I posted the following on another forum. Having received no answer, I'm hoping someone here can help me out. Thanks.

I have an '87 F250 with the 300 six engine. According to the Haynes Manual, on a test for stray voltage at the Ignition Module Harness Pin #2 (the tach/coil ground pin), I should have no voltage shown at all. My voltmeter, though, is showing 1.6 volts. The manual says to correct the short in the harness, but I find no short in a check of the Ignition Module Harness, the coil plug/harness, or the wires in between. Since the tach wire (Pin #2) also runs to the ECM through a resister, could this voltage be coming from the ECM? If so, does that mean the resister is bad, and can it be replaced?

I'm trying to run down the source of an aggravating miss in this truck. I don't know if something like the stray voltage could be responsible, but I want to correct it to see. Any help or advice anyone can give will be appreciated.

 

I am not sure about the voltage reading, but describe your miss a little bit more. Does it at idle only? Only when you are pulling a hill? Only when cruising at light load?

 

I've had a lot of advice about this miss from the Inline Six Forum, but like a bad dream it is still with me, so I keep searching. When the engine is cold, it misses badly. The sound (from many years experience messing with engines) is exactly like one cylinder is not firing. As it warms up, the miss will go away, and it will run perfectly. This has gotten worse over time, by worse I mean that the engine has to warm up more before the miss goes away.

As far as what I have checked/changed, well everything I know to do. All the basics are sound: compression, vacuum, timing, ignition analysis. Plugs, wires, distributor cap, Hall effect, ignition module, coil, oxygen sensor, air temp and water temp sensors, knock sensor, and MAP sensor on the ignition side. The EGR and its connected sensor and control solenoids are okay. The ECM has been replaced. On the fuel side: fuel pressure, filter, pressure regulator, injectors have all checked out. Possible air leaks have been repaired and or checked. There are none. Cats removed and replaced to check for blocks. There's probably more that I haven't remembered.

I've never had an engine problem that I couldn't solve. This one may be the one that knocks the pride out of me. But, before I throw in the towel, I'm going to look for a while more.

If you've got any ideas, please send them on and thanks.

 

Ok. The first thing we can do is verify which cylinder it is, if it is a particular one. As you indicated you already know, a dead cylinder sounds like a consistent thump thump in the exhaust.

If it is consistent, I would put on a glove and pull each sparkplug wire off one at a time. The cylinders that are good will make the engine run rougher. The cylinder that is bad will make no difference.

If you do narrow it down to one particular cylinder, then that will eliminate a lot of the stuff you have been messing with, including the ignition module, since most of the sensors and the ignition module affect all cylinders.