Persistent misfire under load only

[Jimmy Brooks]

Sorry for no feedback, Jimmy. Been out of town. I know I've checked my mafs using the tool, but I don't recall what the fields were anymore - maybe just airflow, which is fine if you can compare the two sides to see if they are about the same. Unusual that both would go bad at once, though both could be dirty if you've been offloading, driving around the forest fire smoke, etc.

Can't remember what else has been covered already. O2 sensors, purge valve, fuel pressure regulator (fuel rail)?

Sorry, just trying to move on this issue, seems to be getting worse daily. I tested the purge valve and it is closed when the car is off. Fuel pressure seemed normal, is there any specific PSI I should be looking for? When I was doing harder pulls it ranged from 2650-2800 psi.

Is there any specific live value I should be pulling up for the O2 sensors? I had bank 2 O2 sensor 3 (furthest one back in drivers side) go bad. Replaced it and the CEL went away. Problem remained and behavior never changed.

On a side note I noticed that when I smoked tested the intake there was no smoke making its way into the crank case, when I opened the oil cap there was no smoke coming out. Not sure if that is normal for these cars.

Haven’t been off-roading in a long time in this car, I can check the air filters and try to clean them. I purchased the sensors just to try to rule it out.

What are the symptoms of a bad low pressure, thinking it could be a part of the issue but it’s hard to say.

 

[jlglr4]

Weak fuel pumps should give you more problems as RPMs/load increases - I think pretty much the same symptoms whether it's the high or low pressure pump going bad. It would create a lean condition that gets worse as fuel demand increases.

That's what your initial symptoms seemed to point towards because you were misfiring under high load as I recall. But the fuel trims suggested the opposite - lean condition that got better under load.

For fuel pressure, I usually just compare desired rail pressure to actual rail pressure and make sure they track pretty close at idle and under load. I think the low pressure pump should put out at least about 72 psi or something like that - there is a separate reading for the low pressure side. It will rise up to 85 or so under load I believe.

O2 sensors - I think I generally check the lambda values. They should fluctuate in kind of a smooth wave pattern as I recall.

MAFs - I run the airflow numbers on both banks and compare them. You can also calculate what your airflow should be at a given RPM, but I figure if one MAF is bad, it's going to show a difference compared to the other.

I believe no smoke in the crankcase is normal. The PCV is closed with the engine off.

 

[Jimmy Brooks]

Weak fuel pumps should give you more problems as RPMs/load increases - I think pretty much the same symptoms whether it's the high or low pressure pump going bad. It would create a lean condition that gets worse as fuel demand increases.

That's what your initial symptoms seemed to point towards because you were misfiring under high load as I recall. But the fuel trims suggested the opposite - lean condition that got better under load.

For fuel pressure, I usually just compare desired rail pressure to actual rail pressure and make sure they track pretty close at idle and under load. I think the low pressure pump should put out at least about 72 psi or something like that - there is a separate reading for the low pressure side. It will rise up to 85 or so under load I believe.

O2 sensors - I think I generally check the lambda values. They should fluctuate in kind of a smooth wave pattern as I recall.

MAFs - I run the airflow numbers on both banks and compare them. You can also calculate what your airflow should be at a given RPM, but I figure if one MAF is bad, it's going to show a difference compared to the other.

I believe no smoke in the crankcase is normal. The PCV is closed with the engine off.

I believe I only have access to O2 sensor #2 on both banks. Should I take a look at the values while the car is warm? Also I looked at the maf values and both of them are in range of each other.

I was on the freeway today driving back home, needed to pass someone so I put my foot down and there was no power, check engine started flashing but went away after I got off the gas. However the car had maybe half of its power for the rest of the trip and when I came to a stop it was idling super rough. Seemed like the engine shut down a couple of cylinders. One thing to note was that the bucking/misfire feeling was gone while I put my foot down a bit with only half power. I turned the car off and back on and it was fine, running normal again. No codes stored, pretty annoying. I looked at live values and it stored misfires for cylinders 4 and 5. I haven’t touched cylinder 5 yet but it seems like the misfire is more random than a specific cylinder.

Also at idle the car will feel a misfire accompanied by a little bump in rpm and when your listening to the engine when it happens it knocks as if the fuel isn’t igniting at the correct time.

 

[scapistron]

If you are getting a misfire on any cylinder, it could be any fuel injector on the whole bank. I had a stuck open injector on 4, driver's side front that would set off a cascade of misfires as the computer tried to adjust from the rich condition this caused. The feedback loop was something along the lines of O2 sensor reads rich, pull fuel, then other cylinders in the bank misfire lean. Eventually the computer would shut down that entire bank to preserve the catalytic converter.

 

[greiswig]

I don’t recall…did you check fuel filter?

 

[Jimmy Brooks]

I don’t recall…did you check fuel filter?

i did not, im guessing it is in the low pressure pump?

If you are getting a misfire on any cylinder, it could be any fuel injector on the whole bank. I had a stuck open injector on 4, driver's side front that would set off a cascade of misfires as the computer tried to adjust from the rich condition this caused. The feedback loop was something along the lines of O2 sensor reads rich, pull fuel, then other cylinders in the bank misfire lean. Eventually the computer would shut down that entire bank to preserve the catalytic converter.

It originally called out an misfire on cylinder 4, replaced everything on cylinder 4 and now it’s saying cylinder 5 and 5. Should I replace cylinder 5? I have all the working spare parts to do it.

 

[Jimmy Brooks]

If you are getting a misfire on any cylinder, it could be any fuel injector on the whole bank. I had a stuck open injector on 4, driver's side front that would set off a cascade of misfires as the computer tried to adjust from the rich condition this caused. The feedback loop was something along the lines of O2 sensor reads rich, pull fuel, then other cylinders in the bank misfire lean. Eventually the computer would shut down that entire bank to preserve the catalytic converter.

Would it make the whole engine read rich or would it make just one bank read rich

Here are the fuel trims. I took 2 screen shots at 1500 ish rpms because the rpms like to fluctuate at that rev range.

These are the fuel trims I pulled

 

[Land Rover Joe]

i did not, im guessing it is in the low pressure pump?


It originally called out an misfire on cylinder 4, replaced everything on cylinder 4 and now it’s saying cylinder 5 and 5. Should I replace cylinder 5? I have all the working spare parts to do it.

i did not, im guessing it is in the low pressure pump?


It originally called out an misfire on cylinder 4, replaced everything on cylinder 4 and now it’s saying cylinder 5 and 5. Should I replace cylinder 5? I have all the working spare parts to do it.

I replaced both the High Pressure and Low Pressure Fuel pumps with my recent build.

This does seem like it could be an issue with the fuel supply and the symptoms are identical to work trucks I drive (land cruisers) which, due to very poor fuel, get the fuel strainer/filter in the fuel tank clogged. That causes a host of issues like this.

Fortunately, in that case, the filters just need to be replaced which requires "only" dropping the tank and doing a service.

Meanwhile, I am working through my own low pressure fuel pump and fuel tank issues.... which you can check out at the thread:

Fuel Gauge Reading Off

Tribe, An update on this job from my side. So I went ahead and ordered “Fuel Sender and Hanger Assembly” - Part Number: LR042974 (Supersession(s): LR015937) from the Team at Land Rover Palm Beach. They were super helpful and did a great job boxing everything up and sending it out. Photos...

www.landroverworld.org

The short of it is that I am replacing the pump and sending unit as well as both fuel level sensors / float assemblies.

Hopefully that will resolve the current issue - which in my case is now the fuel gauge reading zero.

Good luck!

 

[jlglr4]

It's a mystery. Fuel trims really look like a vacuum leak, but the failure under throttle really looks like fuel delivery issues (or spark issues). You also have fuel trim issues on both banks, but a misfire issue that appears to be only on one bank. Maybe there are two different problems happening at once.

Very strange that there is no code after having the check engine light and restricted performance set. If you have another scanner, even a generic one, I'd try it to make sure the Gap tool is not malfunctioning. Or maybe have autozone or oriellys scan it for codes.

I would run the fuel trims again - make sure they really are cleaning up at high RPMs. Maybe also record fuel trims under load, but cruising, not under acceleration. When you accelerate, the car will go back into open loop, but if you are cruising with steady throttle at highway speed, you will be under load and should be in closed loop - at least that's my understanding. If they are worse under load, then it really does point to fuel pumps.

Looking back at your smoke testing post - did you have the smoke test running key on or key off? If the throttle body was closed up, you might not have tested for leaks between the MAFs and the throttle body (I think that purge valve connection is on the engine side of the butterfly valve, thought I'm not sure).

Still other things to consider: Maybe the fuel pressure regulator - we've seen reports of these failing in this forum. Fuel pressure driver module is another point of failure. MAP/MAPT sensors might be another issue (actually two sensors on this car I believe), but I don't know if there is a way to check that on the GAP tool. Electric throttle body...not sure how this would affect fuel trims or how to test exactly, though I think you can get some parameters from the GAP tool.

To answer your earlier question on o2 sensors, car needs to be warm so it's in closed loop.

Misfires on one bank should only affect that bank - fuel control is bank by bank, so it can certainly affect any cylinder on that bank, but it would not explain the fuel trims on both banks.

 

[greiswig]

Very strange that there is no code after having the check engine light and restricted performance set. If you have another scanner, even a generic one, I'd try it to make sure the Gap tool is not malfunctioning. Or maybe have autozone or oriellys scan it for codes.

Mine wasn't throwing codes consistently, either, after restricting performance from the coil failure/misfire issue. Not sure why. CEL light would come on, and the computer seemed to just shut that cylinder down entirely once a misfire on it was detected, but it would only occasionally throw codes.