As auto mechanics we've all heard the line that starts, "Ever since you ....." In this case it was, "Ever since you rebuilt my engine, it starts to run rough and then stalls after about 20 minutes of use." As part of a restoration pro ject for a good friend I'd rebuilt his Boss 302 Mustang engine over 15 years ago.

Having a busy schedule and being convinced beforehand that we had a hot fuel handling problem, I set out to the storage location of this classic 1970 Boss 302 with a bottle of propane and a spray bottle of water. My plan was simple and quick. If the engine started running rough, I'd meter a little propane into the carb and if the engine smoothed out, a lean condition would be indicated. And, if it stalled, I'd cool the fuel lines and pump with water and if it then restarted and ran smooth, a vapor lock would be probable.

When I arrived, I noticed the modified dual point distributor I'd installed in 1983 had been replaced by an OEM specified dual diaphragm/dual point unit. The purpose of the dual diaphragm design is to retard timing at idle and this convinced me ev en more so that we had a hot fuel handling problem on this high compression engine.

Sure enough, things went as planned, when the engine warmed up it began to idle rough. A shot of propane smoothed it right out. Soaking the fuel pump and fuel lines with water also smoothed the engine out for a period of time. I promptly announce d a fuel problem and prided myself on my quickie diagnosis.

As an instructor, I repeatedly advise students against quickie, shortcut diagnostics. Lesson plan after lesson plan involves first understanding the operating principles and then using test procedures which provide measurable data. The diagnosis i s then based on an analysis of the recorded data.

A month later, nobody could find a fuel problem found, so I return to do what I should have done in the first place.

(figure #1)

Attaching a scope to the coil(+) and coil(-) proves this car has ignition primary problems. Referring to figure #1, the 35.8 degrees dwell (79% duty-cycle) tells us the points are closed for too long. We know that if a primary circuit is energized for too long, it will overheat. We can also see that the point close signal rises above the zero line, indicating high voltage drop in the primary ground. Touching the coil(-) to distributor pigtail wire we find it is extremely hot. What is happenin g?

After 20 minutes of operation, the lack of cooling time is taking it's toll on the coil to distributor lead, causing it to overheat. It is basic theory that a hot conductor has more resistance than a cool conductor. This increased resistance shows up as a voltage drop and we all know that resistance in a conductor reduces current flow and reduced current in the primary will cause low ignition secondary energy.

Enrichening the fuel mixture with propane merely lowered the required firing voltages, so the engine smoothed out. Squirting water on the fuel lines resulted in getting water onto the overheating distributor primary lead which is near the fuel lin e. This cooled the wire, thereby restoring higher primary current flow and secondary energy. My quickie diagnostics had misled me.

Having invested extra time in this project, maybe it is time to do some studying. What is that curved signal at the coil(+)? Why does the initial point close signal go down to near zero volts and then rise well above the zero line with the passing of time?

(figure #2)

The curve at the coil(+) represents inductance, or restated, the build-up time of the coil. When the points first close there is very little current flow because of inductance, so there is very little voltage drop even though the distributor prima ry lead is very hot. If we count the evenly spaced grid marks in figure #2 we see that after about 0.006 seconds, the primary current has built to full strength as indicated by the coil(+) signal which has now flattened out because current flow in th e primary has stabilized. This point when

peak primary current is reached is known as the saturation point and please observe that at this point we have an excessively high 2.02 volt drop at the coil(-). Holding the points closed beyond this saturation point is of no purpose, in fact it i s detrimental. The ignition is not getting any stronger, primary components are being overheated, and energy is being wasted.

The fix of course is to widen the point gap (reduce the dwell) and replace the distributor primary wire. It is precisely because of design and service defects like these that engineers were called upon to develop low inductance, fast build-up time , variable dwell, high energy output ignition systems like GM-HEI and Ford TFI.

One lesson to be learned from all of this is that we can monitor the voltage drop at the coil(+) to determine the build-up time in a resistive-inductive (R-L) circuit. We can also measure the voltage drop at the coil(+) to get an indication as to whether current flow in the circuit is normal or not.

Next we’ll look at some Ford TFI IV and GM HEI patterns.

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Copyright Dan Buckley 4/2000 973-472-0328