M56 Engine Knock? Timing Retards during sub 2000 rpm cruise

[Shanehsmp]

What would make my engine timing retard all the way down to -7° then back up to like 30-40° then rinse and repeat during a 4th gear, approx 1500~2000 rpm cruise? The up and down would happen every 5 to 10 seconds or so..... I've included a picture of my an

Long Story:
I recently changed my upstream O2 sensors because I noticed they may not be reporting correctly. All other sensors that can be logged via my cheaper android scan-tool have been checked and they all look normal. I thought this fixed the problem during an idle only log, but it came back during a subsequent drive.
So today, I was checking my camshaft sensors with my scan tool to see if they were reading correctly. I noticed that the Exhaust Camshaft sensors were not reporting the same values throughout the RPM range and at idle, Bank 1 sensor was up and down from -0.5 to 0° whereas Bank 2 sensor was steady at 0°. I even switched the the intake sensors to exhaust to see if that would make a difference and there was no change in either intake or exhaust timing readings. So I moved over to my ignition timing and started driving around, it looked okay, my base timing is retarded by about 3° from what it should be at idle.

I've made a few changes in the past few mths (long tube intake, larger exhaust and gutted primary cats) but the issue has been around prior to these. However it's more noticeable since I now have an audible exhaust note and I can hear something changing in the exhaust as the engine retards down to -7°. It feels as though the engine suddenly gets thrown some huge load at it or as if the accelerator pedal stops working for a brief moment, then starts working again. I can't explain it for find anyone else with a similar story....

Now, there's no codes other than a P0300 that came a couple weeks ago, but hasn't come back. I cleaned and tested the bank 1 vvt solenoid - it was spotless and very clean. I wasn't able to get the bank 2 side out because the alternator is in the way, so I left it as is figuring if one was fine, the other would be too.

Short Story:
Car is modified but not yet tuned. Problem has persisted before any mods.
No Codes
Cars timing retards itself in lower gears, then goes back to normal - then repeats it all again until I get into a higher RPM or Gear (6th or higher)
The noticeable Timing retard doesn't happen in 5th at around 45+ mph, but will happen if I am in the 4th/5th rpm range and under 45mph or so
Camshaft Sensors seem normal
I've done all the relearns and resets you can think of.

Any thoughts? Before I dig into the car to test the knock sensors - I had already brought this to the dealer and they thought it was normal but updated my ECM and TCM anyway $$. This ain't normal at all.

[Shanehsmp]

Took the car on two other drives after this post, one a few hours after - and one at night. The car was doing the bogging in 6th gear as well (over 50mph), I've never really noticed it, or maybe it's just getting worse. I'm starting to wonder if this has to do with heat soak? Could my IAT's cause timing to retard and jump back and forth? I say this because it didn't do this at the beginning of a drive home after it sat for 4 hours and cooled off.

When I got home, Amazon delivered my USB-OBD2 cable so I can run some better diagnostics with Nissan Data Scan 3. I checked last night briefly and was able to get it to connect to the ECM and read live values etc. It also has an option to deactivate each cylinder to discover if the problem is located at a specific cylinder. Hopefully this will help me narrow this down further.

[VStar650CL]

Not sure why it would be affecting the ignition timing, but the fact that it doesn't misbehave cold means it's probably related to closed-loop (warm) versus open-loop (cold) operation. During open loop, the ECM pretty much ignores the A/F's and O2's and riches the mixture to prevent cold stalling.

[Shanehsmp]

Not sure why it would be affecting the ignition timing, but the fact that it doesn't misbehave cold means it's probably related to closed-loop (warm) versus open-loop (cold) operation. During open loop, the ECM pretty much ignores the A/F's and O2's and riches the mixture to prevent cold stalling.

So I've been scouring the internet...found a thread which lead me to check my ignition timing against my vs AFR - whenever my AFR spikes (very high levels ~ 26+) my timing is pulled which makes sense, so now this is leading me back to it being a fueling issue - does that make sense to you?

[VStar650CL]

That looks like a smoking gun to me. Your A/F shouldn't be topping out like that. Hard to say without a voltage scale, but it looks like it's over-responding when the ECM tries to do normal closed-loop oscillation. You might simply have a shot sensor on one of the banks.

[Shanehsmp]

That looks like a smoking gun to me. Your A/F shouldn't be topping out like that. Hard to say without a voltage scale, but it looks like it's over-responding when the ECM tries to do normal closed-loop oscillation. You might simply have a shot sensor on one of the banks.

V Star, my upstream O2's were just changed and the car is staying in closed loop based on my scanner.
I hooked the car up to a better scanner and got some logs of my exhaust valves/intake cams/accelerator pedal & Throttle position - during my drive I made it a point to keep my foot steady on the gas pedal and noted the time on my graph. The findings show that the ECM is cutting my throttle and brining my exhaust valve timing down to around 0°. This would explain the massive and loud change in exhaust note as the car tries to slow itself down. I also noticed that this only happened when the RPM was below 2000 rpm like I had originally stated.

Did some more digging, trying to find the sensor that has a direct correlation to these other sensors and it looks to be the crankshaft position sensor. Does any of this sound plausible?

[VStar650CL]

Yah, a CKP that's missing occasional teeth can cause all sorts of weird mayhem without throwing codes. Certainly worth a shot.

[ken in az]

Only thing I can think of is that the ECU is trying to light off the primary cats by retarding the ignition timing. Sounds like it's doing it on purpose for emissions purposes. How to make it stop doing that - I dunno?? I know my car's cold start sounds a lot more aggressive for the first 15seconds or so, then it goes ack t normal. Not sure if that's the same thing but I have heard that it sounds like that to light off the cats for emissions.

Maybe contact a reputable tuner for the G37/370Z crowd since they use VVEL like us.

[VStar650CL]

Only thing I can think of is that the ECU is trying to light off the primary cats by retarding the ignition timing. Sounds like it's doing it on purpose for emissions purposes. How to make it stop doing that - I dunno?? I know my car's cold start sounds a lot more aggressive for the first 15seconds or so, then it goes ack t normal. Not sure if that's the same thing but I have heard that it sounds like that to light off the cats for emissions.

Maybe contact a reputable tuner for the G37/370Z crowd since they use VVEL like us.

The ECM is supposed to rich-and-lean the mixture cyclically for the cat, that's normal. But they're usually little bumps and they don't make the timing go nuts. You sure the new sensors are the right ones?

[Shanehsmp]

Yea, they're the correct ones. I made sure to verify on a few different sites. Either way, this has been happening prior to the older sensors.. which turns out were good anyway.

[VStar650CL]

Well, the reason the ECM does that is to make the rear O2's react so it can monitor cat function. The rear O2's have no other job -- we have a saying that the fronts are for the engine and the rears are for the government. But like I said, it should go a little rich and a little lean. I've never seen timing fly all over the place while watching it dynamically, so something has to be making the ECM overreact to what should be normal injection cycling.

[Shanehsmp]

Well, the reason the ECM does that is to make the rear O2's react so it can monitor cat function. The rear O2's have no other job -- we have a saying that the fronts are for the engine and the rears are for the government. But like I said, it should go a little rich and a little lean. I've never seen timing fly all over the place while watching it dynamically, so something has to be making the ECM overreact to what should be normal injection cycling.

Yea, this is what is driving me nuts, it seems very much like an ECM issue or at the very least a sensor/circuitry issue. I thought it was the ECM braking feature, but the dealer says it is normal. It definitely feels like its the engine braking feature, but in overdrive.

I am also not sure if this is related, but my RPMS hang at around the same RPM this issue is mostly present at....
I posted this in the FB group and a few others said theirs does the same, one person said this was by design to burn off extra fuel.

[VStar650CL]

Hmm, you know, this just could be a defective knock sensor, or maybe there's enough carbon in your engine to be knocking whenever the ECM tries to lean the mixture. Now that I think about it, it occurs to me that knock response in most ECM's is instantaneous. Timing gets dialed back at the first hint. I know the knocks are impossible to reach on a VK without major disassembly, but if they're on the usual sub-harnesses, maybe you can unpin them at the sub-harness connectors. It'll throw open circuit codes for the knocks, but if the timing quits jumping with them disconnected, you found the culprit.

[VStar650CL]

Yah, that hang happens with a lot of N/I engines, not just VK's. It's just the ECM making sure everything is copacetic before completely closing the throttle plate. The timing wacking out, though... that ain't normal.

[Shanehsmp]

Hmm, you know, this just could be a defective knock sensor, or maybe there's enough carbon in your engine to be knocking whenever the ECM tries to lean the mixture. Now that I think about it, it occurs to me that knock response in most ECM's is instantaneous. Timing gets dialed back at the first hint. I know the knocks are impossible to reach on a VK without major disassembly, but if they're on the usual sub-harnesses, maybe you can unpin them at the sub-harness connectors. It'll throw open circuit codes for the knocks, but if the timing quits jumping with them disconnected, you found the culprit.

Yea I noticed they are way down in the valley. This is a real good suggestion tho, I'll try to dig into it today if I can get the time.

[Shanehsmp]

So I took my manifold off, pulled my plugs and checked the resistance on the knock sensors. Everything seems to be correct, knock sensors were within the correct spec.

So I put everything back in and started the car (I forgot to plug in the passenger side PCV line) - car started rough and immediately threw some codes(plus a P0300):


I plugged the PCV line back in, reset the codes and took the car on a drive. It's still pulling timing when I let off the gas, which leads me to believe that MAYBE the accelerator pedal or throttle position sensors ( or wiring) might be at fault here. I went back into my logs dating to January and my TPS voltage is all over 5V which is the high limit.

Not really sure which way to go from here....

I've read in another thread that this is not an abnormal issue with these cars. This could possibly explain the RPMs hanging as well. I'm tired of firing the parts cannon, but kind of relieved if this is actually the issue I'm facing.

[VStar650CL]

Hate to say it, but resistance on the knocks doesn't mean anything at all. They're ceramic sensors which are mechanically tuned to the audio frequency of knock, and they can easily test healthy on an ohmmeter but have mechanical or frequency response that's off in the weeds. To prove they aren't falsing, they need to be disconnected.

[Shanehsmp]

Hate to say it, but resistance on the knocks doesn't mean anything at all. They're ceramic sensors which are mechanically tuned to the audio frequency of knock, and they can easily test healthy on an ohmmeter but have mechanical or frequency response that's off in the weeds. To prove they aren't falsing, they need to be disconnected.

I had a feeling you'd say this, lol. And I 100% agree, but I don't think the knock sensors are faulty based on the (now confirmed) wide RPM range that this is happening at and the fact that it only happens when I let my foot off the gas or at low/partial throttle. I took the car on a WOT drive to make sure that my RPMs weren't limited to 3500 and under, I was able to get above 4k on a test run.

I just took a look at the TPS section on the manual, I'm going to pull my ECU and start testing the wires to pinpoint the issue. Hopefully it's just a bad ground.

[ken in az]

What kind of drivability problems are you having?

[Shanehsmp]

What kind of drivability problems are you having?

Ken, car has been going into open loop when I take my foot off the gas at any random speed. Every time it goes into open loop, my AFR sky rockets (extreme lean) and my ECU pulls timing to about -7°. The car bogs down unless I blip the throttle. Acceleration and overall performance has been low since this all started months ago.

At first I thought it was staying in closed loop, but I confirmed with my scanner this AM that it is actually switching to open loop. From what I gather the switching on decel is normal - I am not sure if the retarded timing and bogging is normal, it's very very abrupt, which can't be normal.

[Shanehsmp]

So I have some kind of update for this thread and I THINK I have finally pinpointed the issue, but I am not sure exactly what is causing it.

On the graph below, I have my ECU Commanded Fuel Rail Pressure, My RPM and My Actual Fuel Rail Pressure. This happens EVERY time I let my foot off the gas or ease off the throttle - my ECU commanded pressure wants to bring the value down to around 4000 kPa, but as you can see, during the coast, my actual fuel pressure actually raises. In this case, you can see a 3000 kPA difference in pressure towards the end of the decel, then it drops back to normal when the car is stopped/or goes back on the commanded path when I get back on the throttle.

Now I did some signal testing of the circuits between my ECU and the plugs at the High Pressure Fuel Pump. I also back probed the fuel rail pressure sensor with my oscilloscope and the voltage and/or waveforms look like those in the manual. One thing I did notice is that my fuel pressure voltage and fuel pressures don't really match each other.

I ordered a new HPFP, should be here tomorrow. I took the old one out today and checked the resistance between the terminals and it registers 10.4 ohms which is between the 9 & 11 in the service manual. Could the fuel rail pressure sensor be going bad?

My injector pulse width at idle is sometimes higher than the the normal range of 2 - 3.6ms, usually 4ms or higher. Is this supposed to increase with RPM? Because I have seen the injector pulse with get as high as 18ms in some occurrences which seems rather excessive.

Any ideas are appreciated....

[VStar650CL]

I think you're just seeing the ECM dialing-back the injector pulse width when the throttle is dumped. What will actually show up in the rail is an integral of the pressure produced at the pump and the pressure released into the cylinders, so those bumps at the bottom are most likely the injectors being dialed back while the pump's command pressure hasn't been altered. You could put up a graph with injector pulse to confirm it, but I'm pretty sure it isn't abnormal.

[Shanehsmp]

I think you're just seeing the ECM dialing-back the injector pulse width when the throttle is dumped. What will actually show up in the rail is an integral of the pressure produced at the pump and the pressure released into the cylinders, so those bumps at the bottom are most likely the injectors being dialed back while the pump's command pressure hasn't been altered. You could put up a graph with injector pulse to confirm it, but I'm pretty sure it isn't abnormal.

Just an update - I installed the new pump and the my ignition timing has all but disappeared. The deviation that you saw in that graph above was not normal, and it is no longer happening. I believe that the pump was leaking fuel into the crankcase so there is still some issues with my intake and exhaust timing, but I think a few oil changes will fix that. Now I'm just praying my piston rings and cylinders are unscathed, we shall see.

[VStar650CL]

The deviation that you saw in that graph above was not normal, and it is no longer happening. I believe that the pump was leaking fuel into the crankcase

That makes perfect sense, the possibility of a bad seal or diaphragm didn't occur to me. Good job and good info!