This is my first post. I'm sorry if I didn't do a mandatory introduction, but out of frustration i need to dive right into my issues...

I have a 1999 2500 with an 8.0 V10, auto, 4x4. Nothing special, just an old beat up truck. I bought it back in 2016 and at first it was flawless outside of a ticking noise. local shop ruined the original 141,000 mile engine in 2017 so I put in an 88,000 mile replacement. It ran even better than before and got a little better fuel milage. However, about 4 years ago it started bucking. I had other vehicles so it was parked. I'd tinker with it, thinking I'd solved the issue only for it to continue. I must admit, I have made a few farm truck repairs such as not re-looming wires correctly, just putting new loom over them, wrapping them in tape, and zip stripping them out of the way however I don't believe that is the issue.

The issue is, it's over fueling. Bad. When you first start the truck, it idles and runs fine. About 3-8 Minutes later, and its firing through the exhaust, hesitating, and if you shut it off, it has a hard time starting right back up. I started by replacing the MAP sensor, then the IAC, TPS, and camshaft position sensor. After that I replaced the crank position sensor, both ignition coils, the ECU, throttle body, coolant temperature sensor, all 10 injectors, and any other sensor i had missed. Nothing. All of this is happening still and the truck has never thrown any sort of code. We've ran three compression tests, all came back within spec, and shortly after that, we had the heads checked and rebuilt just in case, and still the same things. There are no fluids where they shouldn't be, no knocks, ticks, etc. I am completely lost here. I've taken it to five shops for diagnostics including a somewhat local Dodge dealership and we can't figure this out. After swapping ECU's (PCM? I forget if the Mopar proper term is one or the other), The only thing that changed was with the second PCM I lost cruise control. Could my fuel pump be pushing too much fuel? Why won't a CEL come on even if i pull three plug wires? (CEL will come on if the MAP or IAC are unplugged though).

The other issue is, the coolant system builds an insane amount of pressure, however a dozen new caps and two thermostats later, it's continuing to build pressure and overheat, without blowing any hot air through the vents or puking a drop of antifreeze. I initially suspected a plugged heater core, but atleast when it's hooked up to flush, it seems to be flowing as it should. Over the year's I've replaced the radiator, water pump, T-stat, and all the hoses. I just don't understand why the coolant system is giving me fits.

I apologize if my post is a little all over, it was a long day at work, followed by a long day of trying to solve these mysteries now that I'm in a position where I need to rely on this truck. As crazy as it may seem, I'm attached to the old pig.

 

What kind of plugs? These trucks only like copper plugsWhat brand sensors? These trucks only like MoPar sensors or NTK O2 sensors. Unplug the front O2 sensor and see if it gets any better
As for the overheating you have air in the heater core. You can jack up the passenger side, drill a hole in the thermostat, and/or add a bleeder valve to the heater core outlet hose

 

Plugs are Autolite Coppers, and the sensors are all Standard. Where do you drill a hole on the V10 thermostat? It's the weirdest looking T-stat i've ever seen.
Plugs and wires were replaced two years (15 miles) ago.

 

Standard Motor products? That could be part of your problem especially for the Crank and Cam sensors. Do it on the outside of the thermostat.

Trying unplugging the front O2 sensors and see if it changes

 

Mine built a lot of pressure and consumed coolant. Tore it down, lower row of head bolts was about half torque and you could see were it was sucking coolant in at the head gasket at the rear of the passenger head. Still ran ok though. Curious if you do a leak down test take the radiator cap off and look for bubbles.

 

At least for the dealership I suspect your 20 year old truck was handed to a flat rate technician that likely wasn't even in pre-school when the truck was built. Plus, they won't spend hours trying to figure this out (or not). You already spent 100s of $ replacing parts, time to step back and think this through. From your description it sounds like the truck is starting and running fine when it's running off the fixed open loop tables but has problems when it warms up and tries to go into closed loop. Any halfway decent live data scan tool will have a lot of useful information about open/closed loop, temperatures, fuel trims, O2 sensor readings etc.. Watch the data as the engine warms up and see what changes when it starts to stumble. That would be my first step.

As for your questions:
Very unlikely that too much fuel pressure is the problem. For one, if anything fuel pumps get low on pressure and I've never heard them go bad in a way that causes way too much pressure. Then, the PCM can regulate the amount of fuel through the pulse width and through this can adjust to higher or lower fuel pressure (within limits of course)
A lot of anecdotal evidence is that this generation of Chrysler PCMs seems pretty unconcerned with misfires. The misfire detection mechanism is somewhat questionable anyways, it looks at variation in RPM. With a V8 or V10 the variation from a missing cylinder isn't as noticeable as with a 4 banger.

 

Yeah, I would suspect an O2 sensor problem first...... You should have two pre-cat sensors. Unplug them, drive it. See how it does.

If the cooling system is building pressure before the engine even gets warm, you still have a headgasket issue.

As for misfires...... I had a dead miss on one cylinder on my 8.0, and the PCM never bothered to mention it. Plug wire was burnt in half, so, that cylinder most certainly was not firing....... No codes, no pending codes, no flashing CEL, nothing.

 

What is your fuel pressure, I'm sure it's been checked, but anyways. Wiring issues can cause injectors to be held open. The PCM provides the grounds, but may not be able to tell for sure where the ASD supplied voltage really goes. Poor grounds can cause errors to PCM input from sensors. The PCM fuels two cylinders on compression, fires two cylinders(one on compression, one on exhaust like an old PanHead) simultaneously. If you need to know, lift the upper intake, pull the fuel rail and injectors from the lower. Make sure you have all the injector keepers in place, pressurize the system. Dripping fuel at your +-45psi set point is your sign to start digging/figuring out what's happening.

There's no hole to drill in a V-10 thermostat, there is a fair amount of coolant in constant bypass.

Fuel pressure is not monitored by the PCM. It is fixed by the regulator on the pump assembly and does not vary much at all while running.

Valve sticking is not uncommon. watch your MAP voltage while cranking. A rise after the initial drop could be valves, clogged cats, vacuum leak. Little pulses in MAP voltage while running is uneven pressure in the intake.
A temp gun on the exhaust manifold will show you poorly/overly/non performing cylinders.

DT is right, a scanner that shows real time data, might tell you a lot. Don't know much about them, I'm still in the test light/VOM era. Did the dealer create/provide a running data feed with the DRB tool? They must have, talk to the tech who did the work.

 

I should have clarified that the truck was catless when I bought it, and the upstream O2 sensors had been replaced with the NGK sensors. I made the mistake initially by buying Bosch sensors initially.