My problem started five months ago and was very intermittent until now. The engine would run rough at start up either cold of hot and then run fine after about two or three miles. Now it runs rough all the time. I have installed a new ignition coil, new spark plug wires and new spark plugs. The engine starts ok but it runs rough and when I pull the spark plug wires one at a time,while it is running, the spark is very weak on cylinders 2 and 4. The spark on the other two cylinders looks good. I have checked the connector on the powertrain control module, which is a EEC-V, and there doesn’t seem to be any bad connections there. Thanks for your assistance.
This car uses a ‘wasted spark’ ignition system. What that means is, the coil pack you replaced only has 2 coils in it. One terminal is the negative side of the coil, and the other terminal is the positive side. When the coil fires, it causes both spark plugs on opposing cylinders to spark. The spark for the cylinder on the power stroke is used to light the fuel/air charge. The opposite cylinder is at the end of the exhaust stroke, and the spark is wasted.
For cylinder 2 to have a weak spark, there should be a corresponding weak spark on cylinder 3. Same relationship for cylinders 1 and 4. To have weak spark on only one side means a voltage leak somewhere. With new wires and, plugs, and coils, I wonder if the problem may be the parts you purchased. For spark plugs, I always go with the OEM brand. Some plugs just don’t work well with certain engines. My Ford only gets the Motorcraft spark plugs. I can get those at Autozone, and they are the exact same as at the dealer part counter. My Toyota gets NGK spark plugs and nothing else.
What are the brands of the parts you used?
Do you have a check engine light? If so what are the codes?
You haven’t said enough for me to assume that you have a spark problem. You are saying that you pull the wires one by one - then you just say “weak spark” - what gives you the weak spark conclusion? Are you actually getting wires to arc & watching? Or are you just saying that pulling 2 or 4 doesn’t change how well/poorly the engine is running?
The check engine light was on before I disconnected the battery, it is currently not on. I have not had a code reader connected to it. I am getting the wires to arc as I pull them and I can see that the arc is much weaker on 2 and 4. Much shorter arc and different color than the arc on 1 and 3. Also pulling either 2 or 4 has much less an effect on how the engine is running than pulling 1 or 3.
In that case, BustedKnuckles pretty much said it all. My first suspect would be the wires. (If the weaker arcs did line up 1-4 or 2-3 then I’d say coil or its power supply). I once bought a set of cheapie plug wires for a car - the bottom shelf stuff from a big-box store. Once was enough. They were terrible. I now just use Motorcraft - they’re not expensive.
Other than maybe the replacements being cheap, I’ve also gotten plenty of defective parts new off the shelf.
What kind of plugs did you use? I’ve used both OEM Motorcrafts and Autolites in my '97 without any issue.
Step One with any engine performance problem is to run a compression test, even on a fairly new car with very low miles on it.
If compression is dropping you could throw parts and fret over this for eternity without curing anything.
Switch the coils around if you can…Swap the 1-3 coil with the 2-4 coil and do your test. Same thing? Could be a bad 2-4 coil or the wires…OR the signal wire that tells the coil to fire…wait no scratch that if it it firing but weak then its getting the signal. How about your GROUND CONNECTIONS…please check and clean and tighten any you can find including your batt connection… CLean and grease and tighten please. Let us know what you get when you swap the coil packs…does the problem move?