2008 EX35 Misfire in Cyl2 High fuel trim in bank 1

[paulstaf]

I have a 2008 EX35 that misses at idle but seems to run fine otherwise. It was giving a multiple misfire code and I replaced the coils with Hitachi the spark plugs and that cleared the massive misfire issues. I saw the fuel trims were high so I replaced the injectors with a reman OEM set off Ebay.

I have an Xtool D7, and it is still showing occasional misfires in cyl2, and here is what the current fuel trims look like:

This seems to only be a problem around idle. The idle will drop out once in a while and the cyl2 misfire will tick up. I also see this code once in a while:

I have been chasing this for months now. Does anyone have any ideas or things I can check?

Thanks in advance,
Paul

[VStar650CL]

See if the Bank1 STFT stabilizes lower when you raise the RPM's to 2000 or so. If it does, then it usually means a vacuum leak that's only affecting one cylinder or one bank, typically a plenum or manifold gasket issue.

[paulstaf]

It didn't seem to make a difference.

Here is a link to the video recording of the test.

https://drive.google.com/file/d/1cRbv9k ... drive_link

Paul

[paulstaf]

Oh, and I replaced the upper intake and throttle body gaskets when I replaced the fuel injectors.

Thanks,
Paul

[VStar650CL]

Since it didn't change much with RPM, my second suspect would be your Bank1 A/F sensor (front O2). Flatlined A/F's are easy to diagnose but inaccurate ones aren't. Because they don't oscillate like an HO2S and work off a centering voltage (2.2V in your case), there aren't many easy ways to tell if the centering voltage is "off" for some reason, either wrong resistance in the sensor or resistance in the wiring. You can scan it and see if it seems to respond normally, and make sure the rear sensor (which is HO2S) is oscillating normally at a cruise. If it is then I think I'd try replacing the A/F even if it seems normal, if it isn't then try a rear sensor first. The ECM doesn't use the rear sensor directly for engine operation, but it does use it indirectly as a check on the front sensor, so if the rear "flatlines" but isn't dead, it can cause some weird behaviors from the ECM software.

[paulstaf]

Thanks for the info. I did replace the rear O2 sensor on bank 1. I used an NTK part, hopefully a good brand.

Here is a pic of the air fuel ratio (A/F) Alpha:

Since it is a V, I could do "swaptronics" on the A/F sensors to see if the lean moves to bank 1. I assume they are interchangeable.

Bank 2 seems to be running perfect, sans the occasional misfire in cylinder 2, but I am waiting to clear up bank 1 to see if that helps the miss on 2 any.

Thanks for your help.
Paul

[VStar650CL]

Yep, swapping should tell you assuming the connectors are the same. Double-check that before you wrench anything, they're usually identical but not always.

[paulstaf]

Oh what a night. Broke the "good?" A/F sensor in B2 trying to take it off. Was able to remove the A/F sensor in B1 without issue. Got new Denso A/F sensor. Installed NEW sensor in B1, and Old B1 sensor in B2. B2 went up to 4.7 and 7.0, and B1 STFT went UP 7 points-ish while LTFT stayed about the same at 9.4.

Swapped the sensors... Put the NEW A/F sensor in B2, and the OLD/Original B1 sensor back in B1. Now B2 goes back to NORMAL, and B1 is back down to the 14 / 9.4 range.

Here is what I am not understanding:

Original B1 sensor in B2, B2 goes from normal to +7. This would suggest a bad B1 A/F sensor. BUT, when the NEW sensor is in B1, it goes even higher than with the "BAD" sensor? I thought the NEW sensor could be bad, but when it is installed in B2, B2 goes back to NORMAL, so that would suggest the new sensor is good?

It seems the original "bad" B1 sensor is showing +7 when moved to B2, so it is most likely bad? I don't understand why the new sensor in B1 didn't pull it down by 7 or something.

Am I reading this wrong?

I am wiped out for the evening. Thanks for all your help, it looks like we may be onto something with the A/F sensor. I think I may source a new one for B1 tomorrow.

Paul

[VStar650CL]

Check the wiring for the B1 sensor carefully, you may have a bad pin-fit or high resistance someplace in the circuit. It's also possible the ECM has an issue. If you have resistors laying around, put a 1K across pins 1~2 (Blue and Red on Bank1) with the sensor disconnected and measure drop across the resistor with the key on. If B1 is significantly different from B2, measure both of the pin 1's to ground without the resistor. If it's still different then there's a problem with the bias hardware in the ECM, if they're the same then the problem is resistance in the wiring.

[paulstaf]

Sorry for the break in communication, I really appreciate your help, but I was getting burnt out and my girlfriend needed her car back.

I thought it would be a good opportunity to re-group and make a note of where I am at right now:
------------------- Current parts configuration -----------------------------------
Both original O2 sensors, (Not A/F sensors) are installed in the car.
Both A/F sensors have been replaced with Denso 234-9036
New Bank 1 intake boot
New PCV valve and hose from PCV to intake
"Remanufactured" Denso fuel injectors installed (Got off Ebay)
New Hitachi coils installed on cylinders 2, 4, 5, 6
New Delphi coils installed on cylinders 1, 3
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Results from last test:
Here is what the scantool is showing with the new A/F sensors installed:

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With the resistor installed in B1, the voltage across the resistor is .472v and the voltage to ground is 2.307v.
Here is what the scantool shows during this test:

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With the resistor installed in B2, the voltage across the resistor is .474v and the voltage to ground is 2.376v.
Here is what the scantool shows during this test:

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So all back together, this is what I am now seeing on the scantool for fuel trims:

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I am still kind of concerned about the cylinder 2 misfire. I have replaced plugs a few months back, and all the coils about a month ago yet I still seem to see misfires on cyl 2. I swapped the plug with cylinder 1 and the misfires didn't seem to go away.

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The water pump was replaced in 2019 and I don't know if the car was running rough after that. Here is a movie showing the valve timings and I do see B1 moving a little, but not sure if that is serious or not:

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Thanks again for all your help,
Paul

[VStar650CL]

It's only bouncing half a degree, that's pretty normal when the IVT's are dormant at idle and low RPM. Anything +/- 1 degree is no issue. I'm still concerned that the high compression readings may mean you have a carbon problem. Have you decarboned it?

[paulstaf]

Not sure what you mean by high compression readings. I haven't done a compression test on the engine. I did look down the intake when I was doing the fuel injectors and the valves looked clean enough. I did run a camera down cylinder two and the piston and valves looked clean, see attached pics. I haven't run a camera down any of B2 cylinders because I was more concerned with the misfires in cyl 2.

Thanks,
Paul

[VStar650CL]

My bad, I got you mixed up with another misfire thread. Sorry!

Maybe it's just the light, but those actually look kinda too clean. Have you checked for combustion byproducts in the coolant?

[paulstaf]

It has driven several thousand miles with this miss, and hasn't had any noticeable coolant use. I have not touched the coolant and here is what it looks like in the neck and the reservoir:

Thanks,
Paul

[paulstaf]

Next time I get it in my garage, I'll put a camera down all the cylinders and shoot some pics.

The lean is in B1, but the miss is in CYL2. I have also noticed the every-so-occasional miss in 4 and 6, but thought it may be a blip because of the CYL2 miss.

The car did overheat one time, hence the water pump change, but I don't remember this miss being a problem after the water pump change. Should I do a compression check on the cylinders?

Thanks,
Paul

[VStar650CL]

A compression check is probably worth it to see if maybe there's a valve issue, but the problem probably isn't a head gasket. The VQ's have siamese sleeves, so it is possible to have a leak between cylinders that doesn't reach the jacket or oil gallery, but that should be giving you a miss on 2-4 and not just 2.