Robert Shane Johnson

Been scratching my head for a couple of weeks over a high rpm misfire on my truck. As soon as I hit 3500 rpm it stats bucking and missing like crazy. I’ve swapped the cops, injectors, fuel filter, tps, iac, and all four O2 sensors. Pressure gauge is reading 33psi at idle and climbs to about 38psi when I give it throttle. The MAF was swapped a while back when I had a new PCM harness installed. Truck has long tube headers with a true dual exhaust, high flow cats, and a cold air intake. Thinking maybe fuel pressure regulator, or possibly the pump? TIA.

Aragorn

1. Are you getting codes?
2. What are those codes?
3. Look into getting something like the BAFX bluetooth scanner that can do a live data feed and a laptop to record. See if you can find the cylinder that's misfiring.
4. Exhaust leaks?
5. Vacuum leaks?

Robert Shane Johnson

Last codes I pulled were P0141 and P0161. Had a broken 02 sensor and swapped it. Reading the codes with a superchips programmer. Recently patched up the vacuum leaks and reclamped the exhaust to kill what few leaks I had.

Jbrew

So does this happen while running a custom tune (s) ?

Robert Shane Johnson

Yeah. I’ve tried it with the tunes and it missed, then I reset it to stock, didn’t changed a thing. Gonna take the cops back today for a new set while they’re still under warranty.

akdoggie

If it doesn't miss till 3500, it could be an internal engine problem over a COP or fuel Injector. It could also be fuel related, but that would throw a code. Maybe a bad valve or valve spring? Lash adjuster losing oil pressure at higher RPM's?

Sorry, just thinking out loud for you. While you've got everything out and off, why not do a compression test? It might save you a bunch of time. Before I did anything else, a CT would be first on my list. Good luck.

Robert Shane Johnson

Sorry, should’ve added earlier, engines got 20,000 miles on it. Replaced the stocker with a promar crate engine about a year ago. Shouldn’t have nearly enough miles to be causing me any problems should it? And one of the sensor codes it pulled was the engine leaning out. Pulled the plugs to inspect what it’s doing today and they where an ashy brownish white so I’m thinking it’s a fuel issue. I’ve also got the fuel trim levels written down, I’ll post them up when I get home.

akdoggie

It's not too often that we have a replacement engine pop up, so I'm sorry I assumed that your engine was original and higher mileage.

Your description of the s/plugs sounds like they are burning correctly and about right for 20k miles.

Robert Shane Johnson

It’s all cool. I should have clarified that in the post. I ditched the original motor at 200k since it kept blowing plugs. Figured I’d pull the plug and but a forged motor in it. Just got back with some new coils (again) gonna see if anything changes. I’ll post the trim levels and o2 stuff tonight, and do another fuel pressure test. I’ll let you know how it goes.

Robert Shane Johnson

Got more problems today. **sigh** Swapped the coils out and put a cam sensor in, turned it on and still had the misfire. Turned the truck off to reset the fuel pressure tester and tried to turn it back on, cranked and nothing. Engine sits there turning but won’t turn over. Did a spark test and the spark is bright blue on brand new coils (literally pulled them out of the box today) Checked the fuel pressure tester and it’s showing 20psi but slowly builds up to 30 after I stop cranking it. I swear to god and everything Ford stands for, I’m gonna burn this thing to the ground!