Low Exhaust Back Pressure

[staplesofficewar]

In the O2 data you are looking for the upstream O2 sensors to be between .150V and .750V in closed loop operation. They average target is .490V, but they are moving constantly.
They'll peg at 0.824V -0.845V at WOT once the power enrichment kicks in - about 12 seconds with the stock programming.

First chart: Both Upstream O2 sensors - Seeing alot of them above .750. This is from my 38 min drive home. chart shows from 20 min to about 23 min.

and lower

On the fuel side of things you are looking for long term fuel trim (LTFT) values between -5 and +5 in closed loop (partial throttle). The smaller the better. The STFT measure sent to standard ODBII can be weird, but again you're looking for single digits here most of the time in closed loop.

Here is bank 1 and 2. Trim % axis on left, Throttle on right. Seeing some very high long term trims. Same time about 20 mins into drive to 24 min.

It does feel a little rough when I an stopped and idling.

Also attaching raw data with lots of other things

 

[k1n3k]

Back pressure is 100% a bad thing

This, this again, and 100% this. Anyone that thinks backpressure of any sort is good, hasn't a single clue how exhaust systems work. Including on a 2-stroke.
Banks Power agrees.

 

[staplesofficewar]

Back pressure is 100% a bad thing and there is no small amount your 4.0 "likes" or improves low end torque.

Hot oil pressure should be 12-18psi at idle and somewhere around 40-60'ish driving down the road. If you got higher readings than that, it wasn't warmed all the way up yet. Or you are running thicker oil than you should.

You have an LJ with bigger tires and 3.73's? Those 0-60 times seem right to me. Are you sure your speedometer is accurate?

I'm running 5w-30. Mobile 1 High mileage synthetic. I think oil pressure is great.
I have a TJ, not an LJ. the 0-60 time I posted as 3.73 greats and stock 30x9.5 tires. Rear is a Dana 44 with limited slip if that matters.

 

[staplesofficewar]

This, this again, and 100% this. Anyone that thinks backpressure of any sort is good, hasn't a single clue how exhaust systems work. Including on a 2-stroke.
Banks Power agrees.

Looks like I am good in this department! Almost no back pressure. I had read on some forums that the engine "liked" some back pressure - that is only reason i brough it up.

 

[MikeE024]

Looks like I am good in this department! Almost no back pressure. I had read on some forums that the engine "liked" some back pressure - that is only reason i brough it up.

Curious, what are your compression numbers?

 

[staplesofficewar]

Curious, what are your compression numbers?

That sounds like a Saturday chore. Ill get back to you.

 

[staplesofficewar]

Ill prob go ahead and do it since fast - I am curious now if the main cat is missing.

@GoldenGorilla Replying to myself. Spend 5 min trying to go in the tail pipe. That's not happening. Didn't realize the number of turns back there. Ill take the readings at face value.

 

[SkylinesSuck]

Spend 5 min trying to go in the tail pipe. That's not happening.

Yeah, my wife isn't down either, and I have spent a lot more than 5 min trying before.

 

[staplesofficewar]

Yeah, my wife isn't down either, and I have spent a lot more than 5 min trying before.

Hook line and sinker! I knew someone would bite

 

[GoldenGorilla]

First chart: Both Upstream O2 sensors - Seeing alot of them above .750. This is from my 38 min drive home. chart shows from 20 min to about 23 min.
View attachment 665224

and lower
View attachment 665229



Here is bank 1 and 2. Trim % axis on left, Throttle on right. Seeing some very high long term trims. Same time about 20 mins into drive to 24 min.

It does feel a little rough when I an stopped and idling.

View attachment 665231

Also attaching raw data with lots of other things

Positive LTFT but a generally rich reading on the S1 O2 sensor isva lean condition with the system adding fuel to compensate, but maybe just a cylinder or 2 is lean. I would check 3 things.
#1 possible small vacuum leak on the intake. Injector o-rings are definitely a possibility. Smoke test through the brake booster line if possible.
#2 an injector underdelivering on fuel which you might be able to see zoomed in with a high sampling rate.
#3 fuel pressure outside of expected range.

 

[MikeE024]

Positive LTFT but a generally rich reading on the S1 O2 sensor isva lean condition with the system adding fuel to compensate, but maybe just a cylinder or 2 is lean. I would check 3 things.
#1 possible small vacuum leak on the intake. Injector o-rings are definitely a possibility. Smoke test through the brake booster line if possible.
#2 an injector underdelivering on fuel which you might be able to see zoomed in with a high sampling rate.
#3 fuel pressure outside of expected range.

When I pressure smoke checked my exhaust at the tail pipe, I found leaks at the manifold to the block.

When I pressure smoke checked my intake, I found the throttle body pouring smoke out of the butterfly shaft ends.

I agree that OP should check that stuff per GG's suggestions.

 

[staplesofficewar]

Curious, what are your compression numbers?

Just checked - warm engine - after driving 40 mins home. In Order: 160, 156, 160, 170, 165, 165. I forgot to do WOT, but If anything that would skew my numbers lower.

 

[staplesofficewar]

Positive LTFT but a generally rich reading on the S1 O2 sensor isva lean condition with the system adding fuel to compensate, but maybe just a cylinder or 2 is lean. I would check 3 things.
#1 possible small vacuum leak on the intake. Injector o-rings are definitely a possibility. Smoke test through the brake booster line if possible.
#2 an injector underdelivering on fuel which you might be able to see zoomed in with a high sampling rate.
#3 fuel pressure outside of expected range.

Thanks GG.
1)I looked over everything except the injectors. I did a lot of work a few weeks ago on the Emissions control system to get rid of the "minor leak code". I replaced the LDP and double checked both vac lines that go to that system - power booster hose looks new, nice and tight. only other vac line is the small one for cruise, climate dampers, and the line that leads down to the reservoir in the passenger side fender? I also just replaced the intake / exhaust gasket - proper torque and order followed. Ill do another soap and pressure test on the exhaust to check the gasket .... i wonder if I can do the same thing on the intake? blow air with shop vac into the air inlet, and soap up all the joints.... I don't have a smoke machine.

2) I can increase the sample rate to 100ms - would that be high enough?

3) picked up a fuel system pressure checker. Ill do that tomorrow.

I did order all new o2 sensors. I found that the ones on cylinder 1-3 appear to be NTK, but the ones on 4-6 are different wire colors. Looking online indicated those are likely store brand / off brand (they have a blue wire). Those will be in tomorrow.
Looking at the logs, my fuel system status kept going to fault mode 4 - google convinced me that this was because the o2 sensors are sending data outside tolerance and the computer was ignoring them at that moment.

 

[GoldenGorilla]

Yeah I think you can do shop vacuum outlet to intake to presurize the intake. Soapy water is cheap.

I had to crank down my manifold/exhaust bolts quite a bit above torque spec whwn I installed my header to fully resolve some intake leaks, but I think that had to do a bit with the cardboard felpro that came with my short headers and part with it's slightly thicker flange.

Just the upstream sensors come into play with fueling calc and it does sound like the one was ignored.

Look up Chrysler O2 sensor heater to understand why they're so fickle. The system runs 5V at startup through a circuit that normally sends sub 1V.

 

[staplesofficewar]

Yeah I think you can do shop vacuum outlet to intake to presurize the intake. Soapy water is cheap.

My mind was changed - Shop vac allegedly makes too much pressure for the intake side. So poor man’s smoke machine it is. I’ll update later after fuel system test, smoke test, and finally if O2 sensors change anything.

 

[staplesofficewar]

@GoldenGorilla @MikeE024
- Smoke test - only smoke I see is coming our of the shaft of the throttle body. I opened up other vacuum spots to allow smoke to reach deep into the system.
- Fuel pressure test - about 42-43 psi with car off after ignition on. Running and driving down the road stays between 45-47
- Pressurized the exhaust system again with shop vac. VERY small leak between #3 and #4 cylinder. went a little tighter on all manifold bolts and rechecked good.
- Compression check - (yesterday) In Order: 160, 156, 160, 170, 165, 165.
- Changed all O2 sensors. Will drive and record a few days to see where LTFT ends up.

 

[lBasket]

I still kinda think you're chasing a nothingburger but here's a question, what RPM are you shifting up at?

 

[staplesofficewar]

I still kinda think you're chasing a nothingburger but here's a question, what RPM are you shifting up at?

I am starting to lean that way as well. I'm sure the O2 sensors were at least some of the issue since they were not matched. Under normal driving - around 2-2.2. Probably 3.5-4 or so if I'm trying for test - but I really just listen to the engine.

I have ruled out almost everything at this point.

Let me ask, do you have a slight "lope" which sitting at idle? Almost like the body is wobbling back and forth ever so slightly?

Also - looking at the logs - the jeep speed is about 3-4 under GPS speed - I think I trust the GPS speed more, but not sure.

 

[lBasket]

I am starting to lean that way as well. I'm sure the O2 sensors were at least some of the issue since they were not matched. Under normal driving - around 2-2.2. Probably 3.5-4 or so if I'm trying for test - but I really just listen to the engine.

I have ruled out almost everything at this point.

Let me ask, do you have a slight "lope" which sitting at idle? Almost like the body is wobbling back and forth ever so slightly?

Shifting too early if you're actually trying to see how fast it goes IMO. That's like casual no one is behind me shifting points.

I don't think mine wobbles the body but I know mine has a slight misfire so it doesn't sound great (or correct) at idle. Can hear it in the exhaust but it isn't often enough to trip the CEL so I don't bother with it

 

[srimes]

I am starting to lean that way as well. I'm sure the O2 sensors were at least some of the issue since they were not matched. Under normal driving - around 2-2.2. Probably 3.5-4 or so if I'm trying for test - but I really just listen to the engine.

I have ruled out almost everything at this point.

Let me ask, do you have a slight "lope" which sitting at idle? Almost like the body is wobbling back and forth ever so slightly?

Also - looking at the logs - the jeep speed is about 3-4 under GPS speed - I think I trust the GPS speed more, but not sure.

You're shifting at 4k for an acceleration test? Try taking it to redline.