1998 Dodge Ram 1500 5.2L 4x2 203K miles
First off, this is a 2 owner stock truck. No performance upgrades and only misc replacement parts for maintenance(rebuilt the braking system minus the master cylinder, replaced radiator, replaced timing belt, rebuilt transmission at 175k(replaced soleniods and wiring harness along with tranny)) and a recent tune up(Autolite plugs, PVC, air filter, wires, cap, rotor, and rewired the cables to reflect the "new" diagram).
The truck began having trouble starting last fall. I wasn't driving it much and figured that was a contributing factor. Eventually I killed the battery messing around with it and replaced that and did a tune up. A new set of issues popped up after that, or maybe they were part of the problem all along and I just didn't realize it. The truck was running rough and missing when I got up to about 50 mph. I pulled the plugs(I'd put in platinums) noted #1 was wet and fouled, replaced with Autolites, pulled the wires and re-ran them again. The truck ran better, but seemed to still be rough and misfiring/ hesitating at around 50mph. I rechecked cables but couldn't find anything wrong. No CEL but I borrowed a code reader from Autozone to confirm and it reported nothing.
I noticed there was a hissing sound at start up and remembered reading through the plenum thread about pulling the O2 sensor to test the cat. I did that, but it didn't seem to have an affect except to light my CEL(which I confirmed using the code reader).
I checked for vacuum leaks from broken hoses, and pulled and cleaned the ICV(which may need replacement). But performance didn't change and I didn't want to keep randomly swapping parts. I was forced to take a break from playing with it for over a month and only turned it over once every couple of week without driving it much.
Today I had a break and decided to have a look at it with fresh eyes. Again, rough idle and sluggish acceleration but when I drove it up to and through 50 mph there was never a misfire. I was about to take it out on the highway for an extended drive when the CEL began flashing. I pulled off and circled back home and the flashing stopped on its own before I made it home.

Long story short, I'm a shade tree mechanic and weak electrician who is well past baffled. I attempted to check the plenum gasket, but really couldn't determine whether it was the problem. I debated replacing the MAP sensor, TPS sensor and EGR valve but held off hoping to confirm something instead of just throwing money at it. Bought Seafoam to run through it, but haven't had the time. O2 sensors likely need replacement, along with plenty of other parts that I'm not even thinking of.

My question is, if I head to a mechanic, what specifically should I ask them to check for and/or diagnose? Since the engine is mostly original, should I just consider an engine replacement versus hunting gremlins if I want to keep the truck long term? I don't have a regular mechanic, but there's a couple of established ones I'd be willing to try if I had a good idea of where to tell them to start.

Thanks for any info,
Joe

 

The CEL flashing indicates a misfire. Did you run the wires per the TSB? Copper plugs? Our trucks don't like the rare Earth ones. At 50mph your in OD which directly connects the motor and trans and hence why it feels rough. You could need new injectors. Do you have access to a fuel pressure gauge?

 

I ran and then reran the wires per the TSB to confirm. Moved to copper after reading more here about not using Plats.
Injectors are a definite possibility, I'm fairly sure they've never been serviced. I don't have have access to a fuel pressure gauge, but I can ask around and check the parts house to see if they have one I can rent.

 

Check compression as well. (or, if you have an air compressor, do a leakdown test instead.....)

 

So what HeyYou mentioned plus a fuel pressure test. Also try to pinpoint which cylinder is causing the miss.

 

So, first off I want to apologize for not coming back sooner to comment on this thread. I appreciate that I received good advice, so thanks MoparFanatic21 and HeyYou for giving me positive feedback and a good direction to head in.

I did some searches on checking compression and fuel pressure and knew I didn't have the gear or comprehension to figure out how to do them. I visited Autozone and they weren't helpful.

A helpful neighbor who'd been watching me muck about the last few months, stopped by and we talked. I gave him the story.
One thing that I didn't mention during my initial post was that during the tune up I'd had a problem with miss-fires caused by a bad spark plug wire(a new one). I'd gone through and tested the wires for spark, and one didn't work. I replaced that one and things were much better, but I ended up where the post put me.
My neighbor has an MG along with a Chevy pickup and appears to be an above average shade tree mechanic. With the MG he said he'd dealt with alot of odd stuff. He offered a long, old spark plug wire from his Chevy parts box and suggested going through and checking the spark plug wires again.

Long story short, I bought a set of spark plug wires and 2 were bad...

After replacing the wire, the truck is running solid.

I feel like an idiot for not testing the other wires... but I just figured with my old truck it had to be something else.
Lesson learned, keep your old parts for testing unless you know for sure they're bad and test everything.

Thanks y'all

 

Glad you got it figured out!

 

That really sucks when new parts are bad. You look everywhere else. Glad you got it fixed. Run Seafoam in the next 3 or 4 tanks of gas.

 

Yup, doing that. I really want to Seafoam the throttle body and crankcase too, but not sure on how I can pull it off since I live in a neighborhood. Might have do it out at friends house in the country.

 

I went to a remote area to do it. It produces HUGE clouds of smoke.