Hey guys, I'm starting to think my truck is pretty close to needing a new engine, but thought I'd bounce it off you first. The engine supposedly only has 180k miles on it, but that's according to the PO who's uncle swapped in a motor out of a 98 Durango supposedly. I do see some junk yark type writing on one of the valve covers, so that seems plausible. Either way, the truck currently has 225k miles, the last 30k of which have mostly been towing about 4k lbs. It's never been fast lol (3.55 gears and 265 70 17 tires with a small block) but the 5 speed has made it livable. Anyway, it is incredibly down on power. Without a trailer, you wouldn't know it, but hook a trailer up to it, and it can't maintain 60mph in 5th gear. It used to be able to. I have noticed that sometimes when I first start in in the morning, it seems to have it's old power back for the first few miles before it warms up. To my understanding, before it had warmed up, it runs on basically preprogrammed air fuel tunes, but once it's warmed up, it goes off what the o2 sensor tells it. It does get horrible fuel economy (9.5 with a trailer and 10.5 without, when it used to get about 11 with the trailer and 14 without). Could it be a bad o2 sensor?
It had started burning about a quart of oil between oil every 1k miles, but I switched from 10w30 to 10w40 and now it's only burning maybe a quart every 3k miles. Also, when it's really hot out, or has been being worked hard, the oil pressure guage reads really low. I guess that's what's making me wonder if it's just plain wore out. The included picture is it when it's low.
Oh, and when I go over about half throttle, the fan kicks to defrost. I always figured i just needed that check valve, but I also wondered if maybe a vacuum leak could be causing some of my problems?


I'd appreciate any thoughts y'all have. I like the truck and will either put a rebuilt 360 with aluminum heads/cam/stand alone efi in it or a 12v cummins when the time comes, but I'm just not sure if it's here yet... if I could put it off a little, that'd be nice.

 

Your understanding is correct, initially the engine runs in "open loop" without any adjustments based on O2 readings. A simple test would be to disconnect the upstream O2 sensor and see if it runs with better power once warmed up. Ideally you'd look at the O2 reading but that would require a OBD1 scan tool.

 

First, owners of trucks with an engine type other than what it was manufactured for... FAFO. Good luck.

tldr; read below in bold italics

Oil burning could be the ultra common plenum pan leak, intake air is sucking in oil from the crankcase. That's about all I got to say about that because the internet has 1,453,356,932 sources of info on this.

I'd start with a general health check. Remove all plugs, inspect them, then run compression test. I think you'll be OK as that oil is coming from plenum.

Check distributor cap and rotor (quick an easy). Note with 2nd gen ram, a TSB (I believe) many years old instructed a new routing for plug wires to avoid crossfire.

Don't be dumping money into "tuneup" (coil, plug wires, etc) unless something tests bad. About the cheapest "tuneup" part is above cap and rotor. If the electrodes are ashy and worn... replace it.

Replacing an O2 sensor or some other sensor won't be the magical fix you are looking for. In my opinion, if there is a sensor issue, there's a high chance there's an issue with wiring/connectors.

Asking the internet "what should I replace?" doesn't replace diagnostics. For every time I've pulled out my injector balance tester, a million others have just replaced good injectors with new ones and saw no improvement.

And feeding ChatGPT with symptoms won't produce a magic solution. You could ask ChatGPT/AI what are the most common issues with the 98 durango 5.9L then strategize your diagnostics around that.

When I run into an issue, my fingers get sore from wrenching, not typing on the internet for answers.

ps: still not been into my 2nd gen yet (plenum/water pump/timing chain) ... been fixing up a Honda Del Sol and an endless list of homeowner work. This week I get to wake up (ie work on) a cultivator I haven't started up in years (have work to do with it), resealing roof on my sunroom, fixed my chainsaw (now I have a bunch of work to do with it)... etc.

 

180K is rapidly approach end of life/rebuild time for the motor in any event. Depending on how well it was cared for. I am sure the timing chain is overdue for replacement...... That said, what Kevin says above me is good advice. Take a look at what's there, see how it looks, take a look down the throttle body, see if you see oil in there. (another almost certainty) Truck and rango might have had the same displacement engine, but, I would fully expect the cams to be different. I don't know of dodge recommended towing in overdrive on the manual trans... That's be something to look up, and see.

 

ChatGPT is no replacement for reading comprehension either, the OP stated that the truck ran fine for 30k so the FAFO comment is uncalled for. All the theories about timing chains and plenum and ignition ignore the fact that the truck apparently runs fine until warmed up. Occam's razor says the PCM is making it run bad based on bad sensor data. It might be something completely different but the process of elimination needs to start somewhere, preferably not with tearing the engine apart.

 

Unplugging the O2 sensors......

 

Wow, this engine just needs to be checked over. Being a Mopar, the first thing I would do is the "Key dance" to see what, if any, codes are present. Don't go to start but turn the ignition key on, off, on, off, and then on and leave it on while counting the flashes of the check engine light. Next, I'd listen to the engine for timing slap. It's a rattly noise from the front of the engine. I'd also put a vacuum gauge on the engine to see what vacuum it's pulling. You might just have a stopped up exhaust.