What causes engines to diesel/run on when you shut em off?

Please explain this...I am intrigued.
I myself do have an electric fad, on a simple therm switch/relay.
I wired it to an led indicator, which shows when the fan is actually on. When I turn the ignition off, the light slowly goes out...Kinda like it drops with fan RPM as it spools down.
Huh, Holly sheets... I wonder

 

The joke is on you I dont even have tires!

 

x2
I remember when Mercedes-Benz first came out with their twin cam 6 in the early 1970's they had a horrible problem with engine run-on after the ignition was shut off.They installed electric fuel shut-off valves and the problem worsened.They finally found out that even after the ignition was shut off and the engine was winding down the alternator was still supplying enough current to keep the fuel shut-off valves open.If I recall correctly a diode inserted in the circuit cured the problem.

An electric fan can also cause the problem.We had a brand new 450SL in the shop one time that had a really quirky electrical problem.When you accelerated to about 35 mph the windshield wipers and washers would come on.After much diagnosis and frustration it was found that someone at the factory had inadvertantly connected the windshield washer plug to the auxiliary fan(they both had the same electrical plug and were close together in the harness).When the auxiliary fan windmilled it would create enough current to actuate the washer motor which in turn activated the wipers.

I would also cut back the initial lead on the distributor and make sure you are using a good vacuum source for the vacuum advance/retard.

 

[QUOTE="T'RANTULA";8335908]My timeing is 12 degrees before TDC with the vac***e advance plugged. With the vac***e advance unplugged it wont diesel. With the vac***e advance plugged into manifold vac***e it helps the problem. If that makes any sense, im still a rookie but im learning.[/QUOTE]


first you need to make sure your marks are correct, - via piston stop test.

12* is a bit high but you are describing symptoms of being retarded on the timing.

None of that makes sense until you add in this little tid bit >> the timing light is telling you stories.

If you didn't have a light , and based on what your saying, I'd tell you to advance it some, then back down the idle speed closing the throttle blades. Maybe repeat a few times until it runs, good, starts good, and shuts off good.

 

Back in the 80's, my '55 Chevy with 350V8 would diesel with run on when the ignition switch was turned off. So, the remedy back in the day, was to slowly pour water from a coke bottle down the carb with the engine running. This would knock the carbon build up off the combustion chamber. We thought the carbon build up on the heads would stay red hot when the ign was turned off, and ignite the gases in the combustion chamber causing run on and a rattle noise. The water technique worked but I was always afraid the water would cause a big piece of carbon to damage the valve seat.

 

My timeing tab was originally welded to the timeing chain cover, the old cover was bent so I put a new cover on and a timeing tab that bolts onto the timeing chain cover. The timeing tab is about 3/4 of an inch away from the harmonic balancer. I never tested to see how accurate it was and I agree that was dumb of me to ***ume it was correct..

 

I use a spray bottle with a fine mist...
Rev the motor too about 2500-3000 rpm and spray the the water down the carb... You could see the tanish looking cloud of carbon smoke blow out the exhaust....

 

The dieseling started happening after I replaced my timeing chain. It had a fiber metal gear and whoever before me installed it incorrectly. on TDC both dots pointed up so the cam gear was 180 out. So instead of fixing the timeing chain they flipped the dizzy 180 to compensate. I pulled the dizzy and set everything up according to my chiltons book. I thought I was a tooth off when it dieseled so I took it apart 6 times afterwards and still couldent find any screw up in re ***embly. Im no expert but ive done timeing chains in a few chevys and never had this problem. Could be pilot error.

 

You say that disconnecting vacuum advance stops the dieseling? Then try running ported vacuum instead of manifold vacuum. With the throttle blades almost totally closed at idle, there will be no vacuum to dist diaphram with ported vacuum.

 

Good advice here. Sounds like your engine needs a drink of water. The results will amaze you.

 

If the LED is on the fan side of the relay, no problem, the fan back emf may keep the LED lit for a while but it should be disconnected from the rest of the system by the open relay contacts.

If you use no relay you should put a diode in the + lead to the fan -- I understand the 1N5408 diode will work (cathode band toward fan) -- which will block the back EMF (generated voltage) from the spooling down fan.

 

I tried ported vac***e but the truck wont stay running but it seemed more peppy goin down the road, but then that brings me back to the timeing is too retarded because my timeing tab may be incorrect. First im gonna check my timeing tab, if that is incorrect then im gonna advance the dizzy until it runs good, starts good and shuts off good just as 31 vicky said. Ill let you guys know what I find tomorrow.

 

I think this thread will help people out that are having the same issue as me but im still not sure about the water trick. Somethin about pouring water down a engine raises a red flag for me but ill try it if all else fails.

 

[QUOTE="T'RANTULA";8336293]I think this thread will help people out that are having the same issue as me but im still not sure about the water trick. Somethin about pouring water down a engine raises a red flag for me but ill try it if all else fails. [/QUOTE]

That works (period)
But you have to do it in moderation and has been described in factory service manuals.

 

The only times I had this happen it was related to running 87 octane, never had an issue with the 93 available around here. What octane fuel are you guys running?

 

I'm with him on this!

Sometimes caused by a vacuum leak making the idle-speed increase.

 

[QUOTE="T'RANTULA";8336293]I think this thread will help people out that are having the same issue as me but im still not sure about thewater trick. Somethin about pouring water down a engine raises a red flag for me but ill try it if all else fails. [/QUOTE]

It will not hurt -- rev engine to a high idle and SLOWLY dribble water into the carb. I have done it myself. (It didn't help, because the engine had another issue ).

 

Im runnin 87 octane but never tried any higher octane because its just a work truck.

 

What was the other issue just for the record, just curious? Im here to learn all I can!

 

[QUOTE="T'RANTULA";8336201]The dieseling started happening after I replaced my timeing chain. It had a fiber metal gear and whoever before me installed it incorrectly. on TDC both dots pointed up so the cam gear was 180 out. So instead of fixing the timeing chain they flipped the dizzy 180 to compensate. I pulled the dizzy and set everything up according to my chiltons book. I thought I was a tooth off when it dieseled so I took it apart 6 times afterwards and still couldent find any screw up in re ***embly. Im no expert but ive done timeing chains in a few chevys and never had this problem. Could be pilot error. [/QUOTE]

This sounds like it needs more explanations or thoughts.

This is a 4 cycle engine and the piston comes into top dead center position twice.
The cam timing dots will/ should line up thru the center twice, the first is crank dot @ 12 o'clock and cam @ 6 . Second is crank @ 12 & cam @ 12.

The first position as noted is Tdc@ the end of the compression stroke and when the spark event is taking place. The second position as noted is at the end of the exhaust stroke and when the intake valve opens or the overlap .

What you've described here is Tdc on overlap. In this position a correctly installed distributor would be 180* from firing position.

 

[QUOTE="T'RANTULA";8336293]I think this thread will help people out that are having the same issue as me but im still not sure about the water trick. Somethin about pouring water down a engine raises a red flag for me but ill try it if all else fails. [/QUOTE]

In the 80's, I had a '64 El Camino running a 327 with 11:1 compression. The engine pinged alot under hard acceleration. Ran 91 octane. Installed a water injection unit and most of the pinging went away. The engine used about 1-1/2 quarts of water per 16 gallons of gas. Therefore, I'm not scared of introducing the benefits of water in an engine while running it.

 

Yes and then they flipped the dizzy 180 to compensate for that. I think. They also had walked the plug wires on the distributor and they had number one on the dist. in the back. It took me some thinkin on what the heck they did to make it run and I still aint sure!

 

[QUOTE="T'RANTULA";8336388]Yes and then they flipped the dizzy 180 to compensate for that. I think. They also had walked the plug wires on the distributor and they had number one on the dist. in the back. It took me some thinkin on what the heck they did to make it run and I still aint sure! [/QUOTE]


Technical reasoning- it doesn't matter where or how you install a distributor as long as ....

A. - you have piston properly at tdc for #1 at the end of compression stroke
B- your are correct on what cylinder is #1 in the block.
C- you put the #1 plug wire over the rotor
D- you follow the rotation and firing order.
..... ^^this is not 180* off.

However its much easier if you just install the distributor with rotor pointing @ #1 and follow the world wide standard .

180* off is when the rotor is @ 1 overlap tdc and fires on the overlap-it will not run. Backfire is the best you could hope for.

 

On TDC when I took it apart both dots pointed up in the 12 o clock position and the rotor ****on pointed to number 6 on the dizzy. Thats after the timeing had jumped 3 teeth.

 

Ok that clears up some things!

 

[QUOTE="T'RANTULA";8336422]On TDC when I took it apart both dots pointed up in the 12 o clock position and the rotor ****on pointed to number 6 on the dizzy. Thats after the timeing had jumped 3 teeth.[/QUOTE]

Like I said before -thats Tdc of overlap which is not Tdc of the firing event.
Should you have rotated the crank again 360* to Tdc the dots would have been crank 12/ cam 6 and rotor physically aiming at #1 cyl or drivers side front for SBC in US.

With some variance for the jumped teeth

 

Ok, so when I put the motor back together I turned it until the number 1 piston was all the way up, then put the cam gear on and aligned the dots in the middle, I turned the cam gear to align the dots. My question is did I screw this up?

 

[QUOTE="T'RANTULA";8336449]Ok, so when I put the motor back together I turned it until the number 1 piston was all the way up, then put the cam gear on and aligned the dots in the middle, I turned the cam gear to align the dots. My question is did I screw this up? [/QUOTE]

How did you determine piston was "all the way up" ? If you had heads off and could see it, then its OK. If you used questionable timing marks you need to check that.

Crank @12 and cam @ 6 then the distributor rotor physically pointing to drivers front then its all good if TDC was good.

 

I had the heads off and thats how I determined the piston was up. And the the rest just as you described.

 

The dieseling is prolly caused by hot carbon deposits in the combustion chamber caused by cheap gasoline. No fire to the plug, but the glowing hot carbon deposits will light it off a few times after you shut off the key...