Jeep 4.7L renix fuel trim questions

[knever3]

I have my stroker back together 4.7L Renix and have a Snap On MT2500 scanner and want to be able to understand the data to tune it.

What fuel trim numbers am I looking for? I also have a wide band O2 monitor in the dash to reference.
I have a Hesco AFPR installed, without vacuum 39psi, 24lb/hr injectors, stock throttle body and intake from a '99, Header and 2.5" exhaust

I want to make sure the air/fuel ratio gets tuned in properly so I am looking for your advise of what to look for on the scanner info.

Thanks guys!

[SilverXJ]

Scanner won't tell you any thing except stoich, rich or lean. You need a wideband AFR gauge to determine actual AFR numbers. I prefer the type with data logging.

[knever3]

I have my stroker back together 4.7L Renix and have a Snap On MT2500 scanner and want to be able to understand the data to tune it.

What fuel trim numbers am I looking for? I also have a wide band O2 monitor in the dash to reference.
I have a Hesco AFPR installed, without vacuum 39psi, 24lb/hr injectors, stock throttle body and intake from a '99, Header and 2.5" exhaust

I want to make sure the air/fuel ratio gets tuned in properly so I am looking for your advise of what to look for on the scanner info.

Thanks guys!

I have a wideband O2 monitor and it sits at 14.4-14.7 and it doesn't dip below 13.5 when I put my foot into it. I really don't know how it's supposed to react either, just in the learning stages and need any input I can get! I heard that on the Snap on scanner it is supposed to be 128 on the stft at idle, I don't know what the map or O2 sensors are supposed to be at either. Just need some base guidelines to start tuning it in. Thanks!





knever3 Learning to use the board
Learning to use the board Posts: 34Joined: April 6th, 2012, 3:34 pmThanks Given: 0Thanks Received: 0RSS
Stroker Displacement: 4.7LVehicle Year: 1987Vehicle Make: Jeep

[SilverXJ]

Sorry about that. I did not initially read your post fully. Under load you want the AFR to be 12.5-13. Cruising you want it around 14.7.

I recommend reading through Maud'Dib's build thread as he has done some testing with a few different injectors. http://www.jeepstrokers.com/forum/viewt ... &start=270

[knever3]

Well I read through his thread and it looks to me like I am in the same boat. At idle I can't get the median to get up to 128 even with setting the adjustable fuel pressure regulator to 35psi. With a heavy foot my air/fuel ratio seems to only go to 13.5, but mostly under 50-75% throttle it stays at 14.7-14.4. I am bummed about buying another set of injectors and it seems the SAAB ones are pretty difficult to find a set of 6 that have been reconditioned and flow tested. If someone knows of a set I have missed please shoot me the link, I could use the help.

Here is a screen shot of my scanner of what I have to work with. This is at 39psi if I can recall. Let me know if anything looks out of the ordinary or if my suspicion of needing smaller injectors is accurate.

Thanks

[jeepxj3]

LT doesn't change on my MT2500 so I cant really speak to that, but ST seems too Rich (lower than 128; higher is lean). You want it to be as close to the median (1-256 is the range so 128 is median) as possible. IM the oddity that needs smaller injectors.. but most seem to get by with 24#. 26# would seem way too big to me and it seems to show in the STFT.

I had to look this up again.
STFT = 128 is perfect, Lower is rich and Higher is lean and the ecu has to make corrections.

Your 24 lb/hr injectors and STFT of 89 just means you are a little rich and the ecu is pulling/correcting for this. I guess smaller injectors might help you get closer to the 'desired' 128. But then would your LTFT value go up and and have to correct and add fuel b/c it is lean?

24lb injectors STFT=89, LTFT=130. Maybe 22lb injectors STFT=128 and LTFT=150, IDK. Maybe this would be adjusted out with the adjustable FPR.

[jsawduste]

**** mostly under 50-75% throttle it stays at 14.7-14.4. ****

I AM NOT A RENIX EXPERT BY ANY MEANS

What I understand is;
The ECM will not allow the AFR to rise above your posted numbers for emissions and MPG reasons.
Above 75% and with a few second delay you should see fuel enrichment in the mid 13`s. FP and injector sizing will play a factor in the final open loop deep throttle numbers.

[knever3]

So an update on this. I pinched my fuel return hose and the pressure came up to 60psi and I shut off the engine. Fuel pressure holds great so the pump check valve is good. I called Hesco and Bennie is willing to have me send mine to him and he will rebuild it. That will be excellent. I installed a stock FPR on the rail and the fuel pressure held at 40psi after I turned the truck off. It also primed to 40psi which the Hesco one would never prime to any pressure, it just let the fuel bleed back through the return line.

So after that I think that I will need a bit smaller injectors than the 24lb ones I have installed.

Can someone direct me to a set of remanufactured ones that have been flow tested etc? I don't want to pull ones from the junk yard and really don't think that buying brand new ones is really necessary. I would like the 4 hole ones so am I right by trying to find 93ish injectors? Does anyone know the model numbers I should look for?

Thanks