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I'm going to post a picture from a dyno session. Just looking for opinions and ideas on the possible culprit. Our best plausible guess is valve float. The tuner is very knowledgeable and tried many different things but the issue is at the exact same rpm everytime and we ruled out all common stuff.
Looks like 3800 rpm and major misfire causing rise in AFR. Can't imagine valve float at that rpm. What is the engine combination? Ignition? Has the setup ever ran >3800? What's new with the combo? Possible safeties misconfigured? Interesting, tq flatlines >4k.
We're 99% sure it's valve float. Checked every possible electrical connection etc. Injectors are good. This is a ls2 in my 63 Chevy II wagon swap. I recently got it running and attempted a tune early on so I could safely drive it and shake out any bugs. The engine when I bought it already had a WAY less than desirable cam in it but my thinking was to get running first and go from there. I had a local guy with a very good reputation go through the 243 heads on it and spring accordingly because the springs were unknown. I'm fairly certain that the springs (pac 1211) aren't keeping up with the nasty cam lobes. I questioned the choice in spring and he assured me that they would work well. First trip to the tuner was no different than what I posted above but he made a couple of suggestions to rule out some little simple stuff.
Cam specs 244/242 612/610 112LSA. As I have known for some time this much cam isn't a very good choice for a daily driven street car I've began collecting parts to rectify the situation. I've got a set of used Mamo 223 heads that Tony is reworking freshening up now and I'm going with a cam he spec'd for my setup.
This is what a reputable member here said about the cam. It's on XER lobes which are hard on the valvetrain parts. It's too big for most applications IMO, especially with a stock bottom end, but it sounds good lol.
Just wanted to confirm since I've never seen valve float at 3800. There's always a first. That spring should easily control that lobe at 3800. What is the installed spring height? What computer are you running? If GM PCM, any codes are being recorded?
Engine is out of a 2006 GTO e40 ls2. Trans is a 2007 mn12 t56. The car actually drives decent and if I ease into the throttle it'll run up through the rpm's but if you get on it it'll do exactly like the graph above. I have a reputable engine harness (BP Automotive) and everything works as it should electrical wise. I've rechecked all the engine and chassis grounds hoping it was something easy electrical. My tuner tried several tests/ work arounds sensor wise and he's convinced that it's valve float. He said he personally had this exact issue one time on a C5 Vette and stronger springs fixed it.
Last edited by showdog75; Sep 22, 2020 at 06:00 AM.
It looks more like it’s hitting a rev limiter.
Or possibly has a broken spring?
Cant picture valve float at 3800.....
Tuner said with certainty it's not tune or electrical related. He first suspected a broken spring as well. I pulled the valve covers off and there's no visibly broken spring. They're beehives so it shouldn't be hard to see a broken one.
Just a thought out in left field and you might have covered it already and I missed it, but with that much lift, did you make sure the pushrod are the right length? Have you watched the oil pressure to see if it is effecting the lifters
Just a thought out in left field and you might have covered it already and I missed it, but with that much lift, did you make sure the pushrod are the right length? Have you watched the oil pressure to see if it is effecting the lifters
Pushrods have been checked and rechecked. Oil pressure is actually super high. Not sure what oil pump is in it but it is never below 60 when at running temps.
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