im new to this site, and could use your help. 88 F150 4.9 STRAIGHT 6. The truck will not start if the coil wire is pluged in, but if i leave the coil wire resting near the cap or the coil it arcs and starts perfect, runs great. But when i try to plug the coil wire back in it dies and wont start. I replaced the cap/rotor/coil and it made no difference.Anyone have a clue what it is??? thanks

 

I had the same issue with my Dads 89 F 250 with the 4.9 and I replaced the coil wire and it took care of it. Apparently with the wire pulled loose it causes the coil to produce a hotter spark to overcome high resistance with the coil wire . Hope this helps. Steve

 

nope, i tried changing the coil wire too, that didnt work either.

 

anybodyelse got any ideas????

 

Sounds like a odd one, Have you checked all the grounds. Especially the ground on the engine block for good, clean contact?

Made sure the coil connections are clean and dry, as well as checking all the connections for corrosion.

 

I think 66comet is on the right track - creating the larger "gap" causes the spark at the plug to be a bit hotter. I'd try a general tune-up - plugs and wires, cap rotor. My 97 F250 is very sensitive to bad wires...

 

I would have replaced all the wires first off. It's obviously around the distributor or wire from the coil. There's no real difference between it touching metal to metal and it arcing across. Definitely a strange one. You did say you replaced the coil wire with a new one right? That means you bought all new wires, which you just forgot to mention, or did you just grab a coil wire from somewhere else? I'd replace the wires. Probably due anyway!

 

I know it sounds a bit strange, but causing an arc really does make a difference sometimes - I think what happens is if you have direct contact the electrical potential can discharge never have risen to the point where it will jump the spark plug gap. If you leave a gap, the voltage has to build to a higher potential to discharge. I'm not sure that's the right explanation, but that's what it act's like...

 

screwed with it a bit today, ya i grabed a coil wire from my 57 chevy, all grounds seem fine, cleaned them anyway. still no luck thow.ill try a new set of wires tomarrow, fingers crossed.thanks for the help

 

What kind of voltage are you getting to the coil?

Jim- What you said does actually make sense. It's arcing because it's absorbing time to build up power. If the coil has been replaced... the next thing to check would be voltage to the coil.

 

Ok. New Wplug Wires,replaced Some Ground Wires, The Rest Were Fine. Still Nothing. The Voltage At The Coil Positive Side, Ignition On, 11.2/11.6.the Battery Voltage Is Fine Over 12.im Guessing Coil Voltage Is To Low, Could It Be The Module On The Distributer??

 

also checked, the voltage drops to 8.9 while cranking the motor over???

 

That sounds kinda low, although that's a gut feel answer... Does the battery voltage drop that low, or just the voltage at the coil? I'm assuming you're measuring the hot side?

 

I'd run a hot wire to the coil right from the battery so you know it has full battery power, 12+ volts constant and see if it makes any differance.

If it does and runs with the coil wire in place you know you have a bad connection somewhere.

 

Oh of course that won't help if your truck is running on low voltage to start with.