Persistent misfire under load only

[Jimmy Brooks]

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

 

[Jimmy Brooks]

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

Ok, so if I’m interpreting the information correctly I should have a vaccum leak because short term fuel trims drop significantly at 2500 RPM?

 

[jlglr4]

Sorry for the delayed response - missed the alerts on this thread. Yes, I think that points pretty strongly to a vacuum leak. Looks like it drops at both levels of increasing RPMS. Could be MAFs, so I’d still take a look at those numbers as well, but I believe vacuum leak would be prime suspect.

Smoke test is best, but also a few other easy things to try first if you don’t have a smoke tester available (can build one at home based on YouTube vids): (1) use a stethoscope with the cone end to listen around the engine a bit for hissing, especially around that PCV valve you fixed (though tough to hear sometimes with the engine noise). (2) Check the driver side crankcase ventilation tube - it connects to the air intake plenum and I’ve had mine pop off before. (3) Check the purge valve - pop it off the intake connection and blow through it with the engine off. It should be closed when no voltage is supplied to it. I believe it should be closed at idle, and open up only under certain conditions (but I forgot what those are). You can also disconnect and plug the intake connection (but be careful what you use - don’t want to suck something into the engine) to mimic a closed valve and see if that changes the fuel trims.

One thing on the smoke test - the throttle body should be open at least somewhat. I don’t know if this car closes it tight with the key off, but might test key on to make sure it’s at least cracked open. Can also prop it manually (definitely key off if you do this), but some people think its bad to mess with the butterfly valve on an electric throttle body - I’m not sure myself if that’s true or not. I think this was debated on these forums someplace not long ago, but can’t remember the conclusion.

 

[Jimmy Brooks]

Sorry for the delayed response - missed the alerts on this thread. Yes, I think that points pretty strongly to a vacuum leak. Looks like it drops at both levels of increasing RPMS. Could be MAFs, so I’d still take a look at those numbers as well, but I believe vacuum leak would be prime suspect.

Smoke test is best, but also a few other easy things to try first if you don’t have a smoke tester available (can build one at home based on YouTube vids): (1) use a stethoscope with the cone end to listen around the engine a bit for hissing, especially around that PCV valve you fixed (though tough to hear sometimes with the engine noise). (2) Check the driver side crankcase ventilation tube - it connects to the air intake plenum and I’ve had mine pop off before. (3) Check the purge valve - pop it off the intake connection and blow through it with the engine off. It should be closed when no voltage is supplied to it. I believe it should be closed at idle, and open up only under certain conditions (but I forgot what those are). You can also disconnect and plug the intake connection (but be careful what you use - don’t want to suck something into the engine) to mimic a closed valve and see if that changes the fuel trims.

One thing on the smoke test - the throttle body should be open at least somewhat. I don’t know if this car closes it tight with the key off, but might test key on to make sure it’s at least cracked open. Can also prop it manually (definitely key off if you do this), but some people think its bad to mess with the butterfly valve on an electric throttle body - I’m not sure myself if that’s true or not. I think this was debated on these forums someplace not long ago, but can’t remember the conclusion.

Vacuum leaks are better than fuel issues! I assume the purge valve is working because every time I worked on cylinder 4 (plugs coil and injector) I would undo the fuel thank vaccuum lines and they would suck in a ton of air from being under vacuum so I assume that there is no leaks in that system. I’ll check the purge valve any way.

I was listening around the engine a couple of days ago and I definitely heard hissing from the passenger side of the engine but couldn’t pin point it. The PCV valve was done less than 5k miles ago. The issue was there before and after that job so I am going to rule that out. Still weird to me that the issue was more noticeable when I fixed that PCV issue.

I’ll give the engine a smoke test, I’m pretty sure when you turn the ignition and push the gas the throttle body should actuate so that should work for the smoke test.

Anyone have a diagram of the vacuum system on these cars? I know this is a discovery 5 but even the LR4 version will work fine, I’m sure most of the lines are in the same places since it’s the same motor.

Side note, my tool won’t read the MAF’s voltages. Is there a way to test them other than reading voltages?

Thanks for the reply, excited to get this issue finally sorted

 

[RossN1999]

Hey all,

Getting a lot of similar symptoms with a D5 which has the SCV6. The misfire only happens under load and is mostly noticeable when the car is cold (oil is below 180) when first starting to drive. The misfire only happens around 2500-4500 RPM when under load in any gear from 4th to 6th gear. You can feel it in the form of jerking which gets faster but less present as rpms rise. Each misfire j€rk (it will sensor the normal spelling) you feel you can see it in the rpms tack, the tack will move down about 50-100 rpm each time. I got a code for random misfire and then cylinder 3 misfire a while back but I cleared it when I got an oxygen sensor code and replaced the sensor. The misfire is still there but I haven’t been able to get a code to come back. I replaced the injector spark plug and coil on what the scv6 diagram labels as cylinder 3. It is still there, I am currently in the process of doing the same thing to cylinder 2 (would be cylinder 3 on the V8). But I am not confident that this will fix it.

Would love to hear some thoughts and share more information if anyone could make sense of this. Let me know if anyone has any ideas, I’d like to avoid spending thousands on diagnosis and fixes at an indi shop.

Going through the exact same thing on my 2014 LR4 with the SCV6. Idles super smooth and runs great other than the random misfire/judder it gets around 2500-4000 in really any gear over 4th. The issue is mainly noticeable when accelerating on the interstate but If driven gingerly the issue does not occur. The only other symptom I’ve noticed is occasionally when the engine is warm warm it might bog down slightly in reverse.

I bought the car needing timing work and have done everything associated with that and then to address this issue I’ve done o2 sensors, PCV valve, new plugs, and even serviced the transmission. The only code I ever get is p0430: cat below efficiency on bank 2. Not sure whether or not to go ahead and replace the cat and see if that helps or look else where.

Has anything worked for you?

 

[Jimmy Brooks]

so my IID tool will unfortunately not read the voltages of the MAF sensors. What is the best way that I can check and ensure that they are working properly?

 

[Jimmy Brooks]

so I bought a mini smoke machine off of Amazon and taped up each side of the intake duct and Ran the smoke machine in both PCV lines, in the evap/brake booster lines, and finally let it run through the inlet where the purge valve line connects to. I could not see any smoke coming out of anywhere. When I pulled the smoke machine out a bunch of air would release and smoke poured out so I know it was under pressure. So unless I’m doing something wrong, it looks like I don’t have a vacuum leak.

Any idea how I should read the voltages on my MAFs if I can’t use the IID tool for it?

 

[Jimmy Brooks]

Haven’t gotten much feedback, I’m just going to buy new MAFs for both and return them if it doesn’t fix the issue.

 

[Jimmy Brooks]

Going through the exact same thing on my 2014 LR4 with the SCV6. Idles super smooth and runs great other than the random misfire/judder it gets around 2500-4000 in really any gear over 4th. The issue is mainly noticeable when accelerating on the interstate but If driven gingerly the issue does not occur. The only other symptom I’ve noticed is occasionally when the engine is warm warm it might bog down slightly in reverse.

I bought the car needing timing work and have done everything associated with that and then to address this issue I’ve done o2 sensors, PCV valve, new plugs, and even serviced the transmission. The only code I ever get is p0430: cat below efficiency on bank 2. Not sure whether or not to go ahead and replace the cat and see if that helps or look else where.

Has anything worked for you?

Didn’t notice it till now but it’s starting to run rough at idle, or like you said the bogging down feeling.

 

[jlglr4]

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)?