Hi guys,

I'm yet another poor soul chasing a hesitation/lack of power/backfire/poor fuel mileage problem in my Dakota. My actual question relates to OBDII PIDS for the Dakota (See the last line), but I'll give the rest of my problem for anyone that has a suggestion.

2001 Dodge Dakota 3.9L V6 stock, 2WD, manual transmission

When I start it, the engine idles roughly. The exhaust smells like it's running really rich. If I try to drive it immediately, I'll hit a strong hesitation around 1500 RPM that won't let up until I cross 2700 RPM. When I cross 2700 all of the power returns and the truck jumps like's it's been kicked from behind. Typically in first gear as I near 1500, I get one or two backfires in the exhaust manifold. It can backfire on either side of the engine. When the engine backfires I get fault code P0152 (O2 Sensor Circuit High Voltage bank 2) posted by the engine computer. After the engine warms up for about 3 or 4 minutes, everything is normal except for the degraded gas mileage.

I think the whole problem can be attributed to a running rich condition.

Here's what I'vedone, and what I can rule out due to incompatibility with the symptoms:

I changed the high tension electrical: all plugs, wires, cap, rotor and coil. The coil was done last and that helped, but probably only because it can more effectively cope with the dense fuel mixture being dumped into the engine.

The throttle body has been cleaned.

Air filter was replaced and the air box is clean.

I believe I can rule out the catalytic converter because the engine recovers after warming up. A clogged cat should plague me all day long.

The upstream O2 sensor is probably not an issue, but I'm not convinced enough to rule it out: 1. The error code is probably just indicating that there's very little oxygen in the manifold because the backfire scavenged it all. 2. I'm guessing the engine is probably still running in open loop at the point I'm having problems (how long does it generally take the O2 sensor to reach 600 degrees?)

Whatever the faulty component is, it must have more influence with the engine computer during the first 4 minutes than it does when the engine is warmed up. That probably rules out the throttle position sensor, but maybe not the air mass sensor.


Anyway, I have an inexpensive OBD II scanner that I picked up for 130$. I wanted to clear errors without disconnecting the battery and it also can read the state of the engine sensors at the time of the last error code.

I can see that there's a problem indicated in the data it collected, but I'm not sure how the PIDS names map to actual systems and components in the engine. The scanner documentation is generic, so it won't help me map to the Dakota. Here's the interesting and confusing stuff:


Long Term Fuel Trim 1 = 3.1%
Long Term Fuel Trim 2 = -7.5%
Long Term Fuel Trim 3 = 76.5%
Long Term Fuel Trim 4 = -17.4%

I know these "long term fuel trim" values correspond to injector pulse widths in the engine, but since I only have 4 numbers in a 6 cylinder engine, I have no idea what they point at. A value outside of 10% is apparently a problem. These are the adjustments the computer has made to correct for a perceived rich or lean condition over multiple engine uses. That 76% is probably the source of all the extra fuel for my rich condition.

Fuel system 1 = closed
Fuel system 2 = open

I have no idea what "system 1" versus "system 2" is. They are indicating that one part of the engine is running in open loop while the other is in closed loop. I don't know what management loops these map to in the 3.9L. I would have thought there was only one loop anyway.


*So does anyone know what the OBD II PIDS "Fuel Trim 1-4" and "Fuel System 1 & 2" correspond to in real life for the 3.9L? Does the Dakota service manual cover stuff like this?

Thanks,
-Turnip

 

I would make sure the cat is not clogged first, I have seen that cause motors to run rich. I would also replace the O2's. When was the last time you did a tune up? I would replace the plugs with autolite 3923's (I think someone else will chime in if i am wrong) and the rotor, cap, and wires.

 

your idea that if the cat is clooged at all then its clooged all day sounds good. i have had clooged cats would run fineonemin but not the next. most of the time if the plugs look like they have been in ext manafold is a very heavey indeacator its a clooged cat. allso if its clooged the volumn of air will not go up much when you rev the engine pardon my spelling

 

Cat or o2, you can try and rule it out, but it can still be the problem even with no codes...