Backstory:
1997 Ram 1500, 5.9 Magnum w/ 136xxx miles. I was driving to a neighboring town a few weeks ago and my CEL came on, and the truck started running poorly. By poorly, I mean lack of power, rough idle, stalling on heavy acceleration. Driving at higher RPM / highway speeds was fine, the problem was only noticeable at a stop or under heavy acceleration. After I finish what I am doing and get back to my town and get the code read, P0108 - Manifold Absolute Pressure/Barometric Pressure Circuit High Input (Read more at: https://www.obd-codes.com/p0108). Seemed simple enough, so I replaced the stock MAP sensor with a new one from Autozone. This fixed the problem for all of 25 minutes, until it began doing the same rough operation and the CEL came back on (same code). Next, I took it to a shop and got a diagnostic look at it. They confirmed the code, checked fuel pressure (45, all good), said no vacuum leaks, checked the harness to the MAP sensor for bad wiring. I picked up the truck, and they recommended getting a new ECM. To me, this sounded like they couldn't find the issue, and it is also one of the more expensive fixes. So, I next double checked the Map Sensor harness (complete w back probe test, all came back good). Next replaced the 02 sensors, put a second new MAP sensor, and a new PCV valve (mine was getting worn). After all of this I was test driving and noticed that when flooring it, the engine stalls and "lugs", and then if you let off will leap, and if you press harder into it, will backfire. This lead me to believe I had a clogged cat (along w sulfur-y smell), so I bought and had a new one put on, cleared the code and wa-lah! She drove better than she had in the past 3 weeks....for a few days. The CEL came back on, and the truck runs rough again. I have been avoiding it because they are more labor intensive and less easy to fit into my schedule, but I now believe it is a dying fuel pump (fuel stops, vacuum drops, MAP freaks out, ECM adjusts, etc etc.) or the Plenum is now too far gone.

Symptoms: To recap, overall poor performance, stalls/backfires on heavy acceleration, runs rich, rough idle / wants to die at stops, acts like a dog from a dead stop.

What was changed: To Recap, new Pre/Post Cat 02's, New MAP (2x), New PCV valve, New MF Catalytic Converter

What was checked: Map sensor harness, fuel pressure, vacuum lines

Next Steps:
I believe I have it narrowed down to replacing the plenum gasket (needs to be done irregardless on the truck, but hopefully as a fix) or the fuel pump. If I go the plenum route, I will likely replace the timing chain while I'm at it. I can't rule out the ECM, but I feel it would have many more symptoms. If anyone has advice, similar stories, more knowledge or knows where I have went wrong please let me know!

 

Plenum won't set a code, unless it is REALLY bad..... If you have a scanner, see what the PCM thinks manifold pressure is with just the key on, and then with the engine idling.

 

Could be the cheap MAP sensor. Our trucks don't like the aftermarket sensors

 

HeyYou, I don't have a scanner to test that with. Would a dealership be able to verify if PCM is okay? Also, I'm not sure how to reply under your comment lol.

 

Mopar, what MAP should I go with? AutoZone was around 90

 

Dealer is going to want 100 bucks an hour to diagnose. If you have one of the big chain auto parts stores around, they may have a scanner that can read the data.....

 

Yeah I may try that. In your experience could the fuel pump / plenum actually cause this? or you're pretty certain the ECM is worth testing.

 

Fuel pump won't. Supposedly, a bad enough vacuum leak could cause it.... but, it would have to be pretty bad.

Need to find out what the PCM thinks is going on. Might just be a bad sensor, may be as expensive as a worn out engine.

 

You've already spent a lot of money on that problem but get yourself a cheap OBDII reader with live data. You can find them on Amazon for $50. This will help telling what MAP sensor reading the PCM has.

 

Just ordered a scanner w/ live data, should be here wednesday. What am I checking for exactly? if the Map is reading at all or do I need to be within a certain range?