Terrance Chapman said:
Until last May this motor was running fine. Since then it has never been out of the garage and has my regular garage and local dealership stumped for an answer. One trip can be fine, great acceleration and smooth. Next trip and its shudder shudder, blowing black smoke, cutting out at the lights, no acceleration and don't even think of a hard lock "U" turn. Next trip is OK. Garage mechanics must be lucky. they have changed everything (everything) but have never had the same problems. Almost came to blows. Tried high octane and that seemed to be OK then on the next trip...
Unless it's a diesel black smoke could often indicate too much fuel. So this may be a fueling issue.
Is there any continuity from when it is ok to when it isn't? Such as how much fuel is in the tank? Engine temps, outside temp/humidty, is it raining?
If you can collect some info on when it is or isn't ok then it may help point at the problem.
Have you ever run out of fuel in it? If so then you may have sucked up some crap from the bottom of the tank which is now causing poor fueling.
you said trying a U turn is a no no, does that mean it's worse when cornering? That could indicate a fuel pickup problem or a connector that flops about and breaks contact at times.
I assume you've tried all the regular service items such as plugs, leads, filters, etc.
Is it fowling any of the plugs?
Will it idle ok? If it doesn't try an establish which cylinders are miss firing. You can pull the HT leads off one at a time if it causes a miss fire then it's probably not that cylinder, if it doesn't then there's an issue.
Is the motor burning oil or water?
How many miles has it done?
Basically just try and check as much as you can, such as fuel lines, fuel pump, injectors.
If the fault is intermitent then it most likley isn't anything major it's just diagnosing it is a pain.
Your not running on LPG are you?