Hello again (again, again, again)

Been trying to figure this one out for a while. Weird misfire issue. I can feel it, no codes, misfire monitor says a-okay. No mode $06 data I can get into it seems on my '03 to watch the counters live, but I can feel the damn thing, and hear it.

Vacuum holds solid at 20 inches, compression check has all cylinders between 180 and 190 psi. Fuel pressure is suspect but I'm not sure I used the tester right. Connected the (parts store loaner) tool to the schrader valve on the pass side fuel rail, dead head pressure was somewhere around, well, between 12 and 20ish psi, running it barely moved. Depressing the bleed button on the tool caused the gauge to fall to 0psi, letting go of the button the PSI never recovered. Fuel pump replaced 1.5 years ago due to faulty sender unit, replaced with Precision brand unit, rated 50psi.

Misfire is audible at the exhaust, when it does misfire it'll pop a piece of paper back into the tailpipe. That told me maybe a valve but the vacuum gauge is solid.

Plugs look great, maybe 20k on them.

Ideas? I suspect that fuel pump has some issue but the misfire is only apparent at idle.

2003 Ram 1500 5.7 QC Laramie 4x4

 

If that gauge is correct, your fuel pressure is WAY too low.

 

Hello!

That gauge wasn't correct. Bought a new gauge and it reads 50psi running. While I was at the parts store I talked with my buddy who rang me up a year ago and we discovered I was sold a fuel pump for a 2003 with the 5.9 with a rating of 58 PSI and a flow rate of 20gph. The pump for my specific application with the 5.7 is rated at 50psi and 33gph flow.

We started a warranty exchange because I'd feel more comfortable having the right pump in there, because 13gph is quite a lot more flow. But I don't believe it's a fuel delivery issue anymore.

So, still stumped.

 

If it maintains pressure at high engine RPM/load, then no, it likely won't.

What spark plugs are in it?

 

I didn't test the pump at high load, but I use the truck as a truck, often at WOT towing. It's felt down on power for a while even after my 4.56 gear swap.

Champion coppers for plugs.

 

I recommend pulling some spark plugs from each side. The tip and electrodes should be light tan or light gray, anything else will give you info on the problem.

 

Hello,

Pulled the plugs for the compression test in each cylinder the other day. Electrodes showed no wear, gap was checked, there was, what appeared to be, a small burn mark on the tip insulator on each plug. Not much though. I'll have to pull a plug in a while and share a pic so your imagination doesn't run wild with that.

 

Good compression, spark seems OK, fuel pump fairly new, I think next item is fuel injectors. You could try some injector gas additive, and give a heavy dose on a low tank. There's also videos on cleaning the injectors.

 

Hello!

New fuel pump is installed. The design was even different between the two. This pump is noticeably quieter in the tank.

The two pictures show my Up and downstream o2 sensors before the pump and now after, cruising at the same speed on the same highway. That's a pretty dramatic change there.

I know it seems like I have an answer for everything but i've been going through the motor lol. The injectors were out last week and bench tested. They all flowed well with no obvious occluded spray patterns.

Old fuel pump, for the 5.9 Magnum, highway cruising 65mph

New fuel pump, for the 5.7 Hemi, highway cruising 65mph

Idle is better, still small stumble every once and a while. Only looked at half the plugs when I did the compression test. I might slap a new set in there, they're cheap.

 

That is one nice live data reader you have. Is that integrated into your audio display?
The first picture is what the oscilliscope reading should look like with downstream doing all the work and upstream smooth. In the second the downstream is operating at its limit.
This change is result of new fuel pump?