Help with Carb on computer truck (now wont start at all)

[TacosnBeer]

Here comes the can of worms! Here is much info as I can throw out there to assist....

Bought a 98 GMC SL (originally a 5.0 with TBI) - a shop put a 390hp crate 5.7 motor with Vortec heads, new distributor and an Edelbrock carb with an Edelbrock pressure regulator mounted on the firewall. (I know... why go backward in technology to a carb... dumb).

It runs fine if I keep my foot out and drive normal but if I jump on it hard (WOT) - it falls on its face. Its not a carb like bog though... Its instant cut out.


When I was driving up a steep grade on the way home she almost sounded like she was missing but it was random, not consistant... At that point, it was in D (aka overdrive) and when I put it in 3 it was better but still had a random cut out under load. The gauges usually bounce around and not sure if it is connected but, they did seem to go with the miss/cut out.

All the grounds are there per all the other threads but, the wiring is a bit jenky so I am going to redo all of them with thicker wire/ground straps in hopes to fix some of the electrical issues. The A/C also won't come on at all. Everything is new and hooked up but, doesn't come on if you turn it on.

I know the computer is still hooked up but he bypassed the TBI stuff somehow. The only code that is popping is P0102 *MAF code* - and there is no MAF...

Could it be a voltage issue at WOT? What can I look for to check?

 

[b454rat]

im not great with electronics but know Edelbroc's can be sensitive to fuel pressure. Think it's around 5-6 lbs.

 

[skylark]

Monitor the voltage at the coil and drive it. Also a 98 wasn't tbi, it was central port fuel injection.

 

[Pinger]

If the miss manifests itself at lowish rpm when throttle is opened to about half of WOT than I'd suspect ignition. If takes WOT to provoke it at lowish rpm then something else.

(By half of WOT at lowish rpm the cylinders can fill to full (air) capacity and it is introducing a spark into a cylinder at pressure that catches out a weak ignition system).

 

[TacosnBeer]

Another thing that I noticed is, Looks like the stock higher flow fuel pump connected to the edelbrock regulator and there is no return line hooked up. Not sure if that matters much but, seems like it may be a lot of pressure to drop to the required amount for a carb without a return line yeah?

 

[TacosnBeer]

If the miss manifests itself at lowish rpm when throttle is opened to about half of WOT than I'd suspect ignition. If takes WOT to provoke it at lowish rpm then something else.

(By half of WOT at lowish rpm the cylinders can fill to full (air) capacity and it is introducing a spark into a cylinder at pressure that catches out a weak ignition system).

Supposedly, this is the second distributor in it as the PO was trying to figure out why it was falling on its face. Not sure he changed anything other than that to chase the problem down. Not sure if the ignition module was changed too? It runs pretty smooth all the way up to when you floor it or have it under load going up a steep hill.

Monitor the voltage at the coil and drive it. Also a 98 wasn't tbi, it was central port fuel injection.

Hmmm.. how can I monitor the coil voltage while driving? I am guessing this is the possible issue but not sure. What drives coil voltage? Oh and ya, MPFI... not TBI.

 

[skylark]

Back probe the positive wire at the coil with a straight pin, don't pearce the wire. Mine are from the craft store and they have hearts on the end for 2 reasons, #1 because the heads are bigger meaning that they don't dig into my fingers and #2 so my manly employees don't steal them! Connect a long jumper wire to the pin and run the jumper into the cab, connect to a voltmeter and drive.

 

[Pinger]

Another thing that I noticed is, Looks like the stock higher flow fuel pump connected to the edelbrock regulator and there is no return line hooked up. Not sure if that matters much but, seems like it may be a lot of pressure to drop to the required amount for a carb without a return line yeah?

If I've understood that correctly - agreed.
I'd expect fuel to be pissing out at idle if there's enough pressure to supply WOT at max rpm.

 

[Pinger]

Supposedly, this is the second distributor in it as the PO was trying to figure out why it was falling on its face. Not sure he changed anything other than that to chase the problem down. Not sure if the ignition module was changed too?

As is also being discussed low tension side of system could be weak.

It runs pretty smooth all the way up to when you floor it or have it under load going up a steep hill.

.

You mean pedal on the floor?
If it's ignition related it should be missing by half throttle at mid rpm (half throttle is full throttle at half max rpm). If it's only missing with the pedal on the floor more likely to be fuel related IMO.

 

[Erik the Awful]

Check your accelerator pump squirt.