what is the cause of this problem.my brother is driving my '84 across country and this problem keeps popping up.is it a timing issue or some other hidden problem.has anyone else experienced this problem.any help on this would be great .

By dieseling, do you mean "pinging" while the engine is under load, or do you mean the engine keeps on running after the ignition is cut off?

Both conditions are contributed by low octane gas. Pinging is a bit more serious as it occurrs under load and can damage your engine.

Dieseling" or "run-on" as it is also called, is due in part also to carbon build-up in the combustion chamber. This carbon remains hot enough to act as a "spark plug" so the engine continues to 'run' after the ignition is cut off. Carbuerated engines are more susceptible to this since fuel injected engines shut off the fuel to the engine as well as spark when turning the engine off.

There are a couple of methods to keeping your cylinder top clear of carbon, including specific additives (GM makes one for sure) that are something akin to 'tune up in a can'.It is poured into the carb until the engine chokes on it, then the engine is allowed to sit for a bit before restarting. Some home mechanic types use water poured slowly into the carburator of a fully warmed engine too *although I am not too keen on this* BOTH are temporary measures.

A proprietary water injection systems would probably benefit this condition, but there is a level of maintenance and complexity to installing them.

Making sure your carburator is working properly in the first place is a good start. An overly rich carb is going to create a lot of carbon build up. A mechanic can help get your carb tuned to where it is not too rich or too lean, as well as getting your sparkplug into the right range. You might need a hotter spark. BTW make sure your ignition system is up to par, with wires, plugs, distributor etc all working. A missing cylinder wastes gas, loses power and contributes to 'run on' since the unburned gas creates...carbon build up.

In muscle cars, taking the car out on the highway and "blowing it out' by rapid accelaration and high top speed used to work wonders for the engine, as long as you weren't caught. Not really practical for a motorhome though..

Using a higher octane of gas is important too, or use an octane booster as sold by auto supply houses.

The quick fix? Have your relative fill up with a higher grade of gas and add a can of octane boost until you can have it examined by a good mechanic..

Ed,

If carbon build-up is indeed the cause, and at the risk of a yuk-yuk or two from other members , I'd recommend trying a quart of good ole Marvel Mystery Oil in the tank...Give it 500-800 miles or so and it may just do the job.....Worst case, it can't hurt, and you've added $3.50 to the economy......

I use it religiously every other fill-up....from what could be seen during a recent head gasket R&R, it sure looked clean in there for over 80k miles......

The old practice of running the engine at a fast idle for a few moments before shutting down after a hard run, often does wonders to prevent dieseling.

You can do a pretty good imitation of the "blowing out" operation with a few hard accelerations in a motor home.

Back in the days of leaded gas when engine deposits were really carbon, & not some other hard chemical compound, water injection or pouring water through the carb at fast idle worked well. I haven't had much success with either on modern gas, which, by the way, tends to leave engines a lot cleaner than leaded gadoline did.

Try the fast idle for a few moments before shut-off to let the engine cool down a little, before you do any major surgery.

I would agree with everything posted.

I believe the technical term is "run-on", when an engine continues to run, albeit roughly, after ignition is shut off.

In addition to the previous, all RVs should have a cool-down at normal idle before shutdown. This allows the engine to cool and will cut down on the dreaded heat soak and its attendant damage. Many Chevy RVs require this to assure starting again, as the starter solenoid is very fussy about heat.

After a cool down, put the trans in reverse before turning off the ignition. You can experiment with exactly when you turn off the ignition. Often, doing it just as the chassis jerks when shifted into reverse.

BTW, a new or freshly-rebuilt engine can diesel, too.

Warning: Techno-babble follows.

Many factory air-conditioned GM cars had a fast idle solenoid that raised the throttle arm off the idle stop when the air was energized, to avoid killing at idle. On of these solenoids can be installed on the motor home and wired to the ignition. The normal idle screw should then be backed way down, so the the butterflies close fully. Then the solenoid is adjusted by turning the plunger to provide a normal idle. When all set up, ignition power to the solenoid holds the throttles at idle, and the throttle goes full closed when the ignition is turned off.

If you have only an electric fuel pump, like me, just turn it off and let the carb run dry, like an M48 tank.

quote:

If you have only an electric fuel pump, like me, just turn it off and let the carb run dry, like an M48 tank.

OK Bill, I'm really curious - why is THAT?

quote:

Originally posted by Lee:

OK Bill, I'm really curious - why is THAT?

I have a 502 in my Barth. It does not have a mount for a mechanical pump. It works fine on the original electric pump in back. Because of my hot weather hill towing, all of my tow vehicles/RVs have had electric pumps anyway, for vapor lock prevention. With an electric pump, no need for a mechanical pump. Mechanical pumps don't suck very well on long runs, so many builders added them to longer coaches.

If my electric pump ever fails on the road, I carry enough fuel line to change over to use the genset electric pump to feed the front motor without moving it.

My ski boat also has a 502 with electric pump only, and previous boat had a 454 with same Carter pump as Barth installed on my RV.

Indeed, the correct technical term is "run-on".

With engines using unleaded fuels the deposit buildup is usually insufficent to contribute to run-on; further, it's doubtful that your carb has mixture adjustments. Almost invariably, the problem is incorrect idle adjustment.