I just did my VERY FIRST engine swap/rebuild with a fully machined engine. I had it machined professionally, but I did the pull, breakdown, reassembly, install and hookup for most part, including setting the the valve lash. I finished the install and adjusted the CAM Retard Offset to +1 degree using Torque app using a custom PID with a recommended algorithm.

Fired right up for most part, No other codes except P0300.
Have limited data access to ECU at moment, (waiting on a Foxwell NT510 with GM addon, ETA 2 weeks) but some data includes, at idle and warm engine:

Intake Manifold Pressure at 36 kPa (10.6 in Hg) (seems this is low?)
MAF at 7.22g/s ( I did just clean the MAF)
dont have fuel pressure
dont have any Trim Data
Torque App on ELM327 has clearly indicated misfires on cyl 5 and 7, the others cylinders are solid.
I've only swapped plugs and cap/rotor so far, no change.
Also sprayed carb cleaner...no response.
Tomorrow I was planning on swapping some plug wires.

But any other suggestions to help debug this? Needed specific data? Thanks

 

P0300 is a real bitch sometimes. I'd check your fuel pressures first.

 

So I just hooked up the manual gauge this morning before it got too hot outside: and ran a few tests

Key On, Engine Off, Cold Engine
With key, seems like relay initially triggers and fuel rail pressurizes to about 59/60..but not higher
After few seconds, seems relay turns off and rail pressure drops immediately to about 54/55
Then there is slow decay...in 6 minutes it decayed to 44

Key On, Engine On, Cold Engine
1) at Idle, rail pressure is 51
2) with a slow increase on the throtlle, I dont see much if any change in the rail pressure
3) if I snap the throttle (with immediate release) it jumps to about 58 with an almost immediate recovery. Snapping the throttle almost causes some level of engine cut out, but it doesnt die). I notice during this recovery, the fuel pressure will dip down to 49 but then quickly recovers to 50/51
4) If i do a slow acceleration, the pressure holds at 51, but will dip to about 49 and then recover back to 51 when I let go of the throttle

Are these pressures sufficiently marginal.? and thus could be causing a P0300 with misfires on cyl 5/7 ONLY?
Is the decay indicating a slow leak? Other thoughts? thks

 

51 PSI is just too low for the Central Sequential Fuel Injection (CSFI) fuel injectors to operate correctly, particularly if they have carbon build-up. 55-56 PSI is low tolerance.

60-62 PSI is ideal. Likely have a bad fuel pump, and once it warms up, the fuel pressures will drop as it's going south. In 2002 GM phased CSFI out for the MPFI fuel injection system. Too many issues with the older FI system.

 

This vehicle has multiport on it, I cleaned it mildly when it was apart, but I didnt go to town on it or validate operation.

 

As a last resort, I'd put a full bottle of Chevron Techron Complete down the gas tank. I used to be "In The Biz" of selling Chevron fuels & lubricants, as well as other brands, and I can assure you that Techron does clean up carbon deposits. I am no longer in that biz, so I gain nothing here. Once you get it down the tank and have some Techron on the valves and in the injectors, find a back road and take that vehicle and pound the engine to the floor and rev the hell out of it.

You will only be out $15 dollars so no loss. I still believe your fuel pump or regulator is going bad and Techron will do nothing to help that, only a new fuel pump or a new regulator. There is the option to upgrade to the newer MPFI on your vehicle if you so choose to do so, but it too needs 60 PSI fuel pressure to work properly. You have the old-style "spider" poppet valve fuel injection system which if not maintained well can or will cause issues. Here is a link that will help you to understand CSFI, versus the newer MPFI system:

https://www.fourwheeler.com/how-to/1...e-fueling-fix/

 

I previously put a bottle of seafoam in it, but there was/is alot of fuel in the tank, so it may not be at a concentration to be effective just from mild idling. But no issue sticking a bottle of chevron chemistry. Im a PhD, and did research in the chemical industry so fully appreciate the chemical development process at the consumable suppliers.

Im also confused about your injector type comment. I believe I do NOT have CSFI, but have MPFI. This is the version which does NOT have the black end tips, but has the white tips with the individual electronic controls. But if you are saying even this version requires 60 psi, then it would seem that could very well be an issue there.

I did modify the back cargo floor for ez access to the fuel tank top yesterday, thought i had an extra pump, but turns out that assembly was for a tahoe, not suburban, so i just ordered a new assembly and will have to wait until it shows. Too lazy to just pull the pump from the tahoe assembly and stick it in the suburban assembly...

 

Smart move with access to the fuel pump made easy! I wish they came like that from the factory. If you have the white-tipped injectors, with electronic controls, then you do have the MPFI system. I was not aware that it was available on a 1999 5.7 liter V8 Chevy engine. It's possible that it was an option, or I am uninformed about that F-I
unit change-over. Yeap, it's never fun dropping a gas-tank to change the pump. Back in the day, when carburetors ruled the road, and the mechanical fuel pump was on the motor, it was not unusual for fuel to leak into the engine, diluting the motor oil down with leaded gasoline. Lots of engines went south due to this defect at 100k or less.

 

So at this point, Im just waiting for the new pump and scan tool to arrive, so likely another week and then I will update. Wont do anything more until then. The MPFI may have very well been an upgrade from previous owner. My vague recollection is that my 99 Tahoe that I had awhile back also had the old style when i acquired it, which I ended up upgrading to MPFI, but Im not sure, as Im old and dont remember that well. In any case, wll update around end of month.

 

Yes, please keep us updated on your vehicle. Indeed so, both systems are multi-port injection systems, but the older one uses the poppet valve system. My 1998 K-1500 likely has the old CSFI poppet valve system but I use Chevron with Techron gasoline almost exclusively, so I have had no troubles with it.at all. If it goes south I'll update it to the newer system. I'm old myself, but I can still turn a wrench! lol.

Seems like a relatively easy job do to, from what I can see, but like they say; "If It Ain't Broke, Don't Fix It"