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post #1 of (permalink) Old 12-20-2000, 04:41 PM
brendan
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reverse rotation dieseling?!

when my jeep runs on after i turn off the key, it sometimes is spinning backwards! does this mean anything different than a "normal" run-on? thanks a lot Brendan

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post #2 of (permalink) Old 12-21-2000, 07:57 AM
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Re: reverse rotation dieseling?!

"It's an Excellent day for an Exorcism"

post #3 of (permalink) Old 12-21-2000, 08:43 AM
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Re: reverse rotation dieseling?!

[img]/wwwthreads_images/icons/smile.gif[/img] Dear Dieseled in Maine: Your Jeep is just doing what hot engines with carbon chunks in the combustion chamber do best, and that is continue to ignite fuel after the ignition is shut off. Because the carbon is hot, the ignition point may ocur before the piston has reached and passed top dead center. In that case, the expanding gasses drive the piston back they way it came up and causes reverse rotation. Some of the waggie V8 engines had a throttle stop solenoid to close off the butterflies totally to prevent annoying run-on. By going backwards your engine has lots of opportunities to break rotors, dislodge timing chains, and run without oil pressure lubrication. Isn't THAT nice?[img]/wwwthreads_images/icons/crazy.gif[/img]

CJDave
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post #4 of (permalink) Old 12-21-2000, 09:16 AM
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Re: reverse rotation dieseling?!

Yabut you'd have 4 reverse gears. Yabut season over?

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post #5 of (permalink) Old 12-21-2000, 03:09 PM
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Re: reverse rotation dieseling?!

Thanks Dave. I usually ease the clutch out and stall it when it runs on, not just let it go. The thing runs about as smooth forwards as it does backwards! Brendan

post #6 of (permalink) Old 12-21-2000, 04:17 PM
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Re: reverse rotation dieseling?!

[img]/wwwthreads_images/icons/crazy.gif[/img] Brendan, you could do like a guy from Oaklahoma used to do.....start the engine and gently pour cold water into the carb throat. The cold water hit the hot carbon and knocked it loose. Of course chunks got under the valves and held THEM open sometimes, but it DID get a lot of carbon out of the engine. He had all kinds of auto "physics" that he would perform on used cars.[img]/wwwthreads_images/icons/crazy.gif[/img]

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post #7 of (permalink) Old 12-21-2000, 09:50 PM
 
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Re: reverse rotation dieseling?!

My Jeep used to do this a bit too. I ran a few tanks with a fuel-system cleaner added to it and gave the carb a rebuild and the problem went away. I also ran the snot out of then engine while mudding, which tends to burn off the carbon with the side-effect of blowing gaskets and overheating [img]/wwwthreads_images/icons/crazy.gif[/img]

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post #8 of (permalink) Old 12-21-2000, 11:07 PM
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Re: reverse rotation dieseling?!

Isn't there also a trick with putting a small amount of AFT in the same way to actually clean carbon? Or was that for something else?

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post #9 of (permalink) Old 12-22-2000, 01:38 PM
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Re: reverse rotation dieseling?!

That's a neat tip, but i don't think that i'll bother. It was more of an engine theory question. I have an sweet old 76 that's going to be rebuilt, probably w/ the 4.0 head. Thanks a lot though. Brendan

post #10 of (permalink) Old 12-25-2000, 08:15 PM
 
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Re: reverse rotation dieseling?!

his sounds like another one of those Urban Legends.....

Let me state categorically for the record,
1. I have personally worked on 10's of thousands of engines.
2. I have probably seen every ailment that can befall a gasoline engine.
3. I have worked on hundreds of engines that 'Ran On' after you turned the key off.
AND...
4. NOT ONCE, in the twenty five years I've been doing this for a living, have I EVER seen an engine actually run backwards.

I've hear about it from every tobacco juice dribbling old timer I've ever ran into,
And every 'Other Side Of The Tracks' "Mechanic" (I use the term loosely) has his favorite 'run backwards' story to tell, but I have never actually SEEN an engine turn backwards...

If your 'Mechanic' mentions 'Run Backwards', RUN LIKE HELL!!
Find someone that can diagnose the real problem, instead of trying to fix an imaginary problem, and doing more damage....
---------------------------------

Most likely what is happening is what is more correctly referred to as 'Dieseling'.

The engine CAN NOT run backwards.
Simple physics won't allow it.
Even if you could stop the engine, and crank it backwards in the split second after you turn the key off, which you can not, it would not fire.
No fuel supply in the exhaust tract to make it run.
The few unburned vapors in your exhaust do not supply the oxygen it would take for the combustion process.
--------------------------------

CJ Dave is correct in his description, except for one detail, the engine is actually running forward, or clockwise if you are standing in front of it looking at it.
The 'Rattle' or 'Knocking' you hear is similar to a diesel truck because essentially the same thing is happening in the chamber as a diesel engine.
----------------

Only one problem with that, Gasoline engines are dainty, fragile things compared to a diesel engine, and the pre-ignition or compression combustion that is happening is damaging your engine.

Then there are the other ways to 'Diesel' a gasoline engine...

Pre-Ignition.
The piston is not close enough to the top of the cylinder to have proper combustion in the chamber when an ignition source ignites the fuel mixture, and tries to force the piston back down the way it came, instead of letting it roll ahead over...
That is where the big 'KNOCK' comes from, the piston getting hit with a combustion 'Hammer'.
There is enough momentum in the rotating assembly to have the piston go ahead over, but the burn happened too early, and there is just enough force to get the crank to roll the next cylinder over.... Ect. Ect. Ect.....
KNOCK...KNOCK...KNOCK...

The source of the ignition could be anything from a piece of a broken ring or spark plug stuck in the head of a piston, or a spark plug of too hot of a heat range, or a metal burr from a degrading head gasket or bad machine job of the block or head, or just plain old carbon buildup raising the cylinder compression so high the fuel and air mixture compression combust or detonate, or ridges of soft suit in the chamber that still glow....
(Compression combustion isn't unusual the case in pre-ignition)

Then there are the other ways to 'Diesel' a gasoline engine...
This consists of the combustion being too late, about the time the piston reaches the top of the cylinder. This type of 'Dieseling' is very destructive. It's also called 'Detonation Run'....
The piston reaches Top Dead Center, and the cylinder is full of charge (Gasoline vapor and air),
Then something sets the mixture off...

1. Bleed through of electrical current to the ignition. Just because you turned the key off doesn't mean you have taken all of the power from the ignition. (this is an incomplete, or weak burn, and carbons the crap out of the combustion chamber and plugs up catalytic converters)
2. Red hot suit from a too rich fuel mixture. Lots of ways to have fuel mix too hot.
3. Red hot hard 'carbon' from too rich of a fuel mixture, or incombustible materials in the fuel.
This carbon can build up in the chamber and cause compression fire, see next line.
4. Too low of a flash point of cheap fuel. Simple compression can fire some gasolines.
That's why gasoline engines are about limited to 10:1 mechanical compression, and some diesels have compression ratings of 50:1 or more.
5. Too hot of heat range plugs. Plugs are glowing after you turn the engine off.
6. Debris glowing in the chamber, like broken rings, broken spark plug electrodes, ect.
Those things make nice ignition sources.
7. Bad machine work on the block or heads. Sharp edges will glow red hot, and become an ignition source (like the edge of a knife in a fire, the edge will glow first).
8. Cam timing problems.
Usually flat lobes not letting enough charge in the chamber for complete combustion, and flat lobes will allow charge to 'settle out', meaning that the fuel and air in the intake runner are not used, so they have time to separate, and then you get raw, liquid fuel turning into carbon in the chamber.
With the engines equipped with alloy cam chain gears, or nylon cam gear teeth, the timing chain can slip, or a timing chain with 100K on it, will allow the can timing to be off sufficiently to cause problems with combustion...
--------------------------------

Remedies for the above described problems...

Get your self a windshield washer tank, one of the kind that goes Squirt, Squirt, Squirt....
Mount a nozzle on the air cleaner stem, pointing down the carb throat.
While you are at HIGHWAY SPEED (not idle, like most people say) push the button and add water to the combustion process!
The live steam will scour the inside of your combustion chambers, although it may plug up your catalytic converter and muffler with pieces of hard carbon.
This way is cheap, easy to do, and VERY effective...
Don't stand on the button, just squirt a few times, go down the road a little way, and squirt again.
Do this over three or four days time, or until black water quits running out of the exhaust.
(Black water, not the normal condensation moisture that leaks from cat equipped vehicles)

Moving down the road, at part throttle, the engine and combustion chambers are hot enough to make good use of the water, turning it to steam.
The windshield washer sprayer will keep you from adding too much water and 'hydraulic locking' things up (lots of bent parts when you do that!).
The higher RPM will keep good flow through the engine, and whisk away most of the dislodged carbon and residue from gasoline.

The people behind you won't appreciate the crap that comes out your tailpipe!!
Good remedy for tailgaters!!

Some of the crap will find it's way into your oil (carbon, deposits, acids and moisture), so be sure and change your oil at the end of the week you clean things out.
-----------------------------------

NEVER, EVER USE ANYKIND OF OIL TO 'CLEAN' OUT YOUR CHAMBERS!!
This includes brake fluid, automatic transmission fluid, ect.
All you will do is add more hard carbon, and plug up the exhaust system, causing MORE problems...
Your exhaust system was designed to handle VAPOR.
Not oil, solids or anything like them....
DON'T LISTEN TO ANYONE THAT HAS THE "MAGIC" TRICK TO CLEAN SOMETHING LIKE THIS!!
Also take into consideration that oil does not compress, and the vapor point is much higher than water.
Can you say, BENT RODS? I knew you could.....
----------------------------------

As far as foreign objects stuck in the top of the piston, or bad machine work, there is nothing you can do, short of re-machining the block and replacing pistons.
----------------------------------

Cheap gas, or crap in the gas should be easy to solve, but I'm going to state the obvious...
BY BETTER GAS.

The same with too hot of heat range spark plugs, GET COOLER PLUGS.
----------------------------------

Cam timing problems can cause this problem...
Change your cam, lifters, chain and gears, and check all the rockers, pushrods, valve stems, springs.
No excuse for polluting the atmosphere (MY BREATHING AIR !!) and using WAY too much fuel when all of this is pretty easy to check with a compression check, and a leak down test.
Takes maybe an hour and $100 worth of tools...
---------------------------------

Well, I've spent an hour writing something that no one is going to read....

"I Have The Body Of A God... Buddha"
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