OK...I've got a 3.9 that runs like a bag of you know what. 96 ram 1500 2wd 5 speed.
Very poor idle, and stumble off idle. Almost sounds like it's missing on a cylinder or 2, but it's not(verified on ociliscope)

Here's what I've found it's NOT

-plugs
-cap
-rotor
-wires
-coil
-all sensors seem to be working within thier normal range. O2 sensor is new.
-valves are not sticking
-converter is not plugged
-fuel pressure and volume are good
-Compression is present. 120 psi on the left, around 90 on the right. I've blamed the discrepency on temp varience between the sides while testing.
-Timing chain was completely terrible (1" of deflection) replaced it. runs the same.
-have checked for external vac leaks. None found

I've heard a few guys say the intakes are known to crack on these, but I've never heard of it. Still have to follow up on it.

PCM ?

Any idea's welcomed.

I've never done it, I have a 2000 3.9 and the swap only works up to 1999, but it was quite fashionable to install the the Ford Motorsports 19# injectors in those engines... Direct swap, the improvement was the Ford injector had 4 holes for spray vs. the OEM Mopar's single hole. On the Dakota board guys were reporting much improvement in idle quailty and low speed smoothness in the 3.9's with the injector change, and they were pretty cheap, 8 injectors for less then $100 IIRC. Google "Ford injectors in 3.9 Dodge" should bring up some threads that have more info.



EDIT: Those are the 4.6 V8 injectors

If it has a EGR valve , test it .

I haven't checked the egr valve.
I'll take a peak at that.

Anyone else?

I still think it's probably the ECU.

However you need to double check your compression readings because the 90 is very low even stone cold. Did you check it with all the plugs out?

The intermediate shaft bushing wears out and causes the distributor to jump, can even ruin the distributor. Used to replace them all the time. Remove the distibutor and use a long wide blade screwdriver to see if you have any side play in the gear.

I'm starting to question the ecu as well, but only as a last resort. I have seen lot's of ecus replaced over the years, with only about 1% actually fixing anything.

I'm going to spend some time verifying that the distributor is actually timed right(that no one has messed with it). Would seem to be a good time to pull it out and inspect it too.

Replacing the timing chain with 1" of slack did make a difference in above 2000 rpm performance, but it is still far from fixed. The fact that it changed at all, could ad some credibility to the distributor gone bad theory.

My 98 Dakota has a 3.9L, Its a pig, I cant imagine how slow it must be in a ram, though the 5 speed may help.

I know I've driven a few 3.9's that have had all the power of any 318 rig I've driven.

What I'm getting is worse than just a lack of power. It makes you wonder if you are going to get to where you are going.

In any case, I'll pull the dist next week, and follow up on somemore stuff.

Is the exhaust hissing? The honeycomb brick broke up inside my cat on my old '90 Dak. Pieces of the cat blew downstream and clogged the muffler. Power was very low and exhaust pushing backwards melted my EGR valve a few times before I figured it out. My final clue? Pissed off at no power and not figuring out, I just mashed the pedal to the floor and just held it there. POW!!! Blew a hole in the side of the muffler, then the power came back!

Sounds like a couple things.1.As earlier mentioned replace the dist drive /intermediate shaft 2.The plenum gasket is likely leaking. (It seals the bottom of the intake manifold).BTW they do not crack they blow the gasket out.KRC Performance and Hughes Engines offer a replacement plate that will stop the gasket from failing again.

Sorry to hear your 3.9 dak is bumming you out!! MY 5.2 '94 DAK IS a rocket ship,glad I didn't go with the 3.9.

I had some bad battery cables/connection in my 5.2 once. I haven't had a 3.9, but you might check that out. With these modern ECU driven engines, a little voltage inconsistency can cause all sorts of symptoms.

GO_Fish .... the muffer was completely rusted off, when I got the truck (it was already running like it does now). I replaced it. Then as I became frustrated with the running, I thought it was the converter, so I cut it open to look inside. Already empty so I don't think that's it. Thanks though....

mike_s ..... I'll check some more for vac leaks at the plennum, but I've already looked around there quite a bit. I'm hoping that chcking into the distributor will turn into something.


BIGBLOCK78DOBA ....
"Sorry to hear your 3.9 dak is bumming you out!! MY 5.2 '94 DAK IS a rocket ship,glad I didn't go with the 3.9. "

Actually mine is a Full size, so at least it has THAT going for it

you wont find a vac leak AT THE PLENUM.NOTICE HOW TALL IT IS?to cast that,the bottom is left open.then they machine it and bolt a plate to it.i think larry is right.the intermediate bushing is probably the biggest part of the problem.but i would guess you also have an internal vac leak thanks to a blown plenum cover gasket.new mopar ones are metal and come with new bolts.

If the plenum gasket is bad, the crankcase will be under a slight vacuum instead of a slight pressure. With engine idling in park, remove pcv and crankcase breather cap from valve covers. cover the holes with your hands for about 5 seconds, then lift one hand. Direction of air will tell you if the gasket is bad. If it sucks in, replace the gasket....

Thanks for the further input guys

I see what you mean about the height. It's one piece from the head up to the throttle plates. The bolt on plate is underneath right? so you have to pull the intake to get to it ?

In any case, with the cap off, I cannot feel any play in the distributor.(at least not slop side to side etc).
I can feel some rotational movement, much like I would expect to feel on a v8 in the slot/drive. It doesn't seem excessive.

I pulled the throttle plate assembly this evening and notice there is a ton of carbon in it. Nothing that strikes me as abnormal. Perhaps pulling the intake is the next step for me right now.

Cheers

If the plenum gasket is bad, you should see oil inside the intake.

UPDATE

it still sucks

pulled intake. No evidence of plate leakage, or gasket leakage. Replaced it all anyway. still the same.

Pulled dist at same time. No bushing play at all.

Egr valve is a blockoff plate, so that's not it either.

I'll keep pulling my hair out...er I mean digging.