Can anyone pinpoint my issue on my LT1 swapped C1500?

Hey dudes!

I've been a pretty big fan of this forum for quite some time now, Seems I've found a reason to come out of my shell.

Trying to keep it kind of short but sweet; I have the 1990 C1500 Silverardo, Ext Cab, Short Bed. I evicted the clapped out L05 and dropped in a built 355ci Gen-2 LT1. She runs strong and smooth but has a couple grimlins when you fully give it the beans that I need to figure out but haven't been able to yet.


Before I go on, Y'all should know that my LT1 has been converted from Optispark to an HEI Ignition & the Tuned Port Injection was converted to a 4 barrel Carb using the GM Performance LT1 Aluminum Intake Manifold (#24502692)
(This means no ECM, No stupid standalone wiring headaches, no Optifail horror stories.)


So my problem? When I go to WOT, It stumbles and breaks up after about 5 or 6 seconds for 2-3 seconds at a time and continues this behavior in 3 second intervals. Also, If you stomp down on the accelerator too quickly, It kinda chokes up and even stalls sometimes if you're at a dead stop, You have pedal it to WOT.


Things I ruled out so far. (Or replaced already and is brand new.)

1. Ignition Timing
2. Spark Plugs
3. Vacuum Lines
4. MSD 8mm Plug Wires
5. ACCEL HEI Distrubuter & 14003 Supercoil. (Also tried my other 2 spare HEI's)
6. Fel-Pro PermaTorq Headgaskets
7. Fel-Pro Intake gaskets
8. Edelbrock carb base gasket
9. Freshly lashed valves.


My Hunch(s) -

1. Low Fuel Pressure due to Fuel Pump Relay?
2. Accelerator Pump?
3. Need better fuel? (I'm running 91 octane.)

So; the in tank fuel pump is supposed to push almost 18lbs, I installed an adjustable Holley Fuel Pressure Regulator with a tank return and regulated the pressure to 4 psi before the carb. I do have the Fuel Pump Relay drive wire spliced into the coil's TACH sensor wire. The relay clicks and I hear the fuel pump turn on and off, maybe the pump isn't powering back up fast enough to keep up with the carb?


Any help is truly appreciated, Feel free to share your wisdom!

Carb engines like 5-7psi fuel pressure. put a gauge in line close to the bowl and double check fuel pressure. sounds like its lo . Check fuel bowl levels. Pull the aircleaner off and look into the carb and pump the throttle once to see if the accelerator pump is working You should see steady fuel stream squirting in the barrels.

Madscientist said:

Carb engines like 5-7psi fuel pressure. put a gauge in line close to the bowl and double check fuel pressure. sounds like its lo . Check fuel bowl levels. Pull the aircleaner off and look into the carb and pump the throttle once to see if the accelerator pump is working You should see steady fuel stream squirting in the barrels.

Click to expand...

Hooked my fuel pressure gauge up to check the regulated pressure, Was fluctuating between 3.2psi & 4.1psi just idling, Went WOT and she dropped to almost 0, stalled, Then came back up..... the problem is fuel cut!

I pulled up the wiring schematic for the fuel pump relay again and gave it another look over and I'm 95% sure I found my screw up.

The Oil Pressure Sending Unit apparently works as an in-line fault switch between the Fuel Pump & the Fuel Pump Relay.. (I.E; If you lose oil pressure, The sensor will be "open" and cuts off power to the Fuel Pump Relay and prevents your fuel pump from working.)

Heh.

Going to go pop the Oil Sending Unit in it as soon as the rain lets up, Missouri weather stinks.

Will be SO happy if this fixes it, I just want to go fast and look slow!

BappleApple said:

The Oil Pressure Sending Unit apparently works as an in-line fault switch between the Fuel Pump & the Fuel Pump Relay.. (I.E; If you lose oil pressure, The sensor will be "open" and cuts off power to the Fuel Pump Relay and prevents your fuel pump from working.)

Click to expand...

FWIU, the oil pressure switch will bypass the fuel pump relay. Look a little closer, I think the wire from the switch is connected below the relay. If it were me, I'd hook up the relay to your ignition switch instead, since you deleted the ECM.

I dont think its fuel cut. Your engine is using fuel faster than the pump can supply it. You need 1: a new fuel filter first, 2: pinch off the return line and see if it builds pressure. That will tell you if its the regulator. :3 bad fuel pump and will need to be replaced. my bet is the pump but do a fuel filter first!

If your fuel pressure drops to zero when you WOT it, you're lacking fuel flow.

If your fuel filter isn't fresh, replace it. What pump and how new is it? It's the prime suspect.

What carburetor? The only carburetor I've worked with that liked 4 psi was the old Nikki carburetors on RX-7s. Change out that spring and push 5-7 psi to your carb.

Erik the Awful said:

If your fuel pressure drops to zero when you WOT it, you're lacking fuel flow.

If your fuel filter isn't fresh, replace it. What pump and how new is it? It's the prime suspect.

What carburetor? The only carburetor I've worked with that liked 4 psi was the old Nikki carburetors on RX-7s. Change out that spring and push 5-7 psi to your carb.

Click to expand...

Brand new AC Delco pump, brand new filter & sceen.

Edelbrock Performer 600cfm (1405)

BappleApple said:

Brand new AC Delco pump

Click to expand...

Which one, for a TBI or Vortec? The EP381 is for the Vortec, and flows 155LPH where the stock TBI pump only flows 75LPH which can't keep up with a modded/performance engine,

https://www.summitracing.com/parts/ado-ep381

Think I'm just about to tannerite the entire truck.

Appreciate the help, but I'm about tired of this s***box already lol

5 minutes passed and my temper tantrum has left my body.

I'll throw a good Walboro in it and see how it does.

Gonna try just deleting the regulator for now and seeing if there's any change, No need to regulate fuel pressure from a pump that can't keep up.

Pumps make flow, restrictions to flow make pressure. Without the regulator, you'll have a loop from the tank to the engine and back with no appreciable pressure. That's why the regulator is always between the injectors and the return line to the tank.