LS swap fuel issues

[hath]

Hi guys,

I am currently working on a 73' that has a 6.0 out of a 2008 chevy express 3500. Right now it has an inline fuel pump, but it isn't hooked into the ecu so there's no communication between the fuel pump and ecu so it's running real wacky. I've been told of a few ways to fix this. 1. Go carbureted. 2. put a higher psi fuel pump in and plumb in a return line (right now it's a returnless system). 3. Use a stock (out of the donor vehicle) in tank fuel pump and wire it back into the system. Problem is I can't find the 2 pigtails that clip into the fuel pump. I've read a lot about the different ways to do this, including in line pumps, but I don't get how guys are getting those to communicate with the computer. Any info. would be great. Thanks.

[68c10airstream]

Welcome aboard! Late model gm is using a pulse width modulated control of the fuel pump. The advantages are longer life for the fuel pump and less drag on the alternator at times. As an example, say you are going down a mountain region and you are off the throttle for extended times. Pretty much all manufacturers shut off the injectors (or it would backfire) and on newer engines they shut off the fuel pump. Early ls engines would still run the pump but the return line prevented a dead head condition. The single (returnless) fuel system brought about shutting off the fuel pump during decel. Makes a lot of sense to shut off a pump that is dead headed!

I,ve noted fuel pressure changes on newer gm engines during idle cold and warm and hot restart because before i clean injectors i tap into the system to be able to match the pressure when i shut the pump off and isolate the fuel line and then pressurize my concoction and inject it into the fuel rail. I was surprised the first time i found this out.

[Robznob11]

Sir, im not sure what you mean about communicating with ecm? The only thing the ecm does is turn it on or off. When hooked to the ecm ot will turn on 3 seconds to prime then go off untill the engine starts. If it stays running with the key on its no big deal except that it is dead heading against the regulator when the engine is not running and key is on. Now the computer is controlling the injectors and spark. If you have 58 psi at the fuel rail with the engine running the pump is not the problem. I would hook a cheap gauge to the fuel rail and check it with the engine running under a load. I had to upgrade my fuel pump after the cam. Just ideling i had 58 psi but under a load it was going down to 40 and running lean causing it to backfire through the intake.

[mongocanfly]

Robz right...I'm not sure of your setup..what tank are your using ? If your running a stock tank at the least your gonna need inline or other setup..most inline systems use a walbro 255 pump and a vette style filter reg...yours is returnless and that's fine but the filter reg needs tombe returned back to the tank..as stated ecm doesn't control the pump..it should be run from the fuse block with switch on power..

[Overdriven]

If you look at the stock 3500 express fuel pump module it has 2 electrical connections. One is for the fuel level the other is for power to the fuel pump. The weird thing is the stock fuel pump module has what appears to be a return fitting on it.

Either way, the ecm doesn't need to be connected to the fuel pump. In fact because the ecm uses PWM for the fuel pump it's better to have the fuel pump run off a relay and not the ecm. Most aftermarket fuel pumps don't like pwm power and will burn out, including Walbro's.

Your engine is setup for a returnless so there is no provision for a return on the fuel rails. This means you need a fuel pressure regulator before the stock fuel rails. This can be in the engine compartment or close to the tank. You can go aftermarket or with the popular vette filter/regulator combo. If you're planning on a high HP build, forced induction, you'd be better off swapping the intake or aftermarket fuel rails to go with a return fuel system and the regulator mounted to the rails or at least in the engine compartment.

[Robznob11]

Originally Posted by Overdriven

If you look at the stock 3500 express fuel pump module it has 2 electrical connections. One is for the fuel level the other is for power to the fuel pump. The weird thing is the stock fuel pump module has what appears to be a return fitting on it.

Either way, the ecm doesn't need to be connected to the fuel pump. In fact because the ecm uses PWM for the fuel pump it's better to have the fuel pump run off a relay and not the ecm. Most aftermarket fuel pumps don't like pwm power and will burn out, including Walbro's.

Your engine is setup for a returnless so there is no provision for a return on the fuel rails. This means you need a fuel pressure regulator before the stock fuel rails. This can be in the engine compartment or close to the tank. You can go aftermarket or with the popular vette filter/regulator combo. If you're planning on a high HP build, forced induction, you'd be better off swapping the intake or aftermarket fuel rails to go with a return fuel system and the regulator mounted to the rails or at least in the engine compartment.

In general you should run any electric fuel pump off of a relay and trigger the relay with the ecm or key. There is no way there is enough power to efficiently run a fuel pump off the ecm. Stock setups are pi$$ poor design and looking for a failure from there birth! I prefer to trigger the pump off of the ecm because it cuts power when the engine is not running. I have the racetronix pump with hot wire that will overcome my regulator and stay running if the engine is off and key was on, probably would not hurt much if not left for long but when the ecm already has the provision to control just such a issue, you may as well use it.

[Overdriven]

Originally Posted by Robznob11

In general you should run any electric fuel pump off of a relay and trigger the relay with the ecm or key.

This is what I meant. I guess I wasn't as clear as I could've been. I was trying to say to avoid using whatever the factory uses for fuel pump power as it is PWM (if factory returnless) and most aftermarket pumps don't like PWM power.

What does the factory setup use for fuel pump power? Is it a module, solid state relay that can switch fast enough to do pwm or what?

[hath]

So the mechanic I last took this to thought the in line fuel pump I was using wasn't sufficient. He said that at idle the pressure was good, but when you hit the throttle, pressure would drop. So I attached a chart of the fuel pump specs that I'm running now, and then looked up the specs on stock in tank fuel pumps...the ratings vary quite a bit. From what I see, this inline fuel pump should be sufficient, but I wanted some other people's opinions. See attached.

[mongocanfly]

If your running that first pump on your list then that's your problem...44psi is not enough to make injectors work properly..you should be in the 55-60 psi range..
Get a walbro255 inline and a vette filter/regulator and you should be good..

[68c10airstream]

The comments above regarding the ecm to control the relay is best. If you were in a collision the ecm would shut OFF the fuel pump because the engine died. I could only imagine a front end collision, hot exhaust, engine dies but the fuel pump continues to run from a switched key on relay and feeds a fuel fire to a molten glob on the road. Ouch!!

[hath]

Originally Posted by 68c10airstream

The comments above regarding the ecm to control the relay is best. If you were in a collision the ecm would shut OFF the fuel pump because the engine died. I could only imagine a front end collision, hot exhaust, engine dies but the fuel pump continues to run from a switched key on relay and feeds a fuel fire to a molten glob on the road. Ouch!!

Has anyone ran one of these: https://www.pegasusautoracing.com/pr...sNlhoC17fw_wcB

What are you guys using?

[mongocanfly]

I've seen a few people use something similar and I will be using a similar setup on my 53..cheap insurance...a race car guy I know uses something like that on his race car but I'm not sure of the brand

[87chevy.com]

lol, 5 year old thread..

Originally Posted by hath

Has anyone ran one of these: https://www.pegasusautoracing.com/pr...sNlhoC17fw_wcB

What are you guys using?

clearly states 10 amp load... better check the data sheet on the pump.. most pumps will draw more than that, prob avoid that one

get an 87 style fuel tank, 87 sender, put a walbro 255 in there and your done.

I had these made and sell them on ebay for the 73-87 tanks, only tested on 87, but should be the same size. I had them made to run the higher flow rated pumps that draw more than 14amps.... the factory terminals used on the 87 style fuel pump assemblies is max of 14a..

http://87chevy.com/fuel.htm

if you use the stock 87 sender, a 255 style fits exactly like o.e.
if you use a walbro 450, you need a customer adapter. easy to make or buy if you cant make.

clint

[hath]

And would I need to run a pre pump since as I understand fuel pumps push not pull fuel.

[mongocanfly]

Not intank....inline.. walbro 255...the gsl 392...no other pump needed....https://www.summitracing.com/parts/v...92bx/overview/

[mongocanfly]

It will mount inline like this diagram...

[hath]

And the Walbro GSL392 has an inlet/outlet of 5/16" yet the connection point at the fuel rail is 3/8, shouldn't I run the whole system at 3/8?

[mongocanfly]

I ran my entire system with -6an....equivalent to 3/8"..

[BR3W CITY]

Originally Posted by hath

And the Walbro GSL392 has an inlet/outlet of 5/16" yet the connection point at the fuel rail is 3/8, shouldn't I run the whole system at 3/8?

The pump uses metric I/O which is 10mm, and not a removeable fitting, its part of the pump housing. Depending which way you buy the pump (multiple companies sell the same rebranded pump, some as "kits" with different fittings). Usually it'll come with a pair 10mmx-6 fittings, which is 3/8 feed for the system.