Misfire at higher rpms

95er said:

Ok so I hooked my fuel pressure gauge up with the release valve and the cheap tubing. The gauge read 20psi. When I pressed the relief button it dropped to 10 psi. Which according to a response to me should be remain at around the 20 psi. So if I am understanding everyone right is a fuel pump issue. If this is the case what does everyone recommend for a fuel pump. Is there anything else I should be looking at? The filter is a brand new k&n filter along with the fuel pump being the Delphi pump for the 96+ k1500 which is supposed to handle higher pressure. Thanks again everyone for the advice.

Click to expand...

It depends on how much the release valve flows. Your pump can only flow so much and the RV could be blowing off more fuel than the pump can pump. I think a road test with the gauge hooked up will give you a better reading. Take it out on the road,stick it in second gear and floor it until you hit the rev limiter or reach the max RPM's you're willing to turn it and see what kind of pressure it sustains.
20psi is kinda high to run it at. Back it down to 15psi once you figure out if the pump can keep up.

So interesting change of events. I haven’t driven the truck for a week. Currently the weather is 102*. The only thing I have messed with is I pulled the fuel guage that was installed next to the tbi and installed my test gauge that has a hose connected to it so I could pop it out under the hood so I could read it from the drivers seat. Started the truck it wanted to die as a gave it a little throttle in neutral, that isn’t necessarily normal but it hasn’t been running for a week so ok! I drove it down the street in 2nd then gave it about 3/4 throttle didn’t put it to the floor and she went great. About 4500 started to pop a little bit nearly as bad as before and she had some power did this 3 times exact same result and pressure stayed around 18-19 psi the whole time. I don’t know if this is weather related, though last week it was in the high 80’s low 90’s when it was haveing issues. Or maybe the fuel gauge that the tbi builder installed might have been causing an interference in the fuel line or what. I dunno. Be interested in your thoughts still. Not sure even if this was a fluke or not. Also not sure if this is just cause the motor is still breaking in or not. It only has 95miles on it. Also before I was in 3rd gear when I was punching it don’t know if that has anything to do with it either.

Well my theory about the gauge being part of the culprit I think is not feasible. The more I think about it the gauge is tapped into a 90* elbow. I didn’t remove the elbow just the gauge so it would have no way to be an interference

Sounds like the fuel pump is doing its job although I think that's too much fuel pressure. Gonna have to find out what's up with the tune.
Do you still have the original prom? If he installed a prom adapter the original prom may still be in there,attached to the prom adapter. You should be able to pull the adapter out and remove the MEMCAL from it and pop the MEMCAL back in the ECM for a test to see if the chip he burned is way outta whack.

Scooterwrench said:

Sounds like the fuel pump is doing its job although I think that's too much fuel pressure. Gonna have to find out what's up with the tune.
Do you still have the original prom? If he installed a prom adapter the original prom may still be in there,attached to the prom adapter. You should be able to pull the adapter out and remove the MEMCAL from it and pop the MEMCAL back in the ECM for a test to see if the chip he burned is way outta whack.

Click to expand...

I do have the original prom. I just swapped the 2 chips. I don’t understand how I would tell if the new tune is way out of whack based on the original factory tune. My new motor is nowhere like my old motor. New motor has vortec heads, a cam, different intake and mods to tbi. This motor should be getting close to 300hp which is far from my stock motor. I would suspect the new prom would be nothing compared to the stock. Right?

95er said:

This motor should be getting close to 300hp which is far from my stock motor. I would suspect the new prom would be nothing compared to the stock. Right?

Click to expand...

Yep, IF - Big If (as @Schurkey would say) you are indeed putting out 300 HP, your fuel pressure s/b OK where it's at but, without data logging and optimizing your tune, especially VE and SA, I'd bet you're leaving a bunch on the table.

As an example, I started with the "Harris" Chip and got 9.2 seconds 0-60 (from OE 11.2). After getting a ZIF (Zero Inserting Force) socket and running the SST 27SF512 "Flash" chips, working with Dave W on Gearhead-EFI, I data logged sent the file to him, he sent a modified .bin, I flashed a chip - rinse and repeat. We got it down to 8.0 seconds - a gain of 1.2 seconds!. After getting the EBL (faster with bigger tables than the 7747) it did even better.

Like most everyone will tell you - you need to data log and tune to get the optimum out of your equipment HTH

95er said:

I do have the original prom. I just swapped the 2 chips. I don’t understand how I would tell if the new tune is way out of whack based on the original factory tune. My new motor is nowhere like my old motor. New motor has vortec heads, a cam, different intake and mods to tbi. This motor should be getting close to 300hp which is far from my stock motor. I would suspect the new prom would be nothing compared to the stock. Right?

Click to expand...

No,you won't get optimal performance out of the stock tune but if it runs better than the Harris tune it rules out anything else that could be creating your problem. Like I said,just as a test. If it still has the same problem then you may want to start going over your secondary ignition. Breaking up under hard demand can also be spark getting blown out due to coil,cap,rotor,plugs and wires. If it runs better with the stock prom it rules those parts out.
Playing with TBI is absolutely correct,datalogging is the best way to figure out what is going on.

Scooterwrench said:

No,you won't get optimal performance out of the stock tune but if it runs better than the Harris tune it rules out anything else that could be creating your problem. Like I said,just as a test. If it still has the same problem then you may want to start going over your secondary ignition. Breaking up under hard demand can also be spark getting blown out due to coil,cap,rotor,plugs and wires. If it runs better with the stock prom it rules those parts out.
Playing with TBI is absolutely correct,datalogging is the best way to figure out what is going on.

Click to expand...

Ok thank you. I understand better. I definatly plan on data logging I am just waiting on the equipment from boostednw I just ordered from them on Thursday or Friday.

Well for some reason it runs a little different when I have my normal pressure gauge compared to my test fuel pressure gauge. With the testing gauge it runs better but doesn’t completely fix the issue. I don’t get it but it is an observation. I also pulled a spark plug and this is how it looked looks lean to me but I am seeing some conflicting info on the coloring that this could be ok. Again I only have 100 miles on this motor. I am still waiting on my ALDL cable

If that is with only 100 miles on it you are running a tad rich.
What brand of plug is that? Looks kinda crappy.