I have a 1999 Ram 1500 Sport with the 5.9 that seems to have a lot less power at low RPM than I think it should. I can put the pedal to the floor and I think I'd still lose a drag race to a golf cart. I don't think it's starting in second gear because I can shift manually into first with the same result. It does buck a little bit around the 2000 RPM mark but it shifts too soon to really get a feel for how bad it's bucking. Some more background info:


- I fixed the plenum gasket in an attempt to fix the issue about 10,000 miles ago. It needed done because there was a bunch of oil in there but I didn't notice much of a difference afterwards. There is no oil in the plenum now so I'm pretty confident that's not the issue.

- The truck has 265/75/r16 tires but I'm not sure what the gears are. Is there an easy way to tell? I bought the truck about 30,000 miles ago so there's still a lot of history I don't know about it.

- The previous owner installed an aftermarket Flowmaster dual exhaust system with a cat and up/downstream O2 sensors. The muffler is where it splits to dual (1 pipe in and 2 pipes out). I'm not sure how long ago he did that but it looks pretty clean compared to the rest of the truck so I'd guess it was relatively recent. I have tried six ways from Sunday to get the pre-cat sensor off but can't get it to budge. Heat and PB Blaster don't seem to touch it. I might have to drill it out or hammer a smaller socket on since it's pretty well rounded off by now to check if the cat is plugged.

- I replaced the spark plugs about 20,000 miles ago but honestly can't remember what plugs I put in. I know these trucks can be picky about which plugs they like. I replaced the cap and rotor when I did the plenum.

- There used to be a pretty noticeable spark knock under heavier loads but switching from 87 octane ethanol blend to 91 with no ethanol seems to have helped that quite a bit, although I still notice it every once in a while when going up a big hill or something.

- The truck really doesn't like to stay in 3rd gear. When accelerating it will shift into 3rd just fine and then quickly shift to overdrive. When downshifting (say to pass on the highway or climb a hill) the truck will shift from overdrive to 3rd and then quickly to 2nd and stay there until I let off the gas. If I'm really paying attention I can feather the pedal just right to get it to stay in 3rd but any deviation from that will almost immediately make it shift either down to 2nd or back to OD. This might have nothing to do with the lack of power but thought it wouldn't hurt to mention.


Any suggestions for what else to check would be great; thanks in advance!

 

Do you have a check engine light on? If so, have you scanned the PCM to display any pending or stored codes? I ask, since the bucking is likely related to engine misfires, which tend to produce misfire codes and turn on the check engine light. Changing that O2 sensor at the cat should help things as well.

Has the timing chain been replaced? A slack/stretched timing chain can cause some of the engine sluggishness issues you're experiencing.

Is the truck 2WD or 4WD? As for gearing, there should be a sticker under the hood that can tell you what the gearing is that came in the truck from the factory, along with other options the truck came with (assuming that it hasn't been changed by a prior owner). You can also find that information by providing the VIN here - https://www.faxvin.com/vin-decoder/ram

 

There is no check engine light (and I know the actual bulb works because it's been on for other things in the past). Will there still be pending/stored codes without a light? I'll do a scan tonight and see what it tells me. The engine sounds smooth when accelerating so it seems to me it is either not missing or I have a dead miss. I'll probably start unplugging spark plug wires one at a time to see if there are any dead cylinders. I know a clogged cat can foul the #8 plug so I'll start with that one.

I don't know if the timing chain has been replaced. I know the previous owner replaced the front clip, radiator, etc. from a donor truck after he rear ended someone but I don't think he got far enough into the engine to mess up the timing chain. I have not noticed any metal in the oil any time I've changed it. I assume there's probably not a good way to check that without having to remove the timing chain cover first.

The truck is 4WD. The online VIN decoder says it probably came with the 3.55 gears but I'll double check under the hood when I get home as well.

 

You can also look at the inspection cover on each of the differentials. There should be a metal tag held on by two of the cover bolts that tells the differential's part number and what gear ratio is installed. This assumes that the differentials haven't been modified, however, I'd suspect you have 3.55 gearing front and rear, as this is a common option on these trucks.

 

I got the O2 sensor off with the help of a pipe wrench and a hammer but it didn't seem to make much of a difference. I also checked each cylinder quick with a temp gun and they were all similar so I don't think I have a dead cylinder. I couldn't find any tags on either of the front or rear diffs but I did find one on the transfer case. Pic attached. If the ratio really is 2.72 then that may well be my issue. That would make sense because it runs down the highway at 60 at around 1500 RPM.

Edit: Forgot to mention the hood was replaced by the previous owner after his accident so there's no identifying info on it that would be relevant to this truck.

 

You likely have 3.55 gearing in your front and rear diffs. The ratio on the transfer case has no bearing on your overall gearing, which is determined by the ring and pinions in each of the diffs. They are likely 3.55, based upon your experience with overall speed at 60 MPH. Whoever changed the gear oil likely threw away the diff tags that were affixed to the cover bolts. The next time the diff covers are removed, the gear ratio should be stamped on the side of each ring gear in each diff.

After changing your O2 sensor, did you disconnect the battery so that the PCM could re-learn the new O2 sensor? Also, what brand of O2 sensor did you use?

Something else to check - how is your exhaust flow? If you put your hand over the exhaust pipe(s) at the rear of the truck, does it tend to want to know your hand off of the pipe after briefly trying to use your hand to cover/block exhaust flow out of the pipe? If so, you likely have normal exhaust flow and not a clogged up cat.

 

I haven't actually replaced the O2 sensor yet; I just took the old one out to make sure there wasn't any excess pressure in front of the cat. I put the old one back in for now but I was planning on replacing it with an NTK sensor. I live in a small town in the midwest so I usually either have to order parts online or drive an hour to the "big city" when I get a chance.

I have tried the hand test before and it seems to be flowing fine. It's a little hard to tell since it's dual exhaust so I can't get my hands on both sides at the same time.

 

How many miles on it? Have you done the timing chain/gears? That was THE BEST thing I did for my old 5.9, it felt like a different motor.

Check fuel pressure as well.

 

X2 If you have a scanner that reads live data maybe that could tell you what is off. Have a look at throttle position sensor and see if you have a good sweep from it.

 

The truck has about 175,000 miles. I hooked up my scanner and took pictures of the live data at idle with the engine warmed up. I'll admit, I don't know much about what I'm reading here. I know the fuel pressure should be around 49 psi but I'm not sure why it's giving me percentages for those readings. ​​​​​There was a stored code of P0132 but that may have just been from when I disconnected the O2 sensor. I haven't unplugged the battery to reset since then.