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