Hi all,

I've been having a power loss issue with my 02 3.9 lately. The symptoms are as follows:

-It's more sluggish in general (than usual, I know the 3.9 is no powerhouse to start with);
-It won't hold speed going up any sort of hill, and downshifting will only get me up to 55 at most while going up a hill;
-When it downshifts a gear on the highway, it feels like there's absolutely no additional power;
-The max RPM I can get it up to while driving is ~3,200, at which point it just falls on it's face completely and will not shift out of gear until I let off the gas.

I don't think this is a transmission issue, as there are no check engine codes and the fluid is a good level and color. I replaced the driver's side catalytic converter over the summer, due to it being rusted through, but did not do the passenger's side since it seemed okay.

Does this seem like it would be caused by the third catalytic converter being clogged? If that's what that is, as I've read that it may be a resonator as well but it looks like a large third cat where the y pipe joins.

Anyone have any thoughts or advice? It is really getting to be painful/possibly downright dangerous to drive on the highway, getting passed by big rigs doing 70 mph while I'm over in the slow lane doing 50. I need to fix this, but can't find much on these specific symptoms, especially since I have no codes.

Thank you!

 

Try a cat backpressure test - you pull the front O2, and see how much backpressure is being produced.

You can also do an IR thermometer and see how hot the inlet and outlet of the cats are.

Last but not least, be sure to pick up a competent OBDII reader, and datalog what the truck is doing. It may be something simple (old plugs?) that the ECU is compensating for, that's not bad enough to cause it to toss a code.

RwP

 

If the cat is bolted in pull it out and see how it runs be sure to reset the PCM.


I don't think the fuel filter is clogged usually doesn't happen these days but stranger things have happened.

 

I've read about using an infrared thermometer to measure the heat before and after the cats, I will probably try that here soon.

I really don't suspect that it's anything with the engine itself, as the plugs and wires are not very old and it just doesn't feel like an actual engine issue to me. Though I guess I could be wrong.

Unfortunately, if I want to take any of the cats off, I'd have to cut them and I want to avoid doing that unless I know for sure they are the problem. I think I am going to test the fuel pressure today and see what I get. That seems like one thing that can be either easily eliminated or confirmed that it's a fuel issue.

Thanks guys.

 

Clogged cats would have it idling erratically and running like crap all the time.

 

Not necessarily.

When my cat plugged (both times!) on my 1988 Dakota, neither time did it idle badly.

Then again, it wasn't completely plugged either time. The first time the internal structure had come apart and was blocking exhaust flow.

(The second? Due to the ECU failing and me running on only one of the two injectors, it literally burned out the cat... )

RwP

 

If the cat is not real bad just somewhat plugged up it will only affect top speed performance like he describes in his post.

 

Alright guys so I went to Harbor Freight today and got a fuel pressure gauge and tested my fuel pressure. The gauge was leaking fuel, so I didn't have it on for long, but it was bouncing between 40 and 50 (bouncing because the gauge was leaking). I'm relatively sure that it's the third cat, due to the restricted top end.

That being said, I called up an exhaust shop in town and asked what they'd charge to take that cat out and put in a piece of pipe. I'd normally do it myself and just replace the cat, but all my tools are a few states away right now, and I'm definitely not going to pay what they'll want to put a new one in. They gave a good price on taking the cat out and doing the pipe though, any drawbacks or concerns with doing this?

 

UPDATE:

It's fixed! I decided to pop one of the downstream o2 sensors out and zip tie it up and go for a ride. Aside from being obnoxiously loud, it drove a lot better and shifted nicely and had a lot better performance all around.

I put the sensor back in and drove it straight to the exhaust shop that said they'd take the third cat out and put a pipe in. They did a real nice job with it and it actually doesn't sound terrible. Most importantly, it is so much more driveable now. It now shifts at 5,000 RPM and without me taking my foot off the pedal (hasn't done that since I've owned it). I took it on the highway and was able to accelerate going uphill past 80 mph. It's amazing. Anyone who has issues like I described ought to check their cats, mine was pretty plugged up when the guy showed me afterward.

 

If that one's that plugged, I'll lay dollars to doughnuts that the front two are bad also.

RwP