exact same spot at the end of my street....

You could try moving?


Did you do you harness wiggle like I suggested?

 

[Bammo68]

Update: this time I let the engine warm up at idle for about 3 minutes before I pulled out of the driveway. Absolutely no hint of the hesitation - first time in a long time so the issue certainly appears to have some correlation to the time-from-cold-start parameter.

 

[Bammo68]

Did you do you harness wiggle like I suggested?

Wiggled it like Captain Feathersword trying to get into the Yellow Wiggle's pants at the Christmas party. Same result as he got!

 

[SkylinesSuck]

Wiggled it like Captain Feathersword trying to get into the Yellow Wiggle's pants at the Christmas party. Same result as he got!

 

[Bammo68]

Some of you haven't had kids who watched The Wiggles and it shows....