Misfire at higher rpms

[95er]

I just put a new engine in my 95 k1500 biggest project I have ever done. I followed recommendations from Harris tbi tuning. The engine has vortex heads and a mild cam. I had them flash my chip and I had a guy build me a new tbi bored out to 46mm and with an 18 lb spring. I have been working on breaking the engine in and the other day I decided to give it some juice to see what it had. I have just at 100 miles on the new engine. I didn’t floor it but pushed it harder than I have been. After about 2500 rpm with a heavy foot it misfires and falls on its face. If I feather the throttle I can get it up to 4500 just fine but if I try to get there aggressively it misfires. I am not by any means flooring it. Where should I start to diagnose this issue. I don’t want to just jump in and say it’s a tuning issue but I know that’s not out of the picture but I want to rule everything else out prior to reaching out to the guy that flashed the chip.

 

[df2x4]

You wouldn't be the first here to have problems with Harris/TBIChips. My money is on the tune.

 

[Schurkey]

WHAT intake manifold are you using?

"Tune" is a real possibility. Did they disable EGR in the tune?

Excessive fuel pressure due to the 18-lb. spring is another possibility. Or low fuel pressure due to a failing fuel pump or plugging filter.

Was the camshaft degreed when it was installed, or merely stuffed-in "dot-to-dot"?


The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)
2. How old are the usual “tune-up” parts and procedures? Inspect/replace distributor cap, rotor, plug wires, spark plugs. Make sure the ignition coil will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper initial timing (TBI) and electronic spark advance (TBI and Vortec). Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.
3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle. The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum on this web site.

 

[Supercharged111]

Do you have a wideband? What's it reporting?

 

[PlayingWithTBI]

I had a guy build me a new tbi bored out to 46mm and with an 18 lb spring.

After about 2500 rpm with a heavy foot it misfires and falls on its face. If I feather the throttle I can get it up to 4500 just fine but if I try to get there aggressively it misfires.

An 18LB spring at what pressure? I can adjust mine as low as 15 up to 20PSI. What fuel pump? It sounds like you're getting "Lean Pops" out the TB, not Backfires. In the heat of the summer, you need less timing to prevent KCs and the possibility of Lean Pops.

You wouldn't be the first here to have problems with Harris/TBIChips. My money is on the tune.

^^^Amen - BTDT!

 

[Scooterwrench]

Do you have a wideband? What's it reporting?

If you haven't done so yet it may be time to download TunerPro RT and buy an ALDL cord so you can tune your truck. Buying a "flashed" chip is always going to be a crap shoot. You can build two identical motors and they will tune differently. Once you get some sort of datalogging check your injector duty cycles. Fuel starvation will cause high DC's at WOT and to much fuel will cause low DC's. May want to go over your secondary ignition. Spark can get blown out under hard demand if your plugs,wires,cap,rotor or coil are not up to the task. Make sure all your mechanicals are working properly before you try to do any tuning or you'll be chasing your tail.

 

[95er]

An 18LB spring at what pressure? I can adjust mine as low as 15 up to 20PSI. What fuel pump? It sounds like you're getting "Lean Pops" out the TB, not Backfires. In the heat of the summer, you need less timing to prevent KCs and the possibility of Lean Pops.


^^^Amen - BTDT!

I did upgrade my fuel pump to the 96+ fuel pump the fuel pressure gauge is under the hood at the tbi reading about 16psi so would changing the spring change that pressure?? At this time I can’t see the pressure under load but when I rev the engine in neutral it maintains the pressure. Also it doesn’t have any issues raving it in neutral. Or would this be more of a fuel pump issue? I don’t understand how a spring would change the pressure reading since the gauge is before the tbi Thanks for the response I was not even considering a fuel issue

Also what are “Kc’s”?

 

[95er]

Sorry I didn’t realize I had so many comments. So I got an hp74 engine from ATK. After I ordered it I seen mixed reviews on them. It has a mild cam upgrade with vortec heads. I updated the fuel pump installed new fuel filter. Has new coil and distributor new plugs and wires new water pump I have a jegs

Intake Manifold with Installation Kit for 1996-2002 Small Block Chevy 350 Vortec with L31 Cast Iron Heads​

With a transdapt tbi adapter and spacer. And had spr performance build me a bored out tbi with the spacers new injectors and that spring. I also added a 3 wire o2 sensor. And finally a better flowing exhaust. All these parts were recommendations from Harris tbi. Then had them do the chip since he recommended the entire build.

 

[95er]

If you haven't done so yet it may be time to download TunerPro RT and buy an ALDL cord so you can tune your truck. Buying a "flashed" chip is always going to be a crap shoot. You can build two identical motors and they will tune differently. Once you get some sort of datalogging check your injector duty cycles. Fuel starvation will cause high DC's at WOT and to much fuel will cause low DC's. May want to go over your secondary ignition. Spark can get blown out under hard demand if your plugs,wires,cap,rotor or coil are not up to the task. Make sure all your mechanicals are working properly before you try to do any tuning or you'll be chasing your tail.

I don’t think I have a wide band as I don’t know what it is. I am willing to get stuff though. I just have no idea with this computer stuff. I know tbi is pretty basic it’s just not anything I have ever dealt with.

 

[Schurkey]

the fuel pressure gauge is under the hood at the tbi

Is this a "liquid filled" pressure gauge? Do you burp the gauge before taking a reading?

reading about 16psi so would changing the spring change that pressure??

That's what the spring--in combination with the adjustment--does.

I don’t understand how a spring would change the pressure reading since the gauge is before the tbi

Everything downstream of the regulator--and the regulator spring--back to the fuel pump, will be under the same pressure aside from minor flow restrictions and a potential fuel filter issue. From the regulator back to the tank on the return side, should have nearly zero pressure assuming there's no kinks or other problems with the return plumbing.

Also what are “Kc’s”?

Knock counts. Knock sensor activity.