LS3 cold start problem

[JimLev]

Hi, just joined. Need some advice.
I built a Factory Five 33 hotrod with a new Gen 4 LS3 engine. I bought the engine from GM Connector and Cruise along with the ECU and all the wiring. The ECU was programmed by GM, the engine ran great.
It now has 2400 miles on it and generally has to crank for 5-6 sec when cold. It always use to start almost instantly. Fuel pressure is at 58 psi. The trims look good. After it’s warmed up it always starts almost instantly.
I haven’t bought the HP Tuner yet, not sure if I’d know how to use it, I’m more of a hardware guy than software.
Any recommendation on what to look at?

[RDF1]

Id be looking at crank and cam signals.
If it has good fuel pressure its usually the ECM isnt seeing cam signal and take awhile to fire up.

[JimLev]

Will do, thanks for the quick reply.

[cpd004]

Hook up a fuel pressure gauge to it after you drive it. Leave it on as it cools down. See how much pressure is bled off as it cools. Also, does it start quicker if you double prime the pump? In other words, turn the key to run but not crank for about 3 seconds. Then do this again, but this time start it.

[JimLev]

The regulator has a gauge.
Can’t do much testing today, got company. Will do more tomorrow, like check the cam and crank sensors.
Here’s what I did so far.
Key on, pressure goes up to 58 then the ECU turns the pump off. Pressure immediately drops down to 44.
If I double prime it same results. Tried this in past, doesn’t help it start any quicker.
After 10 min the pressure has bleed down to 30 psi from 44.
The fuel pump is new, it’s below and external to the tank.
There are filters before and after the pump.
The regulator has a return line back to the tank.
There is no check valve in the supply line.

After 30 min the pressure has dropped to 20 psi

[Alvin]

Is this only on hot starts or cold starts also?

I doubt you would have made it this far but it's common for hotrod guys to not wire certain things like the PCM to hot while in start and run. They only get the run part. What happens is the engine doesn't fire until you let off the key a bit.

[JimLev]

It’s only on cold starts that it cranks for ~4 sec before it starts. When it does start it almost stalls then recovers and runs just fine.
Double priming it when cold doesn’t help.
On a hot start it starts right up and runs fine.
For the last 2 years it always started almost instantly and idled fine.

I’ve got a Foxwell NT710 scanner with GM and BMW software loaded on it.
I’m still learning the GM part. I did hook it up and tried to capture live data, cam, and crank sensor data.
I’m sure I did something wrong as I didn’t get any. I’ll keep trying.
Is HP Tuners complicated? Maybe I should buy it.

[edcmat-l1]

businessman-looking-into-crystal-ball.jpg

[Alvin]

It might just need some tuning. The MAF placement makes a tremendous difference in fueling.. It's mounting is huge variable in swaps like yours. It likely could benefit from some tweaking.

[JimLev]

Alvin, thanks for the info, for the first 2 years it started up just fine. Nothing has been changed.
I don’t have many placement options to mount the MAF. I tried to post a jpeg but it keeps saying invalided file type.
Do I need more posts before I can upload a pic?

[lewma]

Jim, could it be taking a while for the fuel to reach the engine from the pump/regulator/tank ? Small leak somewhere in the fuel system ?

[JimLev]

Jim, could it be taking a while for the fuel to reach the engine from the pump/regulator/tank ? Small leak somewhere in the fuel system ?

Don’t think so, the gauge shows 58PSI as soon as I start cranking it.
Just to see if it is a fuel problem or sensor issue I think I’ll spray some starter fluid into the intake and see if it fires right up.

[lewma]

Don’t think so, the gauge shows 58PSI as soon as I start cranking it.
Just to see if it is a fuel problem or sensor issue I think I’ll spray some starter fluid into the intake and see if it fires right up.

Just wondering if something is taking time to prime on a cold start.

[LS ROB]

Did I miss the log and tune?

[gtstorey]

Did I miss the log and tune?

He doesn’t have HPT

[edcmat-l1]

He doesn?t have HPT

businessman-looking-into-crystal-ball.jpg

Good thing I got this ^^^ LOL

[JimLev]

I sprayed a little starter fluid in the intake, it fired right up.
I replaced the gauge on the regulator, it read the same 58 psi.
The Walbro fuel pump is new. There is no check valve in the fuel line, maybe it needs one to hold the pressure.
As soon as I shut the engine off the pressure drops from 58 to 44 psi.

I powered the pump directly without the engine running and could hear the fuel running thru the fuel rails. Sounded almost like there was air bubbling thru with the fuel. Once it starts it runs fine with lots of power.

[sgod1100]

businessman-looking-into-crystal-ball.jpg

When I didn't see a tune/log I was waiting for this...lmao

[pannetron]

HZow long does it take to show good oil pressure? After priming the fuel pump for 2-4 sec. upon key on, I *think* that the ECU doesn't enable the fuel pump again until it sees "good" oil pressure.

[JimLev]

As soon as the engine starts, when cold, the oil pressure jumps up to 50 PSI.
The fuel pump comes on while cranking the engine.