71 302/2V "dieseling"

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Justin71

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All,

We (myself and my son) recently bought a nice driver 71 coupe. 302/2V, C4, pwr brakes/steering. The machine is in good condition, we drive a little bit most days, never too much or to far.

We have seen on many occasions that after turning off the key, the engine will "diesel" a bit (what I learned to be "continuing to run after the switch is off"). No big deal, but probably shouldn't be happening.

I'm in search of the potential causes, hints of "this might be happening" or other reasons this should be traced down and dealt with.

So any help with this? Concern? or annoyance that just doesn't matter?

One other item I have found is that recently I had a heater hose blowout. The water spray from that killed my engine during the event, and after replacing the heater hose, I found some spark jumping (improperly) from the coil wire (just on top of the engine next to carb) to ground on the heater hose clamp (causing some significant stuttering of the engine when it occurred - not often..). I've readjusted the clamp to distance it from the coil wire, but too soon to tell if that has solved the issue or not. I only mention it in case this spark jumping could be related to engine off "dieseling".

Whats your take? What can I do to isolate, and how big of a deal?

Jay

 
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Dieseling is usually one of two things, Either the ignition is not turning off completely and the coil still is getting voltage, but a reduced amount and the car continues to run, albeit badly. This is rare and what you should hope for, as it can be isolated with a multimeter and repaired with some wiring. Unfortunately odds are heavily in favor of the second possibility and the most common cause of dieseling.

The common cause is excessive heat in the engine. Hence the name, the engine begins to operate without spark, based upon heat and compression and generates a self sustaining cycle that unless you cut off fuel or air will continue. Since your car has a mechanical fuel pump and a carburetor, once it starts, only cooling down to where the fuel will not ignite without a spark will stop the cycle if you can't cut off the air supply or the fuel supply.

It is a huge deal and needs to be fixed immediately. Dieseling allows the mixture to ignite without the benefit of spark timing and can destroy an engine.

 
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Justin71

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Interesting. I guess I would add that it only does this dieseling for a second or two after cutting the engine off, so I have not been overly concerned. Also, as I mentioned I have had cooling system issues (old rubber in the radiator/heater hoses) I have replaced now the heater hose and both radiator hoses, and since that time - both the dieseling and the erroneous spark - have disappeared.

So, the big concern then is excessive heat. As this is a new vehicle - I have not verified the continuity and function of the temp sensor, but can tell you that it has not gone off and other gauges are functional, so I have not been overly concerned to this point.

It could be that the cooling system was allowing pressure to escape (leaky hoses), and water temps to be too high. I'll have to keep an ear out for any more dieseling though sounds like. I didn't know this rather innocuous-seeming little issue was a huge deal, so thanks for the feedback.

Jay



back to basics... first tell us the engine timing. Eg 8 degrees advanced?
Unknown at this time. Engine was in good running order when I got it, and I have not made any measurements.

 
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marks73

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Dieseling is usually one of two things, Either the ignition is not turning off completely and the coil still is getting voltage, but a reduced amount and the car continues to run, albeit badly. This is rare and what you should hope for, as it can be isolated with a multimeter and repaired with some wiring. Unfortunately odds are heavily in favor of the second possibility and the most common cause of dieseling.

The common cause is excessive heat in the engine. Hence the name, the engine begins to operate without spark, based upon heat and compression and generates a self sustaining cycle that unless you cut off fuel or air will continue. Since your car has a mechanical fuel pump and a carburetor, once it starts, only cooling down to where the fuel will not ignite without a spark will stop the cycle if you can't cut off the air supply or the fuel supply.

It is a huge deal and needs to be fixed immediately. Dieseling allows the mixture to ignite without the benefit of spark timing and can destroy an engine.
+1 to that. Get a temperature gauge installed to verify operating temperature. Since you also blew out a hose you may want to pressure check your radiator cap as well.

 

Totalled

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Overly rich mixture and too high of an idle will also contribute. In the mean time, shut it off in gear, then put it into park. The added load and lower idle speed will make it harder to diesel.

Carbon buildup can act as a glow plug, and if you have a rich mixture, the remaining fuel in the exhaust system can allow it to run backwards for a bit. Not a good thing.

 
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If you motor is basically stock you may have an anti-diesel solinoid attached to the carb. It's a little plunger thing that many people wrongly believe is used to increase the RPM when the AC is on. Actually what it does is push the idle speed up when the ignition is on. When you turn the car off, it allows the throttle plate to close a little more to help insure there isn't enough air to allow the dieseling to happen.

http://www.misterfixit.com/diesling.htm

I am not sure if your year/model originally came with one. ALso, if I remember correctly, it happens because the idle mixture is lean, not rich.

And welcome to the forum!

 
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MeZapU

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I've seen this from a few cars and in every instance was cured by performing a basic tune-up. Plugs, wires, cap+rotor, points (if required), verifying timing and idle mixture. Fresh fluids never hurt either, especially on a new car purchase.

And yes, being too lean causes detonation/pre-ignition/dieseling.

 
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turtle5353

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I have had a bad intake gasket cause my old f150 to diesel. Just another idea.

 

72HCODE

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engine timing advanced too far.

Carb throttle butterfly open too far at idle.

Spark plug heat range too high.

Carbon build up inside engine.

gas octane too low.

Carb idle mixture too rich.

vacuum leak.

bad ignition switch.

condition of the radiator/cooling system

basically it can be more then one thing.

start by checking the spark plugs condition and where the initial timing is set on the motor. Also is it doing it when the motor is cold or only after driving it for a while and engine warmed up.

 

Justin71

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If you motor is basically stock you may have an anti-diesel solinoid attached to the carb. It's a little plunger thing that many people wrongly believe is used to increase the RPM when the AC is on. Actually what it does is push the idle speed up when the ignition is on. When you turn the car off, it allows the throttle plate to close a little more to help insure there isn't enough air to allow the dieseling to happen.

http://www.misterfixit.com/diesling.htm

I am not sure if your year/model originally came with one. ALso, if I remember correctly, it happens because the idle mixture is lean, not rich.

And welcome to the forum!
Will E-

I am interested in checking the anit-dieseling solenoid if I indeed have one. the PO replaced the orig carb with a Holley. I've attached pics from both sides. Maybe you can help me identify the part?

_MG_0743.jpg

_MG_0742.jpg

 
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You don't have one. I will see if I can find a picture of one installed. Basically it's a magnetic plunger. WHen engergized the plunger pushes against the throttle, just a little.

 
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