dead lean only at 3900 rpm

[jnybbad]

All other rpms the afr is good at 3800 its 14 to 15 range then at 3900 it stumbles and bucks and goes very lean. Then at 4000 to redline it goes from 12.5 to 11.5. No codes. What could cause this. Its the same place everytime. Engine is stock with afp regulator and walbro, 3 inch down pipe to open 3" exhaust. It does this with and without load.

 

[jnybbad]

Well I adjusted the TPS again and it looks dead on to what the specs in the manual say. It is going lean, 18:1 at 3900 rpm. It looks to go into open loop at 4000 as it clears right up. Lower than 4000 and at idle the afr fluctuates a lot. I am thinking maybe the O2 sensor. I checked the O2 at the test port and it is supposed to fluctuate however mine stays steady voltage at 2500 rpm no fluctuation at all.

 

[Nick M]

I am not sure of the specific conditions of when the PCM runs current around the fuel pump resistor to speed up the fuel pump. But that could be your hiccup.

 

[jnybbad]

I have the 12v mod with resistor removed. The FP is good at all rpms.

 

[jnybbad]

I tested all sensors and replaced the o2 . No change. It gets better if I bump up thr base fp to 40 with vacuum. I was wondering if the open exhaust and free breathing intake could cause this?

 

[toyotanos]

I ran into an issue not unlike this- for mine it turned out to be fuel pressure pulsation. Single 1020 pump without a pulsation damper = lean midrange depending on fuel pressure and flow requirements. I put a factory pulsation damper onto the feed for the rail and it was gone! (at least it was in my case)

 

[jnybbad]

Did you have to use the factory steel line that hooks to the fuel rail through the pulsation damper or is there another way?

 

[toyotanos]

Yup, I'm sure there are other ways, but to use the factory one I haven't found an adapter. Steel line was the best, easiest option since I already had the parts.

Alternatively, having a good amount of braided rubber line seems to help- the hose expands and contracts, forming a quasi-damper that gets the job done. Not sure on exactly how much is enough, though.

 

[3p141592654]

Yes, this is why pulsation dampers exist. You can look at adapting a damper from another car if you have size issues.

 

[jnybbad]

I was thinking of putting the pulse damper on the open port of my Aeromotive FPR. It is on the high pressure side so it should work.. Any thoughts? The reason I am doing this different is I have already plumbed the entire fuel stystem with Steel braided lines. I have an aftermarket pulse damper that will thread into my regulator. However this is on the oposite end of the fuel rail that the factory damper was on. Im thinking as long as it is on the high pressure side location shouldn't matter.