New Member
Nov 12, 2002
I have a 1990 Mustang GT Convert. It is basically stock with the exception of a free flow air filter, underdrive pulleys, and MAC 1 5/8" headers. The car has new sparkplugs, new wires, new coil, and new fuel pump, filter and fuel pump solenoids.

The car has been running rough for at least 8 months. I have taken it to various Mech. and nobody can figure it out. Some days it runs awesome...it really pulls...most days it runs like crap...no power..sluggish.

About 1 month ago it started just dying out in 3rd gear..what it does is that the speedo and tach go down to zero and then back up again. Kinda like somebody was turning off the engine. Sometimes ill be lucky and it will go away and I can go on with my day...other days it will just die and I will have to coast to an open lot. At that time, it will take about 3-4 turns of the key and engine turns to start up. Then it may run ok...or like its going to die.

It just came from a mechanic and he says the number 4 piston has a "hole" it it. He also says that that piston had 25% psi on a compression test. He also said that that piston on a "blow by test" had 100% blow by??? I later chacked and noticed that that header bolt near that piston was really loose.

But CORRECT ME IF IM WRONG....yes all that would make the car run bad...but would that make the car's tack and speedo die in 3rd gear?

I dont know...I dont trust the guy. But the bottom line is that I have gone to 5 mechanics and no one can figure out what wrong with my car. I get charged all kinds of money for checking the car and the results are they dont know.

I really need help...thanks guys...If anyone wants to call me please do so at 813-598-2902 thats a Tampa Florida number.

Thanks again

Ernie the Eyeball
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You diden't say if this started before or after the new parts were installed?

I had a problem like that a while ago. I spent a lot of hours following wires to find out that one of the pins in the wireing harness onthe tank pushed in about about a 1/4 to a 1/2 inch. It was just enough that a good bump or turn would cut the power to the tank. there are two connections on the tank. Body to tank and tank to pump. My short was on the tank to pump side. My tank has been out a few times over the years.
If it runs okay in the mornings then runs worse or quits after it warms up, I would be suspicious of the TFI ignition module. I'm not sure what 3rd gear would have to do with in unless that is the gear you are pulling the hardest in. If you had a hole in a piston I'm not sure it would even be driveable, but it would definately be a consistant miss. You would have massive blowby and would probably blow your dipstick out ! The speedometer is a mechanical connection to the transmission, so there should be no reason for it to fall to zero. I've heard of cars that have a bad crank position sensor that causes the tach fall to zero, but never on our cars. Might be the computer as mentioned before. Check all grounds (especially computer and injector grounds).