831xj

here's a little background info
99 4.0 aw4 147k

So I have had this misfire come and go for the last 5 or 6 months. It happens AFTER I get into the high rpm range. Example: gettin mad at slow grandma and flooring it past her. When I come to a stop my jeep stumbles and idles rough. Throws a p0301 after a while. Now usually it will go away after a bit more driving and clearing the code. But this time it's sticking around.. I have a pending p0301 after I cleared the cel the first time. It's still missing but not throwing the cel. I have a good scanner that can tell me lots of live data so if anybody has something I should check with that let me know.

What I've replaced
Both o2s
Champion spark plugs
Wires about 8 months old
Dizzy cap pretty new too
Tps
Injectors 784
Apn header
Compression test came out + - 10 psi between all cylinders

It starts right up. Will just misfire and idle rough around 750 and will still miss until im at about 10-15 mph or 2kish rpms. will drive smooth.The needle will kinda bounce around + or - 50 rpms and I can hear it sputter through the exhaust.

Going to check fuel pressure tonight. But last time I rented one from autozone i couldn't get it to hook up right or read anything on my valve on the rail.

Could fuel pressure cause just ONE cylinder misfire? What else can I check before I resort to paying a shop? I have lots of tools. I just need some guidance

831xj

Bump >_>

Radi

That P0301 is misfire cylinder #1. You already changed the plugs, wires, cap and injectors- how about the wiring harness that feeds the injectors? Any sign of trouble?
Any lifter noise? As in one weak enough to not open a #1 valve correctly until you hit higher rpms (and higher oil pressure?)

831xj

Going to check the harness when I got some daylight tomorrow. Read a thread that mentioned a shorted wire in there.

Fuel pressure wouldn't cause just one cylinder to missfire?

belvedere

You can swap the injector connectors between #1 and #2 to see if the problem stays or moves.

belvedere

No.

When my '98 did this, I stumbled across a TSB for this issue. I think it affected '96-'99. It said to use Mopar Combustion Chamber Cleaner and then replace the valve springs. I ran the cleaner per the TSB and replaced the springs on cyl 1 only, and the problem was fixed.

831xj

I'm thinking its valves maybe a shorted injector wire. I've checked injectors for leaks and they were tested before they were installed by me.

cruiser54

This is what fixed 2 of my buddy's 99s.

Pretty easy.
Get a cup of water, preferably something like a Dixie cup.
With the air cleaner off, hold the engine RPM at 2000 RPM or higher and dribble the water from the Dixie cup, which you've pinched to look like a spout on top, into the carb or throttle body. Go slowly and the engine will bog a bit. Stop and let it recover. Repeat until the cup is empty.

831xj

Done that twice. But with seafoam

cruiser54

My buddies used various cleaners like that to no avail. The water did the trick. Up to you.

831xj

Hmmmf guess it wouldn't hurt. Last night CEL ame back on. And it started idling back to normal. Still a slight miss but not near as bad. Ima try water before I tear the valve cover off

cruiser54

Won't hurt and will likely fix it. I'm 2 for 2 on the ones I was involved in.

Radi

Agreed on trying the water. Seafoam is oil-based. It may clean if left in liquid form, ie: dumped in the fuel tank or crankcase, but I don't think it does much when burned through the intake other than generate a big smoke show.
Water OTOH generates steam in the cylinder and that's what does the actual cleaning.

Ever pulled the head off a vehicle with a head gasket leak? It's obvious which cylinders were leaking- they're the ones clean as the day they were new!

cruiser54

Absolutely correct.

jakecarder

Mine does this exact thing. I bought it back from my parents a year ago and my dad stopped using it as a dd 4 years ago cause of this problem. It was still dd by my mom but only 2 miles up the road. It won't do it all the time but lately its eveytime I drive it. Only ever misses at an idle. I've also done everything you said with replacing parts, tests and seafoam. Im thinking its something with the valves.
I'll give the water a try and see what it'll do for me. My motor is junk tho, no oil pressure, uses antifreeze, 0331 head but no antifreeze in the oil, I believe I have a blown head gasket(there's oil and antifreeze all down the side), leaking valve cover, and this po301.