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Cranking Calibration Parameters - GM Gen3 PCM

EFI Tuning Fundamentals

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Background - Engine is a 1995 GM LT1 upgraded to GM Gen3 PCM with higher performance camshaft, throttle body, injectors, individual coils, headers, 10.5:1 compression, etc. It is in a 1995 Corvette with race suspension and used for Track Days only. I am using HPTuners software and it works great. Overall the car runs great, handles great and is really fun to drive.

I have done quite a bit of tuning and the engine is running really well for warmup, open loop, closed loop and WOT but having issues with cranking/starting. I can get it to start every time but large variability in how easily and how long it takes to start. The fundamental problem is that it either gets too much fuel or not enough. I understand what most of the cranking calibration parameters are for but not how they interact or when they are used.

Specifically the FA Mult Stage 1, FA Mult Stage 2, Soaktime Mult, Prime Pulse, First Pulse, Second Pulse, Delays, etc.

Hi Clark, one of the nice things about reflashing on this platform is that typically if you have the MAF/SD dialled in accurately and your injector data also matches the injectors fitted then the cold start/cranking seldom needs any tweaking. On that basis I have only ever made changes to the cranking fuelling on a handful of occasions so my help here may be limited. It does suggest to me there may be another underlying issue though.

What is the commanded EQ ratio during the cold start process? This could be part of your issue. The other areas I'd ensure are correct first would be your base running airflow (particularly this will need work with a larger cam, and your cranking VE table.

With specific regard to the parameters you've listed the FA Mult Stage 1 (later PCMs don't have all the listed parameters) is where I'd start making changes. I'd make a change of 2-3% and assess the affect before going further.

The engine is running really well for warmup, open loop, closed loop and WOT. Ran a Track Day last Sunday Oct 9th (at least 50 laps around 1:20 each lap) and it ran great. There are no DTCs. LTFTs are about 5% (your webinar on the subject got me started tuning this engine, Thanks!). Most of the time the engine would start instantly and idle just fine. But 1 or 2 starts out of 5, it would not. Key off, retry and same problem, key off and retry and starts. Strong fuel smell and data showed same. It would get a very large fuel pulse on those times when it wouldn't start immediately. I didn't change the cranking VE table because sometimes it would start instantly and didn't seem like the problem was there. Same reason I didn't change the FA parameters. Problem appeared to be related to Soaktime and Prime Pulses.

When I essentially disabled the Prime Pulses and Soaktime parameters, the problem went away but took longer to start (3-4seconds as it wasn't getting fuel until later in the crank cycle. What I'm not clear on is what is the purpose of the Soaktime parameters. When are the fuel pulses supposed to occur - Prime Pulse, First Pulse and Second Pulse? What might cause a very large fuel pulse during cranking 1-2 out of 5 starts?

Really appreciate the help, Thanks!

The fact that you're getting inconsistencies under the same conditions is a little concerning in and of itself. This is going to be tricky but I'd be trying to pin point in your logs the exact conditions that are resulting in your over fuel situation - Easier said than done. As mentioned, I really don't see these kind of issues so I have not seen the need to dive deep into these specific tables. On that note I can't really add much more than you've already found sadly. If you found that disabling the prime pulses and soak time parameters helped the over fuel issue but slowed the starting under normal conditions I'd suggest perhaps making less dramatic changes to the tables - perhaps start by making a 25%-50% reduction and test. I'd also make these changes to one table at a time to get a feel for the affect of each change.

It is odd that the problem only affects cranking and only intermittently while everything else is good. I will take your suggestion and make changes as you describe to see the effects. I have scoured the internet and looked for books covering GM PCMs but actually could not find anything detailing the cranking parameters (or even most of the parameters). The books that I did find do not go into enough detail.

HP Academy's videos are first rate and significantly better than anything else I found anywhere. Thanks again for your help.

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