R32 Skyline Hitting Fuel Cut / Power Cut

[Cormac]

Hi All, second time posting about my R32 Skyline with RB20DET on this forum. I just took the car out of storage for the Summer and I've noticed that some type of fuel cut/ power cut / boost cut has developed which wasn't there last year. 

The boost controller has a high and low boost mode. This issue has only happened in high boost mode (target 1.1 bar). 

- Foot down about 50-60% in 2nd or 3rd gear

- When I get to around 5-6k rpm, the car hits what I can only describe as a fuel cut / rpm limit. A red light flashes once on the dash and the car loses all power. You can feel it fairly violently like the car hits a wall. 

- This doesn't happen every time I hit this RPM, but I would say the harder you drive the more likely it is to happen. 

Ignition system is in good condition with the plugs not long replaced. Car was fully checked over by my tuner last August. I see no fault codes logged. The only thing that changed over the Winter storage period was that I changed the timing belt, pulleys and water pump. 

I have attached a log file of my drive to this post. The issue only happened once in this 11.5 minute log, at approximately 11 minutes (the very last RPM spike on the graph). 

Would appreciate any suggestions on what's happening here. I've had to zip up the log file as its just over 2.55MB at 2.66MB. 

Skyline fuel cut log file.7z

[Vaughan]

You're hitting the MAP limit, looks like your limit is 245kPa with a 10kPa lead in as it starts to cut fuel as soon as you go over 235kPa

 

[Cormac]

Thanks for the reply Vaughan. I was just about to post another log file that I captured a few minutes ago. This one is shorter and I managed to generate the symptom twice at the end of the run, at about 2:28 minutes and again at 2:38 minutes. I'll attach it here for the sake of it. 

Can you explain the MAP limit to me? Is this something my tuner has set as a protection, or is it the limit of the MAP itself?

As far as I understand, the MAP limit is in absolute pressure so 235kPa = 2.35 Bar. So I must be hitting boost pressure of (2.35 bar minus 1.013 bar) = approx 1.34 bar of boost pressure to be triggering the MAP limit?

I've rechecked the boost controller, it has a target of 1.14 bar. 

 

 

 

Skyline fuel cut log file 2.llg

[Vaughan]

It is indeed a safety feature that has been set up in the ECU by the tuner, just like the RPM Limit but for manifold pressure. The limit will be based on what the tuner thought was a safe max boost pressure and how much boost pressure the engine as tuned for, The MAP sensor is most likely a 4Bar one and so should be good up to 400kPa absolute (~300kPa Boost) pressure.

The MAP value in the ECU is in absolute pressure so ~100kPa MAP is atmospheric pressure, this means that a MAP Limit of 235kPa will be approximately 135kPa of Boost pressure or ~1.35 Bar.

You might need to look into what the boost controller is doing.

[Cormac]

That's great thank you. I actually remember my tuner saying he had trouble keeping the boost at the set limit with the boost controller last year.

It's a Gizzmo boost controller. I'm not sure if this issue would be caused by the boost controller itself or just a symptom of something else like the wastegate getting stuck and not opening. Either way it looks like I need to go back to basics and check over the boost control system.  

One last question - where is the MAP sensor usually mounted? I have had a good look around the engine bay but I can't see it. However there are two vacuum hoses going through the firewall on the passenger side. Could the MAP be mounted inside the car near the ECU?

[Vaughan]

If it is the plugin ECU it most likely has the MAP sensor inside of it, the feed for it has to come from inside the manifold (somewhere after the throttle body).

[Confused]

Is there a reason why you're using an external boost controller, and not letting the ECU control boost?

The ECU has all the necessary information to know what's going on with the engine, and can lower boost as one of its protection strategies. I wonder if the tuner introduced the MAP limit to try to work around the limitations of not having boost controlled by the ECU?

[Cormac]

52 minutes ago, Confused said:

Is there a reason why you're using an external boost controller, and not letting the ECU control boost?

The ECU has all the necessary information to know what's going on with the engine, and can lower boost as one of its protection strategies. I wonder if the tuner introduced the MAP limit to try to work around the limitations of not having boost controlled by the ECU?

That's a good question, and it's something I was wondering myself. I bought the car with this setup and the tuner I brought the car to is actually the same guy who set it all up. Unfortunately I forgot to ask him about this. 

Do you know if there is a reason why he wouldn't use the ECU to control boost? The boost controller is set up to have a high and low boost mode and displays the boost as well, so maybe that's its purpose? I know you can do this with the Link ECU as well, but would need to have a tablet connected up or some other type of display. 

[Confused]

Multiple boost targets can be selected using a Digital Input to switch between two, or even use a rotary dial and an Analog Input, and you could select between many, like using this AEM one https://www.aemelectronics.com/products/sensors-connectors-accessories/12-position-trim-pot

 

Displaying current boost, yes, you'd need some kind of additional display - this could be as simple as a mechanical boost gauge, a 52mm configurable CANBUS guage (one example would be http://www.btigauges.com/link--vipec.html) or you could go for a tablet or digital dash. The possibilities are endless, really!