O/T: oxygen sensor/exhaust question

I have a '91 Dakota, 3.9L V6, 5 speed, 4x4. It has a low to mid-range hesitation. Also seems to lack power. I had a '91 Dakota, 3.9L V6, auto, 4x2 that would easily run away from this one. Otherwise, the truck runs smoothly, no misfiring, no smoke out the tailpipes, etc. Would be nice if it got slightly better mileage.

Cycling the key and flashing the "check engine" light gives me a code 21, referring to the oxygen sensor.

The truck has been converted to dual exhaust and does have the O2 sensor located in the passenger side exhaust pipe.

Does this modification in any way affect the operation of the O2 sensor? Factory location was behind the Y-pipe, so it monitored the combined volume of both sides. Now it's only checking the right bank. How sensitive is this sensor? Will it recognize the reduction in flow volume and, possibly because of that, incorrect proportions of exhaust gases?

Or will replacing the O2 sensor cures all it's ailments?

Additional info: it had a Marshall Engines reman engine installed around March 2002. Before I bought it this spring, it had sat for 2 years because the pick-up coil in the distributor was bad.

I appreciate your time, and thank you for sharing your knowledge.

Anthony

I have a full size 91 with the same motor and a 5 speed and it is not very powerful either. It runs great and uses no oil but it just isn't very economical or powerful.

The oxygen sensor reads how much oxygen is in the exhaust by volume. If you change the amount of space that the exhaust gas ocuppies without changing the amount of exhaust you will get a different reading of the amount of oxygen in the exhaust. Simply put, changing the configuration of the exhaust is likely to change what the O2 sensor tells the ECM is the amount of oxygen in the exhaust resulting in a MIL.

Simply put, the O2 sensor ain't your problem.

I'm no Dodge guy that's for darned sure, but I tend to agree with Bob.

Think you're a little cross-threaded on your codes.

Allan

How long has your modification been on the truck? If it happened at the same time as your modification it might have something to do with the modification but probably not. O2 sensors are wear items which lose their ability to accurately send the O2 reading to the PCM over time. If you do replace it, OEM are the best sensors to buy. Nobody can tell you it is or it isn't the O2 sensor without diagnostics. Yes it can cause the symptoms you describe, but so can other things. Asking if it will cure all your ailments is asking a lot since driveability is so complicated. The O2 sensor range is approximately 1 volt, so yes, it is sensitive. Since the combustion event is suppose to be equal in all cylinders theoretically the proportion of oxygen is suppose to be roughly equal whether it's one bank or two. That of course assumes you have no issues on either bank which would cause a change in oxygen. Hope this helps. Gerard

The O2 sensor should be as close as possible to the engine. It requires heat to operate and to stay clean.
As previously stated O2 sensors with age and time produce less voltage for a given amount of O2 and fuel vapours. A "tired" O2 sensor will fool the computer into running the engine too rich.

engine codes are cause and effect ! Just because you get an O2 sensor code does notalways mean it's bad. Lean /rich mixture will trip em . Low fuel pressure , vac leak , fouled plug etc ... Spend lil $$ on diagnois's will save you in the long run .

ASEguy: the exhaust job was already done when I bought the truck, so I do not know if any of the problems coincided with that installation.

I nosed around under the hood a little more and noticed a couple things:

1. The air pump system is disconnected; bolts broken off where EGR pipes attach to manifolds, etc. Also sounds like exhaust manifold gasket leaks. Not really surprising, since manifold is missing one of the bolts that holds it to the head. Maybe this is the cause of the O2 sensor code?

2. What is the item in the picture? What does it do? Why would someone have removed it? It is located in the rear of the passenger fender, near the firewall. It's vacuum related, but I have no idea of it's function beyond that.

I think I'll start with a tuneup, because I have no idea when it was last done. Also plan to check lines/hoses for cracks, leaks, loose connections, etc. Maybe pull the throttle body and clean it out. Possibly something got gummed up/stuck because it sat for a couple years? My ex-FIL has a fuel pressure tester, so I can do that too, although I don't think that's my problem.

Once again, thank you.

Anthony

Any little leak up stream of the O2 sensor will cause trouble codes.
The air pump likely injected downstream of the O2 sensor so it should not matter.
Maybe vacuum lines to operate hvac and/or cruise control?

Anthony, Usually HVAC controls only use one vacuum line for all modes. My memory is fading on that system. Sorry. Yes a leaking exhaust manifold can pull in outside oxygen after the exhaust pulse passes the crack and fool the PCM as stated. When you pull the throttle body to clean it, remove the AIS motor and clean the pintle. This can help with tip in hesitations. If fuel pressure/volume is suspected, we used to tape a fuel pressure guage to the windshield and find a steep hill to climb and keep a close eye on pressure. Only do this if you are comfortable doing so. There are other ways to check pressure and volume output and professional shops can do this for you. Check the distributer cap terminals for corrosion too. It will be a whitish powder and change it if it's there. Hope this helps. Gerard