Any cures for dieseling/run-on?

[MCfan]

One annoying characteristic of my Monte's BB is that it often experiences "dieseling" or "run-on" for a few seconds after I kill the ignition. It is especially predictable and lengthy when the engine is "hot" from higher speed operation.

 

I have always associated this with the higher-than-stock compression ratio of my engine (11:1 or more). Years ago, my brother-in-law had a '62 Impala with a 409 and it did exactly the same thing. It was bone stock but also had higher compression. I have never suspected other factors or modifications like ignition timing, cam profile/lifters or carb but I may be naive.

 

I have always understood that "dieseling" occurs when the more highly compressed (hence hotter) air/fuel mixture is partially ignited by the temperatures in the combustion chamber even though the spark plugs are not arcing (i.e. when the ignition is off).

 

Today, I stopped by my local salvage guy's place to pick up a rally wheel for my spare and he heard the engine diesel when I shut it down. He immediately commented, "Your timing is off!" Well, that may be - but it's news to me! This guy lives and breathes cars so I had to at least consider his opinion/diagnosis. Of course, he didn't volunteer HOW the timing was off and I didn't ask (primarily because the engine currently idles well, accelerates strongly and does not ping).

 

I spent several weeks after I first got the car messing with my timing settings, vacuum advance on/off, switching centrifugal advance springs, etc. In the many test drives I took with all types of timing settings, the engine ALWAYS dieseled when I shut it down after an energetic run so it is difficult for me to accept that timing (at least in a normal operating range) is the cause or cure of dieseling.

 

So what do you engine tuning pros think? Is dieseling something I just live with due to the compression ratio of my engine or is there some timing related cause or solution? BTW, it is not enough of an annoyance or embarrassment to cause me to run timing that adversely affects street or cruising performance. Any ideas or suggestions? Many thanks!

[Winston Wolf]

11:1 is nothing. I agree with the salvage guy. Back the timing off (or make sure it's set correctly) and make sure the idle is at the correct speed. Simple as that.

[Winston Wolf]

and just because it idles find, runs fine, and is ping free doesn't mean you have the timing set correctly or optimally. The higher the timing, the better it will idle. Get a timing light on it.

[Sam (Bones)]

 

 

too high of idle speed can cause and/or compound the problem if the timing is off

 

also if your engine is 11:1 compression, and you're running just pump gas, dump a few gallons of race fuel in it and see if it helps

[wallaby]

They do tend to do that.

Most of the time the problem is the throttle position is open too wide. It would be simpler to say the idle is too high, but your engine idle speed may be correct. Chevrolet had a fix for it with the original idle solenoid...it would drop out of the way when you shut off the key and close the throttle plates in the carb.

Focus on seeing if you can get the car to run with a slower idle speed. You can back out the idle speed adjustment to make it slower, then adjust your timing and/or idle mixture screws a bit to compensate. You can do this in stages and be surprised at how far you can back off the idle speed screw and still retain a decent idle. The goal is to get a smaller throttle opening at idle, and still have the engine happily running.

 

Or you can do what I do: shut off the engine while it's still in gear.

[Canuck]

My 72 350 engine does the same thing. I shut it off in gear to avoid the embarassing "kick, kick, kick, ghasp" LOL. It did it before I tuned the carb/timing and it still does it after several adjustments.

[MCfan]

Thanks for the quick replies and suggestions, guys.

 

Sorry, I should have given the current timing specs instead of just referring to the end results. I have and use a new Equus 5568 timing light and I know my current timing specs (which doesn't necessarily make them "correct", of course):

 

Idles evenly at 800 rpm (after fast idle cam drops out)

Static advance (low idle, vacuum advance blocked): 4 degree BTDC

Vacuum advance: adds 17-19 degrees at idle

Mechanical advance: adds 32 degrees, all in at 2600 rpm

 

Again, just knowing the ignition timing settings does not make them "correct" but, whatever they are, they only have a DIRECT affect on engine operation when the ignition is ON, right?

 

So, what is the INDIRECT or secondary effect of ignition timing that causes dieseling when the ignition is OFF???

 

I can see that ignition timing probably has a big effect on combustion chamber temperature which surely contributes to dieseling at shut down. Since the engine is always idling just before shut down, perhaps timing at idle is the likely contributor. If so, my total advance at idle (static plus vacuum) is currently only 21 - 23 degrees. That doesn't seem excessive to me, but maybe it is for this engine.

 

On the other hand, the dieseling is alway "worse" (lasts longer) when the engine had been working harder/running faster so maybe it's the timing curve that's contributing to higher combustion chamber temperatures just before shut down. With my 3.31 rear gear and 28" tires, I get exactly 2.5 mph per 100 rpm so 65 mph is 2600 rpm and 70 is 2800 rpm, either of which should have the mechanical all in for a total of 36 degrees. Again, that doesn't sound excessive, but it may be for this engine.

 

Sam, when you say 11:1 compression ratio is "nothing", do you mean it should not be a factor or contributor to dieseling?

 

With no spark in the comubstion chamber, operating heat is the only other cause of combustion I can think of. The more compression, the hotter the compressed gas, of course (from Boyle's Law if I remember my physics correctly). I suspect that the combination of higher compression AND higher than normal/desired operational temperatures in the combustion chamber create the environment conducive to dieseling.

 

Unlike electronic fuel injection, cutting off the fuel supply at shut down is not possible with the idle circuit still available mechanically. This engine simply does not idle well under 800 rpm but there may be something I could do with the idle circuit on the Edlebrock 1806 - need to read the manual.

 

An excessively high operating temperature could be a factor but I'm running a brand new aluminum radiator, a 160 degree thermostat and a fixed factory fan blade driven off a 7 3/4" diameter crank pulley. My water temperature gauge confirms operating temperatures well below 200 degrees but maybe it was installed or calibrated incorrectly.

 

There may be a simple fix, but if not, I'm just going to live with it. At least half the time it occurs in my driveway or garage anyway.

[MCfan]

Thanks, Mark, I hadn't seen your reply yet. I will focus on adjusting the idle circuit as you suggested.

 

I always shift into reverse before killing the engine because it's the only way to remove the key. So when I tried releasing the clutch to stop the dieseling the other night (with my right foot firmly on the brake, of course), the controlled lurch scared the $#%@ out of my wife - not a wise move under the conditions.

[kc8oye]

ok, first things first.. you may have some carbon buildup in the engine.. i had a 305 (smallblock) that used to drive me crazy with this too. this sounds odd, but it works

 

with the engine warm and idling, slowly pour a can of "Sea Foam" down the throat of the carb.. pour it slowly, you don't want to kill the engine right away, (You'll starve it of air before it hydrolocks if you pour slowly) once you get a good amount in there.. pour it just fast enough that it does stall.. let it sit for 15 minutes.. then fire it back up.. it will be stubborn, and it will smoke like you've never seen smoke before.. but it does a great job. I've heard you can do the same with water.. but the sea foam is good stuff.

 

I'm not familliar with BBC's, but 4° sounds rather tame. I would suggest maybe adding a couple of degrees timing, which will let you close the idle speed screw down a bit more. a lot of times, run-on is caused by too much throttle opening, more then it is any thing else.

 

also try adding an 1/8 or 1/4 turn to your idle mixture screws towards the rich side.. lean idle can cause run on too.

[MCfan]

Hey, Tim, thanks for your suggestions.

 

Color me out of touch, but I'd never heard of "Sea Foam" until I read your reply! I just read about it on their website - sounds a bit like "snake oil" but, as you say, it apparently works great! (If not, they sure have fooled a lot of people over the past 65 years!)

 

I haven't solved this problem yet but I'm already learning a lot in the process. I can see this is a classic case of "barking up the wrong tree". For run-on to occur, there has to be combustion, however incomplete it may be. Even for partial combustion to occur, there has to be air, fuel and an ignition source. Without electric spark, the ignition source has to be combustion chamber heat. BUT, the heat won't matter IF there is no or insufficient fuel in the air to burn.

 

So, instead of focusing on eliminating heat as the ignition source, I need to focus on reducing or eliminating the fuel entering the combustion chamber after the ignition is killed and the engine is still turning from rotational intetia.

 

I appreciate the relationship between static timing and idle speed you suggested. I have always thought 4 degrees was low but that is what my GM service manual specified and I had no expereince or other specifications to the contrary. Early on, I had erroneously set the static advance between 8 and 10 degrees and I was getting a lot of pinging on acceleration. But that was before I worked on the advance curve with various spring combinations.

 

This is probably all leading to a whole new tuning exercise with the objective to reduce the fuel coming through the idle circuit so when the ignition is killed there is less fuel to support ongoing combustion. Or something like that ...

 

Thanks again for your suggestions. I'll probably try some Sea Foam first just to see what that does. I've never had much use for snake oil type products, but since it really works why not try it? Maybe I'll save a little to pour on my cereal in the morning ...

[kc8oye]

I had never heard of it either.. but the store manager at my favorite Advance Autoparts told me about it.. he's a good guy who has always done right by me so I trusted him on that stuff.

 

apparently it's good stuff to put in the fuel tank and well as the crank case for different things.

[Mike 57]

One source of ignition during dieseling/run on are spark plugs. you might try a cooler plug.

[wallaby]

it sounds as if you are doing your homework. You have all the principles understood...now you just need to act on them.

I don't know if anyone has tried this, but have you ever tried to shut off a fully-warmed engine running at high speed? maybe your fast-idle is stuck, or throttle return isn't working well, or maybe you have your foot on the gas...but let me tell you the engine is going to do everything in its power to resist turning off. it will do the "clunk, clunk, clunk...Wheeeeeze" thing. That's bad news, by the way. The "wheeze" part of the process is often the engine spinning backward, and exhausting a cloud of gas vapor out through the carburetor. The pressure the carb sees will blow power valves, and the backward running can be awful on timing gears/chains, and never mind the oil pump sucking oil away from your bearings..

 

Heat IS a factor. The flamability of your fuel is a factor, too. You can try going to a higher octane fuel and those are slower to burn...not quite as volatile, so they are less apt to burn without a real ignition source. Your problem is "dieseling". It's called that because the combustion occurs without ingition; it only needs the heat generated by the compression of the gasses to initiate combustion.

 

With a gasoline engine, your only real defense is to limit the amount of air/fuel the engine can take in. As Tim was saying, you can advance your timing a bit and that will raise your idle speed, and then you can turn down the idle speed with the screw on the carb. I was trying to explain this earlier also. The net result is still the same idle speed, but with a smaller throttle opening. That's what you want.

[kc8oye]

yeah higher octane helps a bunch too.

 

marc: been there... my electric choke wire came unplugged.. so I had to shut the engine down with it on fast idle.. that was..."interesting" shutting it down from 2000+ rpm :>

[Kliegl]

.

[kc8oye]

if you have A/C there is a setup from later cars (late 70's) that would turn the a/c compressor on as your turn the key off, to help drag the engine to a stop.

 

the other solution is to shut if off in gear.. THEN put it in park.

[wallaby]

the other solution is to shut if off in gear.. THEN put it in park.

That's a kinda crummy option if you have a stick shift.

[kc8oye]

generally speaking mark, you don't put a stick shift into park...