Back fire through intake when floored

[Connorpod]

Hey crew!

Just finished putting together my gmt400 with Vortec Heads and Summit 8000 cam. Runs pretty awesome except I have a little hiccup. When the truck is in motion and I floor it, the RPM will drop and there is a back fire through the intake. I just replaced all the plug wires, spark plugs, Vacuum lines and Distributor. My mechanic has said that the fuel pressure is OK. I also put in a brand new vortec pump. This was also a problem before a re did the top end. I data logged and the only thing that comes up that is concerning is that my MAT = -40 degrees. (Not even sure that I have a MAT sensor) Thanks for the help!

 

[PlayingWithTBI]

Is this a TBI or Vortec intake? Check your fuel pressure again when accelerating. It may be "lean pops" you're getting.

 

[Connorpod]

It’s a Vortec intake. Would you recommend moving to a AFPR? I’m currently just using a 14 psi spring. 18PSI spring I originally was using was way too rich…

 

[Schurkey]

WHAT VEHICLE??? Vortec heads, TBI SYSTEM?

 

[Connorpod]

Sorry, wasn’t thinking. 1995 chevy k2500 350 tbi system

 

[White Trash]

Check your ignition timing at idle, and have someone rev the engine up and check advance timing? Additionally, recheck your plug wires and make sure you do not have two crossed up.

 

[Schurkey]

1995 chevy k2500 350 tbi system

finished putting together my gmt400 with Vortec Heads and Summit 8000 cam.

Link? I can't find an "8000" cam on the Summit web site.

There's an 8800, though.

CHEVROLET Summit Racing SUM-8800 Summit Racing™ Pro SBC Camshafts | Summit Racing

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www.summitracing.com

When the truck is in motion and I floor it, the RPM will drop and there is a back fire through the intake.

I just replaced all the plug wires,

Are the plug wires routed exactly like factory? Including the original plug wire looms?

spark plugs,

Which plugs, and what is the gap set to?

and Distributor.

Cheap Chinese distributor with a crappy ignition module and cap/rotor?

My mechanic has said that the fuel pressure is OK.

What is the actual pressure, under load?

This was also a problem before a re did the top end.

Seems like a long-term problem involving parts (including sensors and wire harnesses) that were not changed.

I data logged and the only thing that comes up that is concerning is that my MAT = -40 degrees. (Not even sure that I have a MAT sensor)

I don't think the TBI trucks have a MAT sensor. My '88 K1500 5.7L doesn't.

What are your fuel trims when the engine backfires through the intake?

What are the cranking compression pressures of all eight?

Do you have EGR connected? If not, is it tuned-out in the computer program?

How old is the O2 sensor? What are the cross-counts at idle, and at moderate speed?

 

[Connorpod]

Thanks for the questions! Cam is the 8800. Plugs are routed the same way with the exception of the stock retainers. No egr but deleted on the chip. I did not set the gap on the plugs. Distributor is a Davis Unified. I do not have a AFPR but I slapped in a 14psi spring. The 02 sensor is an ACDelco with about 2 years on it. Three wire 02. For fuel trims and cross counts I’m new to Datalogging but I can see what I can dig up.

 

[Connorpod]

How old is the O2 sensor? What are the cross-counts at idle, and at moderate speed?

I took a couple screen shots of idle and moderate speed 02 readouts. Thanks for having a peek!

 

[Connorpod]

What are your fuel trims when the engine backfires through the intake?

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Do you know where I could find that in tuner pro? I've included the TPS and MAP data with the RPM drop.