posted 08-03-2013 02:25 PM ET (US)            

--"plugs 1 and 4 had weak spark compared to strong spark on 2 and 3"

The circuit that generates the spark has several elements. One of those elements is the external spark coils. I assume you took the spark coils from cylinders 2 and 3, and moved them to cylinders 1 and 4, and, at the same move, put the ones from 1 and 4 onto 2 and 3. Now we have everything the same as before, except the location of the spark coils.

Then you repeated your test. On the second test, there was evidence of weak spark on different cylinders. Instead of 1 and 4 being weak, you saw weak spark on 2 and 3:

--"2 and 3 were weak after the swap"

I interpret this test as demonstrating that there is some defect in the spark coils that were originally on 1 and 4 and presently are on 2 and 3. Let's call them the suspect coils.

The part's counter salesman contradicts your diagnosis. He says the problem is likely in the "trigger coils". This makes no sense, because in your test the "trigger coils" did not change location. The same ones are still driving the same cylinders.

Regarding the "trigger coils", that naming is a bit confusing. In the typical ignition system there is an arrangement of coils under the flywheel which respond to magnets in the flywheel passing by them by generating electrical pulses. There are usual one or two of these coils, perhaps more, that are intended to produce a pulse that will be used to initiate the spark plug to fire. These coils generate only a low voltage pulse, too weak to be used for much, except to operate a solid-state device with a lot of current gain. The solid state device then controls the flow of battery current into the primary winding of the spark coil. The battery can provide a very high pulse of current into the primary winding of the spark coil. The turn-ratio of the spark coil is very high, so that an enormous step-up in voltage occurs, generating many thousands of Volts for spark.

If there were a problem in the under-flywheel coil assembly associated with the spark--which is sometimes called the timer base--this would always affect the same cylinders.

The signals from the timer base coils are amplified in an electronic assembly. Various names are used for this assembly, like switch box or Power-pack. Inside the assembly are solid-state devices that amplify the current of the timer base pulses and condition the pulse so that it can drive the spark coil primary. These circuits are dedicated for their cylinders.

In the test you made, the timer base coils and the Power-pack circuits did not change cylinders; they all stayed right where they were. The suggestion that your test indicated a defect in either the timer base coils or the Power-pack (or whatever those components are called on your engine) makes no sense to me. The test indicated the problem followed the spark coils.

posted 08-04-2013 09:00 AM ET (US)            

I thought my test was sound and thought the same thing about the parts counter guys's comments - that if the problem was under the flywheel, it would impact all cylinders. He also suggested a fuel problem, but again I thought it would be a broader problem. I felt I ruled that out in the beginning with a new tank of clean treated fuel and clean fuel filter.