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Thread: Pre-VVT Hemi - Dialing in Fueling - VE Correction or LTFT+STFT?

  1. #1

    Pre-VVT Hemi - Dialing in Fueling - VE Correction or LTFT+STFT?

    Hello everyone, working on my 2008 Dodge Ram 5.7. Rebuilt engine and put in a new cam, so I am trying to get it to idle right and adjust so the fuel trims are closer to zero. Right now, it is pulling an excessive amount of LTFT+STFT. Even after pulling 20% VE in the idle areas, it is still pulling more.

    It's my understanding that to get a good idle, I should increase the spark in the idle areas. Nevertheless, I am trying to get the fuel trims under control. I don't see a way to directly adjust fueling, but I believe it is done by adjusting VE. After logging a bit, I noticed that there is a VE Correction parameter. Should I be using 1-((VE Correction B1 + VE Correction B2)/2) instead of my current 1-((LTFT B1 + LTFT B2 + STFT B1 + STFT B2)/4) for corrections to the VE table? (I have it calculated that way so I can copy/paste into the VE Tables.)

    I do notice the VE Corrections follow the fuel adjustments fairly closely, but I want to make sure I'm doing this the best way.

    I'm also noticing the fuel trims for one bank is a few percent higher than the other, would that indicate an exhaust leak? Keep in mind I'm pulling fueling, so I didn't think it was that since it was still rich. Engine does have a tick to it, that does follow RPM, but I think it might be an error in pushrod length calculation on my part -- slightly too short. I scoped the exhaust manifold area with the tube-in-the-ear method and didn't hear any leaks.

    Thank you for your thoughts!
    Sorry for asking a lot of probably stupid questions, I'm trying to learn. Thank you in advance for any information you have provided me

  2. #2
    Advanced Tuner
    Join Date
    Feb 2019
    Location
    Owensboro, KY
    Posts
    266
    Just a thought but why not just make the ve correction 1. I'm assuming they are a multiplier so that would negate it having any effect on your fuelling. Then just tune ve table to your liking.

  3. #3
    Tuner in Training
    Join Date
    Sep 2020
    Posts
    12
    I have the same issue trying to tune my buddy's Charger. Its a 6.1 but problems you describe are the same. LTFT B1 is adding nearly 20% and I'm coming to the conclusion that its an exhaust leak around the O2 sensors on the drivers side. Until my buddy can get his car into a shop to verify, that's the only reason I can come up with. Regardless, I'm interested in hearing any theories apart from mine. Sounds identical to your issue! Maybe it's something with these old gens?

  4. #4
    Thank you for the replies. I've went over a bunch of different possibilities used from some posts on here, as well as some tuners on youtube. I had a VE of around 41% in order to make it idle right, and it's not a crazy big cam 220/230 .598 lift, 113 LSA. Even then, despite not showing any misfires, the engine shook quite poorly. I did some leak tests by listening around the exhaust components with a tube in my ear, and I even wanted to check the intake side of things so I carefully sprayed brake cleaner around the intake manifold and fuel injector bosses, looking for an idle bump (nope). I pulled the valve covers and I couldn't feel any pushrod lash, although I could ever-so-slightly slide the rocker on the tube. I don't know if that means I have too much lash -- I measured the zero lash length and ordered pushrods with about 0.067 preload on the intake and 0.075 on the exhaust. At that point, I decided to do a compression test. This engine was running fairly well before the rebuild, but was tired with over 200k miles. The cylinder compression was give or take 150-155. Now that I've rebuilt it -- using the same rods, same head although decked 0.006", but different pistons (Sold as stock replacement/CR), I'm seeing between 120-130 on the cylinders. Tested all of them. I've seen lots of numbers thrown around for a pre-VVT hemi, but I think the consensus is about 170psi is expected for a new engine.

    I'm praying I got the cam timing off by a tooth, but if it's not that, then I think I have to tear into the engine and check the pistons. I measured ring gap on all of the pistons and they were a little on the high side, but still within spec.

    This weekend, I'm going to pull the front timing cover off and see what's what. I really hope I can get it off without pulling the pan (Again).

    On some of my first tunes, where I had already pulled 10% VE, I was still seeing LTFT's pulling 38%. I didn't realize it could even go that far. I even swapped out injectors with the old ones, to see if the cleaned/OE set I got were the wrong flow rates -- nope.

    I went as far as holding the timing with a minimum of +15*, even when my fuel trims were correct (With those terribly low VE's) and it was shaking/bucking pretty bad.

    When I pulled the plugs (Only have about 3 hours and maybe 30 break-in miles on them) and they all looked good.

    I also tried creating a vacuum leak by yanking the vacuum line to the booster, and the rpm's climbed, and it ran a little bit better, which to me means that there probably wasn't a vacuum leak. My guess is it leaned it out even more, which made it run better.
    Sorry for asking a lot of probably stupid questions, I'm trying to learn. Thank you in advance for any information you have provided me

  5. #5
    Update: tore back into engine to check timing and other things. Turns out pushrod was too short. After I used a zip tie to extend the distance from the lifter to the rocker (Effectively shortening the pushrod), I saw 170 psi. So I had measured my intake pushrod wrong, and wasn't fully sealing. The timing was spot on. I also looked at the parts bucket of old parts and I was shocked that there was 7 of the pre-vvt pistons and 1 that was a post-vvt piston! I weighed the pistons, the pre-vvt were about 414g and the post was 422g. I'm surprised the engine ran as smooth as it did before. I don't have a good way to measure compression height, but the actual post-vvt pistons are slightly taller as a whole. The dome begins much closer to the OD of the piston on the pre-vvt. The post-vvt is essentially a flat top for the 1st half inch OD. I think it's because of the squish pads which extend further inward on the post-vvt engines.

    I ordered a borescope because I want to see if I had piston to valve contact.
    Sorry for asking a lot of probably stupid questions, I'm trying to learn. Thank you in advance for any information you have provided me