Having problems with the car running lean, what can cause this?


New Member
Nov 20, 2003
NW Burbs, IL
When I got the car dyno'd about a week ago, the fuel curve was way off. I started at 42 psi of fuel and 15* of timing and ran about 16/1. We bumped the Fuel to 52 psi and dropped the timing to 12* and the fuel curve was still 16/1!!!!

So I'm either having fuel issues or a vacuum leak that I can't find... I've sprayed carb cleaner over every gasket surface and there was no change in idle RPM's.

I'm running a new H/C/I combo with a FTI cam. I'm pulling 10 lbs of vacuum at idle (Normal for aftermarket cam???). I also don't have the smog pump on the car anymore and I'm pulling codes 311 and 314, which have to do with a valve on the lines having to do with the thermacator tubing.

311 - AIR System inoperative during KOER (Bank number 1 with dual heated oxygen sensors
314 - AIR System inoperative during KOER (Bank number 2 with dual heated oxygen sensors

I've installed a new 255 lph Walbro Fuel pump, a set of 24# injectors (used) and an adjustable fuel pressure regulator (used).

Anywhere you can guide me to check I'd appreciate it. I'm pretty close to yanking the intake and re-doing that install even though I can't find a vacuum leak.

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I would suspect the "used" injectors might not being flowing what they did brand new. You can have them flowed to see for sure. Definitley something wrong, because you shouldn't have to run 52psi with 24# injectors for you setup.

311 - AIR System inoperative during KOER (Bank number 1 with dual heated oxygen sensors
314 - AIR System inoperative during KOER (Bank number 2 with dual heated oxygen sensors

THose codes you got with engine runing right, I have that too, you are take out smog pump and now you have those codes, don't worry nothing gonna happening, those codes are just there, and nothing do with your air/fuel ratio

About vaccum leak, I have that probem and thats is IAC , try clean IAC with carb cleaner , if nothing happening try bought new one
Seth I'm also having lean problems, but just above 4-5K, and they are severe problems. See my other post about "being alive/dyno results" and there is some more info about it, I'm also really hoping to get this worked out soon.
Very strange, I have almost the same combo with Walbro 255lph pump (GSS340), used 24lbs and an AFPR. I was at 16:1 with 38psi, but after bumping it up to 46psi, I got it down to 13.5:1.

Don't have any ideas, but you should not have any problems getting enough fuel for that combo with those fuel system components, so you definitely have a hidden problem lying somewhere.

I've looked further into this problem and I'm leaning towards the fact that I had a good sized exhaust leak which was causing more air to be sucked into the exhaust stream... I dunno though. I also found a crack in my lower intake so there's another $300 that I have to spend now.

I've also sent out the injectors to be professionally cleaned and flow tested just to be double sure. We'll see when its all back together.

Oh yeah, one last thing, is your lower intake ported Jeremy?

Thanks for the replies!
Nope...it is not ported. Box stock Cobra Intake with casting flash and all.

That along with the 100% stock tune is hiding more hp in my combo, I'm sure of it. Hopefully with maybe a Tmoss job and a TwEECer, I could see another 10-15rwhp and 10rwtq.