Hi guys. I have a an 04 FX35 in the shop with a constant misfire. I have a P0300 but can't distinguish which cylinder(s) are misfiring using the power balance test. I took some waveforms with a pressure pulse sensor and was hoping you guys can take a look and help me figure out which cylinder is misfiring and if I might be looking at a valve sealing problem. Ignition coils, spark, and injectors already tested good. I took running exhaust, running intake, and cranking intake. Any feedback will be much appreciated.
Compression test to help isolate affected cylinder
Check for a stretched chain or belt.Perform a leakdown test as well.
Can you shut off individual injectors to help figure out the problem cylinder?
Alex- How did you test the coils, spark and injectors?
This is the capture with the intake pulses while cranking and there is 1 cylinder with a repeating deep vacuum pull.
Thanks Ray. Can you shed a bit more light on what the deep pull means in relation to the other cylinders? My RC test showed #4 being lower than the others. If the deep pull is happening during #1 power stroke, that would put #4 on it's intake stroke. The firing order is 123456.
Measure the compression and do a cylinder leakage test in cylinder 4. Put rubber gloves over the tailpipes and a rubber glove over the throttle body.
Here's my cranking compression test on cyl 4. Compression looks low and I don't see an exhaust valve event.
Alex I don't see the cranking compression test on cyl 4. Was that an in-cylinder pressure waveform test? Based on the information provided I was leaning towards a intake valve problem on cylinder 4. I looked at the suspect cylinder 4 and compared what it was doing during each of the intake pulse events and how a open intake would effect each. I am still learning so I might have missed something…
Sorry I guess I didn't attach it. Here it is. And a running compression test from the same cylinder. If anyone can point out what I might be missing in these waveforms, that'd be really helpful. Also, about the no EVO event, it didn't show up on the cranking compression test but it did on the running. The known good cylinder I did the cranking test on also did not show a distinct EVO event.
Thanks for doing that analysis with the overlay. It does make sense. However, when doing the leak down test, it blew the rubber gloves off the tailpipes, no action at glove on the throttle body.
These are famous for pulling the head bolt threads and letting the head gaskets leak into the cylinders causing misfires. Make sure and do a coolant/exhaust leakage test.
Walter- I cant say Ive ever heard that before. And Ive owned 3 of these(Current is a 2015 QX70) Is this something you've seen a lot?
Several. Timesrt has the special helicoil kits for these and often the first helicoil will not hold and they have a double oversize one. Not fun, but they are aluminum blocks.
Thanks to everyone who replied. I'm trying to get better at misfire diagnostics using the pressure pulse sensor which was the main purpose of this post. Aside from seeing low compression with the pressure transducer, I didn't gain any valuable information from that waveform on the cranking compression test. I'll do the test again with the engine running and see if anything is different. I…
Thanks for sharing this one. Do you have a borescope you could feed in the cylinder to take a peak at the valves/piston tops?
I don't right now. I sold the one I had with intentions to buy an ablescope 980.
A little information, when using a pressure transducer for cranking, I found the first look sensor very accurate over the Pico version, crank engine , save data, set your rulers on right green dots, both On # 1 one ignition signals , click right lower ruler , it should say 360, change it to 720, go to bottom of screen and add 4,6 or 8 (depends on how many cylinders) Gets tricky now, 4&8…
I’ve only used this method once myself but you can use a lab scope and the crank signal along with a trigger to identify a misfiring cylinder if you have no counters. Super Mario Diagnostics has a video on the set up and use on youtube.
Upon further analysis here's what I gathered comparing the running compression tests. The cylinder with the leaking exhaust valve appears to go into a slightly deeper vacuum after the exhaust valve closes (-8.5 psi). It also doesn't have as much vacuum before the EVO event (-7.5 psi). On the known good, I see that after the exhaust valve closes, the vacuum level is the same as before the EVO…
I made these GIfs to compare the known good with the misfiring cylinder.
I had to play with the scope settings to try and measure when the exhaust pressure change direction- exhaust valve opened and it looks approximately 24 degrees btdc. What caught my eye was the amount of time the expansion stroke stayed low before changing direction. My first thought was a flat cam lobe. Also, looking at your exhaust pulses it looks like one bank has an issue compared to the…
I did notice that the pocket at the bottom of the expansion stroke looked different on the misfiring cylinder vs the "good" cylinder. Also, I had the pulse sensor in only 1 tailpipe so I don't know if that's okay or not. And for the intake pulse waveform, I probably could have obtained a better waveform had I isolated the manifold from the crankcase and kept the throttle plate closed.
The pulse sensor is ok in only 1 tailpipe. The test should be done while cranking to help find mechanical valve/cylinder problems and i like to stuff sponges in both tailpipes to get a cleaner signal.
It's my understanding and experience that intake waveforms are probably the hardest to decipher given all the variables- sample point at/on manifold, manifold design, length of tubing to transducer, the more cylinders/overlap. What rpm are your running compression shots taken at? For some reason I'm not getting an rpm auto calculation in demo mode when I measure time between events. Back…
The known good GIF shows the running compression at 1364 rpm and one exhaust pulse measures 1.6 psi. The misfiring cylinder is running at 1203 rpm and one exhaust pulse measures 1.4 psi.
When cranking the engine yes throttle closed , when the piston comes up on compression the pulse will increase with sensor in intake if there is an intake valve issue, if there is an exhaust valve leak the pulse will be high with sensor in the exhaust , on the affected cylinder, due to compression pressure leaking past valves