This weekend I went to street wars at E-Town dragway. I made the runs on street tires. First run was 13.48 at 109.8. POS mbc was set at ~14 psi on 94 octane. All is good.
Go out for a second run. Launch... boost hit 9 psi, no more boost and a wonderful clanking sound. 19.4 at 65, after letting off the 1/8 mile. I do a quick check on the vacuum lines thinking my external wastegate was not opening becuause of leak somewhere and added C-16 to the mix for about a 99 octane.
That should've taken care of the pinging.

Well the boost controller has a small relieve hole in it so I took it off to be a little safer.
Run 3, again same thing 17.0 at 79. Only able to boost 6 pounds with race gas, 9 pounds of boost max.

Pull off in the pits, notice that fuel pressure DROPPED by 6 psi !? I didn't even check last time :eek: never having this problem. So I raise the fuel pressure to 49psi (I'm running the 4 bar program). I took out the plugs this time. Yup.. detonation. Cylinder 1 and 4 plugs were crispy clean with little silver specs on it. Cylinder 2 and 3 plugs had burned the eletrodes off! :eek: I was running on 2 cylinders !

Put new plugs in that I just happened to have, bosch ones but better than burned ones and went for another run. At this point I also turned down the timing which was set rather high at 18 degrees.

13.7 at 9psi at 108mph. Car is ok..slicks would be nice..

My question is why would my fuel pressure drop all of a sudden ? No visible leaks, changed fuel filter about 2,000 miles ago.

Can the ECU pull back the fuel pressure since the car was running on 2 cylinder for a while ? I thought it only read fuel pressure that was preset manually ? That's why the car ran lean on the 2nd and 3rd run...

I've never had this happen and I make it almost a ritual to check fuel pressure every morning when the car is warming up.

Lessons learned, check plugs afterevery run .
Run a little bit more conservative boost and timing when the weather is as hot as e-town was. Car was boosting wonderfully on the ride back home on the cool night btw.

Please try to help me figure out why the fuel pressure dropped and the car ran lean, thank you for any suggestions and input.

Fuel pressure is regulated by engine vacuum/boost. The ECU does not know what fuel pressure you are running. It is preprogrammed and whatever static pressure your supposed to have, it expects to be there.

BUT, if you are closed loop (engine warm, range between idling and WOT) it looks at O2 and adjusts accordingly. So if you have too little fuel pressure or too much it will vary injector pulse to compensate but it only does this because it's trying to judge by the O2 reading, not because of FP, again because it does not know it.

I suspect you either have a fuel leak, fuel obstruction in line (filter clogged), loose vacuum hose/leak, bad regulator, or bad fuel pump.

Are you pulling the vacuum hose to the regulator and setting the pressure to what the JWT ECU sticker says it should be at static? I suspect not if you are talking 49psi. I have 3 bar but I am pretty sure 4 bar setting should be 59psi. Again go by the JWT sticker or call Ben at JWT tomorrow and ask him for the exact setting.

And people call *me* crazy for being one of the only few in the turbo SE-R community that runs a real-time digital in dash fuel gauge! I ALWAYS want to make sure that pressure is correct. At WOT (open loop) the ecu does not look at 02 so it EXPECTS the fuel pressure to be right, IT CANNOT COMPENSATE AT ALL IN OPEN LOOP. This is a serious problem if you pressure is extremely lower than it should be.

The above information is as far as I understand it, from reading lots of books extensively, listening to some smart SR20 gurus, and just plain reading the FSM.

Good luck.

1 bar is about 1 atm which is equal to about 14.7 psi. So the 4bar should be ~60 (59psi is what I've read). So that would make the 3 bar around 44psi.

When you hit WOT, the ecu goes into open loop; so it doesn't look at the O2 sensor to detect rich or lean. So if you're fuel pressue is low during WOT, the car is going to run lean.

bad fuel pump, pressure regulator: those could be reasons why the fuel pressure was low.

Khiem

Been there done that. Vacuum leak to your FPR.

Originally posted by Thomas Reynolds
Fuel pressure is regulated by engine vacuum/boost. The ECU does not know what fuel pressure you are running. It is preprogrammed and whatever static pressure your supposed to have, it expects to be there.

BUT, if you are closed loop (engine warm, range between idling and WOT) it looks at O2 and adjusts accordingly. So if you have too little fuel pressure or too much it will vary injector pulse to compensate but it only does this because it's trying to judge by the O2 reading, not because of FP, again because it does not know it.

I suspect you either have a fuel leak, fuel obstruction in line (filter clogged), loose vacuum hose/leak, bad regulator, or bad fuel pump.

Are you pulling the vacuum hose to the regulator and setting the pressure to what the JWT ECU sticker says it should be at static? I suspect not if you are talking 49psi. I have 3 bar but I am pretty sure 4 bar setting should be 59psi. Again go by the JWT sticker or call Ben at JWT tomorrow and ask him for the exact setting.

And people call *me* crazy for being one of the only few in the turbo SE-R community that runs a real-time digital in dash fuel gauge! I ALWAYS want to make sure that pressure is correct. At WOT (open loop) the ecu does not look at 02 so it EXPECTS the fuel pressure to be right, IT CANNOT COMPENSATE AT ALL IN OPEN LOOP. This is a serious problem if you pressure is extremely lower than it should be.

The above information is as far as I understand it, from reading lots of books extensively, listening to some smart SR20 gurus, and just plain reading the FSM.

Good luck.

Good post Thomas Reynolds.

You LMs might want to look into this as are car a pretty old (Classic Sentra SE-R). You never know I bet hardly anyone has thought of this.

Link
http://www.roadraceengineering.com/fuelpumptechtip.htm

Thanks for the reply's. Keep any more suggestions coming.