Dieseling 427
 

Just got the engine put together after a bent valve problem which required new valve, new cam (Comp 282S) , new solid lifters and reground conrod and crankshaft bearing surfaces.
Heads are Dove F5 aluminum with a total of 9,6 compression ratio, intake is Edelbrock Performer RPM with a 850cfm all mechanical Holley 4barrel.
Advance timing with MSD 6AL is 10-13º at idle (850-900 rpm)static and 35-38º at 3500rpm.
Engine runs ok , temp is ok , but when shut off it diesels, sputters and reverses rotation very frequently.
What is the problem?Carbon deposits to glow are not possible , because the heads went in absolutely clean, mixture is on the rich side, Spark plugs are Champion RC9YCC with a 0.035 gap.
I hooked up a os2 sensor with a digital gauge and mixture is sligtly rich at idle and richer while driving at 2000-3000 rpm. Vacuum is about 15 at idle and even higher when cruising.
Water Temp is about 175ºF while driving at 90ºF ext.temp and reaches 195ºF when idling in this hot weather.
Does anybody know what else to check in order to get rid of the dieseling problem?

Eljaro,

At this point, it's impractical to change anything internal. There's probably a hot spot somewhere. My first approach would be to lower the idle speed as much as practical. If that doesn't work, lean it out a bit, although I'm not sure what effect that might have. **)

Next - a colder plug. You could also add some gap to the plug. With the MSD, .045" is OK.

I have a similar problem but altough with another engine maybe my recent discovery could put you on a track you have not checked yet. My engine is a chevy 383 stroker and would keep on running after shut of. Its a recent rebuild and carbon accumulation has to be put away as a possibility. The problem would occur only when the engine is fully hot. I have a remote timing mechanism and retarding it from its regular 10 degrees to as much as 3 or 4 degrees was not doing nothing. The idle speed then down to around 500 would not affect it positively neither. I had the ignition switch letting passing by some current as the instruments would stay on even at "off" position. I've solved that problem by installing an in-line switch on the instruments main positive wire but the expected curing of my run on did not solve then neither!

I am also running rich (edelbrock carb./electric pump combo) and modifying the jets/rods do not bring satisfactory...

Suspecting a too high pressure delivered to the carb. from the pump I tried to stopping the pump working a few seconds before switching off and IT WORKED the motor would stop immediately!

So I will install a pressure regulator on my fuel line and hopefully this will be the solution. Now the edelbrock needs to be ran at a 6 pounds maximum pressure and mostelikely the electric pump works at seven ...your set-up while different,holley carb. and mechanical (?) pump ,from my experience, could be suffering from a gaz problem also.The apparent incurable too rich situation in my case could very well also caused by a too high feeding pressure overwhelming the carb. but this I will be able to confirm only when my regulator will be on.

Coming back to my case it looks like the carb under overwhelming pressure was couldn't deliver the right mixture (too rich) and was not stopping dripping raw gaz at motor shutt of.....

One of the solutions you may want to consider is a 100 A diode in line n your alternator line.

The claim is the alternator will continue to supply voltage as the engine runs.

Just a thought.

Sorry I've sent my forward message while not having resumed completely.

So you could have something similar...check to see if the carb. you have works within the recommended pressure . If this is positive I would keep cheking into the carb. itself: float (s) adjustement state of those small pointer that are actually blocking the fuel admission under the floats pressure. In short from my understanding a carburator that keeps spitting in some drops AFTER engine shut off can be a relative common source of that kind of trouble and that is all fuel delivery system related...but of course it can be something else also...just telling you what it was for me.

regards, Ronald.

P.S. I remember trying shutting of my car immediately after a rapid slow down on the road (normal braking from 60 mph to full stop) and then it stopped completely right away not giving a second of idling rest....it must have been becuause then ,in my case of electrical pump, the fuel pressure was down because of the higher fuel consummation at higher revs..another proof in the pudding of my theory!

I too have the same issue but only when combined with a certain set of circumstances. If I am cruising at a rather constant rpm (2500-3000) for an extended period of time and do not rev the car with load (not just blip the throttle) my car sill diesel and then kick back some smoke through the carb. If I run it up to 5-6K just one time before shutting down it seams to really help. My engine is also a fresh rebuild but in those so excessive carbon in the combustion chambers is out but that doesn't mean there isn's a little accumulation on the plugs as I would suspect under tame driving. Best wishes!

P.S. - If anyone asks why just tell them that's how race engines work.

The strange thing with this whole matter is that the engine didn´t do the dieseling before , with the same heads, and the same pistons, the same plugs and the same fuel pump. Pressure at the fuel pres.gauge is between 5-7psi, and the transparent plug on the carb doesn´t show gasoline level to be high.
Only new camshaft (sharper though !), springs and intake manifold are different from the previous setup.
The old manifold , a Blue Thunder dual plane had to be taken off because water was seeping into the engine through some pore , which welding and rewelding didn´t cure.
I will check on the gas pressure issue though, and increase the spark plug gap to 0.050 as recomeded by MSD.
An adittional problem is that the outside temp here is about 95ºF with a humidity of 80%. That is no good temp to go out and test a Cobra with its permanent footwell heating.
( By the way, does anyone know of a duct system to channel cool air from the outside to the footwell?)
Let you all know what happened after my test drive late this afternoon.

The "duct" system your referring to does exist and is very popular on Cobra's here in the States. It typically uses a blower to force air to cool off the toes. Any of the major Cobra parts groups stock them.

PS - In your photo gallery the picture of your rear suspension looks as though the lower control armsa are aluminum? Is that how your car came or did you change/upgrade those? Thanks!

Eljaro,

The way I found out the excessive fuel pressure was a simple no risk one: I took out the electrical fuse of the pump out just few seconds before actually shutting the engine off letting the motor running with the pump desactivated for 3-4 seconds....I've tried it twice and it stopped immediate while it would keep running always since I had the car on the road since last may....since I really know how irritating and time consuming and frustrating a problem I really wish you find it!

But as long the fuel regulator won't be on I cannot be 100 percent sure of the issue but surely looks like it.

good luck.

to Cracker,
the original rear suspension on my Contemporary Cobra was a Jaguar-Salisbury setup with inboard brakes.
There was no way of keeping those brake disks free from oil mist from the differential, even after replacing several times the output shaft seals. Got bloody hands every time when taking the rear inboard calipers off and on, so I decided to replace the rear end with an AMP setup , which takes the brakes outboard. The lower aluminum control arm is an extra to this modification and works fine in my car .I also got the Watts linkage upper support, which bis great for cornering.
If you wish pictures of how the setup looks like, let me know.
Check out AMP web page at:
(www.cobracountry.com/amp/home.html)

Eljaro,

I ran into the same problem with diff leaking but have seemed to solve this problem by applying some "gasket maker" around the outside edge of the O-Ring on the hub. I only have about 700 miles on it since the application but so far not even a drop has escaped! Thanks and let us know whats happens.

An engine idle of 850 to 900 rpm means that the throttle blades are open wide enough to let excess air pass the blades when you shut off the engine. This causes "run-on". Youy can install an anti-diesel solinoid on the carb which will close the throttle blades completely when the key is turned off. I would try to set the idle as low as possible to combat the problem. Also, heavier steel or cast iron flywheels can accsentuate the problem with their extra inertial mass. An aluminum flywheel can help cure your "running on".
HTH,
--Mike

Dieseling
 

Eljaro,

How do you have your electric fan wired? If you are coming off your ignition direct to your toggle switch and then to your fan you could be getting some feed back through the ignition connection.

Clois

That's exactly how the "run-on" problem I had was solved!

YMMV,
Randy R...

Cracker,
is "gasket maker" mean Permatex type Silicone? or is it something different?

BTW, I LOVE the look of your tires on the car!
What are they???

(Cool, I think this may be my first Thread Hi-Jacking:D :D )

Shaps

New member so be easy with me.

I have a 460 Ford crate motor that used to diesel. I installed an anti-diesel selenoid and it didn't cure things. Then I just shut down the car with it in gear and ease off the clutch to put a load on the motor so it wouldn't diesel. But then what really fixed it is that I increased my initial timing to 18-20 degrees which increased my idle speed. I then adjusted the idle on the carb back to 900 rpms, which of course meant my butterflies were closed more allowing idle through the idle slots only. No more problems.

Ronnie

Ronnie

well, problem solved !
You don´t believe this, but it´s in the MSD manual, I have read it in many web pages , especially in this clubcobra page, and nevertheless I kept on ignoring it.
THE missing DIODE BETWEEN THE VOLTAGE REGULATOR AND THE CHARGING LIGHT was the culprit. I put one in, and see there, no dieseling any more!
Wow am I a happy man now !!
Thanks to Clois , who gave me push to go ahead and wire the diode into the circuit.
Thank to all of you.
Steve

Glad I could help. I had the same problem...I think fixit or Roscoe encouraged me.

Clois

...
 

Eljaro,
once you installed the diode, did it COMPLETELY eliminate the run-on problem? What exact RPM is your engine idling at now, once warmed up? Did you do the idle adjustment and the diode at the same time, or at different times?

decooney,
it was just the diode. The idle is about 800-900 and I did not do any adjustment to that.
As a matter of fact the dieseling would occur with an almost cold engine as well.
The only drawback now is that the red "charging light" on my Contemporary Cobra does not come on, but the charging dial shows healthy charging going on.
Does anybody have an electric schematic
of the Contemporary Cobra ? The only thing I have is the loom schematic, but there is no way of knowing where the pink cable from "I" on the voltage regulator goes to and how the charging light is wired into this circuit.
As for the dieseling , I often also see the suggestion to check for the fan wiring in the car electrics and wiring a diode in there, and I believe this is not taken seriously enough. At least in my case an electric problem caused the dieseling.