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Lq4 won’t stay running

chris85

chris85

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Finally fired up last week after the build, no problems except a big trans fluid leak that I’ve since fixed.

This morning I started it up and it went off without a hitch. Ran strong for a good 3-5 min then I shut it down to tighten an oil cooler line. Go to start it up again and it fires up strong, then starts to stutter and die over about 10 seconds. Tried it about 3 more times and the same thing.

Not sure what else to check. Has a new fuel filter/regulator, pump still runs as it’s dying, I would think the injectors and all are good since it has fired up before without problems.

Searched and many people say it’s the VATS but it sounds like theirs cranks and immediately dies in 2-3 seconds without the stutter. Had the ECM flashed through LT1swap.com so I’m assuming that’s not the issue.

I have the O2 sensors hooked up with just open headers, is it possible to be running too rich?
 
Shouldn't be vats, wouldn't run for that long. Let it get completely cool and try again. Do you have a scanner can you watch what the ecm is seeing? Do you have any codes?
 
Just tried again. Same thing but ran just a little bit longer. Got a video if anyone knows how to post it.

I got the Dakota digital BIM-01-2 OBD2 reader. It’s not showing any codes. I got another scanner in the mail that should be here tomorrow that I can use with Torquepro software.

I’m about to check the voltage on the sending unit bc the guages are saying I’m low on fuel, but I know I have 8 gallons in there, should be plenty.
I don’t think that would cause it not to run, just make the guages inaccurate?
 
Sending unit should have the power it needs since its sending signal. They work off varying the impedance to the gauge. Make sure you set it to the correct GM range on the gauge set up. I assume you know, but just in case.
 
So sending unit is showing 5 volts which is about on par, but the pump isn’t getting its 12 volts. The pump is still turning on though.

I’ve already had one bad relay with this harness, is it possible this one could be going out as well?
 
Does the fuel pump relay input show correct voltage?

You’ll usually have evidence when an engine is running too rich to stay started. Strong smell, smoke, uncombusted fuel, etc.

I think you’re on the right track with fuel starvation.
 
86 is ground
85 is 12v switched
30 is your 12v constant input
87/87A is your accessory depending if you want normally open or closed
 
Abusfullofnuns said:
86 is ground
85 is 12v switched
30 is your 12v constant input
87/87A is your accessory depending if you want normally open or closed
Click to expand...

So that should mean the relay is good.

The only other things I can think of is I got the wrong (smaller) sending unit in the tank(31g) bc the guages are still saying low fuel when I should be at at least 1/4 tank, or something in the ECM.

I guess I could rig up a fuel pressure gauge to see what’s happening there.

Ugh, I’m gonna go drink a beer and mow the yard.
 
Fuel pump, filter, pick up, or pressure would definitely be my plan to check. Definitely seems like it didn’t have the fuel to keep the fire burning.
 
Fuel pressure. Injectors can't do the job below 50 psi on an ls. Put a gauge on it and watch what it does as it stalls. If the pressure drops you either have a pump or a regulator issue.

Are you running a returnless system or return fuel line setup?

Post up what the gauge is doing once you hook it up.

It should jump right to 55-60 psi with turning the key on. Once running it should stay around 55psi. Watch if the needle is fluctuating a lot or smooth.

It sure sounds like it's running out of fuel.
 
You need a fuel pressure gauge on it. If it's dropping then you can check the pump to see if it's an electrical or mechanical issue. If it's not dropping you likely have a sensor issue. Divide and conquer and all that.
 
ZooMad75 said:
Fuel pressure. Injectors can't do the job below 50 psi on an ls. Put a gauge on it and watch what it does as it stalls. If the pressure drops you either have a pump or a regulator issue.

Are you running a returnless system or return fuel line setup?

Post up what the gauge is doing once you hook it up.

It should jump right to 55-60 psi with turning the key on. Once running it should stay around 55psi. Watch if the needle is fluctuating a lot or smooth.

It sure sounds like it's running out of fuel.
Click to expand...

Returnless with the corvette filter/regulator.
 
So I’m only getting about 30 psi at the rail. That’s with the initial pump prime and I went ahead and cranked it to see if it would increase at all and it didn’t.

I guess the best way to go about is replace the cheaper part first? Haha
 
Had this happen to me in utah on a job. I was convinced it was the fuel pump. Ended up being the map sensor at the back and top of the intake manifold.
 
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