I read on the forum today that the PCM ignores any misfire codes when the fuel gauge reads 1/4 tank (or less) fuel -- basically when the Low Fuel light is illuminated. Do you know if this is true? If so, could we wire the PCM to read fuel level always at 1/4 or below to solve our MIL issue with the random misfires?

Could this be the solution to your "White Whale?"

Let me know what you think, or if the whole 1/4 tank relationship to the PCM is a myth.

 

I've never heard of this or read anything that would support it. I suppose it makes sense since sucking air into the fuel system could cause significant misfiring. Then when people put fuel in, they would be stuck with the CEL being on.

However, the opposite argument is that it take two consecutive trips to turn on the CEL over misfires. That makes the chances of the CEL turning on rather slim.

 

From my 1997 Dodge Ram Factory Service Manual in the section (14-4) under Fuel Gauge Sending Unit:
For OBD II emission requirements: The voltage signal is sent from the resistor track to the Powertrain Control Module (PCM) to indicate fuel level. The purpose of this feature is to prevent a false setting of misfire and fuel system monitor trouble codes if the fuel level in the tank is less than approximately 15 percent of its rated capacity.

 

OK. So if one wires a signal replicating the signal from the resistor track to the PCM it will read low fuel. Therefore the PCM should never record misfires or fuel system trouble codes. Therefore no MIL.

 

True, in theory it should do as you say.

Problem with that though, if you do have a fault in the ignition circuit or a failing injector you will not know that you need to check it out either. It may not throw a necessary code that can help prevent engine damage. Also would it go into engine limp mode if something serious developed with the false information from the manipulated fuel sending level to the PCM?

 

Quoted from Alloro [However, the opposite argument is that it take two consecutive trips to turn on the CEL over misfires.]

Not always true. October last year when I did the plenum gasket on my intake and ran a Seafoam treatment through the intake, when I took it out on the first drive test I set several misfire codes within one mile while the Seafoam was cleaning out the combustion chamber.

 

So what...are looking to pick a fight or something? Your Seafoam experience is not the same kind of misfire as a low fuel situation. What you experienced was a catalyst damaging misfire, commonly associated with no spark in a cylinder causing raw fuel to be dumped into the exhaust system. A misfire due to low fuel is just the opposite situation, a lack of fuel going through the system. This is the kind of misfire that is the subject at hand and it will take two consecutive trips to turn the CEL on.

I don't mind being corrected when I'm wrong, but I really hate being corrected when I'm not.

 

No, not trying to pick a fight, just trying to help all whom see this thread to know that there are times when a situation the PCM monitors may cause it to trip the misfire code sooner than two consecutive trips. Most active forum members know about yours and Rusty93RamVan's misfire but some new members or lurkers may not.

 

I understand the value of having the OBDII system monitor engine functions, and I would never want to permanently bypass that function. I would wire the resistor track replication signal to a switch mounted somewhere under the dash prior to going to the PCM. This way, I can go back to factory mode at any time by flipping the switch.

My van (and Alloro's too at last discussion) is running Tip-Top. Even though we have changed all of the electronic engine controls we still have the MIL. We both probably have head issues but neither of us wants to tackle that job right now for various reasons.

My only interest for the present moment is getting the van through PA emissions inspection come February -- and that certainly isn't going to happen with my Check Engine light on. So the question becomes do I change my heads or take on a small wiring project?

With Thanksgiving, Christmas and freezing weather coming I'll have better and more important things to do rather than wrenching on my engine.

 

I can't see how or why if the only situation being monitored by the PCM on that wire is low fuel level. I mean, if a person does lots of in-town driving they could conceivably stay at 15% fuel capacity (and subsequently less) for many days. Mopar wouldn't have the limp mode activate in this circumstance.