Possible fuel issue?

[L8RH8R]

I will try to be as brief as possible. 91 Silverado with 02 4.8 swap. After first several attempts to start, engine ran (each time I tried to crank it, took several tries. Low fuel pressue). Replaced fuel pump, upgraded fuel lines, filter, injectors. I would later discover bad head gasket. Rebuilt engine. Installed ls6 cam and had heads ported with larger valves (ls1 valves). Now the hard start is not as bad but it runs really rich. Throwing all sorts of codes. Multiple engine misfire, 02 sensors, knock sensor etc. (all those sensors are new) my nephew, whose a mechanic, scanned it and discovered cylinders 2 and 7 misfiring nearly constantly. We swapped the #2 and #4 coils to see if that was the problem, however 2 and 7 still had the worst misfires. He also discovered the fuel trim learn is disabled both long and short term. I know I need to add another ground, but could the fuel trim being disabled explain the hard start, running rich and misfires. Btw fuel pressure is 50psi once ignition is turned. Thanks in advance for any help.

[farmtruc]

extended cranking after a long period of sitting is def fuel related. hard start, then pressure builds.

the ls6 cam will show slight misfires while scanning, but if two are standing out then it's an issue.
determine if the misfire is no/little spark or no fuel. or could be compression related.
valve spring, compression test, wiring, injectors, plug, coil, wire, etc need to be checked.

perform a vac leak using a smoke machine also.

with all those codes, o2's especially, the pcm will never go into closed loop so ltrims/strims will not register.
using the scanner, what does the o2 data look like? what are they doing? mv, graph, wave, etc.

[L8RH8R]

My nephew remembers the O2 sensors reading between 440 and 460mv. He also mentioned His Autel scan tool was reading four O2 sensors, however, it only has two Wired into the harness. They are both connected and installed just before the collector on shortly headers. The exhaust is not complete at this point. Why would the CPU say the fuel trims are disabled. I assume this would cause the rich condition, rough idle, hard start, ce codes etc. The valve springs are new. Came with crane cam kit. Fuel pressure is immediate once the key is in the on position. What is interesting is that the truck ran about the same with both of the problem plug wires,2&7, disconnected. The truck does start back up after running for a while and warming up. Although it does take a few start attempts. Thank you for the response.

[farmtruc]

The O2 sensors should be toggling up and down from 50-900mv. If they are at 450 that's a lean condition and the pcm will dump fuel but never satisfy the request.
Ur better off unplugging them.
Be sure the o2's are case grounded style. Sounds like the wrong o2's maybe.
No o2's working means no closed loop means no trims working.
What do u mean exhaust not complete?
If the o2's are too close to the open exhaust they will read lean.

[L8RH8R]

I read on another site that the O2 sensors could be wired on the wrong sides. Is that a possibility? The harness that was made drops them in the right spots. I've never heard of such a thing. This is what I read. "Sounds like you have the wiring for the o2s switched from side to side. One side will read lean and try to compensate rich, the other will read rich and try to compensate lean"

[L8RH8R]

Yes they are close to the collectors which are open at this point. Dumb it down for me a bit. Case grounded style. They are for a02 Silverado which is what the engine came from. They plugged right up. Is there a better type to get. They are 4 wire as I recall.

[vortecyota]

You'll have to ignore anything thing related to fuel trims until the misfires are sorted out and the exhaust is complete. GM uses about a 445 milli volt bias on the O2 sensor signal circuits so if the O2s are the wrong type, disconnected or the harness is wired incorrectly for the type O2s used, the O2 pods will read around 445 mv. You have an O2 issue for sure BUT since you have two confirmed cylinders down you must attack that first.

Another cause of extended cranks is lack of cam signal. It will run without a CMP signal but will take longer and often several attempts to start. I've seen 3 bad LS CMP aftermarket sensors right out of the box. Have your nephew check for a CMP signal either thru a scan data pid OR if he is a scope user it will be a 0-12v digital square wave. This won't cause your misfires though.

Start with the two misfiring sensors. Check spark, then put a noid light or scope on those two injector circuits. If those are both good, check compression on those two holes both cranking and running then check a known good hole to compare against. If fuel, spark and compression check out you'll need to check that the coil control circuits aren't crossed by the harness builder.

I know you're hoping for a silver bullet that will explain all of your symptoms and often times that is the case but I can promise you that in this case, I'm certain you have multiple root causes.

So go in this order or you will be chasing your tail.

1. Solve both constant misfires
2. solve extended crank
3. Finish exhaust
4. Solve O2 non response

[foggy]

The misfire could be as simple as 2 bad spark plugs. I've had 2 engines that had the spark plugs fail. 1 engine had less than 7000 miles and 2 plugs failed. The other engine had less than 40 miles and a plug failed in it.

[farmtruc]

get that exhaust finished, open exhaust will mess up the o2 readings.

[L8RH8R]

get that exhaust finished, open exhaust will mess up the o2 readings.

When my nephew scanned the CPU he ran a cycle of the injectors. All of them responded. Should I still use a noid light? I plan to fix the exhaust this weekend and rescan it. Thanks again for everyone's input.