What causes the engine to keep on going after turning off the key?

Is it strictly a timing issue? Or is it related to carb/choke tuning?

My engine seems only to do it when the truck is on "fast idle".
It sounds very harsh when it does this (pinging/very noisy) for the 5 seconds that it is doing it. Is this harmful to the engine? (It doesn't sound healthy)

Matt

1978 F250 Ranger 4x4 - 351M

 

Oft times this is due to an overly advanced ignition. Try retarding it slightly and see if that helps.

If your knuckles ain't bleeding you did something wrong.

'72 F-250 "Hi-Boy" 4x4, Dana 60/HD44, FE390 @ 400hp(purt near!), 4-speed, custom suspension w 4" lift, mud on black.

 

You are talking about dieseling. The main cause of this problem is the build up of carbon in the combustion chamber. This carbon holds enough heat to continue igniting gas. This is some times caused buy running cheap gas. Run some kind of carbon gas treatment. Chemtool works well



http://www.angelfire.com/ok/jwtimmerman



Jw

 

The buldup of carbon shouldn't occur on a brand new engine should it?
It has less than 700 miles on it!

Can't afford anything but cheap gas!

1978 F250 Ranger 4x4 - 351M

 

It is related to carb. choke tuning. That is why you never here of it on fuel injected vehicles. The fuel supply is shut off with the key, carbs still have fuel available. You need to find out why the choke is staying on so long (keeping it on a step of the fast idle cam) or lower base idle speed if it is happening when the fast idle cam is all the way open.


2000 XLT Flare Supercab Sport 5.4-Auto-355LS-Tow pkg-Bedrug-Xtang Tonneau

 

Matt, There are a couple of things that cause this more than others. Timing advanced too much, and idle set too high.

John
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66F100s Rule
In the cool still quite of night you can hear chevies rusting away.

 

I had a car that used to do this and it was all because of the gas. try using 92 octane. Mine quit when i started using it.


Jason
77 F100

 

Matt:

Check to see if you have an elecrtic operated solenoid mounted where the idle adjustment screw contacts the stepped lever on the throttle shaft. It has one wire connected to it.

It operates this way; When key is on, solenoid energizes center plunger rod to engage (abut) stepped lever so idle speed stays at 600-700 rpm or wherever it's supposed to be.

The solenoid has 2 locknuts on the threaded shaft, allowing adjustment of idle speed.

When key is turned off, solenoid plunger retracts so throttle butterfly closes completely, shutting off fuel to engine. If this isn't adjusted properly, engine will diesel at shut-off.


94 E-150 High Top Conversion Van 4.9/C-6
89 Mustang LX 5.0 - 5 speed

 

Thanks for all the good advice boys!

I will make sure that this is fixed asap.
I am not willing to switch to 92 octane from 87 for any extended period of time, 92 is way too expensive and the truck ran perfect before the rebuild on 87, with not enough changed to affect the octane needed.

Matt
1978 F250 Ranger 4x4 - 351M

 

Nito94 & 94Van are both correct on this one. Get your choke working properly first. You shouldn't need much, if any, choke after about 5min of running. You also may not need as high of a cold (fast) idle that you have now. This can be adjusted independantly of the warm idle ajdustment. The solenoid that 94Van described is the most effective way to cure the problem if it continues. If wou don't have one, you may be able to build a bracket at the carb to make one work. They started putting them on some emmision controled vehicles and some vehicles with A.C. to bump the throttle up from the higher load of these devices.

Ignition timing has little or no effect on this since there is no electrical ignition taking place if the key is off. Decreasing advance on the ignition will lower your idle RPM thus reducing the chance of dieseling, but you would be better of to change the idle at the carb and acconplish the same effect without lowering normal running performance. Unless you bumped your compression way up on the rebuild and you are getting pinging, then you shouldn't need 92 Octane gas. It might help make the dieseling stop, but the additional cost of this gas will quickly pay for a solenoid. Until then, try shutting the engine off while slightly applying the clutch, or shut it off while still in gear if it is an auto. The extra load and lower RPM may be enough to keep it from dieseling.

 

If you cant afford to change octanes try switching brands, that can occasionally make a big difference

 

Ok... I have had this problem many times on my truck.. And 9 times out 10 its just bad fuel... I now religiously use Union 76 gas. And now I have no problems. You may want to buy some kind of gas treatment.. Has always helped in those times my truck was doing it. My 2 cents...

 

when I've had this problem, it's turned out to be a rich mixture/too high idle speed problem. Try turning down the idle speed and check out your mixture.
Should fix the problem.

Greg
1967 F100 CCS 390 cruise-o

 

your idle is too fast. my brother set the idle up on his truck and it diesled some he set it back and it was fixed.

 

my problem was the air/fuel mixture too rich, back the adj screws two full turns as a base...

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77'FORD F150 XLT 400