could a bad ignition coil make a truck run bad?

 

Short answer: yes. The coil is a special type of transformer known as an auto-transformer because the primary and secondary windings inside share the same core. A transformer's job (one of them) is to transform a low voltage to a high voltage. This means taking the 12V the truck runs off of and turning it into tens and thousands of volts to make a spark jump across the spark plug gap.

If there is a problem with the effective ratio of number of turns between the two windings inside the coil, the output voltage will not be as high as it should be. If the voltage is not high enough, there won't be a spark for a given plug gap. The windings can short, or increase in resistance due to age and not produce the proper voltage, or not fire at all.

In my experience, the coil usually works as well as it needs to, or doesn't work at all, without a "rough running" region in between. If any part of the ignition system in any vehicle begins to fail, the truck can run poorly. If you tell us what year\engine you are working on specifically then we can give you what things to actually look for in your particular case, as Ford used a variety of setups throughout the years.

 

As fmc400 says the coil is usually a "go/no go" item. I did have one that had a crack in the case and it would short circuit to the block during wet weather. Bad wires or plugs will tend to make the coil work harder and I would check them first. Coil failures were more common on my older GMs than my Fords.
regards
rikard

 

Many things can make an engine run badly. First thing to do is pull the codes to see what they can tell you.

Running badly is a very broad statement. Any more clues?

Chuck

 

89 ford f 250 4.9 and after it gets hot, turn it off and then occasionally it will run like crap until u turn it off and let it cool off..

 

Is there a miss, backfire, or won't pick up any speed? How new is the catalytic converter? Do you get a rotten egg smell when it gets hot? Have you pulled any codes?

Chuck

 

i guess i should say i've changed the distributer cap, soilinod and ingnition control module and only thing electrical i havent changed is the ignition coil...just the way it acts and runs, and stummbling i just think its something to do with the ignition

 

yep i would say deffently backfiring and missing i have no idea on the cat converter and no rotten egg smell

 

A new coil would be a fairly inexpensive thing to change. If that doesn't do it there is a chance that the power supply for the computer is going bad and when it gets hot it switches the computer into "limp home mode". I know of no way to test it and just replaced mine in my 86. It is attached to the computer, on the passenger side of the computer. At least it is on mine.

Pull the codes and see what it tells you.

Chuck

 

thanks .. maybe i can get some more advice

 

For sure pull codes if at all possible. MANY things that can cause the problems you describe, but the codes will usually point you in the right direction. Also, when replacing the module, did you make sure the mounting surface was clean, and install with plenty of dielectric grease? Heard of overheating module due to no grease causing this kind of behavior. Also, something that people often overlook because they rarely go bad: the condenser. Had one give me fits before, and it acted like carburetor problems. popped on a new one and ran like a top. REAL cheap too, so you aren't out much money if that's not it.

 

Is that the same thing as the computer running in "open-loop?"

Your problem doesn't sound like a coil. My money is on a vacuum leak or improper timing. I'd put even more money on a vacuum leak. I can almost gaurantee you that it is not ignition related, unless you have a bad plug wire or you got the firing order off when you put the new distributor cap on. A new coil will probably be a waste of your money.

 

if i replaced the distributer rotor would i have to retime it? its just ticking me off now...

 

Nope, just pull the cap and replace the rotor. It only goes on one way.

 

i noticed when i had the distributer off that the rotor had a bit of crap on... kinda like black greese... that might be the problem...