Hey there experts, have I got one for you. I am sure some of you have had this happen, but I figured I would not waste time or money trying to guess this one.
My car just hit 113k miles and now it stumbles/misses. Here is when it happens and when it doesn't:
1) Car in park, everything is fine while reving the engine, even up to 6k RPM's.
2) At part throttle or normal acceleration, she chugs and stumbles, but not in any set pattern, it is random.
3) Stumbling gets worse above 40 MPH until about 90 MPH.
4) Going up hill, really bad stumbling, feels like a train.
5) At coast, she is good, only seems to do it under a load.
6) Seems to be worse when the engine is warmed up.
7) Car was dyno'd about two weeks ago, saw a puff of blackish smoke come out at over 5k RPM's and the engine stumbled for a brief second at the same time.

OK, what do you all think I should check first? Starting with the cheapest item on up. If it is fuel related, what PSI should I have at the rail port at idle? Should it remain the same through all RPM ranges? How can I check it while driving (I do have CATS, what do I look for with that?)? Could it be a fuel filter (has been changed in the last 20k miles)?
If it is electrical, my coil has only about 10k miles on it (Hypertech 45k volt unit), wires are original, Opti is original, water pump is original and no leaks. Engine has not been wet. How can I tell exactly what it is if it is electrical in nature? What can I test or check out before tearing into things?

OK, am I missing anything? What do you all think? Give me any and all ideas, and maybe assume the Opti could be good at first, I really do not want to have to tackle that one yet, however, I have read here that AutoZone sells the whole unit for around $150, is that right?

Thanks and please give me a hand with this one, really!

 

sounds like it could be plugs/wires/and/or opti. can you see anything burned?

 

i would look at coil/coil wire, fuel filter, plugs. Then go on to the opti . Check for a burnt wire.

 

oh, how is your top-end?

 

Nothing looks burned at all, but I will give it a closer look for sure. I also already have a new fuel filter on the shelf, she will go in tomorrow for sure.
I did change the plugs with Bosch +4's at 93k miles, so they should be fine.

Thanks to you people up late! So, anyone else have some input? Even give me the strangest things you may have found for these types of issues, knowing my luck, it would be something I'd never look for! LOL!

Thanks again!

 

Top end?

If you mean the intake and the like, she has had the intake leak fixed and new FMS 24# injectors at around 93k miles. Everything is cleaned, including the throttle body. I power washed the intake when it was off and even replaced the EGR valve and oil pressure sender since they mount on the back of the intake (much easier to change with the intake off for sure).

If that is not what you meant, sorry. I have not done anything internal to this engine since I bought it at 86k miles. History before me is unknown.

 

No, i meant, how does your power feel up top. Does it drop off, or feel like you hit a brick wall?

 

It does seem to drop off a bit....

 

i had a similiar problem as yours so i changed the coil/coil wire and fuel filter and it cured it. Also, it might be better to go back to the stock coil.

 

i had a stumble/miss only when it was hot, i replaced plugs and wires and it went away

 

Trouble shooting on an internet forum can be pretty hit or miss.. A few suggestions -
Invest in a Haynes manual, a fuel pressure guage, digital volt meter, and a spark tester. The manual will have the procedures to test your sensors, coil, injectors, wires, distributor, and fuel pressure with these tools. A means of scanning is extremely useful as well.

It won't take that long to perform the tests, and will help to keep you from throwing parts at it until the problem is resolved.

I remember a while back Shoebox recommended these items as being a necessity in all of our tool boxes - he was dead on. I've found numerous problems (IAT wire soldering broke, coil wire shot, sensors not reading right, opti not distributing, etc) as well as verified other parts are in spec with this stuff.

 

I do have the Hanes manual as well as the three book Factory set for the F-Body. I will go through those tests and see where I end up.
I will also at least change the fuel filter and check for any burnt wires. I really hope it is that simple to fix, but we will just have to see, eh?

Thanks everyone, I will post my results here in this thread!

Anyone else got something different to offer, just in case?

 

I tried all this yesterday, and still no luck at all, she is the same way!

1) Checked all wires, none burned or cracking, very flexable in fact.
2) Changed fuel filter
3) Did Seafoam in gas tank and intake and filled with Premium fuel (as usual).
4) Never had an SES light, not once!

I have noticed 3 other things, maybe not related.

1) The fuel gauge reads higher than usual for a longer period of time before dropping off. I am about 240 miles before she drops below the half tank mark. It used to be that it did that at around 200 miles.
2) Gas mileage dropped from an average of 23 MPG to 19 MPG.
3) On the Opti, the bottom vent line (closest to the crank) is broken off, cannot fix.

So, now that this is where I am at, what do you all think should be the next thing to check? How shall I go about it? Please help me if you can, thanks!