[ARS97Pathfinder]

Did a pressurized fuel cleaning on injectors. Didn’t help the issue where I drive and come back to the vehicle anywhere from 20 minutes to an hour and half on a hot restart where it stumble for a few seconds.

Sometimes it would even surge at a very low rpm for those few seconds. One time it even died when it did this. After that vehicle is perfectly fine. No drivability issues or problem with cold start and starting after few hours.

I know many have said could possibly be leaky fuel injectors but I want to make sure first before pulling everything apart. Did a fuel pressure test and lost 10 psi in 25 minutes.

that picture data is at idle. When Erving vehicle the short term fuel trims drop down to around 0 or even less sometimes.

Can someone help with the diagnosis

I included some other data.

 

[VStar650CL]

STFT with RPM is what's meaningful. From what I can see there, it's going way rich at idle. If it's normalizing above idle as you say, it means you have a small vacuum leak.

 

[ARS97Pathfinder]

STFT with RPM is what's meaningful. From what I can see there, it's going way rich at idle. If it's normalizing above idle as you say, it means you have a small vacuum leak.

Does that contribute toward the warm restart stumbling/low rpm surge?


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[VStar650CL]

Sure. With a cold engine the injectors are running enriched in open loop. On warm start there's no enrichment and the ECM doesn't know it has a leak, so it takes a minute for it to ramp up the FT to compensate. Until it does, you get stalling and stumble.

 

[ARS97Pathfinder]

Sure. With a cold engine the injectors are running enriched in open loop. On warm start there's no enrichment and the ECM doesn't know it has a leak, so it takes a minute for it to ramp up the FT to compensate. Until it does, you get stalling and stumble.

Gotcha, but if I may ask, why would it not occur if I shut the engine off and restart in a minute or two. But more specifically only that time frame.


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[VStar650CL]

My guess would be the ECM firmware. The ECM doesn't shut itself down immediately when you turn off the key, and it's likely it retains ECT data for awhile after shutdown. It never sleeps completely, it always has a subsystem awake to perform EONV on the fuel tank as needed. So if the ECT reading is still the same within limits when it wakes up, it may re-use the last STFT data. That's only an educated guess, and I'd be interested to see if your scanner shows STFT already elevated when you restart under those circumstances.

Regardless of that, rich STFT at idle that goes away when RPM's rise means a vacuum leak for certain. So if that isn't causing your restart issue, then you actually have two issues.

 

[ARS97Pathfinder]

My guess would be the ECM firmware. The ECM doesn't shut itself down immediately when you turn off the key, and it's likely it retains ECT data for awhile after shutdown. It never sleeps completely, it always has a subsystem awake to perform EONV on the fuel tank as needed. So if the ECT reading is still the same within limits when it wakes up, it may re-use the last STFT data. That's only an educated guess, and I'd be interested to see if your scanner shows STFT already elevated when you restart under those circumstances.

Regardless of that, rich STFT at idle that goes away when RPM's rise means a vacuum leak for certain. So if that isn't causing your restart issue, then you actually have two issues.

Gotcha, I’ll check for any vacuum leaks. Most of the vac lines, at least all the ones you can access from eyesight have been changed a couple months ago.

There was a smoke test done on the engine through the intake and the only place smoke came out of was the egr bpt valve. But that is normal if the vehicle is off due to no vacuum on the diaphragm on it.


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[VStar650CL]

There's precious little else that can cause STFT to do that, but it's also a small leak. If it was larger the STFT would be topping out at +25% and it would throw a code.

 

[ARS97Pathfinder]

There's precious little else that can cause STFT to do that, but it's also a small leak. If it was larger the STFT would be topping out at +25% and it would throw a code.

Hopefully I get a code in the meantime I check around. But it’s been about a year now I’m dealing with this intermittent warm start stumbling and can’t find the cause of the problem.

Have taken the vehicle to a longtime Nissan tech I know. He checked it out and isn’t sure. Though I don’t believe he put too much effort into checking thoroughly. To begin with its hard to just replicate the issue since it’s in the time frame I spoke of. It did happen to occur when he www looking into it. I saw he try to adjust the IACV with engine on and I didn’t see any idle speed change though.

He did recommend the ECT sensor which was replaced and no change in issue. New distributor, leads, plugs, maf, air filter, fuel injection pressure clean.


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