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  May 16, 2010 10:55 p.m. Nitroracer Dork

  I am looking for a few extra brain cells to diagnose the problem with my camaro running lean again. The car is a 98' LS1 camaro and it will throw codes for a lean mixture (p131/p151) only under hard acceleration.

  I have checked for vacuum leaks and found none (reads 20 inches at idle), run heavy duty fuel system cleaners (BG 44k), replaced the fuel filter, replaced the pcv valve, and cleaned the MAF.
  

  Yesterday I reset the computer after a short burst where the code was set and decided to gradually accelerate going up to 6k rpm in first and second. Under more gradual acceleration to the same high rpm no codes are set.

  What would be different about the demands of the engine stabbing the throttle versus gradual accel? I am thinking the fuel pump is running out of steam up top.

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  May 16, 2010 11:33 p.m. RXBeetle Reader

  Acceleration enrichment in EFI is like the smart version of an accelerator pump on a carb. You need a lot of extra fuel on a stab of the throttle because manifold pressure jumps and fuel drops out of suspension. The amount of needed enrichment over base fueling is dependent on the rate of throttle change and RPM so the difference you are seeing from throttle stab to rolling on the throttle and letting the RPM come up points to a fueling supply problem. It could be a pressure regulator problem (do you have a vac referenced regulator? is the vac line intact? Pull the regulator vac line while the car is idling if it has one. If it goes way rich and stumbles or dies it's doing it's job. If it doesn't change at all T in a mechanical gauge and find what the pressure should be.

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  May 31, 2010 6:36 p.m. Nitroracer Dork

  I finally had a chance to check the fuel pressure while running down the road this evening and I wanted to update this post.
  

  Running at idle, up to temperature 59psi.

  Normal driving, smooth acceleration 59psi.

  Wide open throttle 55psi.

  Shut off after running 50psi.

  Would these figures point to a weak fuel pump for an LS1 camaro?

  1968 Fairlane 500 - 1998 Camaro Z/28

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  May 31, 2010 6:53 p.m. zomby woof HalfDork

  I doubt it. Maybe you have an O2 sensor problem, not a lean problem.

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  May 31, 2010 7:00 p.m. Knurled Reader

  59psi at IDLE? Brain fart: Is it a single-rail system where fuel pressure is not dependent on manifold vacuum?

  If it is, it should hold the set pressure all the way to redline at WOT.

  If it is NOT, then anything not WOT should be base pressure minus manifold vacuum - generally about 7-10psi. But, it might be.

  I fought a strange one a while back with a V6 Lexus with drive by wire. It was throwing codes for airflow discrepancy or something. By the time it came to me, this was checked, that was checked, everything was checked checked. So, figuring all the bases were covered, I went in and started manually scoping all of the redundant sensors and the MAF and so forth.

  Poking around under the hood for an unrelated reason, I noticed that the throttle elbow was loose. If was fine in the shop, but under load on the road, engine torque pulled the boot off of the throttle body...

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  May 31, 2010 7:42 p.m. carguy123 SuperDork

  When was the last time you cleaned the injectors? If they aren't clean they might be able to deliver enough fuel at idle but not enough at WOT.

  Same thing goes for the fuel filter, but that should show a drop in fuel pressure, of wait you do have a drop in fuel pressure.

  "mobilito ergo sum" I drive therefore I am!

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  June 3, 2010 10:36 a.m. Nitroracer Dork

  Finally got this thing fixed, and the actual problem was remedied for a total of $0.

  Apparently the mass air flow sensor that I carefully cleaned, dried, and re-installed was so covered in K&N filter juice that it wasn't able to give a proper reading - but it was also never so far out of spec that it set a code in the computer.
  

  Scored a free used maf ten minutes from my house and bolted it up last night. I've had the car WOT quite a few times and no check engine light.
  

  So now I have a really clean fuel system, a new oxygen sensor, and I want to go back to the factory air box so this crap never drives me crazy again.

  To the previous owner, dont douse the filter in oil!

  1968 Fairlane 500 - 1998 Camaro Z/28

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  June 3, 2010 11:42 a.m. Jensenman SuperDork

  Glad you got this straightened out.

  Saw this same code on various vehicles over the years, it comes from nailing the throttle with low fuel. Duh.

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