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I have an '05 4.7L Dodge Dakota that has had an issue with rough idle for over a year now. It seems to only happen after coming to a stop at a traffic light. It seems to happen more often after having driven a long distance but occasionally occurs during shorter trips. The behavior is basically that the RPMs drop to around 500 and it acts like it might stall but doesn't. If I put it in neutral or park the idle returns to normal.

I've also had very sporadic behavior where the engine just dies completely. This doesn't seem to coincide with the rough idle but it might be related. I've also had it seem to just lose power while driving slowly after starting up and then behave normally after that.

I've changed the spark plugs, replaced the O2, TPS & MAP sensors and the IAC valve. I finally got the bright idea to get a data logger and see if I can get some better insight into what is going on. It happened today while I was logging and it looks like the voltage on all four O2 sensors goes to zero at the same time that the RPMs dip down to ~500. The only other obvious thing I noticed is a jump in the intake manifold absolute pressure as the rpms drop.

Here is a link to a google spreadsheet with the sensor data for the trip: https://goo.gl/2Uuq9u. The data of interest is around row 750 (1226 seconds into the trip).

Anyone help on figuring out what might be going on would be awesome. I'm wondering if perhaps the problem is fuel pressure related (the data logger doesn't seem to be able to measure that). On another forum, there were several suggestions that the transmission might be the culprit.

  • You can pick up a decent fuel pressure gauge for about $35 off of Amazon. – Robert S. Barnes Mar 31 '16 at 11:40
  • Since the issue is transient and occurs only certain conditions, would checking the fuel pressure provide any useful information? – kldavis4 Mar 31 '16 at 13:00
  • @kldavis4 Is this problem completely intermittent? – I have no idea what I'm doing Apr 1 '16 at 10:28
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    Looks like STFT is jumping a bit - double digits. There are a few idle (by reving in neutral) you can do to check how that affects fuel trims. This could be a leak in exhaust manifold before the O2 sensor. – Nicko Po Apr 13 '16 at 23:11
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    @kldavis4 It was intermittent, depending on whether the engine had been under load just before the idling; so it wouldn't do it on the driveway, but if I drove around the block, and then left it idling on the driveway, it would start running very rough and, after a while, stall. I think it was because the engine was trying to pull the timing back by vacuum, not succeeding, but it adjusted /other stuff/ (conjecture - I don't know what), and then causing a stall. – PeteCon Apr 18 '16 at 14:29
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O2 sensors are interesting critters to diagnose. The ones at the upstream side of the Cats are the ones that send data to the ECM module continuously for keeping the air/fuel ratios optimum. The downstream O2 sensors for the most part do absolutely nothing throughout their service life. Their purpose is to monitor the condition of the Cats. If the Cats are defective the downstream O2 sensor detects O2 and a check engine light lights up. O2 sensors have heating elements in them that are activated by a relay. The elements are activated to ensure that the O2 sensor elements work properly when the engine is cold. Once the engine is sufficiently warm the heaters are deactivated. Maybe it the O2 sensor relay-you mention that voltage on all 4 O2 sensors went to 0 ......

Just had a another thought on the rough idle and stalling: Possible EVAP system issue.

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You guys have the o2 sensor backwards. When their is a lean condition, the voltage drops, when there is a rich condition, the voltage increases. An o2 sensor (not Wideband o2) cannot accurately measure HOW rich or HOW lean the exhaust is. 0.500v is considered stoichmetric (the proper amount of fuel). Since the sensor cannot read how rich or lean the mixture is, the computer continually increases and decreases injector pulse width (the amount of time the injector(s) is on, and so, how much fuel it injects) in an effort to achieve and average of 0.500v. This is the job of the upstream (before the catalytic converter) O2.

The downstream (after the catalytic converter) o2 is used primarily to check the condition of the catalytic converter. Some systems have the ability to use them to control the injectors in the event of an upstream o2 failure, but this is rarely the case. The engine computer (ecm/pcm) checks the condition of the cat by counting the ratio of switches (voltage moving up or down across the 0.500v threshold) between the upstream and downstream sensors. A catalytic converter acts as an oxygen battery. When there is an abundance of oxygen (lean) the catalytic converter stores it, and also releases oxygen when there is a lack of it (rich). If the downstream O2 sensor switches too often, it indicates that when the there is a lack of oxygen in the exhaust stream, the cat has none stored to release. The cat's ability to store oxygen is important as it is needed as part of the catalyzing process to convert harmful emissions into non harmful emissions.

I know this doesn't help you solve your issue, but I wanted to clarify, as a misunderstanding of O2 voltages and how they work could cause you great grief.

It is important to note the a Wideband O2 does not work this way. The effect of rich and lean on output voltage is inverse, and also not in the 0-1v range (range varies, dependent on application).

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