I have a 2004 non SC SS.

For the past 2 weeks I've had a really random misfire, or what feel like a misfire, problem

I drive 160 miles to work and back. One morning while in some light traffic about halfway to work I feel something not normal. As I come to the next traffic light and I'm stopped I can feel a slight mis and I can see the tach move down 100-200 rpms when I feel the mis. I turn off the radio and AC and keep driving the car with the mis so I can find a good place to pull off the road. within the next 2 miles the mis go away. I pulled onto the highway and drove 70mph the next 40 miles to work with no problems. Even turned the AC and radio back on.

On the way home from work about 10 mines in I hit stop and go traffic on the highway. Once again I feel the mis. This time it doesn't go away. For the next 5 miles I'm stuck in traffic for about 20 min until it opens up. the entire time the mis is getting worst. By the time traffic is really moving I can't accelerate past 50 mph. I end up pulled over on the side of the highway. Radio and AC off and it's missing bad. I try to rev the motor while in park and it has a hard time reaching 3000rpm. After sitting there for 3-4 min it dawns on me it's it's acting sort of the same they way it did 6 months ago when the mass air meter went bad. I remember the test for that was to unplug the MAF and see if the engine would start. So I leave the car running, pop the hood, unplug the maf, jump back in the car and the mis is gone. I put it in gear and drive home 50 miles with the MAF unplugged. Seems to run just fine. I let the car sit for 2 days then last weekend I decided to check the car out. Plug the maf back in, start the car and it's like it never had any problem. So all of last week I drive it to work putting 500+ miles on it with no problem. Then yesterday a week later I'm driving it to get coffee and back (6 miles round trip) and 2 miles from home at 45mph it starts missing again. It does this for about 30 seconds then clears up.

At no time during any of this did the check engine light come on.

I drove it to advance auto today to see if they can read any computer codes and they told me if the check engine light is not on they can't read any codes.

I'm stumped on what to even start checking. I did check the output from the maf sensor and it's reading 2.4-2.5 hz at idle and climbs normally when you rev the engine.

any ideas?

 

I got a Bluetooth OBD2 connector and installed the app Torque Light. No codes found. MAF appears to be working properly.

When I look at the O2 sensors bank 1 seems to move up and down pretty quickly. Bank 2 moves but not nearly as quick as bank 2. I wonder if this is maybe an O2 sensor getting ready to fail. I think I will change them out. Car has 140k on it and I got it with 82k on it so I doubt they have ever been changed.

 

My first thought: Spark Plugs and Wires.
Factory plugs are to be changed at 100K miles, pushed to the limit, may run until 150K. Get some NGK platinum plugs and AC Delco or BOSCH wires from RockAuto.com and get them changed.
It is possible as they begin to fail to be intermittent.

You bought the car at 82K, it has 140K now, if you did not change them, odds are no one did.

O2 sensor swaps (at least the upstream) is also not bad, as those also wear out.

You may want to check this thread and see if there is other maintenance you should consider:
https://montecarloforum.com/forum/fw...0-miles-46078/

 

Factory plugs and wires were replaced with AC Delco OEM parts at 98k miles. I did look at the plug wires to make sure they were not rubbing on something. The problem happens so infrequently that I can't troubleshoot it but don't trust it to drive it 80 miles to work. I'll put a pair of upstream O2 sensors on it next weekend. For now I'll just have to drive the mustang to work.

I looked on here for any tips on replacing the O2 sensors but didn't find any. If you have any good tips please share.

 

I'd only replace the upstream O2 (the downstream doesn't do anything with fueling on this car, just measures the "health" of the cat). Use AC Delco or Delphi for this job (not all O2's have the same OEM specs).
As long as you don't have the California emissions AIR pump setup, it's EASY! Since you are changing it, cut the cord as close to the O2 as you can, put a deep well socket on it (I think it's a 19mm, and if you have to bust the sensor to do it, so be it). Use a couple extensions so that you are not under the hoodline trying to bust it free. Use a breaker bar or a ratchet with a pipe (like a jack handle) and bust it free. I've done that a couple of times successfully (never had it fail me yet). The key is to be able to apply leverage and the best way to do it is if you have the ratchet un-obstructed by parts of the car. Also, use a 6 point socket, not a 12point, it will grip the old sensor better, less chance of rounding. Do not use an O2 sensor to take it off, due to the split for the cord, it can allow the socket to flex open as you try to remove the old O2. Just use the O2 socket for installing the new one.

Another item to look at for random mis-fires:
- If you have a scan tool that can read mis-fire data, see if it's linked to a couple of cylinders. Coil packs do not fail often, but it does happen. If two cylinders are failing and on the same coil pack, then odds are high that is the problem. Could also be a failing injector if it's only one cylinder.
- Have the ICM (Ignition Control Module) tested. They don't fail often, normally then they do the car just won't run, but get it tested.
- Fuel Pressure regular is another potential source.
- Plugged fuel filter could be a source.
- Normally a fuel pump completely quits, but it could be intermittent failing on the pump.

Anything fuel related, I don't have a best answer to diagnose.

 

Thanks for all the advise. I've done O2 sensors on other cars and I always cut the wire and put a socket or closed end wrench on it of the proper size. Also used the jack handle trick for leverage. Since I've had the scan tool the car has yet to have any problems. I'll just keep driving it around town to see if the problem shows up again. I'll get the O2's changed this weekend. Too much going on this weekend to mess with it.

 

I had the same symptoms, long ago.
I replaced the Plugs with Iridium Plugs, ... just to eliminate the hassle of getting to the Rear Bank to change Plugs so often.

I replaced my Plug Wires with Bosch Stainless Core Wires and splurged for a set of Plug Wire Guides to keep things neat.

When you see a "Misfire" displayed by the Tach, ... that usually indicates a Coil isn't performing right. Check the Plug Wires are securely affixed to the Coil Posts and both Coil Post and Plug Wire attachments are clean and tight.

 

Yea checked the plug wires to make sure they were tight on the coils. The plugs that are in it are AC Delco Iridium plugs just like the ones that came out of it. They only have 40k miles on them.

Also the MAF sensor is 6 months old and I did the upper and lower intake manifold gaskets 6 months ago as I was loosing some coolant. No coolant loss since I changed them out. Replaced those gaskets with the good Felpro ones. I suppose some of the coolant going out the tailpipe could have had a negative effect on the O2 sensors but it's strange for it to show up 6 months later.


On a side note. I've read on here by several people that the rear spark plugs are a real hassle to change out. It only took me about an hour to change out all 6 when I did it. I'm not sure why it's a hassle? Maybe since I'm tall I can reach back there easier than other people?

 

I agree with this statement. I read a lot of people "rock" the engine forward. The hardest problem I ever had was if the boot was sticking on the plug and you have those heat shields on the boot, you have nothing to really get a good grip on the boot with. I've used vice grips to grab the part of the boot sticking out of the heat shield and give it a good twist and then I'm all good. And I re-use the heat shields with new plug wires.

3400's are even easier (I can normally do all the plugs/wires on a 3400 n or w body car in about 30 minutes).

 

Yea that's why it took me an hour. I had 2 of the plug boots stuck on the back side and I had to "persuade" them off with a pair of vice grips. The new ones got plenty of grease so that won't happen again.