So I got my first cummins not too long ago, and I just have a few questions I was gonna see if y'all could answer for me.
First off, it has a stock injection pump with the stock fuel plate and stock timing. I checked fuel pressure at the banjo bolt going into the pump, and the needle jumps around like crazy, i got a fluid filled one and it helped a little bit, but it still vibrates. Is this normal? It seems to be at a safe fuel pressure (around 20 idle and about 25 cruising) but it is pretty slow, especially cold. Is this just because its a stock 12v cummins? It has an HX40, but the only mods are a 4" exhaust and an intake, no fueling mods.
Also, when its cold it does miss sometimes when i give it some throttle, but once it warms up it drives great. It does start up instantly in cold weather, hardly even turns over one full rotation before its running.
Are stock 12v's slow stock? Its a 160hp model (automatic 94) so i assume its just gonna be slow, but it feels almost as slow as my old 6.2 diesel.
If anyone could let me know if this is normal for the fuel pressure gauge to vibrate, run rough when its cold (40-50 degrees) and it being slow, that would be awesome.
Im not sure if i should try to fix it because i don't know if anything is wrong with it.
If it sounds like something could be wrong please let me know what i should check, or if this sounds normal for an old 12v cummins.
Thanks for the help!

 

Ya that pressure for stock is fairly normal I'd say. As for being slow ya a stock truck is slow and if you have an hx40 on a stock 160 truck it will be even slower due to the long spool up. There are plenty of mods that can change that depending on your budget and goals. As for the miss, when idling I'd start by loosening one injector line at a time at the injectors and see if the idle changes.

 

The FP needle bounces around because of the mechanical lift pump. Mechanical gauges aren't quite so bad for this, because the long tube full of fuel to the gauge dampens the violence of the pump. You can simulate this with a rubber fuel line(a grease gun whip works well for this, and has the right 1/8" fittings and a good length too) and screw the sensor into a coupler on the end of the whip instead of directly into the IP. If you get a Tork Tech snubber to put between the line and the pump it will be even better. Some people are fine with just the snubber. I wasn't. I have the snubber screwed into the injector pump, a needle valve in the snubber, a fuel line into the needle valve and the sensor on the other end of the fuel line. I'm pretty happy with it. It's not quick to get up to pressure on startup, but it's very quick and smooth while driving.

Also: that pressure is normal. It's slow because it's a stock automatic. If you want it to be anything but slow (and I don't mean fast, I just mean not slow) it's going to require significant transmission modifications and a lot more fuel. The trans work is essential, all you will do with a stock trans and non-stock power is not really go any faster and blow up the trans sooner. If I were you, I'd plug it in overnight when it's cold. I'd be very surprised if your "miss" doesn't vanish.