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

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

 
Old 10-21-2018, 02:52 PM
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Default 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?



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Old 10-21-2018, 04:31 PM
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Try replacing the mass air flow sensor. I had a similar problem and that was the culprit.
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Old 10-21-2018, 04:36 PM
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Why is your fuel pressure set at 43.5 lbs?
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Old 10-21-2018, 04:54 PM
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Originally Posted by Che70velle View Post
Why is your fuel pressure set at 43.5 lbs?
Ya why so low? Is there a fuel pressure reg?
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Old 10-21-2018, 04:56 PM
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Originally Posted by qweedqwag View Post
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.
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Old 10-21-2018, 05:09 PM
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It has deka 80s. I set it that way because at the time I wasnít sure how to set them up in hp tuners scaling wise. It doesnít have the maf installed. Itís in speed density open loop. My previous motor ran fine this way until the miss started. But itís moved from one engine to the other. The springs are the only thing I can think of to carry the problem but I donít see how one or two etc. would go bad. They rated for a lot more lift than what their getting. I disabled burst knock and itís still doing it. My coils checkout good according to scan. Of course Iíll have to research on how to fully test them. When it misses it comes and goes really quick but sounds horrible like itís preignition. My timing is around 10 degrees though and when it started with the 4.8 I know my timing was a little higher. I guessing something electrical but really donít know.
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Old 10-21-2018, 06:47 PM
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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?
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Old 10-21-2018, 07:53 PM
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Yes the crank sensor was also reused. The springs are less than a year old and was still driving the truck around town but just not running the rpms up. Also on my scan Iím showing misfires but with the 4.8 when it did run fine I would always log some misfires just not a crazy stumbling.
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Old 10-21-2018, 07:56 PM
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It always hits at the same spot about 3 to 3500 rpms. Slow throttle or fast it does it the same. My timing is 7 degrees advance at this point.
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Old 10-21-2018, 08:00 PM
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Does it quit after 3500 or keep doing it?
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Old 10-21-2018, 09:42 PM
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why only 7ļ ?
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Old 10-22-2018, 06:23 AM
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It will continue doing it past 3500. It will clear up for split seconds like itís trying to work. I pulled timing back because I wasnít sure if it was preignition. It was recording as spark knock on the scanner. But that low of timing I donít see how.
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Old 10-22-2018, 12:18 PM
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Originally Posted by jon1440 View Post
It will continue doing it past 3500.
Check your springs before doing anything else.
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Old 10-23-2018, 03:42 PM
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Iíve looked at pac 1219 springs but wasnít sure they are a direct drop in. The 1218 was. Although Iím close to the maximum lift with them but everyone has been running them I suppose with good luck. Lift on my cam is .585 and the spring is rated at .600. Should be good to go. I would consider the 1219 because their good to .625. I donít know whatís best at this point. Try anotherís set of 1218 or get the 1219. The 1219 cost a little more at least where Iíve looked and also would want to know for sure they are a direct drop in.
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Old 10-23-2018, 04:15 PM
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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.
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Old 10-23-2018, 08:11 PM
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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 **** 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.
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Old 10-24-2018, 10:29 AM
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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.
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Old 10-24-2018, 12:30 PM
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Originally Posted by jon1440 View Post
It has deka 80s. I set it that way because at the time I wasnít sure how to set them up in hp tuners scaling wise.
The stock PCM is scaled for 58 PSI, not 43.5. You're off to a bad start already.


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Old 10-25-2018, 11:43 AM
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How's your coolant sensor?? Could be bad
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