MISFIRE UPDATE

[scottac]

I talked to the guy I bought the truck from and mentioned the problem to him and he said that I year ago he replaced the air filter with a reusable K&N filter. when he did this he said that he forgot to plug the O2 sensor back into the intake tube, before he started the truck back up. he said he immediately noticed the mistake because itwas slamming into gear really hard from park to reverse. and then when he put it in drive it would slam into gear into each successive gear.

 

I read how a misfire could potentially damage your cats and the o2 sensor and was wondering if this could have anything to do with the problem.

 

 

 

here is the link to the prior post

 

PRIOR POST

 

thanks in advance for all advice

[scottac]

bump this to the top

[JayMan]

I'm sorry I'm confused:

 

The previous owner forgot to plug the O2 connector back in? There are four O2 sensors, two for each bank of the motor.

 

Are you refering to the MAF (Mass Air Flow) connector?? Please clarify.

 

If you are indeed refering to MAF, that would go a long way to explaining your misfire history. According to page 6-2181 to 6-2183 of my Helm shop manual, if anything happens to the MAF, the PCM is commanded to do the following:

 

-immediately assume a misfire and disable the torque converter lockup clutch

-immediately change baseline timing to minimize misfire damage

-flash and/or set MIL (Malfunction Indicator Lamp)

 

When the MAF is out of order, the PCM is forced to use Speed Density operation (Page 6-2197). So it relies on the ignition subsystem to derive engine speed, and it relies on IAT (Intake Air Temp) and MAP (Manifold Absolute Pressure) to derive air density.

 

Head's up: the MAF also contains the IAT sensor. If you unplug the MAF connector, this will REALLY confuse the PCM!

 

Using an aftermarket high-flow air filter can also cause problems. The airflow is more turbulent and this confuses the MAF and MAP. If you're still using the stock aircleaner, put in a new stock air filter, clear the codes, and perform the Idle Learn Procedure.

[scottac]

first off I don't konw much about cars. I have never really worked on them myslf. It may be called the MAF. It is whichever plugs in right there next to the air cleaner.

 

Maybe this is all related to the misfires. I hope so.

 

I just want it fixed.

[JayMan]

Yep, the connector right next to the air cleaner box is the MAF (Mass Air Flow) This also contains the IAT (Intake Air Temp). It's a Weatherpak connector and the thing it plugs into usually has a "Delphi" label on it.

 

There are numerous fail-safe modes of operation for the motor, so unless something critical like a Crank sensor fails, you're not stranded in BFE. Once the computer loses communication with O2 sensors or the MAF, it defaults to Speed Density operation.

 

Sure this will cause bizarre operation. If the PCM also assumes that misfires are taking place, or about to take place, it changes the timing and/or shift points. That can cause really weird shift characteristics.

 

I would get a new stock airfilter, clear the codes, then do the Idle Learn Procedure. Make sure the MAF isn't put in backwards! Sometimes when a person puts in a high-flow conversion, they reassemble the MAF backwards and you REALLY have problems. Although it's hard to see, there is a flow arrow on top of the meter. And the Delphi logo faces towards the Driver side.

 

Let's cross our fingers on this

[scottac]

I am really glad the previous owner was honest about the mistake he made. this may actually help me out in the longrun.

 

as far as long term damage to the vehicle, what concerns should I have? Remember the mistake with the MAF was made about a year ago. and it only misfires after long road trips (the previous ownoer also mentioned that he too had a misfire after and during a long road trip). I just hope the truck isn't permanently damaged.

 

anyhow. you ahve been great jayman lots of helpful advice. at least I will have an idea of what is wrong before I take it in.

 

thanks.

[JayMan]

Since the truck has been doing this for a year, I guess we're into "long term!"

 

I would worry about the O2 sensors and the cat's, for starters. Depending on how many miles it has, you may end up needing new O2 sensors, especially with misfires over that long period of time.

 

The basics like PCV valve and fuel filter need to be done. Depending on miles, new spark plugs might also be needed. Make sure to use the OEM AC Delco plugs, as the Coil Per Cylinder ignition system is calibrated on this.

 

Once the basics are done, remove the negative battery cable for 10-30 minutes to erase the short-term memory of the PCM. Then do the Idle Learn Procedure and put on about 20-100 good highway miles before checking for more codes.

 

Since the truck is capable of storing Per Cylinder Misfire History - per OBD2 requirements - you should be able to see if the misfire is constant across all cylinders or confined to a few.

 

Until you see the codes, it's really hard to guestimate any further. All I can suggest is checking the basics. Good luck with this!