necrocannibal

iTrader: (5)

I am having a similar issue so I will chime in, my issue seems to only be under 2000 rpms though. A few weeks ago I started having voltage issues where my voltage would drop to 11-12 after an hour or so of driving. That would cause my pumps to not function efficiently, I have two, and cause lean idle and cruising. I found I had stripped battery terminals on both positive and negative terminals so I bought some new ones and tightened them down good and tight. Now my voltage doesn't want to drop below 13, usually stays around 14 except when I let off of the gas it will drop under 13 for a second I believe due to under drive pulleys. When I let off of the gas in third or fourth gear and I am around 2000 rpms I get a jerky bucking and my afrs are in the low 12s to high 11s. My thought is it was tuned with those terminals like that not making good contact and now with the new terminals it has thrown my tune off because my pumps are getting more voltage and causing this rich low rpm bucking. I would definitely look into that tune.

ILuvPizzaTimes10

iTrader: (1)

Use cracker not follower to smooth out low the rpm cruise. I used to have the same issue. I changed the injector timing event to after EVC. And increased cracker at low rpm. Went with big jumps to see change. Once you get a" cruise control" feel back it off a bit. Should help allot. I have a 242/252 @ .050 cam and had a fidanza in there.

murphinator

iTrader: (2)

for what he ^ is saying search for "EIOT" on here

I have never done any tuning on anything cammed but when a friend bought a z06 with bolt ons and asked me to check the tune to see if it was ever modified injector timing in relation to valve timing was one of the things changed that I hadn't ever messed with before so I went in search of why and found discussion on eiot's on here and let my buddy know his car is also cammed lol

AdsoYo

iTrader: (1)

I'm starting to feel pretty reassured about looking elsewhere for the cause of this problem now haha

When I'm cruising around town, rpm's are about 1800-1900. The CC503 is one of the most common LT1 cams and I haven't heard of anyone else dealing with this. Makes it even more frustrating.

Since getting hptuners I've tried adjusting cracker and follower tables with limited success. I've also adjusted EOIT so the fuel injects after EVC. That actually seemed to improve things slightly. Since then I've adjusted other things and returned EOIT to stockish timing. Recently within the last week I feel like I've had somewhat of a breakthrough. I increased spark in cruising cells from 41ish to 44ish, threw a bunch more air at it with the cracker table, and fattened up the mixture to about 14.3. This combo produced a noticeable dampening in the bucking. The guy helping me from overseas said he's tuned engines with more cubes than mine, with way more wild cams than mine, and this is easily the most temperamental engine he's ever dealt with. We both suspect something is wrong in the long block+intake somewhere. Now the questions are: what inside the engine would be allowing it to feel perfectly smooth for the first 1-2 minutes after a cold start, then start bucking after it gets a little warm? And what would cause the bucking to feel worse after going WOT, driving on the highway, or just driving for an extended period (like 45+ minutes)?

Yeah sometimes guys like to retard the injection timing so none of the fuel gets sucked out the exhaust valve during overlap. The only downside really is that atomization of the fuel isn't as good since it doesn't get to sit on the hot valve for a second.

I've also suspected maybe there is an electrical gremlin somewhere contributing to this. I know all my main grounds are good but maybe there's some little obscure wire or ground somewhere that's giving the car trouble. The engine wire harness is brand new and my big 3 are upgraded, but all the other wire harnesses are 22 years old at this point. It seems possible to me since the car usually feels great in the first 2 minutes after a cold start and once it gets warm the heat could be irritating a bad wire somewhere. I've been dealing with this problem for so long though, sometimes my ideas get a little out of control.

Darth_V8r

iTrader: (4)

One thing that really helped me stabilize the IAC was to edit the derivative air tables and zero out the 0.00-0.05 fields. Set the next one to 1.75, and the rest to 2.

This way, when the cam is naturally chopping a bit, the controller doesn't see a slope and pre-emptively open or close the IAC. Stays much more stable this way.

AdsoYo

iTrader: (1)

That actually helped a lot. The car felt great for about 7 1/2 minutes instead of 1-2 minutes this time. Then the bucking gradually worsened from that point. Email sent.

ILuvPizzaTimes10

iTrader: (1)

Also read egts across the engine after a long cruise. I had the iac bypasses on my TB. So i corrected that as well. Along with a small hole drilled threw the iac inlet. This all helped smooth out idle egts and cruise egts. I was getting a ton of air to front 4 cylinders and little to rear 4. So i was over 400 egt on front cylinders and under 300 on rears. Not anymore after tb fix.

AdsoYo

iTrader: (1)

figured I'd update for you folks that helped. Took it to a shop whose owner is considered THE authority on LT1's. He's a member on this board with an 8 second LT1 Camaro. I gave him the entire background story of the car and told him everything I'd done and tried. Long story short, he said it's the rings. So now I get to decide what the next step is. I'll be starting a new thread about this but wanted to say thanks for the help.

showdog75

iTrader: (8)

The rings are making your car buck? That's a new one for me.

BALLSS

WOW...not saying THE expert is wrong and know this is the "short"" version of the story.....but the "why" is way above my pay grade if rings can cause this

after re-reading your initial post it seems the car behaves fine under certain RPM's/speed so having bad rings, to me, would exhibit the bucking thing all the time....but again that reason is something I don't understand the "why" part

I would have thought a tuning issue and/or IAC not happy because of a lager TB but you did the drill mod IIRC.....very weird

for your wallets sake I hope rings are not the issue.

TheJDMan

Rings? I would like to hear the reasoning behind that diagnosis. Seems to me a compression test and leak down test are in order just to prove or disprove it. But IMO, at 1800-1900 rpm you are very close to lugging the engine which is likely why it starts bucking combined with the light FW which just makes it more pronounced. A simple test that will cost you nothing, try adjusting your driving style for a week or two to keep the rpm above 2000 while you are driving around and see if it smooths out. I personally like to be between 2000 and 2500 most of the time I'm driving.

stevieturbo

These are large V8's, I could drive mine for a thousand miles and never even go above 2000rpm ! And still manage illegal speeds with ease.

Absolutely no need to operate above 2k for just knocking about unless it's got some crazy cam or really short gearing

jmd

iTrader: (4)

You're refusing to try the easiest diagnostic test that would 100% answer your question, and very potentially, solve the problem.

The absolute best part is . . . You can avoid being a bullshit jockey and validate 3 of the below 5 items by doing before and after dyno test and use of a scale:

Have fun!

lsx55

I highly recommend taking someone for a ride who can diagnose the bucking firsthand - you should easily be able to tell if it's the flywheel or not.

Jonesin10

I have the same issue. Low rpm in 5-6 gear the car gets jerky/bucking feeling. Also if you don't let the clutch out perfect with a lot of rpm she will buck hard. Clutch is spec3 plus with fidanza aluminum flywheel. Talked to my tuner he said get rid of flywheel. I'm gonna throw a ls7 flywheel in it an see what she does.

kingtal0n

iTrader: (1)

I just went through this with 2 cars in a row. A very slight, almost imperceptible jerking while at very light highway loads.

In car #1 it turned out to be faulty VR sensor shielding / noise in the signal wire. I caught this by logging the tooth count and found out every so often it would fail to catch the missing teeth on the trigger wheel. Solved it by adding resistance to the transistor base in the ECU responsible for setting the low voltage limit for 'going high'. This is on a megasquirt ECU which has a potentiometer specifically for this reason.

Car #2 it was injector phasing issue. The fuel injector was opening too early( or late, depending how you look at it ) and spraying through overlap session. Solved it by correctly phasing the injector to the cylinder #1 tdc (so it opens slightly after TDC exhaust stroke while cruising rpm). After this 'fix' the engine ran richer at idle, and leaner at WOT, it seems that the fuel was leaving the cylinder through the exhaust valve, so I expect an economy boost. This was on 2jz-gte engine and AEM ECU.

I would say, if the ECU you are working in does not have a phasing setting, try playing with the injector dead-time (open delay vs voltage) adding some value here might do the trick.
Another thing I have seen cause a slight buck around cruise is the cam timing. "big camshaft" sometimes have just the right valve timing to give this behavior. I had an intake camshaft in a 2.0L that when it was advanced 20* using electronic valve timing the car would feel like that- slight buck/misfire feeling at light loads. When I turned off the 20* advance solenoid the feeling went away. So cam timing, injector phasing, trigger sensor noise are what I am pointing at.

Any imbalance in the drivetrain rotating parts like tires or driveshaft/flywheel should get worse with rpm/speed. Also you could be having an ignition fault where the high resistance/lean cruise condition causes a misfire, sometimes people use the wrong plugs or plug wires and this can happen, or if the combustion chamber itself is somehow misshapen from modifications that make it harder for the flame to... propagate... or whatever the terminology these days is..