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Another thing that sprang to mind is a hot-spot or oil related compression ratio pre-ignition. You can try spraying water during a cruise (or just let a vacuum line take water while you cruise) to see if the drop in temperature helps smooth the engine, diagnosing such hot-spot preignition related problems.
Your O2's may give you 128 BLM's, but if they are targeting the wrong AFR for your cam overlap (O2 voltage target), and/or if they are timing the AFR reading to the wrong BLM cell (O2 delay), you'll THINK it's all good because the PCM does. But, in reality it's giving you bad information because the PCM is calibrated to the engine and the sensors and their locations incorrectly. Injector delays need to be spot on too. \
One thing kingtal0n got right was hitting the Dyno again and loading it up in the cruise range (you don't need WOT tuning for bucking and drivability tuning, but most Dyno shops are about getting you the Dyno number, not actual drivability) that is bucking and make small adjustments.
What I did was drive from one parking lot to another a couple miles away (a 115V inverter for your laptop is nice to have for this). I'd idle the car until it was in closed loop and drive to the other parking lot in the BLM cell I was adjusting, then review that datalog and make adjustments to one part of the tune (either O2 delay, O2 mV target, BLM cell in the VE table, etc.), load that tune, idle the car to closed loop, then drive to the other parking lot, repeat. It took a LOT of driving, but it paid off in the end. Patience is a virtue when tuning for drivability.
The problem is I feel like I've done this already. I flashed my PCM so many times it broke and I had to get a new one!
Originally Posted by NewOrleansLT1
Did you check for cracked spark plugs? I had a dead miss and later found a hairline crack in one spark plug.
Yes I've had multiple sets of plugs in the car.
Originally Posted by kingtal0n
So to re-cap, Disable closed loop, run the wideband at various af ratios in open loop to see if you can stop the bucking at different A/F ratios between 14.2 and 15.5:1 more or less.
Then adjust ignition timing for cruise situation, again in the same open loop, at various A/F ratios. Try 24* and 36* btdc and anything between that for cruise situations above 2500rpm, with 13.5, 14.2, and 15.2:1 air fuel ratios. If I were there I would have you drive while I added 1* at a time with the laptop for example. If none of those help it to get better, then its probably the ignition related (because there is no misfire/miss, we know fuel is going in sufficient to fire the cylinder, that leaves compression and ignition, which we ruled out the compression, back to ignition) which brings us to: mark the pulley, run a timing gun, watch it when the buck happens.
final note:
bucking to me sounds like the plug is firing early, when it should not be. It sounds like something is interfering, or enabling the spark when there should not be any spark. it could be a faulty wire, especially a frayed or loose end signal wire that leads to a coil. There needs to be a thorough inspection of all electronics related to the ignition to find any fray or trouble wires. You can also try wiggling them with the engine running trying to detect the subtle cough/sputter/misfire that would occur when the plug sparks at the wrong time. It could also be magnetic interference by something you have too close to an ignition component, making it more sensitive to fire a plug under some situations. Do you have anything magnetic or electronic extra on the engine, near the ignition system? Perhaps an aftermarket amplifier or something....
I've tried everything you mentioned except the timing light. Also remember this happened with the stock engine management so I'm ruling out all ignition components. Nothing magnetic anywhere near the front of the car.
Originally Posted by ACE1252
Do you show any increasing knock counts as the surging is happening?
no knock ever during surging. Only rarely at high load on the freeway (still tuning that one out). When I was still running the stock PCM, I would see burst knock ALL THE TIME. Car never felt like it lost power but it would pull anywhere from 0.1 to 10 degrees and give it right back. Rarely did it behave like you'd expect where the PCM pulls a certain amount of timing then gradually gives it back. That's one of the problems that went away with 24x.
For the motor to cough/buck, without there being an electronic/mixture problem, then it is a mechnical problem. Such as a valve bouncing open or a pre-ignition hot spot. You tried water while cruising?
Another thing that sprang to mind is a hot-spot or oil related compression ratio pre-ignition. You can try spraying water during a cruise (or just let a vacuum line take water while you cruise) to see if the drop in temperature helps smooth the engine, diagnosing such hot-spot preignition related problems.
Originally Posted by kingtal0n
For the motor to cough/buck, without there being an electronic/mixture problem, then it is a mechnical problem. Such as a valve bouncing open or a pre-ignition hot spot. You tried water while cruising?
Haven't tried that. Although it runs smoothest during the first minute after a cold start. I've tried changing the tune to mimic those conditions after the car is warm (more fuel, more IAC, earlier EOIT) but it doesn't work so I've suspected the smoothness in that first minute is temperature related.
Originally Posted by SS RRR
Does the engine have an operational EGR and solenoid?
No I've removed EGR and installed block off plates. It did have EGR up until about 1 year ago though and getting rid of it didn't change anything.
I didn't see it but did you try running in SD? When my WS6 was bucking a light throttle, I couldn't get rid of it until I went in SD. I don't know if the MAF was bad, but I had tried another and it did the same. Don't know if the airflow was screwing it up or not.
Spark and fuel was perfect and it would still buck at low/no throttle. When I went to SD, it drove great.
I didn't see it but did you try running in SD? When my WS6 was bucking a light throttle, I couldn't get rid of it until I went in SD. I don't know if the MAF was bad, but I had tried another and it did the same. Don't know if the airflow was screwing it up or not.
Spark and fuel was perfect and it would still buck at low/no throttle. When I went to SD, it drove great.
Have you tried degreeing the cam to see if the timing is right? Hopefully they didn't send you the wrong cam or grind it wrong(or maybe installed with the timing off a tooth or two). I think it's rare for a timing chain to jump, but I think I recall it happening once or twice over the years.
Have you tried degreeing the cam to see if the timing is right? Hopefully they didn't send you the wrong cam or grind it wrong(or maybe installed with the timing off a tooth or two). I think it's rare for a timing chain to jump, but I think I recall it happening once or twice over the years.
Haven't degreed the cam, that's something I've never done before and seems like it would require a lot of reading. The LT1 guy I visited also said he didn't think it felt cam related if that still means anything. I had to remove the timing gears for 24x and reinstalled dot to dot so I don't think it's off any teeth. Whenever I'm brainstorming what the problem could be I keep coming back to the question: what would cause the engine to run noticeably worse after being driven on the freeway or ran at WOT a couple times? That seems like such a unique symptom I've never seen anyone else complain about I feel like it's gotta point to something.
EDIT: Just realized this is my 500th post, woohoo!
Given that the trouble seems to not be electrical given you replaced about everything under the sun, I think verifying the cam timing would be a good thing to do.....if you plan to stick it out with this engine. I got to admit, looking at some of the LS3 hp numbers with just a mild cam.....that route would be very tempting.
Is your knock sensor the right one for the LS1 PCM? I think the OBDI vs OBDII sensors are different.
When I first did 24x I never saw any knock ever. Thinking this couldn't be right I tried an OBD2 LT1 sensor then I had knock all the time. Now I'm using a 2001-2002 Chevy Express van sensor and it works great. Fits the LT1 block and was made for the 0411 PCM.
EDIT 23FEB17: Making an edit here so this thread doesn't return to the front page. I ended up doing a cam swap and that smoothed the car out beautifully. New cam is a custom Lloyd Elliot grind: 219/227 .549/.565 114+3. Also got new Lunati 73925K5 springs recommended by Lloyd.
I can see how you want to give up on this thing. But, I'm convinced it's in the tune. Your compression numbers are good, the leak down numbers are good. The displacement, cam, and SCR combination you have is good, especially for a very daily driveable car. Unless you're running 3.23 or numerically lower gears on 27" tires in 6th gear at 45 MPH you shouldn't be bucking at all.
See sig for my details, no bucking. 3.42 10 bolt Posi with 25.7" Tires. It took a lot of tuning to get here. Small adjustments over months. But I got it there. Also, my gas mileage improved as I made the adjustments to get rid of the bucking. Probably from using the taller gears. What isn't shown here, and you can't really hear it either, but once I get to 6th, I roll into and out of the throttle from 0% to 100%.
Responding to this thread because I think this fits best here. I recently sold the engine because the stock timing chain broke and the guy who bought it tore the whole thing down and sent pics. In an earlier post in this thread I mentioned the rings being a suspect for my bucking problem. Well the guy said the rings were gapped at .024. Not sure if that would cause bucking but he seemed to think it was quite large for a NA engine. He also sent pics of a particularly scraped up cylinder wall and a bearing that looked like it went through some serious pre-detonation.
I just got another message from my buyer who's still tearing down the engine. He said it doesn't have Mahle pistons as advertised, it has SRP pistons. It's starting to sound like Golen just slapped together a long block with whatever he needed to get rid of.
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