my 3.9 still sucks..UPDATED

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.

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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.

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My 98 Dakota runs very well, I was in W.Va. with a 700 lb. quad in the back and my friend had his 97 Chebie 1500 4X4 with a 350 and a 700 lb. quad, we stopped at a light side by side starting up a large grade, he asked if I wanted to race and I said your on, expecting to get stomped. By the time I hit third gear he was 8 cars back. Mine is a 5 spd and his a heavier 4X4 butt the difference was unbelivable. Another freinds 97 Dodge 1500 4X4 with a 360 Magnum isn't much quicker than the Chevie.