I have a 99 extended cab Dodge Dakota with the 5.2 V8 and 3.55 gears. When I got the truck 5 years ago it got around 15 MPG in town and 18+ on the highway. 2 years ago I installed a set of aggressive mud tires and the mileage gradually dropped to around 9 MPG. I naturally thought it was the tires. But 2 weeks ago I went back to a small (31x10.5) highway tire and mile mileage is still around 11 MPG on highway and around town.

With this in mind, I got a bottle of sea foam and added 1/3 bottle to the in-take and 2/3 to the gas. I really did not see any change in how the engine sounded or ran, but it did smoke dramatically for 5-8 minutes. After a week or so, I noticed the truck became very slugish and required excessive pressure on the gas peddle to get up to speed. Once I get around 2500 RPM, the truck runs, but it just seems weak.

I checked the Air Filter and its only a few months old and looked clean. Plugs and wires were done about 6 months ago, Engine oil and Front/Rear end oil was changed 1 month ago.

The truck runs smooth and does not have any issues. Actually sounds great, but it just seems very weak. A few months ago I could smoke the tires in first and bark in second, now a 18-wheeler could get me off the line. LOL

Looking for suggestions...

1999 Dodge Dakota extended cab
5.2 V8
Dual Flowmaster exhaust
170,000
3.55 rear with limited slip
31x10.5 highway tires
9-10.5 MPG

 

the 5.2 never had great mpg but 10 is low. make sure there is no exhaust leak or manifold. i think if your cat is starting to go the exhaust fumes will smell like rotten eggs.

 

There was a recall on the cat (a long time ago) so I am surprised your original cat is still functioning at your mileage. The recall was good to (I think) 80,000 miles, and mine crapped out at 90,000. And then I got the recall notice. Go figure.

Anyhow, when mine went, it actually developed a rattle. The stuff inside apparently fused together and shrunk and started to rattle around. No rotten egg smell or anything like that. All of that being said, it could be plugged, and your symptoms would seem to match a plugged cat. But...

You said "I got a bottle of sea foam and added 1/3 bottle to the in-take and 2/3 to the gas. I really did not see any change in how the engine sounded or ran, but it did smoke dramatically for 5-8 minutes."

Sounds like you were maybe burning off some oil? Do you use a lot of oil? A quart every 1000 or so?

If you are NOT using quite a bit of oil, I might be tempted to say it is the cat.

If you ARE using a ton of oil, then perhaps you have a leaky belly pan gasket.

 

No, Seafoam removes the carbon buildup in the intake and valves. Thats what was causing the smoke. Its normal when using it.
At that mileage the cat is due for replacement IMO. I have a cheap cat from summit on mine, around $60. Couple that with a new upstream 02.

 

Hmmmmm...

From the Sea Foam website:

"Safely frees sticky lifters and piston rings, removes carbon build-up and oil residue"

http://www.seafoamsales.com/motor-treatment.html

Looks like they claim it cleans OIL RESIDUE. Which is exactly what you get with a leaking plenum gasket. I have put SeaFoam into various intakes and would not say it "smoked dramatically." Sure, it smoked a little. But dramatically for 8 minutes? Of course, maybe the OP's idea of smoking dramatically is different than mine. Which is why I actually highlighted that phrase when I quoted it in my response.

Oil, on the other hand, will "smoke dramatically." At least in my limited experience and by my definition of dramatically.

All that being said, I did not say for sure this was a plenum gasket leak. I simply said, based on what the OP posted, it seemed like a possibility. Ordinarily, I would say to pull off the throttle body and look to see if there is any oil residue in the intake. But, if there was, it is presumably burned up as of now. Since the Sea Foam removed it, as per their claim.

Furthermore, he didn't mention any change in exhaust tone. A plugged cat will generally sound differently than a free-flowing one will. In fact, he said the truck "sounds great." Cats that are plugged sound, to me at least, like they are plugged. Kind of thuddy-poof-poofy if that makes any sense at all. Nowhere close to what I would describe as great. In fact, IMHO, a plugged cat sounds crappy.

Now, there seems to be at least two equally plausible causes for this problem. Both have similar symptoms. I see no way to rule either one out at this point. Additional information is needed before it is possible to say "part X is the problem." Sure, at that mileage the cat could be bad. But, at that mileage it seems just as likely that the plenum is leaking.

So, prior to spending $50 for an O2 sensor and another $60 for a cat, plus the time to tear the exhaust apart, which will likely entail cutting and welding given its age, I would try to make darn sure I knew what the cause of my problem was.

Does the truck use oil?
Does the cat rattle at all?

 

Yeah, it could be a cat. Make sure your new tires aren't running on like 8 pounds of air or anything crazy like, because that will definitly make it seem sluggish. Most likely not the problem, but worth a shot.

On a side note, I guess I'm not the only one up now... It's 5:15 and I FINALLY finished my final structural deisgn project! Woo!

Ok sorry back on track, I just had to get that I've been working on it for weeks and its due in less than 3 hours haha.

 

The truck does not use much oil. I usually change the oil religiously every 3000-4500 since I don't give my truck any slack when I drive. But usually when I change the oil, it is only 1/8-1/4 quart low. The worst I have ever seen it was 1/2 quart in 4000 miles and that was when I was doing alot of towing in the mountains.

The CAT does NOT rattle. I crawled under the trunk the other night and hit it with a hammer a few times and it had the solid metal ting.

I have looked the system over and did not see or hear any exhaust leaks. Even looked it over a few months ago when it was cold. Usually you can see steam on a early morning cold start, but again the dual exhaust system looked fine.

Tires only have around 1000 miles on them and they are at 34 PSI Front and 36 PSI Back. I really expected to see big improvements on mileage since these tires are highway tread in comparison to the MUD tires I had on before.

Again the truck runs smooth, ideas smooth, and sounds good. No weird smells or sounds. But I have noticed a reduction in gas mileage and power over the last few months. Other than being a gas hog, it sounds solid. Check Engine light use to come on if I would let the truck idea a long time (15+ mins), but that has not happen since I changed the battery (I guess it reset the computer). Other than that, the Check Engine light has never even come on.

As for the smoke with the sea-foam, I have treated about 6 different vehicles and all of them but one has smoke dramatically. My truck smoked about average compared to the rest. So I feel the comment I made about the white sea-foam smoke was within reason of other sea-foam experiences.

I have a exhaust bracket that needs to be welded back up. When its at the shop I may get them to do a pressure check an see if the CAT has any issues. My CAT looks to be welded solid on both ends and my welder is on the blitz at the moment.

 

When I Sea Foamed my truck, I smoked out the neighborhood. Much worse than any other vehicle I have seen. Smoke was thicker and not really very white at all. You ever been behind a car that spews out smoke as it drives instead of colorless exhaust fumes? The kind where it becomes difficult to see? And you think to yourself "Man, there sure won't be any mosquitoes in this neighborhood this year." That is what my truck did when I used the Sea Foam in it. Only vehicle that has ever happened on for me. It was BAD.

OK - back to the point -

Since you are not using oil, I am leaning more toward the cat than the intake gasket. When my plenum gasket crapped out (which was roughly the time I had the "smoke out the neighborhood Sea Foam experience") I had an almsot immediate, dramatic increase in oil consumption. It was like one oil change I used a half of a quart in 3000 miles and the next time I checked the oil (about 800 miles later) I was already well over a quart low.

That being said, your description of the problem still makes me hesitant to say it is the cat. I just have a hard time figuring how a 2 1/2 pipe could be plugged and not make the sound exiting that pipe change from when the pipe was open. It just seems weird, to me...

I looked over the list of maintenance stuff you did. Have you replaced the fuel filter?

Other than that, I am personally at a loss. Have the muffler shop check the cat.