Misfiring at low RPM when cold... ?

[designie]

I have a 1992 SHO, which just recently turned over 200,000 miles When the engine is COLD, it runs fine, plenty of power. After it warms up, it starts misfiring under acceleration, at low RPM, with any amount of throttle. If you floor it, it will miss, pop and backfire, but as soon as it reaches about 3,500 RPM (real close to when the secondaries open) it roars to life. It will run right up to redline with no loss of power.

Maintainance I've done:

New Motorcraft AGP32 plugs, new CPS, fixed the spark plug wells that were leaking (they still are fine), new timing belt, new water pump.

It ran great after all that, this problem just developed over the past couple of weeks. It's really hard to drive around town. It has made it 200,000, I'd hate to have to part with it now. Any ideas?

Thanks!

 

[Rob94]

You changed the plugs and the plug well seals. Did you change the wires? If not, change the spark plug wires. I'd also run your codes, just to be safe.

 

[designie]

Can the wires work fine at high RPM but do what I described at low RPM? I thought about changing them, they are original.

 

[shoteen95]

congrats on a 200,000 mile SHO

original plug wires can't be in the best of condition being almost 13 years old.

 

[Rob94]

If they are original at 200,000 miles, then by all means, CHANGE THEM!! And yes, I suppose it is possible that they could cause a misfire at lower RPMs. The higher the engine speed, the faster the current moves through them. The faster the current moves, the less is loss to a breach.

 

[designie]

I'll try that first, thanks guys.

 

[NotSoSlowSHO]

your definately in for a pleasant surprise if those wires are original

 

[SHOSIG]

Check each end of the connectors on the DIS module. Mine were cracked and had the same symtoms. If the connections are not cracked, check the dielectric grease between the DIS and the crossover tube. But of course if the plug wires are old.......change them. Good luck

 

[Whiner]

Could it be the coolant temp sensor? Maybe running to rich when the rpms are down because the computer is telling it othewise? My Beretta had the same problem.

 

[SHO--ripper]

Look under your hood at night when your car is at idle. If you see fireworks coming from your plug wires you need new wires

 

[Fourcats]

How about the fuel system, condensation, filter, ect....?

 

[sdpatt]

A misfire without a CHECK ENGINE light is usually either the plugs and/or plug wires. Since you replaced the plugs with appropriate parts and the wires are original, The suggestion to replace the wires is on the mark.

 

[shoteen95]

Look under your hood at night when your car is at idle. If you see fireworks coming from your plug wires you need new wires

haha, i'd like to see that someday...

just not on my car

 

[AutoSHO]

Yep, the problem is definitely plug wires. Replace your plugs again as well, they will have carbon tracks on them which can cause the misfire to come back even with new plug wires.

 

[shoteen95]

while were on the topic of plug wires..the ones in my car are probably 10 years old as well, and i'm due for new plugs soon..so...

what kind of options do we have for wires? high performance..low noise..uhm..whatever? just oem replacements from autozone?

 

[SHO--ripper]

Get motorcraft or Taylor.

 

[AutoSHO]

Or AC Delco. They're as good as OEM and cost considerably less.

 

[HopefulSHO]

AC/Delco's. Great at half the cost of the Motorcrafts.