misfire at about 3500 rpms and up. Need some help. Running out of things to replace.
 

I'm getting a misfire somewhere around 3500 rpms. I do have a turbo on but I unhooked the charge pipe to see if it was spark blowout but it starts missing NA or Boosted. Previously I had a 4.8 and now I have the 6.0. Same problem. Before with the 4.8 I thought maybe a had a broken ring land that was causing the missing. I did swap out the cam and valve springs from the 4.8. Springs and cam are pretty new. Would a weak valve spring cause this? When I installed them they all felt strong. I've never had a weak spring problem so I wouldn't know the symptoms.

I just installed new plugs. Gapped to .024. New plug wires. Still have the problem.

Fuel pump is a Walbro 450.

Also have a fuel pressure regulator. Boost referenced. set at 43.5

I made a post earlier about upgrading wiring to the fuel pump. Would voltage loss cause this misfire?

I've explored engine knock. But its doing this without boost. I just put 93 octane in it and its still doing this.

Any thoughts?

Try replacing the mass air flow sensor. I had a similar problem and that was the culprit.

Ya why so low? Is there a fuel pressure reg?

He said he had a boost referenced regulator set at that pressure. Did not say why.

Yes a valve spring can absolutely do that. It's far more common for it to happen at a little higher rpm but yes a weak spring will cause higher rpm breakup. If the springs have ever sat for a long period of time compressed (like storing an engine with the rockers still installed) can do it. Or it may just be a funky spring, it happens.

Some people have had that issue and a crank sensor helped. Did you also re use that?

Is it under heavy throttle only, or will it also do it under mild acceleration/throttle?

Does it quit after 3500 or keep doing it?

why only 7º ?

Check your springs before doing anything else.

You are better with the spring with lift capacity closer to actual lift. You want it just short of coil bind to prevent "lofting" the valve. Too much lift capacity does you no good. You only need a little more than the actual lift.

It's unlikely it's the springs, but not impossible. I say unlikely considering the spring you used isn't known to have issues, they aren't very old and didn't set for long periods of time compressed. But shit happens and every once in a while you get a bad one.

How many cylinders are misfiring? I would have a hard time seeing multiple PAC 1218's on a 585 lift cam float at 3500 rpm.

Try a crank sensor and see if that helps, get a good one not a cheapy. Dealer one or AC Delco if possible.

You stated the motor has the same miss as it did on the earlier motor. That means it is something you transferred from the first motor. So, lets break it down from there.
1) It sounds like a spring problem but you said the springs were good.
2) High speed misses are usually caused by the ignition.
3) Its not the plugs because bad plugs miss at any speed.
4) Start with the easy stuff and work back.
5) Its also not fuel because lack of fuel would cause the motor to role over and stumble but not miss.
6) Check the plugs wires for cracks, broken terminals, arcing, and do an OHMs test.
7) Check the coils and their wires for cracks, shorts and grounding problems.
8) Stay focused on the parts you tranferred from the other motor.

High speed misses are usually from the plug wires breaking down under stress. I know these wires are about a foot long but check the OHMs resistance anyway. They should be 1000 OHMs or less. Wire resistance used to be more common problem on Gen I motors because of their 2-3 foot lengths. If the wires are OK, then move to the coils.

Also, try to isolate the cylinder. With the motor running remove a plug wire and note the difference. The miss will stop when the bad cylinder is unplugged. If the miss does not stop, then repeat the procedure on the injectors.

Another useful tool is a heat detector. Take the temp on each header pipe. The cooler one is the culprit. Coils also break down under load and heat, although they usually produce a steady miss.

If what you said is correct, that the springs are good, the CKS is new, and the old motor had the same miss, the problem has to be in the ignition system with the wires and coils being the primary culprits.

Good luck and post the results.

The stock PCM is scaled for 58 PSI, not 43.5. You're off to a bad start already.

How's your coolant sensor?? Could be bad