Ok guys, I got my car dyno tuned today for my H/C. Something very strange is happening that is affecting my low rpm, full throttle timing (hell, part throttle too!). In the log on HPTuners is seems to be MAF related, but we switched meters, & the problem was still there.

The tuner would start the pull at 1800 rpm in 4th (of course), & the MAF readings would fluctuate wildly, which would cause the timing to be retarded to 16.5 total. This would happen until 2500+- when it would straighten up, & the run would go flawlessly. No matter what he tried, he couldn't get it to stop doing this. We made 7 runs, & it repeated itself, identically, every time. Once I get a picture or a scan of the dyno graph, you can see it.

This is of course effecting my in town driving. Sluggish isn't a strong enough word to describe low rpms. You can actually feel when the timing goes back up. It almost feels like a small shot of nitrous, that's how dramatic it is. I feel like I'm driving a 4cyl. that sounds like a V8, in low rpms.

He worked on it for hours, but we ran out of time today. He said that he would continue researching it, but I thought I would do some of my own. Peak numbers were low, because whats the point in tuning for max hp, when low rpms are so screwed up. This looks to be the begining stages of doing my own tuning. Might as well start learning about this stuff now.

IDEAS??

 

was the problem there prior to the tune????????

 

I'm not gonna say this is perfect but I have installed and tuned a number of cams and this is the process I go through to first get it to idle then work into a basic tune. I'm sure others may have some more elequent methods but this works for me. Perhaps your tuner has already done all of these to no avail. If so, then I would say something else is going on here.

- did you drll a hole in the throttle body with the cam installed? Ussually a 3/16" or 13/64th hole is enough.

- Set the Engine/Idle/Target Idle Speed vs.Engine Coolant temp up to about 1350 across the board to around 133 degrees and 900 above that (depending on cam size and desired rumble while not surging).

- Play with the VE table (Volumetric Efficiency) between 400 and around 4000 rpm. Multply those columns by .95 or so until she starts to smooth out (Hopefully). You're probably getting some surging/bucking in that range. Keep going by increments until it smooths out but doesn't hesitate

- For now, back the timing off about 2-4 degrees below 4000rpm in the High Octane Table until you get it to idle properly.

- Reset the LTFT's (long term fuel trim cells) through the HPT-Scanner VCM Control function under the tools menu and then run the engine and watch the LTFT's through the table view. If they are positive (lean) for a quick adjustment lower the Injector Flow Rates in about .50 increments until they are in the 0 to -8 range. Lowering the injector rates tricks the PCM into thinking you have smaller injectors and has to work harder so it actually has the reverse affect. Due the opposite if the LTFT's are very negative by increasing the injector flow rates. Ideally you would want to run the engine at part and mid-throttle over different loads for a period of time to get accurate LTFT readings.

- If you have an oil based air filter (K&N or Blackwing) make sure the MAF is clean.

Each change can affect other tables so make adjustments in small increments and only update one area/table at a time and check the results.

My .02 and good luck.

 

The problem was not there before the h/c swap, but was there after the swap but before the tune.


78Vette-SA, thank you very much for taking the time to go through that. I'll pass it along to my tuner.

 

Let me know how you make out and what your tuner has to say.

Good luck...

 

Well the car has been at the tuner since Monday, no change. It's really strange, as they've never had this problem on any car they've tuned. They even put a few calls into LGM. They did manage to straighten out the idle, & hot starts, but the timing issue is still there.

 

Could this be a problem with the cam itself? Maybe the cam sensor "wheel" thats made into the cam is off a little and only shows up at lower rpms where its not spinning so fast...?? Who knows? Weird..

 

what is the power range of the cam? My cam is a 2000/6500 and it acts sluggish anywhere under 2000.

 

You stated that the maf readings are all over the place at low rpm's. Did you disconnect the maf and see if there was a difference in low rpm performance?

 

Any codes??
What do you mean by the MAF readings would fluctuate wildly??? Can you post a scan?
Has your Tuner tuned a C5 before? If the VE table/Dynamic Airflow and MAF are out of range it will usually (not always) set a code (MAF related) and drop to the lowest available timing value at that g/cyl and RPM.

Looking at the tables for a 2000 C5 it looks like the low octane table is lowest available at or near WOT/ (g/cyl). Did you try copying your High Octane table to the Low Octane Table - which would give you 25 - 29 * of commanded Spark?

If the Tuner has EFILive/FlashScan he scan with Spark PIDs and determine the source of the spark advance -16*.

 

No codes since the tune, before I was getting random cylinder misfire, but no MAF related codes. He has absolutely tuned other vettes. Just recently a triple black very w/ a 408, & a much bigger cam than mine.

"Did you try copying your High Octane table to the Low Octane Table - which would give you 25 - 29 * of commanded Spark?" He specifically mentioned doing this, with no change before 2750 rpm, it still only shows 14* timing.

Running it in SD mode clears it up. With the MAF connected, the weird thing is, that no matter what he does, & he assures me he has tried everything, it will not get over 14* of timing before 2750 rpm, EVERYTIME.

Another thing that just started is that the dash will randomly quit. It did it twice last week. No lights, no gauge sweep, the DIC doesn't work, press the traction control button = nothing, the door ajar chime doesn't work, etc... Disconnect the battery, & it starts working again. THe car will turn on, but the gauges don't work, it's weird. Could this be a ground, or BCM PCM issue?

I don't have a scan of the MAF readings, but here is the dyno graph. Check it out before 2750.

 

Maybe. If it happens immediately after a flash, probably not. (What you describe sounds like something commonly observed with a variety of programming tools.) OTOH, if it really is random, then yes.

 

No MAF codes thrown? I had a MAF wire eaten by the PS pulley that was on the bottom of the bundle that feeds MAF. Intermittent and drove me nuts.

 

When I initially put my H/C in (before the days of VE/NoGo/ChrisB/gameover/SD tuning) it would "fall off" the MAF at lower RPM and drop to the lowest available timing - I spent months trying to eliminate it. There were no MAF related codes - None!! GMPX/Ross at EFILive (PCMTech) helped me quite a bit in deciphering the problem. If the MAF and VE were off by a substantial % then it would drop off the MAF and pull timing . Timing would drop to the Lowest available value - In my case this value was often the Closed Throttle/Parked table. I could prop up the Timing by increasing the lowest table value at a given RPM g/cyl. Unfortunately, I did not know how to correct the VE values back then.
My cam is similar to yours 239/251, 106 lsa.
Not saying this is the same problem...just trying to add some insight - especially on a lack of codes.
FWIW.
Good Luck, keep us posted.

 

Nope, no MAF codes at all.

 

I would bet this is the cause of your problem. Have the tuner adjust the MAF table to agree with ve and see if that solves the problem.

 

Any updates ???

 

Newbie Here, Just tuning in so I can learn from this. Hope you're able to find what's causing the problem.

Good Luck

 

No updates yet, car hasn't been touched since Friday. Still sitting at the shop.

 

Could torque management be causing this?