I recently inherited this truck. It a 96 Dakota, 3.9l 121k. Was running poorly so the previous owner rarely if ever drove it. Ive done a full tuneup, plugs, wires, cap, rotor button, waterpump, clutch. It cranks and idles great but wont rev up very good, and has little to no power when under load.

Ive read about the fuel system problems on these, but Ive checked the pressure and its at 45 psi which seems to be in a good range. I plan to replace the injector seals as well as clean and replace the gasket on the throttle body, but just dont see that being the issue that will fully correct the lack of power situation.

Any thoughts out there appreciated.

 

From your description it sounds like it could have blockage in the catalytic convertor, or maybe a clogged fuel filter. You can try removing the O2 sensor in the exhaust prior to the cat temporarily and see if changes at all. I'm not sure it's enough to make a difference, but maybe.

Also, I've heard you can check the pressure at the O2 sensor bung, where the sensor screws in. I'd have to check, but I think it's supposed to be less than about 4-5 psi.

I know you said fuel pressure looks good (at idle I assume), but have you swapped out the fuel filter?

 

Well I gave that a thought, but PO had a new converter installed on it not to long ago.

 

Walker, the manufacturer of my new catalytic convertor, says that new catalytic convertors are supposed to be broken in. If not done properly the catalyst will not seat to the inside of the convertor and can then break, causing a blockage. It's a short process but skipping apparently risks damaging the convertor. I would check, just to rule it out.

If it wasn't driven much there may be gunk in the fuel tank or lines. If after changing the filter it runs better and then the problem returns, you may have gunk build-up from old fuel.

After thinking about this some more a third possibility comes to mind. You say the owner replaced the cat not long ago, but 120K is on the early side for a cat to fail under normal circumstances. If, however, the truck developed a plenum gasket leak, then it would start to run poorly, lose power, burn oil, foul plugs, and in turn, destroy the cat. If the plenum leak isn't fixed, the new cat will eventually be damaged as well, so you end up doing tune ups and replacing things to no avail. If you're not familiar with the plenum failure problem, you can find more about it by searching the forum. There's a kit available to repair the belly pan under the intake manifold, which is the source of the problem.

You can do a quick check for the problem by removing the intake tube, opening up the throttle body plates (engine off), and peering inside the manifold with a good light. If you see oil on the bottom of the intake, toward the rear, its a good sign your plenum gasket has failed.

Keep us posted on what you find.

 

Do you have any codes? Look at the FAQ page 1 for a lot of common problems solutions and just good to know stuff. There is a section that explains how to find codes without a scanner.
Could be your plenum gasket. Look for oil in the intake or on the bottom of the butterfly.
I had the same problem on my 94 3.9. I did a lot of things. The best improvements came from doing the plenum ( I went for the permanent fix of a kit), new crank sensor and fixing the exhaust manifold leaks. Look for vacuum leaks and check all the electrical connections are clean & tight.
I ended up replacing all the sensors in the intake and TB. IDK that it actually gave me any improvements but it was already more than 20 years old and looked like maintenance was haphazard.

 

In talking with the family, it had this poor power thing for some time. It may be that they were convinced to install a converter leading them to think it would fix the issue. Obviously it did not. But you can clearly see under the truck that the converter does in fact look fairly new not to include you can see where the old was cut out and this one welded in.

No I have not done the fuel filter yet. from what I can tell this one only has the one that sits on top of the fuel pump and also acts as a regulator. As I did check the pressure and saw it to be quite high I figured fuel flow as probably not the problem.
Plenum? what are you referring to the intake manifold gasket? I see nothing in parts listings called a Plenum gasket but I have read some posts here about the topic. Still searching for in depth details on the topic. However, when I pulled the plugs to replace they were pristine clean just quite old. That being said did not get the impression it was using or burning any oil. But when next chance I get to work on it, I'll surely check for that. I was planning to next pull the throttle body, clean and regasket as well as to check and clean the IAC. IN doing that can surely check the plenum condition you both mention.

No its not tripping any codes as all. I have several scanners of all types.

This loss of power to me is more like a lack of fuel. You go to pull off and can near take the pedal to the floor with little to no results. Then on occasion will rev up ok.

 

The intake plenum is the belly pan bolted on the bottom of the intake. There is a gasket in between and then between it and the block. The intake is aluminum and the pan is steel. differential in expansion contraction rates destroys the gaskets allowing oil into the intake and fouling the system. That may be why the previous owner changed the CAT. There is a test you can do with vacuum gauges to find if there is a clog in the exhaust. I had very little sign aside from oil on the butterfly but it made a major change in how it ran. Turns out the other two major improvements were what I already mentioned. I did a lot more but those things didn't really cause much improvement so I figure they were dead ends. This is the website I got the kit from http://www.hughesengines.com/Index/p...=&partid=26194 They do good phone help and the link was also helpful. I found I had to remove the fan to get the shroud off in spite of what that link says.

 

Thanks for the continued input and detailed explanation. yesterday afternoon checked the bottom of the intake via throttle body butterfly and it is most definately covered in oil. After a bit more research here, figured the Hughes kit is probably a better long term approach than leaving the old plate and just replacing the gasket. Therefore, ordered on of the Hughes kits this morning. While all that is down will go ahead and check all sensors on the throttle body as well as replace the injector seals.

 

I took the opportunity to replace the timing chain and add a tensioner (there is a TSB on that). I also had the intake tanked and replaced all of the sensors in it. I bought a gallon of carb cleaner and tanked the TB in it myself. I also changed all those sensors. You should replace the short bent coolant hose since you'll have everything off and I also did a new WP because I had it apart and age. The ticking wasn't my Timing Chain it was a leaky exhaust manifold, broken studs on both sides. Good Luck!

 

Yeah, if you're seeing oil pooled on the plenum belly pan you've definitely got a leaking gasket, and an explanation for the issues the truck has been having.

I like the Hughes kit, but I heard they now include gaskets made in China, instead of the Fel Pro gaskets the kit used to come with. Never the less, I haven't seen any complaints from folks that have used the newer kits so they must be working OK. Don't try to re-use the manifold bolts, once they've been torqued down they stretch and shouldn't be re-used.

I think you're gonna be real happy with your Dak after you put that kit in. ☺