EFI Puzzler
 

Mike Norris and I were messing around with II Much yesterday, and one of the things I wanted to do was to increase the fuel pressure in the fuel rails. I've been running at 42 psi (or 38 depending on which gauge you believe).

I wanted to run at 58-60 and see if there was any more power or if the idle would be smoother due to increased fuel atomization.

I bumped the pressure to 60 (Aeromotive rails, regulator, A1000 pump) with no problem. However, the motor went lean. Mike started tossing in fuel and it made no difference. Thinking the o2 sensor might be the culprit, we swapped it over to Mike's dyno sensor, and saw the same results.

Next, we reset the pressure to 42, and reloaded the original fuel map that Mike had saved (he's a smart one). Everything went back to normal. Then we increased the fuel pressure with the engine running. The engine went progressively leaner as we did that.

Not knowing what else to do, we put everything back and I drove the car home.

The only thing we could think of is that the injectors (Holley 42 lb) aren't rated at pressure higher than 42 (45?) and the high pressure prevents them from allowing more fuel by. Does that make any sense at all? Is there a different brand injector that we should try? Or is it even worth trying to get to 60 psi?

Thoughts?

jp

don't think you'll gain much if any by running the higher pressure. GM did it because they started running returnless fuel systems (or at least they didn't return from the engine compartment, and the higher pressure helped against possible vapor lock issues in the steel fuel rails.

Jody

Thanks, Jody. I've heard that from others too. The car runs fine, makes great power, but has this annoying cold start issue. I was hoping better fuel atomization might help it.

But 60 psi sure didn't have the effect I thought it would, lol.

OK then.

Let's talk about the part that Mike and I have both tried to figure out: my cold start issue.

When the car sits for a day or so, the engine cranks for a long time before starting. 5-6 seconds or so. When warm, it starts in a couple of revolutions, like < 2s.

I've looked at the fuel enrichment, timing tables, etc., and all seems reasonable. Mike has looked it over too. I'm using a FAST XFI, FAST eDIST, and FAST Crank Sensor Convertor (that emulates a crank trigger setup from the factory LS1 crank sensor). The engine will never start like a stock GM ECM, but it ought to start cold as quickly as hot, right?

Thoughts anyone?

jp

it should start as well cold, but is usually one of the hardest things to get perfect for me. Add in the fact that you only get one shot at it each day, so it can take a while to get dialed in.

It's usually a balance of cranking fuel and iac position. I generally play with one thing at a time so I don't get lost.

Hmm. I can't say I've done very much with iac. That's a good idea of something to play with that I haven't done much. I'll check it out. Thanks.

jp

it's critical to get that right. Where it parks determines how much bypassed air is happening while it's cranking. The balance between that and cranking fuel determines how hard it is to start at all temps. Of course cold starts need more fuel and more iac.

If you find it starts easier with the throttle cracked a bit, open the iac start position more at that temp.

Good luck.

Jody

Here's the current IAC vs Coolant Temp values. Look reasonable?

jp

looks reasonable. How much have you bumped the cranking fuel in the colder areas?

The cranking fuel is up about 20% at 70, though it is closer to 15% at 85-90 degrees.

jp

JP-
what are your fuel #s during cranking? You might try closing the IAC a bit more for the same amount of fuel, then see what happens to idle quality & stability. How well does it transition from cold-warm-running temp?

IRT the leaning issue w/ greater pressure- was this open-loop, or closed? The injectors are supposed to be able to handle 75 PSI before overcoming the pintle or disc, so you should have seen a fattening, given all else being equal.

The car starts after about 6 seconds ... then nearly dies (or actually dies). Then I restart it, and it nearly dies ... then catches ... and then runs fine.

Listen to how the car starts. This video captures it very well:

https://www.youtube.com/watch?v=qAZPVNMh1SU

You don't have to watch the whole thing ... but that's what the car does when starting.

Yes, the leaning at 60 psi was very odd. It was at open loop. We did another experiment: we had the car hooked to the dyno A/F meter, started the car, and slowly increased fuel pressure. As fuel pressure went up, the mixture leaned out. 14.5, 15.0, 15.5, 16, etc.

jp

Cranking fuel (pulse width in ms) vs Coolant temp

20.1 ms 55 degrees
20.1 68 degrees
18.9 85 degrees
16.8 102 degrees
15.0 119 degrees
12.2 136 and up

Was that what you were asking?

It still sounds to me that the IAC is cranked out to far- fattening it up might get it to catch a bit better. Can you see the IAC #s fluctuating while you're starting it, or does it stay static (slaved to the ECT)?

Still weird about the leaning issue!

I don't know. I think the thing to do next is log the IAC numbers and watch what the ECM does to get the motor to the target idle speed. That might be a clue to see what is going on.

About the leaning problem, something just occurred to me: I wonder if the issue was that we didn't change the fuel calc parameter for Injector Flow Rate. At 60 psi, the injectors flow more fuel right? So the number should have been more increased quite a bit ... maybe to 55 psi or something. Perhaps the ECM got confused, and was calculating the fuel load based on the smaller number? I can't quite wrap my mind around that (it's 2 am here on the East Coast, lol).

But I know we didn't change that value.

jp

Does your XFI 'see' the extra fuel pressure, and have the ability to compensate?

I did that on my S/C toyota- I got the sweet spots golden, then forgot to tune the rest of the table! It popped like a madman...

It doesn't have a connection to any sensor. We sure didn't tell it. That's gotta be the problem.

But, as Jody says, if GM only went to higher pressures for their newer deadhead plumbing, then I don't need higher pressure. It won't resolve my slow cold start issue.

jp

Agreed- and with your manifold, I don't think it's a problem anyway.

Just pump the pedal a few times to get the choke to close!! ;)

An update:

Yesterday, I swapped the engine over to SEFI (it was bank to bank because the original tuner couldn't get the car to start that way). Now I know why! He had put in the wrong firing order!!!!!

Anyway, now that I'm running SEFI the car seems smoother at idle and part throttle accel.

Today I increased the IAC start position about 20%, and allowed it to move faster and farther. The car started much quicker, with no sag after initial fire. Much much better.

Tomorrow, I'll increase the cranking pulse width about 10% and see if that helps too.

jp

that's great John. That's how you find the balance. The IAC park has to be enough so when it does fire it doesn't try to die immediately. Then figure out what pulse width makes it happy at that opening.

Jody

Yeah, it fires with a "burst" now, rather than a sag. I think a little boost to the fuel might just get it started a half-beat quicker. Then I may try to lower the IAC a bit, but I'm trying to only do one thing at a time. :)

Part of this problem was because I originally wired the IAC backwards (I had the wrong pinouts), and so the original tuner (not Mike Norris) tried to tune for easy starts with no IAC. I got the IAC working a few weeks later, and now it's time to get the start tuned right.

The new owner should have a car that lights right off and goes anywhere, lol.

jp

that is definitely the idea............... :rofl: Congrats on your progress.

Jody

Anything brewing on finding a new owner? I was going to make a smart comment about getting it right for me, but so far, no bags of money from the sky.

No... nothing brewing. Sigh.

My next project seems as far away as ever.

jp

Well if you're bored You can always come help assemble mine!! All the fun without writing all the checks!

You'll find an owner! Good to hear you are figuring out the starting issue!

Today's update: I sorted out the sync signal from the cam. The firing order is 18726543, with the cam sync signal preceding cylinder 5.

Now when I retard timing on cylinder 1, it actually retards timing on cylinder 1, lol.

Today's cold start was more of a sag, even though I didn't make any changes to the cranking fuel or IAC. Today's change was to change the run/crank rpm number to 150 (from 400), and to have it start the fuel enrichment decay at 6 revolutions rather than 12.

I'll have another go at it tomorrow.

jp

II Much cold start: an update
 

Recently I put one of Norris' catch cans on the car and went back to the factory PCV. (my car was sucking big quantities of oil out of the crankcase with the racing PCV setup). One lesson there.

I also replaced all my Russell hose with hardline and Earl's hose. Another lesson there.

I also removed the stereo as I'm working on packaging A/C. A third lesson there. :)

Anyway, with those changes, and with no change at all to the EFI tune, the car starts way better. 2 seconds max. It fires with a burst, then quickly drops to a fast idle, then a normal idle as the engine warms up. It's perfect.

Theories:
1. The stereo was causing a big voltage drop, causing the XFI to reset.
2. My plugs were coated in oil, and kept the engine from firing easily.
3. My car likes playing jokes on me.
4. I'm going insane.

jp

Don't forget Murphy! He is a real a$$ and loves to invite all his family over to jack with you causing questions to your own sanity. I have meet a few of his other family members as I am certain everyone here has. I believe he has a half brother named "Interferance". A close cousin "Clarence" that rarely shows up when you need him. :lol:

?
 

lower you iac at crank for cold temps and raise your low temp cranking fuel up ..


also is your pump wired up to prime when you key on or is it just constant after sitting for a decent amount of time without a priming pump this can happen as you lose fuel preasure in the rail.

leaning out

2 things
1 do you have the wide band setup so you are in closed loop .. your raising the fuel preasure and can be fatenting it up causing the ecu to start corecting the other way.
2 are you sure your fuel pump can handle the load of the higher preasure without losing a ton of volume.

I think Mike Norris finally got the cold start figured out. And with my latest changes it starts faster than my truck! Amazing, but true.

But to answer your questions:

1. Yes, I have a priming sequence. At key "on" the pump primes for 10s, and I get fuel pressure after about 4s. I then start the car. It starts immediately now!

2. I'm using an Aeromotive A1000. I'm sure it can deal with volume at higher pressure.

3. I've got a wideband and run closed loop after the engine warms up.

jp