PGDimpala

Hi,

I have an LT1 running Gen 3 PCM 24x. Need to diagnose an annoying starting issue. Hoping to engage those who have the experience and wiser than I.

If I turn key to on, fuel pump kicks on and primes. When I go to start the engine it will start to fire cylinders immediately but runs like crap, low rpm and rough/sputtering. Then if I give it some gas it will start to run smooth, then I can let of gas and it will idle fine. From there car runs, drives, idles, and has great power. As long as i keep it running it is great. Just as I would expect.

Now to correct starting issue I do the following.
If I press and hold the pedal ever so slightly with foot, maybe give it ~5-10% throttle, during cranking....It will fire right up strong. Of course it revs really quick to 1500-2000rpm becuase I gave it some throttle. Then I let of throttle and it will return to idle. From there totally normal behavior in all operating conditions.

In either instance when I give it gas I usually let it return to idle over a 5 second period by slowly releasing pedal. If I quickly drop too fast it will stumble again. After this slow return and it goes to normal idle, i can then open and close throttle as fast as I want and the stumble never happens again.

Any idea why the starting is doing this? Based on the car running great after this initial start I feel like my parts are all good. Leaning me towards a start/idle tuning issue? I have HP Tuners what should I log for diagnosis? What is everyone's best guess for why this is happening? Should I clean IAC, throttle body, and MAF?

Its just a starting/cranking issue seems.

Thanks, Paul

ctd

iTrader: (1)

Does this help at all? It's my start up config.

PGDimpala

Yes this will help. I will setup hp tuners in the same way and log. Then do a compare and post results.

MontecarloDrag

iTrader: (3)

It needs more fuel and maybe a little more airflow. Most afterstart problems are caused by lack of fuel

PGDimpala

Im going to check my fuel regulator health, but are your additional fuel and air suspicions from a tuning perspective? As in i need to change startup fuel and airflow tables?

MontecarloDrag

iTrader: (3)

Yes in the tune. If the engine runs good and only the startup is the problem, change cranking and afterstart air and fuel, it will cure your problems. I have fixed several without problems, even a few stock cars

AdsoYo

iTrader: (1)

What did you use as your starting file to get your 24x up and running? I used a stock 2002 F-body tune and it starts fine for the most part.

JakeFusion

iTrader: (7)

Check your fuel pressure. I added a ton to the after stat enrichment and pulse tables. Turns out my hose from the pump to the top of the basket was slightly loose, and I was allowing fuel to seep out between starts. I returned all the tables to stock and it fires fine.

In my opinion, you shouldn't have to touch those tables if your injector data is correct and your fuel pressure is verified and your check valve and/or FPR are operating correctly.

PGDimpala

So have done some tests.

Please see my attached scan log.

In this log I turned the car on and let it do its low rpm start stumbling. Over the course of a minute it increases rpm until it finally comes to happy idle point.

Again, with this very same tune if I give it 2% TPS throttle it will fire to say 1500rpm right away and i can let it down slowly to happy idle.

If i let it start by itself with no help from throttle. It does like the scan shows and idles at 500rpm for quite some time, stumbling and such before it starts to slowly increase rpm all by itself.

I did check my fuel pressure and it is good. Bleed down take many hrs so i think my check valve is also good. I have 41psi at idle key on and idle (LT1).

Something to note is that if i let it warm up to normal temp (say 85 degrees C) and turn it off, then turn it back on it starts MUCH better and almost "normal" like it should. So seems to be cold start thing.

Also I tried adding some airflow to startup airflow table in the low ECT range and it started even worse.

Other useful info is that once warmed up and idleing my iac counts are between 40-55 while in park.

Thanks, Paul

AdsoYo

iTrader: (1)

So what start up tune are you using? Fbody? Express? Your own creation? It looks like the PCM is trying to achieve your commanded rpm of 1,050 with a wide open IAC valve and low afr. For cold start, try setting friction airflow to 0.7 and startup airflow to 0.132. Afterstart enrichment adder set to 0.250, initial adder vs IAT at 0, p/n drive enrichment at 0.1. That's all I can think of for now

PGDimpala

Hi,

Did some more fiddling today. Still on-going.

My tune for this conversion is from my tuner who I have used for my LT1 pcm for years. Mail order tune, always worked well. for the 24x Conversion I simply requested an equivalent tune from my LT1 pcm into a 411' gen 3 pcm. With the necessar injector banks updated, firing order, etc, etc. All in all once started and I make it idle the tune seems to work similar to old tune. Just have this new starting issue i never had before. Perhaps there is more control tables in the gen 3 pcm and something is amiss.

Anyways AdsoYo I tried your tips and did not improve much, worse in-fact.

So then i reverted back to my standard tune and simply increased the OLFA main table in the startup temp - map zone by 10%. Started much worse. So then I reduced it by 10% in this same area and tried again. Started better, still weak but now stumbled at approx 600rpm. So then i reduced more, another 10%, it then stumble starts at 700rpms.

Now my OLFA table is wrong i know, simply very low EQ ratios where there should so i am guessing i need to reduce my MAF calibration and/or VE tables in this corresponding zone by say 20%? I think its running much to rich on startup?

Before when i didnt modify fuel, and just added more startup/frictional air it ran worse. Kinda makes me think....sure i was asking for more air, but that would also ask for more fuel right. Which if was rich issue, woudl just continue to be richer issue.

This is probably why if i open throttle with pedal 2% it fires right up. Gives it an initial extra flood of air via TB to match the rich fueling condition.

Does this make sense?

Think I need to work on low ends of MAF and VE tables....concur? Or based on this is there something else people think might be going on?

Thanks, Paul

PGDimpala

Thought i would post my tune

Also posted is my startup config and latest log based on increasing RAF by +.100lb/min in the cold start regions.

ctd

iTrader: (1)

You always need good basics in your tune to work from. Your IAC is wide open as mentioned, so by holding the throttle slightly open you are doing what the IAC is suppose do. Why is the IAC wide open, the basics are not correct. The basics are correct injector data, even more so with low speed characteristic's chasing idle & drive ability isuue's. If that data is correct then you move on.

You need correct AFR's in the MAF & VE tables, timing & then get the BRAF table correct.

Hopefully the IAC falls into place, if not you may have to adjust & modify it.

Now you work on the start up tables, if needed.

You can see from your start up log you have issue's, STIT's are out of range, based on the O2's the AFR is wrong, IAC is pegged & the BRAF is off.

Turning ***** blindly isn't the right approach.

Maybe start by getting your FT's logging.

PGDimpala

Yes I totally agree.

When i originally started to get into tuning the startup issue I thought since the tune was made by my long time mail order tuner and made from my previous tune in my old LT1 PCM that it was fairly "close". Didnt expect anything majorly different. Then i came to realized it Just had this startup issue that might need some addressing in the various tables that affect startup.

I have a couple more things to try, however just seeing how the car behaves with certain changes I am certain my 400, 800 and 1200rpm Main VE values are suspect as well as potentially the MAF. With these wrong, all bets are off everywhere else.

My impala has a 222/228 111LSA cam, so i think my VE's need to be dropped pretty good (20+%) in the low low rpm ranges.

The changes that make the car start the best (not good but better) are when i reduce VE in these low rpm ranges by 10% and reduce the EQ ratio in OLFA table (lean it up). Everytime i dont change these things and just try to add more air it starts weaker and idles lower (500rpm and less).

I have one more test to try based on some help from another member. However my new main plan is to get a wideband like I was going to do anyways to improve my tune and start to dial in my MAF, and VE as a first priority. Before this I will confirm all my IFR, offsets, etc (I am confident these are right, as low odds my tuner would have made mistake there). From there I will think my efforts will be much more logical and progress will be made as it should.

Paul

ctd

iTrader: (1)

A WB is nice but not necessary for what you are doing, you are going to run in CL so what is wrong now can be corrected with your FT's.

Your start up is already lean, so removing more fuel by reducing the VE & OLFA table is more evidence the base tune isn't correct. It needs air to start, you've verified that by pressing the pedal.

Try & get your STFT & LTFT's in the table & on the chart & post them up.

PGDimpala

Ok, I am just throwing new driveshaft in it right now. Once i get that buttoned up I will have a look at logging closed loop STFT and LTFT. Hopefully next day or so.

From there I will try and do as you suggested and adjust MAF and VE with my narrowbands in closed loop. Before that I will check IFR, etc. I have stock GM 24lb injectors in my Impala, where do i get the offset, and short pulse adder info? Is there a database somewhere.

Paul

AdsoYo

iTrader: (1)

Hptuners has a tune repository. You could use stock 01-02 Fbody injector data. Even though they are rated at 28#, they are 24# at the 43.5 psi the LT1 runs at. Just copy paste the data then set Flow Rate vs. KPA to 24 all the way across. Leave short pulse adder alone and copy the '0' KPA column in the offset table and paste it in all the other columns.