turabo87

Hi folks,

Brief background first:

I just finished my "big C5 project" last weekend. This involved going from basically bone stock to installing a mild cam (JEGS .585/.585/228/230 112LSA), top end rebuild with my stock 853 heads (rocker arm trunnion upgrade, BTR pushrods, LS7 lifters, dual valve spring kit, cleaning valves and piston tops), new Melling oil pump, resized 42#@58PSI injectors, LS6 intake, K&N Cold air intake and airbridge, and full catless exhaust with Texas Speed long tube headers 1-3/4 with X pipe and Magnaflow XL mufflers, among other "while youre there" things. Did it all myself too . Anyhow, finally took it to tuning to a reputable local tuner and he did a great job and making torque, power (397WHP), good idle and very good cruising drivability, BUT I have two issues. I already contacted him and he's going to take care of it but he's not the talking type nor to give a lot of details while I'm the learning and curious type. Anyway, I want to post my issues and understand what features, settings, and tables are responsible for this behavior. Maybe I'll get HP Tuners one day and fine tune things little by little:

1) As I coast down on slow speeds (5-20mph) and I press down clutch, the RPM's raise up to 1400 and won't settle unless I come to a complete stop, then it settle to idle at 850RPM which is my set idle speed. Although it helps not to stall, but it's a little bit too high for my taste and gets annoying to hear such high RPMs when coasting, when all I want is to hear that choppy idle.

2) On a morning cold start, somewhere between 50-60F of ambient air temp, the car would struggle to start unless I throttled the gas while cranking. Seems to be a like a temp related fuel enrichment table or setting is off for this temp, but I don't know about the HP Tuners software and what tables and settings it accesses. I was able to recreate it again this morning on another true cold start. It was around 55F this morning and did the same thing. I made sure the pump primed the injectors before cranking. It cranked and you could here a couple of combustion events but not enough to start. Then again I tried cranking and cranking without throttle and nothing happened. Then, I tried priming fuel with key on again and this time I throttled maybe 25% or more in pulses while I cranked. Took a couple of seconds and started, and I had to keep it alive by keeping it throttled at 2k rpm for a couple seconds, until it could keep itself from stalling. Once it warmed up for a couple of seconds, I was good to go and had no issues afterwards.

Just trying to learn here of the process and how the C5 ECU functions. I have some tuning experience I had once with a Megasquirt project (fully programmable ECU) on a 600cc engine, so I can understand basic tuning concepts and ECU parameters.

FYI: My C5 was tuned with HP tuners on MAF strategy, for 93 octane.

aaronc7

1.) Sounds like "throttle cracker" table. I have similar mods and since I moved to high elevation this suddenly became an issue on my tune too.

2.) Sounds like cranking VE table, Car runs in open loop for xx amount of time depending on CLT or CLT temp.... surely at 55F it's going to be open loop for a while. There is an open loop fueling table, but I doubt many people mess with that, which is why I would suspect the VE table-- values need to be lowered with a more aggressive cam since it is not as efficient at low rpm now.

I had a turbocharged miata that ran on megasquirt 2 that I built and tuned, sounds like we have a similar background. I have EFIlive for my car and kinda tinkering and learning as I go. I would recommend HPtuners though, they still continue to develop a bit for the LS platform and have some neat new products. I got EFIlive only for a convenient handheld flash tuner and black box logging at the time.

turabo87

aaronc7

BTW you can download the HPtuners or EFILive software and install, look at various stock tunes for free. You can begin your learning before purchasing anything or tuning your actual car. Both softwares actually have pretty good table descriptions and lots of great info on both site's forums (much more active DIY community on hp tuners).

Throttle cracker "cracks" the throttle open by X amount when above Y speed (tables for both).

turabo87

Hey Aaron, so I took my car back to the tuner and he fixed the throttle cracker as best as he could. It's pretty good now, however he believes I might have a small vacuum leak somewhere that's keeping the car from being at idle when coasting. Only a few hundred RPM higher so it doesn't bother me. Anyhow, he also said car needed more airflow on cold start on cold temps so he tweaked some values. Made it better, but still needs my throttle to keep it from stalling in a cold 50-60F morning for first 10-15 seconds or so of the cold start.

I figured it would be easier and more of a learning experience to get HP tuners and an OBDII dongle. How much is the HP tuners download? Then what is it that I need besides the software and the dongle that actually costs?

aaronc7

I don't have hptuners so I'm not really an expert, but I have been looking at the website out of my own curiosity... The MPVI2 is the new dongle/interface. There is standard and Pro.... standard is $300, Pro if $550. Pro adds blackbox logging (no laptop), bluetooth connectivity, expander hub which allows external analog and serial inputs (think Wideband). Then credits are $50 per.... you either need or one or two for a GM vehicle, not sure.

The pro is a decent price jump, but I would get that if you have the desire to wire in a permanent wideband to the car. Also the BT connectivity would be convenient at times for me and works with their own "track addict" app etc. If you get standard, but still want a wideband, there is a workaround via routing analog signal to the unused EGR port on the PCM. I used to do that, but now I have wideband connected via serial cable and have fuel pressure going into EGR port.

https://www.hptuners.com/mpvi2/

Hope that helps

turabo87

Aaron, my tuner fixed pretty well the "throttle cracker", but I still have issues with problem #2). To give him the benefit of the doubt, it's been trial and error because I haven't been able to drop the car at his house or shop over a cold weekend for him to fix with the coldstart conditions. It's been more of me just taking the car already warmed up and him tweaking some values. I'm going to give him one more shot before I just get the HPTuners hardware and fix it myself. I have more observations on the cold start issue: The car starts when old (just a couple of combustion cycles) but would stall IMMEDIATELY unless I hold throttle down anywhere between 5-10% ballpark, and it's basically like this until Coolant Temp hits EXACTLY 100F. When it hits 100F, it's like another table or logic takes over and immediately takes control of the throttle and raises it until it continues to warm up and slowly tapers down to idle on its own. I've been able to recreate it every cold morning of this week and it behaves the same: it won't fire up without gas and it won't idle on it's own until EXACTLY 100F, and the change of idle control/throttle at 100F is huge. Do you still believe it's Cranking VE table?

C5 Diag

Have you looked at your ECT sensor ??...you can check the resistance of the sensor with an ohmmeter...if the car "thinks" it's warm you won't get that increased injection pulse you need for cold starts !!....your tuner should know this....if you want the temp vs. resistance table I can pass it along.

turabo87

I can get my ohmmeter and check but I have no reason to believe it's not the right resistance. For example, my Oil Temp and Coolant temp match exactly on a cold start at any given condition. If it's 60F outside, my ECT will mark 60F and oil temp sensor will mark 60F. Same thing at 70F or 80F or whatever. These are readings from the DIC.

aaronc7

Yeah, the DIC gets its info from the PCM, if that is correct I would not be suspect of the coolant temp sensor.

Cranking VE table may be part of the issue-- the initial cranking/starting issues. But after the car is running the Cranking VE table is N/A. The car will generally run open loop for a certain amount of time and/or coolant temp is reached. Once that is reached, then it targets 14.7:1 AFR and the narrowband sensors get to work. It may be running lean while in open loop. Generally the purpose is to run a little rich during this open loop warmup period, but if some values are off, it could be the opposite. We really don't have any way of knowing without a wideband. You can look at narrowband output voltage but that is a bit of a crapshoot. But I believe it will be able to tell you if you are running leaner or richer than stoich-- that could at least confirm or deny the lean theory.

Sorry don't have the software in front of me right now to give specific advice and my car has never had these issues, so I have never really dug into this on the LS platform. But the idea is the same as it would be on megasquirt or about any ECU. there's a few differnet tables that define things like coolant vs rpm, open loop fueling AFR, how long to stay in open loop, etc.

It sounds like you are a DIY guy... the best thing may be for you to get a wideband and hptuners so you can start tinkering on your own. I said I wouldn't touch the tuning on this car, because surely it has to be down to a science by now...... but ultimately I got my own tuning solution due to not being happy with end results and I just like to tinker anyways.

turabo87

Narrowband? I thought our C5's primary 02's were wideband.

C5 Diag

Ok, so we will assume your ECT sensor is OK. Don't know if you have a scan tool but I look at ECT vs. IAT first on the scan tool...they should be within 5 degrees F of each other...try disconnecting the MAF sensor and see what happens....the car will initially cough but then it should run OK.The PCM looks at ECT, TP, and MAP for your starting A/F ratio...you should look at the MAP sensor also...and yes, you might have a vacuum leak too as your tuner had suggested....once again you'll need a scan tool to look at your fuel trims...if you have a vacuum leak your long terms will be elevated...run the engine at 3,000 RPM....if it's a vacuum leak your short terms will go negative and long terms will come down a little slower...I'd also check your throttle body and see if it's clean...I usually don't touch "tuned" cars...I'll tell the owner let the tuner fix it !!

C5 Diag

No, they are narrowband !!

turabo87

Thought I'd provide an update and set some closure on this thread. Took the car back to tuner and this time I left it as his house. It was best for him so he could have it a couple of consecutive days to fix the cold starts. Apparently after tinkering with it a couple of days, he came to the conclusion that the tune became corrupted at some point because just so many tables were wacky. He started from scratch again and reflashed the first tune he did after the dyno tuning and did his magic from there. Long story short, got the car back and it's dialed in perfectly. It even idles less rich now and doesn't stink up my clothing anymore.

aaronc7

I find the idea that the table just became "corrupt" to be highly unlikely.... If the car had been previously tuned by someone else, there may have been some changes done that this tuner wasn't aware of and was causing conflict...I find that to be the more likely scenario.

I like to start with a stock tune when going to a different tuner for this reason, or at least give them the history so they are aware to look for anything weird. Usually still takes a couple revisions to find a few more missed tables etc, in my experience.

Glad to hear you got it sorted.

turabo87

It was same tuner, beginning to end. Regardless, he solved it. Car drives better than ever and funny enough the best thing for me is that it's not idling too rich to stink my clothes.