Hi guys,
I have 1996 Dakota Sport 5 speed with the 3.9 v6. 120,000mi. The engine seemed over the last few month to be idling lower and lower first 600 rpm then 400 , then 200 and now it just wont idle whatso ever. New plugs in it. Starts easily and revs great, no misfire. A small amount of hesitation and sputtering when cold. I replaced a lot of the vacuum line that looked corroded. Another strange thing is that a few months ago the catalytic converter clogged and i cut it out, since i couldnt afford a new one i just ran a straight pipe in untill i get the money to buy another. The check engine light of course turned on then and hasnt been off untill today and now the truck doesnt idle. Have I screwed my upstream O2 sensor? Should I have removed the downstream O2 sensor? Not too sure what could be going on. Any ideas?
Thanks
Kevin

 

Let me clear up some confusion:
About the oxygen sensors. Pretty much, the only reason there is a rear oxygen sensor is to alert the computer that the cat. isn't working. The front oxygen sensor is the one of primary importance (i.e. the one the computer will base fuel ratio off of).
With that being said, when you turn they key on a "cold" vehicle, the computer takes readings from a few sensors, and goes into "warm-up" mode (which is an open-loop mode). Only until certain criteria are ment will you go into "idle mode" (which is a closed-loop mode). The oxygen sensor readings are ignored by the computer during any open-loop mode, and only taken into consideration while in "closed-loop mode."
Simply put, I doubt its an oxygen sensor problem.

What I would believe is the problem is a fault Idle Air Control motor. This is mounted on the back of the throttle body. It has a pintle that moves in and out (to control air flow to the engine when the engine is idling). My guess would be that this piece is either dirty, or faulty. You can take it off and clean it up and see if that helps. Also, while you have it out, have a helper turn the key on (but do NOT start the motor) while you look at it. The pintle should move (be "set" in place).

Post back with your findings.

*post edited in light of what erock92kota said*

 

P.s. do not.....I repeat do not start the truck with the idle control out. Upon starting your your truck, the rpms will skyrocket almost immediately which may result in engine damage and a few other things going wrong.

Just a lil F.Y.I.

In case you were wondering.......yeah I did it once........not really knowing what would happen.

 

HI
I have something similar and the open/closed loop seems to be a possible issue.
97 dakota; 5.2L 5speed, 4x4, 140 000 mi,
Here is the problems that I see, and it is very reproducible:
- check engine light has been coming on due to 'downstream O2 high'. The sensor was replaced several times and is clearly not the problem.
- when I drive the truck for the first few minutes it runs ok, but then starts to run rough.
- after about 5 min, on my way to work, I get stopped at a traffic light. When I accelerate the engine bogs down severely, and detonation can happen, and I also get unburned fuel out the exhaust.
- also, at idle the rpm will go to 800, drop to 400 and then bounce back up to 800.
- If I drive through this for a minute or two, by playing with the clutch/throttle, the problem goes away and the truck drives ok. Check engine light stays on.
- cylinder 8 and the next one are very rich, cylinder 5 was very lean by spark plug check

I have hooked up an ODBII scanner and following is what I have seen:
- on startup engine is in open loop. After about 15 sec I backed up and the truck went immediately into closed loop. The coolant was still cold, below 100F.
- under the rough idle condition, at 800 rpm the oxygen sensor is reading 0.8 V (rich and fluctuating slowly a bit); engine advance at 0; when the RPM drops to 400, the O2 sensor reads down to 0.12 or 0.08V (very lean, with some slow, fluctuation). The engine advance goes to 4 then 14 degrees, as the engine speeds back up to 800 RPM and the O2 sensor reads 0.8V again.
- when the Truck warms up and the rough idle problem goes away, the O2 sensor will fluctuate rapidly between 0.1V and 0.8 V and the short term Fuel trim 2 tracks this: 1-4% while the O2 sensor is at 0.1V and downto about 0 or negative when the O2 sensor goes up to 0.8V.

My guess is that the engine is getting to much fuel into the two cylinders that are showing very rich. Possibly 2 injectors are dirty and getting stuck open? The O2 sensor reads this and compensates by cutting the fuel which results in the RPM drop.

I am planning on checking those fuel injectors but I am looking for any other suggestions. Is the behaviour of the open/closed loop consistent? My understanding is that the truck should stay in open loop until the engine temp is at around 190F.

Thanks in advance

 

Hey Kevin,

If you had a (steamed engine) check the the to temp sencers by the thermastate. there are 2 in that area of the moter and there on the computer uses to read off of is back be hind there. i have a problem with idle and it turned out the sencer came unpluged then i was i replaced my thermostate. Its hard to see with out a flash light. Also Do you know the trick about gettin a code of the computer. If not with the key in it cysle it with out starting it. go from off to on 3 times then on the forth turn it on and watch your check engine light. first number will fash first then a pause the second number. if you get a code that reads 55 then there is nothing that the computer is pickin up other wise it will give you a code. If you have a repair manual it will define the problem area of whats going on. hope that helps you out at all