If it's too far advanced the engine will keep turning by centrifical force.

I've also had my 300 turn over and diesel, albeit backwards, for a moment with the battery disconnected after an hour of it sitting in the driveway, while I was working on it. Due to one of the cylinders being on top of the compression stroke most likely.

 

If there is nothing lighting the fire, and no fuel/oxygen for the fire, then the engine is not going to run on or diesel.

Most modern engines do have carbon or a thread on a sparkplug that is hot enough to to light the air/fuel mixture. So the most sure fire way to keep it from happening is take the fuel/oxygen away, which means shutting the carb down as much as possible. That means slowing the idle speed down, and or using a throttle stop solenoid.

 

to wagonerkl:
It sounds like your EEC is totally disconnected. I have an 86 F150 300 with the EEC-IV and the feedback carb; when I bought it the EEC power was disconnected and the emissions light bulb was removed. If the EEC is in the system the emissions light will lite when starting but quickly go out. It also dieselled and got very crappy mileage (about 11mpg). I replaced the O2 and the EGR, plugged in the computer, rebuilt the carb, and it worked great, passed emissions, and hit the road.

The EEC-IV needs the O2, the MAP, and the TPS inputs to work, and the throttle stepper and feedback solenoid outputs to work, but the air pump is not needed to make it run closed loop or pass emissions provided the cat is 3-way (it will however set a code for thermactor system failure but this code does not disable closed loop).

The reason your truck diesels when shutting down? When you shut down the truck, the EEC shoud stay energized for 10 seconds. It activates the throttle stepper motor to close the throttle completely. If the computer is dead it will diesel, period. Listen for a click inside the cab 10 seconds after shutting off the key, if you don't hear it then the EEC relay is not being de-energized and your computer is most likely disconnected (therefore the throttle plate is not being closed at shutdown).

Previous posters left out this note. If you convert to Duraspark the carb will need a throttle shutdown solenoid or something to drop the plates closed when the engine is turned off.

I don't completely agree with the posters that say you can't make the feedback carb system run right, but I do agree its harder than making a Duraspark system run. Mine runs and passes emissions, gets almost 17mpg on hiway 70mph driving (with the 3-speed no overdrive). No air pump, but I did install a 3-way cat to replace the original 2-way cat that required air input to work.

 

The Emissions light on 1984-1986 trucks are not hooked to the EEC. They are hooked to a timer. This timer causes the light to light up at aproximately 60,000 miles. It lights up if the there is a problem with the truck or not. The Emission light on these trucks are useless. The Emission light will flash on when you start the truck as a confirmation that the bulb hasn't burned out. If the bulb doesn't flash on startup. the bulb is blown, or disconnected. The same thing happens to the brake warning lamp.
1988 is the first year of a "CHECK ENGINE" lamp, and that lamp is hooked to the EEC.


Not all Feedback Carbs have this "throttle stepper motor" known as the Feedback stepper motor on the 7200, and sole-vac throttle positioner on the 2150. It depends on calibration. The later CARTER YFA for instance has a electronic Dashpot only.

Another reason why the truck will diesel with the feedback carb disconnected, or the EEC-IV disconnected, is the Feedback solenoid is of an electronic "pulse" type. The EEC or MCU sends pulses down the Feedback solenoids wires, and depending on how fast or slow the pulses, depends on how the solenoid controls the fuel mixture. You will hear the solenoid click while operating normally. Some feedback solenoids are spring loaded, these click all the time, and some are controlled by +/- the polarity during each pulse, these click at random. Both use an electromagnent to move the valves inside. With no power to the solenoid, or if the spring inside breaks, it will be "stuck" in wide open, fully closed, or somewhere inbetween. This will cause a lean or rich condition wherever the solenoid "openings" happen to fall the last time it was energized. When you shut off the truck, the EEC or MCU sends a pulse to the feedback solenoid, or the spring returns it, to shut off the fuel mixture. When this system is not operating properly, the solenoid does not shut off, and fuel can still enter the engine causing dieseling.

I disagree here, the carb should be completely replaced with a non-feedback carb, if going to Duraspark II ignition without the computer. The feedback carb will not run right without the EEC-IV to adjust the air/fuel ratio.

You should try to work on the system I have. 1981 300-I6 california, with MCU, feedback carb, and duraspark II ignition, with vacuum advance distributor. And california does not give you the choice to make it simpler.

At least it gets 21MPG average highway with a t-18 four speed. 2.75 gears.

 

Well, I was a sceptic too but here is what worked:
- Reset timing (manually) timing mark set on 10 deg BTDC (per sticker under hood); TFI pickup centered on distributor node and rotor pointing at #1 on rotor cap. This caused truck to idle very high and it also "pinged" terribly on acceleration. Remember this engine has had all the airpump equipment jerked out in a not-very-professional manner, so the EEC-IV is not getting good information. At this point it still dieseled on shutdown.

-I then began to incrementally retard the timing to reduce pinging to an accceptable level. Truck was still idling high. As soon as I adjusted the idle screw on the feedback card to minimum, the dieseling stopped.

I understand that one is not supposed to adjust the timing on a TFI system in the conventional way as the TFI and EEC (when working properly and recieving all correct inputs from the sensors) should take care of that. But, I don't have a correctly working (or even complete) system.

Anyway it isn't dieseling now and I am now a believer in your list of Five Things. Thank you for the help and insight.

 

I hope I didn't come off as too knowing - sometimes I do that and my apologies. I have learned a lot from this board since I got my truck. I do see why most people prefer to convert to the Duraspark, but my bias is toward the computer. My previous truck was an 87 EFI using the same computer, I've fooled with the EEC-IV for about 20 years now. So I am probably more inclined to make the EEC-IV run rather than remove it.

You are right about the 'emissions' light; but I noted when I bought my currrent truck the EEC power relay was disconnected, and the bulb was out - replaced the bulb and it stayed ON all the time. When I reconnected the EEC relay it functioned as you describe. So I suspect it does indicate if the EEC power relay was disconnected.

You are incorrect about the Carter YFA (at least my 1986 model); it has a stepper motor, not a dashpot, and the EEC pulls it down at key-off for the 10 seconds it stays energized afterward. Otherwise it changes the hi and lo idle during warmup (instead of using a dashpot or step cog on the choke).

"When you shut off the truck, the EEC or MCU sends a pulse to the feedback solenoid, or the spring returns it, to shut off the fuel mixture. When this system is not operating properly, the solenoid does not shut off, and fuel can still enter the engine causing dieseling." The feedback solenoid simply powers down. It is spring-loaded to the full-open position. The dropping of the throttle plates (and closing of the metering rod) is what stops the dieselling. Feedback solenoid unpowered is full-open otherwise unplugging the EEC would prevent the engine from running. Instead it runs full-rich (making dieselling worse).


When you convert to Duraspark you will need a throttle shutdown solenoid...
I could have left that one out, as you are right they need a new carb...and the older-style non-feedback carb should have the electric dashpot or vacuum throttle positioner. If it does not then you may need to provide it if you cannot adjust the engine to not diesel on shutdown. Without it there may be a bit of compromise between adjustment for diesel prevent and adjustment for idle. I prefer a higher idle (run AC a lot)and seem to be right 'on the line' with dieselling without the drop-down throttle plate at key-off.

21mpg.....??? Now that may be a reason to convert to Duraspark in itself. I was thrilled with 17. I probably won't do any better than that without getting some OD transmission and laying off the AC *(FYI I like my junkyard AC - $100 for the parts and a weekend of work, and I drive cool).

 

It's called a electronic dashpot in the service manual on the Carter YFA. Otherwise it does exactly as you describe above. But calling it tomato, or tomahto, is a matter of semantics really.

This depends on calibration. some are as you describe, some are as I describe. This is the reason why you cant use just any feedback solenoid off of any carter YFA, on other Carter YFA, it will cause calibration issues with the carb, and the computer. The numbers have to match.

Again, it depends on calibration, mine has neither from the factory. Only a standard dashpot to keep it from stalling. The idle adjustment screw on the drivers side of the carb, touches the carb housing, it adjusts the throttle plate by turning the screw, up or down. The plate does not go below the curb idle adjustment of this screw.


There should be a seperate throttle positioner for the A/C controls, called a "Carburetor Throttle Emission Control Solenoid".

This compensates for the load on the engine with either the a/c on or off. With the A/C off this solenoid is de-energized, and normal idle and throttle plate position is realised. When the a/c is turned on, this solenoid energizes and opens up the throttle plate to compensate for the extra load, thus giving you a consistant idle speed with the a/c on or off.

I have no a/c but I do have a Borg/Warner T-18 4 speed 2.75 gears.

My friend had a simular 1981 truck as mine, same engine and california computer controls, but he had a/c, and the clark 4 speed overdrive. He got 26 MPG highway, a/c off.

And to explain what computer controls, and distributor, and ignition system I have...

I have a california hybrid system, feedback carb and MCU Computer with duraspark II. The computer controls the vacuum advance with a solenoid. there is constant vacuum to the distributor vacuum advance at all times, even idle, however it increases with engine speed as normal. The distributor and carb is calibrated to compensate for this. When the computer wants to retard the timing, it bleeds vacuum off with a EGR solenoid, (AKA Spark Retard Solenoid).

Why I keep stating, Calibration is so important. There are numerous different carbs, distributors, and controls.