16 replies to this topic

[mini_newboy]

Hey all,

After a journey over 5 minutes, the engine wants to carry on running when you turn it off, anything under 5 minutes - it's fine. What's causing this & how can I stop it?

Chris

[tommy13]

This problem is usually called "Running on" or "Dieseling". A description of the common causes is here,
http://en.wikipedia.org/wiki/Dieseling
Check your idle speed is not too high as this is one of the common causes.
Sometimes the only cure is to fit an electric solenoid which allows air to be drawn into the inlet manifold when the ignition is switched off, rather than a fuel air mix being drawn in through the carb.

Edited by tommy13, 26 March 2011 - 06:06 PM.

[huw_jenks]

Could be due to carbon build up in the combustion chamber. I've got an MG Metro (which are notorious for running on) and even with an anti run on valve fitted it would still keep dieseling! However, since removing the head and cleaning out all the carbon it hasn't run on once!

If you can't be bothered with taking the head off, which is a bit of a faff, then I can recommend this stuff: Ecotek

[bmcecosse]

Some just run some water through the fast-idling (and hot) engine. One way to do this is to use the little vacuum tube that connects to the dizzy. Pull it off the dizzy and dip the end in a jar of water.

[chazzybaby]

will this not kill the engine? sounds dangerous to me. can you explain your thinking here?

[pdaykin]

I would guess that your timing is out.

Does it only happen with the choke out ? I've had the vacuum advance unit go and cause the car to run on.

[Cooperman]

It's always been a problem, especially with high comp-ratio 1275 engines.
I just put the car in 1st gear, switch off and immediately slip the clutch up which stalls it as it tries to run-on.
It used to be the practice to pour 'Redex' upper cylinder lubricant oil into the carb mouth with the engine running at around 4000+ revs to clear the 'muck' out of the combustion chambers.
The idea of adding a small quantity of water is excellent as it will turn into pressurised steam which will clean the combustion chamber surfaces and piston top as well. In fact, water injection is an accepted way of increasing power, but that is another whole subject.

[maggies_minder]

mine runs on and the chap at the rollers reckons its need "decoking" i dont fancy taking the head off (knowing my lucky it'll never go back on).

with the water method how much should be used??

[dklawson]

As Cooperman and bmcecosse said, water is easy and no it is not dangerous. You are not pouring water directly in.

One of the more common manual methods is to remove the air cleaner, bring the engine to operating temperature, then raise the idle RPM to about 3000. Then use a hand pump-sprayer (like for cleaning solvents) and spray water mist into the intake as fast as possible. You only need to slow down when you hear the RPM start to fall off quickly. Keep doing this as long as your hand can do it. Eventually the car will belch a cloud of heavy smoke out of the exhaust which indicates the bulk of the carbon has been dislodged.

My work around for run-on is what Cooperman described, stalling the engine. The only thing I do differently is to select 4th gear so the engine delivers less torque to the driving wheels. For this method to work you have to get in the habit of stopping your engine by stalling EVERY time you switch it off. While it will work to stop an engine that is running-on, you don't want it to start running on in the first place. Be proactive, not reactive.

[mini_newboy]

So it need to clear the carbon out of the system. Which way is easier water into the filter or thro the vacuum fees?

[dklawson]

Easier is a relative term.

First, you would not be putting water through the filter. The filter would be off and you would be spraying water into the carb throat at elevated RPM. Its advantage is that it is quickly done and the job is over.

With the method Roy recommended, you disconnect the vacuum hose at the dizzy, put it in a container of water (making sure the hose and the water are as "high as possible" but just below the carb) and you drive for as long as you want/need. The process will be slower (days, not minutes) but equally effective. The water needs to be kept "high" so the carb vacuum doesn't need to lift the water WAY up get sucked into the intake.

Again, both methods are effective. It's really just a question of available time.

[starletman]

So it need to clear the carbon out of the system. Which way is easier water into the filter or thro the vacuum fees?

Isnt your engine newly built,therfore its not likely to be carbon build up.Mine does the same,i will be fitting an anti run on valve shortly to see if it improves it.

[MRA]

It could be poor tuning, or sharp edges within the chamber area.

Have you used the head or is it a new head as well ? what about tuning has it been rolling roaded yet ??

[chazzybaby]

i thought run-on was more commonly seen when the fuel mixture was excessively rich?

[dklawson]

The mixture by itself is not enough to cause a problem without there being an ignition source. During run-on the ignition source is a hot spot in the combustion chamber (valve heads, carbon deposits, sharp edges, overheated plugs, etc).