4.0L Stuck In Open Loop - Almost Stalls In Closed Loop

[boxyjeep]

I've been troubleshooting this for awhile now and have done a lot of the suggestions on Cruiser54's site.  The key issue is that the engine has a very rough/low idle and comes very close to stalling once warmed up when coming to a stop or not depressing the accelerator.  I have a SnapOn MT2500, so I can see that when in OPEN LOOP mode it runs reasonably well.  I've replaced the IAC, adjusted the TPS, replaced most of the vacuum lines, grounds, etc.  Based on the Jeep Renix Fuel Injection manual, I believe the O2 sensor is likely bad (since it's running terribly in CLOSED LOOP mode and always in a LEAN condition), but wanted to get some advice here before I start swapping parts.  I don't have an analog voltmeter to test the O2 sensor.

 

Here's a recording of a cold start.  Jump to 4:40 to see it under load and then almost stall.

 

 

[87MJTIM]

My truck has a similar issue.  I have Nickintime's REM to monitor my engine.  After starting up the engine, it will switch to closed loop and the STFT starts to climb up to 255 (running lean).  After 2 seconds, it switches back to open loop.  Once the engine warms up, it will switch back to closed loop.  But, the STFT shows low numbers - 30 to <100 (rich condition).  When i give it gas, the STFT climbs up above 128.  I believe my issue may be an intake leak somewhere from the manifold to the injectors.  I have not had the time to diagnose it yet.

 

Back to your issue, with it in a constant lean condition, I would think you may have an intake leak.  With the MT2500 visible, spray carb cleaner/throttle body cleaner at various points on the intake system: around the injectors, manifold gasket, throttle body.  See how the O2 sensor responds; the STFT response; open v. closed loop response.

 

Replacing the O2 sensor is not too expensive, nor that difficult.  

[fiatslug87]

On a side note, which cartridge are you using?

[boxyjeep]

It was my understanding that a vacuum or intake leak would cause a higher idle.  I have replaced the intake/exhaust gasket and all bolts are properly tightened.

9 minutes ago, 87MJTIM said:

My truck has a similar issue.  I have Nickintime's REM to monitor my engine.  After starting up the engine, it will switch to closed loop and the STFT starts to climb up to 255 (running lean).  After 2 seconds, it switches back to open loop.  Once the engine warms up, it will switch back to closed loop.  But, the STFT shows low numbers - 30 to <100 (rich condition).  When i give it gas, the STFT climbs up above 128.  I believe my issue may be an intake leak somewhere from the manifold to the injectors.  I have not had the time to diagnose it yet.

 

Back to your issue, with it in a constant lean condition, I would think you may have an intake leak.  With the MT2500 visible, spray carb cleaner/throttle body cleaner at various points on the intake system: around the injectors, manifold gasket, throttle body.  See how the O2 sensor responds; the STFT response; open v. closed loop response.

 

Replacing the O2 sensor is not too expensive, nor that difficult.  

It was my understanding that a vacuum or intake leak would cause a higher idle.  I have replaced the intake/exhaust gasket and all bolts are properly tightened.

[boxyjeep]

1 minute ago, fiatslug87 said:

On a side note, which cartridge are you using?

I’ll have to check.  I think it’s the pre-97 Jeep troubleshooting cartridge.  I bought the bundle on eBay recently for a fraction of what it cost new 😁

[Ωhm]

Preliminary checks should involve 12vdc at O2S (C229_A) and O2S (C229_B) to ground on the harness side. On the component side, check for open circuit (heater element) between A and B.

[boxyjeep]

10 minutes ago, Ωhm said:

Preliminary checks should involve 12vdc at O2S (C229_A) and O2S (C229_B) to ground on the harness side. On the component side, check for open circuit (heater element) between A and B.

Ok will check that.

[87MJTIM]

8 minutes ago, Chargum85 said:

It was my understanding that a vacuum or intake leak would cause a higher idle.  I have replaced the intake/exhaust gasket and all bolts are properly tightened.

 

If a leak affects one of the sensors, like the MAP, then yes, it may cause a high idle.  If the leak is away from a sensor, like an improperly set injector, the air intake is un-monitored.  More air in the cylinder will give a lean condition.  According to the Renix FI manual (p.3), if the STFT is at either extreme 0 (rich) or 255 (lean), the ECU will default to open loop.

[boxyjeep]

12 minutes ago, 87MJTIM said:

 

If a leak affects one of the sensors, like the MAP, then yes, it may cause a high idle.  If the leak is away from a sensor, like an improperly set injector, the air intake is un-monitored.  More air in the cylinder will give a lean condition.  According to the Renix FI manual (p.3), if the STFT is at either extreme 0 (rich) or 255 (lean), the ECU will default to open loop.

 

The injector o-rings were replaced when I swapped out the exhaust manifold and gasket.  The MT2500 is showing both STFT and LTFT at 128.  It's just odd that the AFR never changes from LEAN the entire time the engine runs/starts, and only enters CLOSED loop for very short periods of time and quickly flips back to OPEN loop after running super rough.

 

I don't have a fuel pressor gauge, but the fuel pump was recently replaced (this could be good or bad).  The truck also had the fuel pump ballast resistor bypassed when I got it.  I fixed that and it is now NOT bypassed.

[87MJTIM]

Try testing the O2 sensor first.  See Cruiser's test procedures or Another Freakin' Cherokee Website for Renix help.

 

Swapping the O2 sensor may be a simple fix.

[boxyjeep]

51 minutes ago, 87MJTIM said:

Try testing the O2 sensor first.  See Cruiser's test procedures or Another Freakin' Cherokee Website for Renix help.

 

Swapping the O2 sensor may be a simple fix.

 

Ok, just tested the O2 sensor.  B to C terminals read a solid 5v while running and A to B read around 14.3v with +-.05v while running.  I believe that would point to a faulty O2 sensor.  I’ll get one ordered.

[Ωhm]

1 hour ago, Chargum85 said:

The MT2500 is showing both STFT and LTFT at 128.  It's just odd that the AFR never changes from LEAN the entire time the engine runs/starts, and only enters CLOSED loop for very short periods of time and quickly flips back to OPEN loop after running super rough.

Note STFT climbs when in CLOSED loop. INJ PW also climbs to about 11mS.

Time 4:40

 

2 hours ago, Ωhm said:

On the component side, check for open circuit (heater element) between A and B.

Do this test with harness disconnected.

[boxyjeep]

6 minutes ago, Ωhm said:

Note STFT climbs when in CLOSED loop. INJ PW also climbs to about 11mS.

Time 4:40

 

Do this test with harness disconnected.

 

O2 sensor resistance is infinite when disconnected from the harness and measured across A/B.  I'm assuming that confirms O2?

[Ωhm]

Confirmed.

[boxyjeep]

2 minutes ago, Ωhm said:

Confirmed.

 

Thank you sir.  Will let you know how it goes when I receive it.  Went with the Bosch part from Amazon.

[cruiser54]

Doesn't the MT2500 allow you to watch the O2 sensor in real time? 

[JMO413]

24 minutes ago, cruiser54 said:

Doesn't the MT2500 allow you to watch the O2 sensor in real time? 

It is displaying the sensor it's just not switching like it should. It's in the middle top row.

[JMO413]

Mine did the same thing with a bad oxygen sensor. 

[boxyjeep]

19 minutes ago, JMO413 said:

It is displaying the sensor it's just not switching like it should. It's in the middle top row.

 

Yes, but I’ve heard varying reports that this is not an accurate value on the MT2500.

[JMO413]

This is mine switching like it should.

[boxyjeep]

1 hour ago, JMO413 said:

This is mine switching like it should.

 

Haha well there we go.  Should get the new sensor in a few days and be able to report back!

[cruiser54]

10 hours ago, JMO413 said:

It is displaying the sensor it's just not switching like it should. It's in the middle top row.

That's the answer. It should switch very fast. 

[boxyjeep]

Quick update--got the new O2 sensor installed this morning and it's running about 10 times better.  Now it's properly switching between LEAN/RICH, and CLOSED loop doesn't cause the near-stall condition like it did before.  The O2 sensor reading is properly ranging between 0-5v on the MT2500 (as expected).  I didn't have a chance to drive it or let it fully warm up, but the results were pretty immediate.  The idle is still a bit low, but overall seems much improved.

 

Thanks everyone for helping on this.  Super pleased that this MJ that I bought a few months ago is finally starting to run better.

[cruiser54]

Go drive it.

 

Ever clean your throttle body?

 

Of my Tips on www.cruiser54.com, which have you completed? 

[boxyjeep]

28 minutes ago, cruiser54 said:

Go drive it.

 

Ever clean your throttle body?

 

Of my Tips on www.cruiser54.com, which have you completed? 

 

Just took it for a drive and let it fully warm up.  Unfortunately the low idle condition is still mostly there.  The truck runs notably better with the new O2 sensor, but idle is still super low.  I started adjusting the TPS for the second time and am seeing some big variances in idle speed.  I'm also seeing a large variance in TPS return voltage using a backported voltmeter vs. MT2500.  Not sure if you have any advice on which one is more accurate for calculating the 17% basis.  Also, what % throttle should we be at for idle?  18% is what I am seeing.

 

Regarding your site, it's been super helpful and I've even printed it out in a binder in my garage--thanks!  I've completely refreshed the grounds, cleaned the throttle body, replaced the throttle-body-to-map hose, and done a handful of others.  I still need to complete the C101 elimination and verify all sensor grounds.

 

Additionally, disconnecting the fuel pressure regulator vacuum hose has no noticeable effect on idle.  Previous owner replaced the fuel pump with a few months before I got the truck.