Restart Stumble is back?

[A1218]

I wrote about this months ago and I thought I solved the problem but I guess not.

On the 1997 Pathfinder, this past spring I pulled the upper plenum and lower intake to change the knock sensor, fuel injectors, IACV, coolant hoses, intake gaskets, fuel rail to intake rubber grommets, and egr gasket.

It was for a warm restart stumble where whenever I go to restart the vehicle after running in the store or such for about 30 min to hour it would stumble on restart then normal out. After doing this work everything was fine throughout the whole summer until now.

I'm not sure if its due to the weather being colder now coming into winter or if its due to driving very short trips at times. Like today I started up the vehicle, warmed it up for about 3 min and drove it about a mile to the pharmacy where I spent about 15-20 min. When I restarted it to drive back it started up perfectly fine then about a few seconds later the idle stumbled down to about 400 to 500rpms and normalized back. Its in the 40s today.

But yesterday I made a 150 mile highway trip about 70-80 mph most of the time. After I reached my destination I decided to start it up after about 30 or 45 minutes and the problem didn't occur.

Is the engine flooding somehow still? A weak fuel pump? EGR?

I am truly stumped here.

[A1218]

Any suggestions?

[VStar650CL]

Try a crank sensor. They often misbehave with temperature and/or heat-soaking, and the problems usually show up at low or cranking RPM's because that's where reluctance is lowest.

[A1218]

Try a crank sensor. They often misbehave with temperature and/or heat-soaking, and the problems usually show up at low or cranking RPM's because that's where reluctance is lowest.

Okay, thanks.

Is it possible to reach that from under the vehicle to change?

In your experience have you seen them go bad without throwing a code?

It seems as if this problem is only occurring in the late fall winter months. Like today is was in the high 20s low 30s and the issue occurred all three times after stopping for a short time then restarting. Each drive was only a few miles city. Didn’t experience this in the summer time after doing the work I stated during late spring.

It restarts fine with the fast idle and everything on the first or second crank. But a second or two later its as if the engine wants to cut off but catches itself back. A few weeks ago right after a restart it cut off.

Just trying to find a solution here. I’ve asked Nissan techs and they aren’t sure. I’ve been told to wait until it gets worse then I’ll find out for sure, but I don’t think it’s such a great idea to do so and it hasn’t gotten worse.

[VStar650CL]

Yah, crank sensors often misbehave without throwing codes. It's a matter of the reduced reluctance at low RPM's, it can cause "missed teeth" which stop missing once the RPM's increase. Until they stop missing the ECM sort of goes into "duh" mode and runs the engine on the cam sensors while it tries to sort out the problem. But then the crank speed picks up and the ECM calls it good, so no code gets thrown. The same thing can happen from a bad cam sensor, but it's less common because the ECM has the other cam sensor to compare to. That usually leads to a code right away, unlike a misbehaving crank sensor.

The problems can also be temperature related because a cold engine causes mechanical shrinkage while a warm one causes decreased electrical efficiency. The combined factors can make for some inexplicable behaviors with no codes.

[A1218]

Yah, crank sensors often misbehave without throwing codes. It's a matter of the reduced reluctance at low RPM's, it can cause "missed teeth" which stop missing once the RPM's increase. Until they stop missing the ECM sort of goes into "duh" mode and runs the engine on the cam sensors while it tries to sort out the problem. But then the crank speed picks up and the ECM calls it good, so no code gets thrown. The same thing can happen from a bad cam sensor, but it's less common because the ECM has the other cam sensor to compare to. That usually leads to a code right away, unlike a misbehaving crank sensor.

The problems can also be temperature related because a cold engine causes mechanical shrinkage while a warm one causes decreased electrical efficiency. The combined factors can make for some inexplicable behaviors with no codes.

Here’s a video of the issue after making a 30 min stop.

https://youtu.be/xKlsF_v4KdQ

Also, the manual says that this sensor is only used for diagnostic purposes and does not affect engine operation?

[VStar650CL]

Also, the manual says that this sensor is only used for diagnostic purposes and does not affect engine operation?

Yep, my bad. The CKP on yours can confuse the ECM and cause a long crank, but it won't cause stumble once the engine starts. The cam sensor (opto wheel inside the distributor) is all the ECM uses at run time. The opto components rarely fail per se, but take a look inside your distributor and make sure there isn't play in the shaft. That can cause weird issues from the wheel "precessing" when the distributor first spins up, until gyroscopic force stabilizes the shaft. Check for oil in the distributor too, that can happen from obstructions in the PCV system and can also cause weird transient behavior.

[A1218]

Also, the manual says that this sensor is only used for diagnostic purposes and does not affect engine operation?

Yep, my bad. The CKP on yours can confuse the ECM and cause a long crank, but it won't cause stumble once the engine starts. The cam sensor (opto wheel inside the distributor) is all the ECM uses at run time. The opto components rarely fail per se, but take a look inside your distributor and make sure there isn't play in the shaft. That can cause weird issues from the wheel "precessing" when the distributor first spins up, until gyroscopic force stabilizes the shaft. Check for oil in the distributor too, that can happen from obstructions in the PCV system and can also cause weird transient behavior.

Okay, now that you mention distributor this is where my dilemma gets interesting.

I replaced the original distributor with one from AutoZone back in 2019 for an idle misfire when engine warm (solved that problem). In December 2019 I installed a remote start in the vehicle. But whenever using the key takeover function it would randomly shutdown when driving. Thought it was the AutoZone distributor so I luckily got it refunded and ordered one from Nissan and installed that in March 2020. Turns out it was simply my ignition switch causing the random shutdown.

But.... coming back to this distributor that was replaced in March 2020 and looking back at my posts on this forum and over at Nissan forums, I started asking about this issue in April 2020. Looks like it started after putting in the Nissan distributor.

Could I have possibly received a faulty Nissan distributor? Why didn't I think of this before

[VStar650CL]

Even factory parts can be faulty, or it may be a tangential issue like oil, or it could even be resistance in the cam sensor wiring or connections. I'd certainly start by popping the cap and giving the distributor guts a look and a wiggle.

[A1218]

So I decided to check the fuel pressure regulator out. I pulled the vac hose coming from it out of the plenum and when I gave the hose a squeeze, gasoline came squirting out. I did this after letting the vehicle sit for for about 30 mins, turned the key to ignition once without starting, came back and gave the small hose a squeeze. Then gas…

Isn’t gas not supposed to be in that hose?

[VStar650CL]

Nope, you're correct, that should be vacuum only. The regulator has a blown diaphragm.

[A1218]

Nope, you're correct, that should be vacuum only. The regulator has a blown diaphragm.

So that’s probably the culprit right? Shooting gas into the intake when it starts up.

[VStar650CL]

Yep. If it fills up with a puddle from residual pressure after the engine stops, that will all get sucked into the plenum at once right after she starts. Sounds very likely. Good diag!

[A1218]

Yep. If it fills up with a puddle from residual pressure after the engine stops, that will all get sucked into the plenum at once right after she starts. Sounds very likely. Good diag!

Thanks! I appreciate your input in helping me figure this out.

Do you think it’s necessary to go original Nissan with this part? Or a fuel pressure regulator like Delphi would be okay?

[VStar650CL]

Unless I'm mistaken, I think Delphi made most of the OE regulators for Bosch, and for Nissan by proxy. So I expect a Delphi would be fine.

[A1218]

Okay, thanks for that info

[A1218]

Just to report back, the FPR solved the problem I was having.

Also ended up replacing the egr and egr pipe to the exhaust manifold as when trying to lossen the flange nut, it broke the threads right off the egr valve. Couldn’t get the broken piece out the nut either, so had to get the pipe.

I used all Nissan parts to avoid potential issues.

[VStar650CL]

Happy New Year!

[A1218]

Happy New Year!

Happy New Year!!