I hooked up a very expensive scanner to my 98 Trans Am M6 originally to see any past codes that were stored in the computer. I have had troubles with my vehicle misfiring when it rains out and I drive over a puddle.

I found that, even dry, multiple cylinders are misfiring miscellaneously- and it moves from cylinder to cylinder. They all do it at different times. 1,3,4,5,7,8. I believe 6 was the only one that NEVER misfired. (It was either 5 or 6) This misfire DOES NOT SET OFF A CODE unless it rains a lot. The scanner was monitoring the system while running and was getting random counts of misfires.

The miss is very small but it idles rough. It seemed the go away while driving. My mods are listed below. It was supposedly tuned by SpeedInc before I had the vehicle.

My plugs and wires are brand new as of this winter. I have cleaned every single connection on top of the engine including injector wiring harnesses, coil harnesses, and virtually anything you can see. MAF was cleaned this winter as well.

We are thinking it may be something to do with the main harness? Please let me know if you have any idea where I should start to try to figure out this thing. It seems as if it has a small misfire at idle, sometimes under load, that gets AMPLIFIED by rain. What do I do?!

 

Anyone?

 

i had an issue very close to this. the car would only misfire in the rain on random cylinders, turned out to be the oxygen sensor, water was getting into the area where the wires attach to sensor, use you scanner to graph your oxygen sensors, the sensors could be switching very slowly or not at all. other than that i would monitor all of you PID values very closely to see what the sensors are reading

 

Not going to be affected .by water... But I have seen some crazy misfire issues on ls trucks .. Fixed. By performing a crank relearn....
...........

 

I may start by changing the o2's just because i'm not sure how old they are anyways. I keep being told that o2's wouldn't cause a misfire but i'm really not sure. @armyinfantry what are PID values? (The scanner was my friend's so i'm not up to par on all the readings and all that.) @imnotguilty how is a crank relearn done?

I am not sure if the water misfire has the same cause as the dry misfire but it sure seems like it. The misfire is very minor but you can feel it if your hand is on the car that it's just not completely right.

 

I had a weak coil give me problems at part throttle and idle but worked fine at full throttle.Never did set a code.

 

"The generic OBD II protocol works on a request system, which means that the scan tool has to actually ask the ECM for each PID (In OBD II a PID stands for parameter identification). This OBD II request operation contrasts with the OEM communications protocols that work using data packets, whereby, the data stream PIDs is sent in bursts or packets. However, generic OBD II standardized the whole communications process and made it possible to, at least, have access to a minimum of data for diagnostics regardless of make and model. In generic OBD II, by simply reducing the amount of PIDs on the screen a faster data rate can be obtained, since the scanner has to request less data. By combining different and faster data PIDs to form a relationship, a signal correlation can be arrived at. An example is an EGR valve that is commanded on (manually or otherwise) which should have an effect on the MAP sensor. A lack of MAP sensor response is a good indication of a defective or clogged EGR valve, since an opening EGR valve should create a drop in intake vacuum. The same PID strategy is also employed by the ECM when running each drive cycle. The difference is that we can also use these techniques to our advantage when diagnosing a vehicle."

I hope this helps explain a litte. for more info check out this site http://www.autodiagnosticsandpublish..._parameter.htm

 

Hmm I just ran it open loop (disconnected the o2's) and it seemed to run way better. Hoping to get my friend over here to use the scanner and see if it is still missing. But it feels better...

 

just make sure when you hook up the scanner you watch the O2's...at a MINIMUM they should switch from .01v to .09v three times in a 10 sec period of time. if they dont then replace them, if they are hovering around .09 then they are showing a lean condition and if reading .01v show a rich condition, check you long term fuel trims as well that will give you a good indication of whats going on. ill help you as much as i can

 

Hey man I think you got youo2 readings backwards...
Low voltage is lean
High voltage is rich

 

yeah your right, i was typing really fast and watching family guy at the same time...thanks for catching that!

 

Thanks guys! I think it's all starting to make sense now. A few observations I made...

1. It has 2 diff o2 sensors on it (different color wires coming off) meaning 1 was changed at a diff time than the other. (I deff wanna change them both and have 2 new, good ones in there)

2. Ran better open loop. The mis seemed present still but only judging by the small vibrations in the car at idle but i'm thinking that could be motor mounts or something. I reconnected the o2's and it definitely lost low-end power, and misfire was more distinct.

3. This misfire was/is amplified by rain. It would miss reallll bad when wet. It would drive fine once I hit the throttle hard but I wouldn't do that very often (didn't wanna hurt my motor) Which leads me to this thought. That problem definitely could have been in the o2's because if the car runs open loop at "WOT" then in the rain when I open it up and it accelerates fine, then it may very well have to do with o2's. Bc if it's cancelling out the o2 readings at more throttle... you get it.

I will be getting the o2's hopefully tomorrow or monday-tuesday. i'll change them and let you guys know what happens.

I'm not sure how much access I have to my friend's scanner but i'll try to get readings on everything if/when possible!!

One more thing. The previous owner didn't tell me much about the car but I do know he was hiding this problem (misfire in rain issue especially.) He said it used to get bad gas mileage, they got it tuned and then it did better on gas.... but I don't think they ever even thought about the o2's, and they had the car for about 2 years (they weren't mechanically inclined but knew a decent amount about the car)

Just pointing out some thoughts and observations!

 

Changed the o2's. Didn't do anything. Sprayed around the whole engine bay with carb/choke cleaner. Nothing. No vacuum leaks seem apparent. Reset my PCM- didn't help.

I am starting to think that my Crank position sensor or cam position sensor is/are going? Or my PCM maybe?

So confused.... any suggestions?

 

Bump.

 

My o2's seem to be switching okay, they switch at different rates. Trying to figure out how to check PID levels on this scanner and what to do with that....

I am getting misfires sometimes mainly on cyliners 1, 3, 7. They almost disappear any time the throttle is being pressed.

 

I am getting counts between 2 and 7 on 2-3 cylinders. Should I ohm out the coils on these cylinders? I am running out of ideas.

I removed my PCM, cleaned the pins, the misfire is still there but it's not really that bad... but I dont think I am supposed to get any counts of misfires..

 

One cylinder will start missing a little, then the engine will be perfect, then another one starts a little, perfect, then another..

 

did you ever do the crank relearn procedure,

 

I have no clue how to do it, ive been trying to look it up but it looks like I have to take it to a dealership?

 

I'm done worrying about it. I've spent 12 hours in the last 2 days trying to figure it out. I hooked up the scanner today, both o2's were switching perfectly.

I switched coils around, and took it for a drive with the computer hooked up to find this:

While driving, there are absolutely no misfires unless I hammer it sometimes i'll get a count of 2-4 on cylinder 1.

When it is idling at a light, I get random counts of 1-2 misfires on different cylinders. No check engine light even comes on.

I will take it to the people who tuned my car while the last owner had it and see if they have any thoughts but I don't think this miss is harming my engine too much and obviously I have tried everything.