So I just installed my Hallman MBC Pro Rx and I tunned it to around 24psi. On one hard pull in 3rd gear my CEL came on and when I slowed down to park my RPM's wouldn't go below 2,000. I ended up turning off the car and turning it back on. My RPM's stabalized but my CEL light was still on. Then I disconnected the negative side of the battery for about 1 min and when i turned it back on the CEL came off. It ran fine back home and nothing seems to be damaged. I've heard of fuel cutting before but what excatly is that? Can I make it so the ECU doens't fuel cut me anymore? anybody knows what happened? any good input is appreciated!

 

limp mode. search it.

 

Yup, you went into limp mode..

Is that your only mod on the car?

 

Actually he didn't go into limp mode. If he went into limp mode he couldn't go pass 20 mph (around there). Are you sure it was a CEL and not a SES light. This is why i had to disconnect my EBC. When you put the MBC or EBC it hold a lot more boost closer to red line there for messing with the ECU. The ECU know how much air it is suppost to be getting and when you put the MBC on you are getting way more air, there for popping a SES light, and it makes it hold a idle at like 2000 - 3000rpm until you turn the car off.

I popped many SES light when i had mine on but only went into limp mode once. You didnt go into limp mode, just messing with you ECU. The only way to fix this is to either get it disconnected or get a TUNE. I am not going to be connecting mine back until i can get a tune.

 

now that i think about it, it was a SES so i guess that is what happened then. What is limp mode though? Only way to fix my problem is to get a tune, bigger injectors, and basically better fuel all around right?

 

have you tried lowering the boost? maybe 21-22psi? 24 seems kinda high.

 

^No need for fuel system upgrades, not yet at least. All that fuel cut means is that the ecu cuts fuel to the injectors to prevent possible damage if it senses that things are getting dangerous, or what it thinks could be dangerous; it doesn't mean that your fuel system can't handle the demands of your motor. You actually didn't even hit fuel cut anyway, you just threw a cel.

Usually the code that MBCs throw on the X is a P1235 code, which is an overboost code. The most common way of throwing this code is at partial throttle where MBCs tend to hit 80% of the max boost at like 40% throttle. The ecu sees the low level of load and low amount of fuel for this kind of scenario and freaks out when it sees huge boost pressures when there shouldn't be.

By getting your ecu "tuned" (reflashed) you can prevent overboosting at partial throttle (by tuning the boost with the ecu, NOT the mbc) because the ecu uses boost correction. No offense but MBCs are junk unless your car is for drag and you are never going to be at partial throttle. However if your car is for road racing or especially daily driving then the driveability issues of MBCs are not worth it, and in addition to giving a more throttle depandant boost level, the ecu can do an as good or better job of controlling boost anyway.

 

With this new CANbus system, there are different versions of limp mode. The one you describe above sounds like a severely restricting one.

Still, the P1235 code is _definitely_ a full fledged limp mode. I hit it one time with ECU boost when I forgot to turn boost correction back on post tuning, and in addition to idling around 2000rpm, it also restricts maximum boost to 12psi. Its a limp mode, because it "limps" onto one of the safety maps.

BMW limp modes on the N54 are freaking cool. If you get a misfire, the ECU responds with a spark limp mode that makes the car feel like it is running on 5 cylinders. If you get an overboost trigger, the ECU responds with cutting boost to something ridiculous like 2psi.

These newer limp modes are more restrictive than ever, but they are in force to protect your engine when the ECU sees something it detects as dangerous. If you hit a cell at 25% throttle and you are seeing 21psi, the ECU perceives that as very bad, hence the code. It happens when you are opening the throttle part of the way and the car passes through certain cells. Welcome to drive by wire!

Solution 1: Dump the manual boost controller and just wait for ECU boost control if you get tuned or self tune in the future.

Solution 2: Reduce maximum boost in fifth gear on your MBC to 22psi. It won't partial throttle boost high enough to trigger this code.

Happy motoring.