M38A1 sometimes wants to continue running after shutdown.

[Chris]

Hi folks,

I was wondering what I can check to find out why my 55 M38A1 wants to continue running for a few seconds after shuttdown. Its not always, just once in a while, and its usually after I forget that it does it...surprises me again...
Now, I've had some people say it may be timing, but if the engine ignition is off, would there be any timing issue? It also starts fine and runs like a raped ape...
Someone else said it might be bad gas, but its the same gas I stick into the 51 Cj3A and it runs fine.

Any experienced mechanics want to give me some advice, I would greatly appreciate it.

Thanks and have a good day!

Chris

[Zane]

your timing is off , adjust it and it will stop

[wesk]

Check your timing for 5 BTDC on #1. If it is not off significantly then check for a pre-ignition or burned cylinder component issue. What you are experiencing is run-on or some folks call it after firing. The timing is the usual culprit but if it is an intermittent issue it could be any source of ignition in a cylinder that is still hot after shut down like a red hot plug electrode or the burned edge of a valve or a very heavy carbon buildup on the tops of the piston. Most of these should also make their presence known with a slight pinging when accelerating under load.

The important thing here is to keep an open mind and problems like these do not always have one cut and dried solution.

[Chris]

Thanks Wes,

I appreciate it.

Three more questions?

Would it start and run fine if the timing was off?
How would I take care of the carbon build up?
Would replacing the plugs take care of the "red hot plug electrode" ?

I'm 99 percent sure we got the timing right this past summer, so I guess I'm just looking for other ideas...

Thanks again for the help, it is appreciated!

Chris

[wesk]

1-Would it start and run fine if the timing was off?
2-How would I take care of the carbon build up?
3-Would replacing the plugs take care of the "red hot plug electrode" ?

1-Yes but timing settings are a compromise from the engineers so you don't work the starter too hard, you don't run the engine temps to high and you don't get poor performance or pre-ignition and other problem. Running too far advanced often gives you great acceleration and cruise performance but can cause the other issues.

2-There are a lot of old tricks and ideas but pulling the head and cleaning is the real answer.

3-Depends on why the plug tip is red hot. Cold be a plug defect or it could be a overly lean cylinder from a valve issue or induction system leak.

This brings you to another test you should perform which is a compression check and if results are questionably follow up with a leak down test.

[Chris]

Thanks Wes, I'll get on the compression test first thing.

Very very much appreciated!

Chris

[rdsar2k]

I can't wrap my brain around the theory that timing would cause this issue. If the ignition is turned off there is no spark. In my mind timing is now out of the equation. The only thing I can think of that would agree with the timing being the issue is that there is a higher residual heat in the head area that is caused by the timing being off.
Is that the theory?

[wesk]

Is that the theory?

Yes and not just theory. Old time fact.

If anything, including timing, has left red hot glowing metal in the cylinder it can cause the engine to run on or Diesel. The high cylinder temps heat the small metal edges red hot and they behave just like a glow plug.

If an intake valve is not closing all the way the remaining flame in the exhaust manifold can ignite that cylinders fuel charge.

An overly rich carb can dump enough raw gas into the intake to allow an unusually hot cylinder to fire for several more cycles.

It pays to troubleshoot in an orderly fashion and cover all the bases before buying parts or changing parts.

[Chris]

So its not the actual timing, its the effect that it has on the engine, if not timed correctly, causing the actual "dieseling" by overheating the engine.

That makes sense to me finally too. I had a hard time with that, as there should have been no timing as the engine was turned off...Okay, I finally have it clear in my head...I think.

Or perhaps it could be my rebuilt carb is running too rich too?

So many variables, like Wes states, I'm going to do a checklist in an orderly fashion....

Step 1. Have to go to Napa and get a compression tester... today.


Thanks again Wes and all, its all starting to sink into my mushy brain.

[wesk]

If you have not done a compression check on anything before try to remember:

1- The battery should be fully charged.
2- The choke and throttle plate must be held wide open.
3- The engine should be warmed up to normal operating temps.
4- All 4 spark plugs should be removed.
5- Only crank until the pressure on the gage quits rising.

If you have a single low reading cylinder whose adjacent cylinder(s) are ok then squirt some 30 weight engine oil in the spark plug hole and check that one again. If it increases significantly the problem is usually worn or broken rings. If there is no or very little increase then the problem is usually a leaking valve. If two cylinders adjacent to each other are low together that usually indicates a head gasket leak between them.

If you do detect problems the next step is to do a leak down test. In this test you apply air pressure to one cylinder at a time and look for the location of the air loss. For example a blown head gasket between cylinders will reveal itself when you apply air pressure to one of the cylinders you will hear and feel that air escaping from the adjacent cylinder's spark plug hole. There are high dollar leak down test sets out there or you can simply use a threaded spark plug hole adapter and a gage and a regulator to regulate the air pressure in to 80 PSI. Use caution because this air pressure will rotate the crank if the piston is any where other than TDC or BDC.

[Oldsalt]

Is you idle speed set above normal? Setting the idle speed down used to be the the way we stopped dieseling in cars in the 80s.