Are the parameters that determine a/f on a cold start during open loop fixed? In other words, ya got what ya got till someone goes in and make a tuning change (via SCT), and that there are no adaptives that will alter your a/f during that run time?
I was half-wondering about this too. My car has a nice, lumpy idle with a cold start, but once it warms up it gets all goofy. Does the PCM get feedback from the rear O2s once it switches to closed loop? I'm wondering if an O2 ate shit but the SCT has been told not to rat them out.
Yeap, open loop (cold start/operation) is all fixed parameters. Once the motor is warm, it starts listening to the front O2 sensors.
The rear O2's are ONLY used to monitor catalytic converter health. O2 sensors oscillate from rich to lean, because the ecm constantly makes changes in closed loop to get you as close to stoich as possible. If the rear O2's have the same oscillation pattern as the front O2's, the PCM knows that the catalyst is fuct. The rear O2's should be closer to a straight line than a waveform.
I'm not that familiar with the Gen2 PCM's, but I know our C6 calms the shit out of the new cam when it's warm. I can fix it in the tune, but just haven't gotten around to it.
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Andrew MacPherson
1995 TT RT/10
Why these issues tuning these cars in with the SCT? I thought the SCT was supposed to be the hot ticket for tuning. My VEC II tuned car runs great.
I like and have a VEC 3 in my TT car, but it is only good at tuning WOT or close to it. When you go into WOT, the car goes into open loop and ignores the o2 feedback and such. When your car is in closed loop cruising around, you can make all the changes you want in the VEC, but the stock PCM is going to fight it back to what it "thinks it should be".
The great thing about the SCT is you can tell the stock PCM what things "should be" lol.. I would love to be able to use a SCT.
Can't you just buy a Gen. II PCM and redo the entire fucking wiring harness? I mean, it's that simple, right?
lol, I don't mind the work, and I like wiring, but for all the $$$ and effort, I would go the Megasquirt route and would be MUCH further ahead. Believe me, I've thought about it.
Yeap, open loop (cold start/operation) is all fixed parameters. Once the motor is warm, it starts listening to the front O2 sensors.
The rear O2's are ONLY used to monitor catalytic converter health. O2 sensors oscillate from rich to lean, because the ecm constantly makes changes in closed loop to get you as close to stoich as possible. If the rear O2's have the same oscillation pattern as the front O2's, the PCM knows that the catalyst is fuct. The rear O2's should be closer to a straight line than a waveform.
I'm not that familiar with the Gen2 PCM's, but I know our C6 calms the shit out of the new cam when it's warm. I can fix it in the tune, but just haven't gotten around to it.
The way catalysts work is that they need to experience a range of A/F to remain active and viable long term. You will never see close loop AFR exactly at 14.7...but an oscillating range. I don't have time to get into the specific chemistry regarding what a rich AFR does to the catalyst and then what a lean AFR does, but together, they keep the catalyst active and able to do its job. So what I'm saying is that by design, AFR in closed loop will not stay 'fixed' at 14.7...even though it is technically possible to tune to exactly that AFR. Cat convertors wouldn't last long if that was the case.
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Why these issues tuning these cars in with the SCT? I thought the SCT was supposed to be the hot ticket for tuning. My VEC II tuned car runs great.
Based on the inability (so far; 3 tunes have not fixed it) to tune my Roe car with the SCT, and the explanation to me of the limitations of SCT tuning a Roe car, I would say that SCT is not a good platform for the Roe. I have however been talking to some other sources that have explained how to make it work correctly.
I'm pretty sure SCT has parameters that would alter cold startup open loop behavior.
I thought I saw things like:
Battery voltage/temp fuel enrichment compensation, IAT, ECT spark/fuel multipliers and so on.
UPDATE: I can't think of a reason why SCT wouldn't work with a ROE unless the VEC is needed for a more advanced feature like individual cylinder trimming.
Everyone is correct, the PCM doesn't learn on cold startup, trimming/adaptives shouldn't be occurring during this time.
Last edited by dirtbag; November 15th, 2012 at 12:16 PM.
I'm pretty sure SCT has parameters that would alter cold startup open loop behavior.
Yes they do. My car dying multiple times during cold starts is one of the 3 tuning related issues it has. Trying to die when accelerating and trying to die at intersections being the other two. I was just trying to confirm that the cold start one would never change unless someone did something i.e. no adaptives at play here.
Yes they do. My car dying multiple times during cold starts is one of the 3 tuning related issues it has. Trying to die when accelerating and trying to die at intersections being the other two. I was just trying to confirm that the cold start one would never change unless someone did something i.e. no adaptives at play here.
Dave:
Maybe what follows means nothing and you have a mechanical or electrical issue. Anything is possible, but, dying during idle would typically indicate a malfunction not a fuel table issue on a NA car, that SC complicates the diagnosis. What level of vacuum or pressure do you show during cold idle, that could tell us a lot.
Make sure the injector pulse values have been modified correctly (for the larger injectors) in the base fuel table, that would cause a stumble for sure. There is also more than one fuel map table you have to modify. As it relates to stalling at intersections. if you have raised the idle rpm with SCT and have not raised the deaccel rpm. you could force the car to stall. One problem with SCT, it is easy to enter an incorrect value where you are modifying multiple cells. It is always best to use the compare feature and review the new with the old.
Remapping of Fuel Tables Versus RPM and Load:
Have they tried to generate a table of a/f versus rpm and load. That is the only way modify the base closed loop fuel table to get driveability.
1. With SCT turn off the long. term fuel trim (LTFT).
2. With SCT increase the rpm limit for closed loop. I believe open loop is now forced by the PCM at approx 3500 in my PCM version. I changed it for the table building to 4500.
3. Use an OBD input to your laptop with a program such as Palmer Engineering. That program allows you to define custom logging tables. The idea is to construct a table that logs the a/f for rpm versus load (vacuum).
4. Take the car out and drive at various speeds and engine loads. As you drive, the table cells will be populated (multiple times) with a/f values in those cells covered by your rpm and and load range. Typically a 30 minute drive will give you enough data to make changes.
5. When you review the table, you might show the cell at 2000 rpm and a given vacuum value shows lean, you then use the SCT to add fuel at that cell in the pcm fuel table.
6. I had a bad stumble at 2000 rpm and partial throttle, it took three session to get the fuel table close to where it should be. In my case the fuel table was off due to heads/cam.
7.There are also places in SCT where you can modify the response of the injectors to changes in load parameters, I found that reducing most of the delays to lesser values made the pedal transition smoother. In your case that probably is more important due to the forced induction.
You have a compound problem because the pcm does not know what to do with manifold pressure above one atmosphere. In your case if you build that custom table and drive the car, in partial throttle cases you will be able to record the offending rpm/load. Again, this is easy for an NA car, it may be far more difficult with forced induction, especially with the ROE style charger.
When the table is finished the LTFT and open loop rpm have to be returned to oem values.
Last edited by Jack B; November 16th, 2012 at 09:38 PM.
Good info there Jack. I will say, that if there is a mechanical problem, I paid for an installation, and would have expected mechanically for it to have been either installed right, or issues discovered and corrected by the installers. So, regardless of why, if it's a mechanical issue, I put the blame on the people I paid to deliver my car back to me in running condition for that problem. Based however on conversations with other tuners, I am leaning towards a tuning problem.
A little better description of the 3 problems my car has had since I got it back. At least how they remain now, on the 3rd tune.
-> Cold start / open loop. Lugs and chugs and dies several times before closed loop kicks in. Pretty sure that's the easiest one to fix. They made an adjustment to richen it with the last tune, and it made it worse. Probably need to go the other way, but I'm not the tuner.
-> Accelerating from idle. In other words, taking off and driving. If you try to casually give it gas and clutch together, it will lug and chug and nothing you do short of slipping the clutch and getting the rpm's up will address that. Simply giving it more gas when it bogs results in making it worse. Well, until it finally recovers and all of a sudden you launch lol. So, the way I have to drive the car now is doing the 3000 rpm slip clutch launch. Yeah, it's badass at times, but sucks in stop n go traffic, and I about killed some kids and adults at a school crossing the other day. A 3K launch with my side exhaust is deafening, and as they stood there covering their ears before I even went, it was like this is gonna sting a little...
-> Regarding it wanting to die at intersections, it hasn't yet, but it sure acts like it's going to. Not once you've stopped per se, but as you're rolling up and dump the clutch, it will drop the rpms well below normal, surge and recover, and drop them down a few more times before finding some level of running that works for itself.
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