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NG900 & OG9-3 Performance, Mods & Tuning Covers Tuning & Performance modifications for the NG900 (1994 to 1998) & OG9-3 (1999-2002) & '03 Convertible

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  #1  
Old 22nd December 2009
nickfjields nickfjields is offline
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Exclamation Lean Running, at a loss for solutions

1994 900Se Turbo, I've got a oxygen sensor low voltage code, lean mixture. I pulled the plugs and they are definitely white at the tips, the ring around the bottom of the electrode is sooty however. Checked the FPR, wasn't holding quite enough pressure (35psi) so that was replaced. With the new regulator pressure held, and increased and decreased as it should with vacuum or pressure applied to the regulator. After replacing the FPR I checked the fuel pump capacity as per the WIS procedure, I only had a 500mL container, still it overflowed in about 17 seconds which works out to about 850 mL in 30 seconds, well over the WIS number of at least 700mL in 30 sec.

My last guess is the Jak Stoll stage 4 tune I have but my buddy is running that too with stock injectors and has no problems with lean running. I've tried injector cleaner with no results. I'm not considering going much higher with the engine tuning, but maybe I should invest in some higher flow injectors anyway?

Thanks for the help
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Old 22nd December 2009
John Z Williams John Z Williams is offline
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You need to check fuel pressure under load! You need to be able to tell what the fuel pressure is doing while under boost. There are many people that have had to replace the fuel pump. Put in a walbro 255 high flow pump and that will take care of any fuel pump issues.

I had a weak fuel pump and my pressure was fine until about 20psi of boost and then my fuel pressure would fall back the other way and go from 65psi back to 40psi and I would lean out.

I put in a new walbro 255 fuel pump and my problems were solved and I then held 65psi all the way to 25psi of boost and all the way to redline.

A kinked fuel line will cause issues, clogged fuel filter, bad injector, fpr as you stated and of course a weak pump.

I have seen where saab indy shops check fuel pressure at idle and then say yes, fuel pump is fine, but they do not test under load!!!

John
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Old 22nd December 2009
nickfjields nickfjields is offline
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I check all the fuel lines as far as I could see them, nothing looks kinked, and my fuel filter is brand new.

In WIS the procedure for checking the FPR instructs you to attach a vacuum/pressure pump to the FPR vacuum line to change the pressure on the regulator. I figured this was to simulate the different loads the system would see when driving.

I guess I just wanna be as sure as I can be that this is a fuel pump issue before I start the process to replace it.

One more thing I was thinking about. My pump provides at least 3 bar, so if i remove the hose from the FPR then the pressure in the fuel rail should stay at 3 bar, unaffected by the intake pressure. At idle this is too high of a pressure, which would cause the engine to run rich, or at least richer than it was. I was thinking I'd backpin the lambda sensor connection and see if there was a change in the voltage from the extremely low voltage I got before I replaced the FPR. This make sense?

Last edited by nickfjields; 22nd December 2009 at 02:49 PM.
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Old 22nd December 2009
John Z Williams John Z Williams is offline
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That is the same way shops do it, that does not take into account flow, the injectors have to be wide open and the engine under load to know what the actual pressure loss is in real conditions.

Like said, my system tested fine under those conditions, but once I put a fuel pressure gage on the fuel system and tested it under wot and full boost, the fuel pressure fell over 20PSI as I passed 20psi of boost.

The only way to know for sure is to test the fuel pressure under full boost and load at wot.

You need to determine if the pump is keeping up with the injectors wide open. Many tuners recommend an uprated fuel pump for this reason,

John
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  #5  
Old 23rd December 2009
nickfjields nickfjields is offline
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alright looks like a fuel pump it is then. While I'm at it I'm definitely going to upgrade to the 255lph. I was looking at http://www.fuel-pumps.net/12products.html and I can't figure out what pump I need. searching this forum there's a lot of talk about model 341, which I found on that page, but that appears to be the stock pump. Can anyone tell me what pump number I should be looking for to get the correct 255lph pump, and anyone with recommendations about where to get it please tell me.

Thanks for the help
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