Torvus

4.8L (was supposed to be a 5.3) into a '69 chevelle. Everything is wired up and just about ready to go. First crank of the car and it appears that cylinders 1,2,7,8 are not firing correctly. I've traced out just about everything. Cylinders 3,4,5,6 seem to be running a bit rich.
- Pulled the plugs on 1,2,7,8 wet with fuel. Header tubes were fairly cold.
- Plugs 3,4,5,6 blackened apparently running rich. Header tubes warm on driver side very hot on passenger side 3,5.

Stock 2005 Silverado 4.8L DBW. Modified stock harness and PCM from the original donor vehicle. Walbro pump with corvette regulator. Transmission is a 4L80E but not wired up yet although sensors are connected. New wires and plugs.

- Checked all grounds and leads.
- Swapped coils.
- Swapped injectors.
- Checked crank sensor
- Showing spark with spark tester on the missfiring cylinders.
- Checked cylinders for compression.
- DTC's below. No missfire codes.

Strange that it's only running on the inner cylinders and although I've tried searching I have not been able to come up with a solution.


With MAF disconected for this particular capture. No codes while running with the exception of the Fuel Tank Pressure Sensor Circuit. These codes only showed up with key off.

[PCM] P0452 - Fuel Tank Pressure Sensor Circuit Low Voltage (SES) (Pending) (Old) (History) (Current) (Immature)
[PCM] P0758 - 2-3 Shift Solenoid Circuit Electrical (SES) (Old) (History) (Current) (Immature)
[PCM] P0102 - Mass Air Flow (MAF) Sensor Circuit Low Frequency (Old) (Current) (Immature)
[PCM] P0443 - EVAP Purge Solenoid Valve 1 Control CKT (Old) (Current) (Immature)
[PCM] P0449 - Evaporative Emission (EVAP) Vent Solenoid Control Circuit (Old) (Current) (Immature)
[PCM] P0480 - Cooling Fan Relay 1 Control Circuit (Old) (Current) (Immature)
[PCM] P0481 - Cooling Fan Relay 2 Control Circuit (Old) (Current) (Immature)
[PCM] P0522 - Engine Oil Pressure Sensor/Switch Circuit Low Voltage (Old) (History) (Current) (Immature)
[PCM] P0650 - Malfunction Indicator Lamp (MIL) Control Circuit (Old) (Current) (Immature)
[PCM] P0654 - Engine RPM Output Circuit Malfunction (Old) (History) (Current) (Immature)
[PCM] P0740 - TCC Enable Solenoid Circuit Electrical (Old) (Current) (Immature)
[PCM] P0753 - 1-2 Shift Solenoid Circuit Electrical (Old) (Current) (Immature)
[PCM] P0785 - 3-2 Shift Solenoid Circuit Electrical (Old) (Current) (Immature)
[PCM] P2761 - Torque Converter Clutch Pressure Control Solenoid Control Circuit/Open (Old) (Current) (Immature)

Any help would be greatly appreciated.

99VetteFRC

What tune are you using? I put a 4.8 in my Chevelle and it started and ran without issue with the stock tune? Did you rework the harness yourself or buy a standalone? Yo have no spark on those 4 cylinders if the plugs were wet so check to see if the coils are firing. If not check the bat+ side of the coils and make sure they are getting power. If they are then use a LEd test light, connect to bat+ and probe the other side of the coils and see if the light blinks when cranking. If not check continuity of the wires to the PCM for each nonworking coil. If the wires are good suspect the PCM(hardware or possibly software) as not firing the coils. You could very easily throw a new set of plugs at it just for good measure. Hope this helps.

Torvus

Thanks for the reply.

Using the stock 4.8 tune minus the VATS. Modified stock harnes, I tried not to cut any of the stock wiring as much as possible to help avoid as many variables as possible.

Pulling one of the 'bad' cylinders I can set it on a spark tester and am seeing a spark thus the coil is at least firing. Not sure if it's possibly a weak spark or not I just see it sparking. Might swap a couple of plugs as you suggested since I have all the plugs pulled currently.

I'm all ears if anyone comes up with anything else to try as well.

pdsq98gt

Injectors on those cylinders gummed up from sitting?

Torvus

I thought the injectors might be gummed so swapped with the cylinders that are firing and the problem followed the cylinder. Found that the wires for the B1 and B2 O2 sensors were crossed although I don't think that is going to change much since O2 levels were pretty steady.

Torvus

The only logical issue to me was timing or compression. Pulled the valve covers and found that some genius loosened all the rockers for whatever reason. I can't believe it ran at all. Tightened them all up and it fired right up. I pulled the valve covers when I bought the motor but never thought about checking the rockers for being loose. No clue why someone would do this on a complete and stock motor but on to other issues now. Thanks for the help regardless. It's appreciated.

99VetteFRC

Yeah I had the o2 connectors crossed side to side the when I first got mine running. Car ran like crap. Had on side full rich and the other full lean! Switched them and runs like a new car.

Felon454

Please! Someone help me out here. I have a 66 Chevelle and I work at a boneyard. Going to put a 4.8L into it. What all will I need to run that with a turbo 350 or a powerglide trans. I need a legit build list so I know if I'm over my head in money or not. I have all of the big parts. Motor/Trans/ECM from the truck the engine came out of and stuff. I keep trying to search a list of parts sort of but I can't find anything substantial. Thanks you guys.

spot_55

felon454 you have a pm

oldgoat69

I am willing to bet that the PCM is now in limp home, full rich, full retard. If I recall correctly some of the DTC's you see will cause that.

gMAG

iTrader: (1)

I had the exact same symptoms.
It is possible that you have your coil harnesses reversed.
For the driver's and passenger's coil harnesses, you have the one main, common, large plug.
From there, it is an easy matter to connect the smaller plugs backwards...that is, the plug with the correct color coded wires for #1 , is going to coil #7. Then, the color-coded wires & plug for #3 is actually plugged in to coil #5, and vice-versa.
So, instead of having the color-coded plugs correct... ie 1, 3, 5, 7, you have 7, 5, 3, 1.
Likewise for the passenger side coil harness.
It is possible that only one (pass OR driver's) harness is reversed.
1-8-7-2-6-5-4-3 is the correct firing order.
If both coil harnesses were reversed, your firing order would be, 7-2-1-8-4-3-6-5...thus causing the coil to fire when that particular cylinder is not at TDC/firing. You'll have raw fuel on certain cyls, partial ignition on other cyls.

gMAG

iTrader: (1)

Have you had any success locating the trouble?

wildcamaro

iTrader: (15)

Power probe!

1bad03snake

iTrader: (16)

My 4.8 started to misfire today. It seems like the driver side harness is dead. I have a stand alone harness