351C dieselling

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danoreilly

Well-known member
Joined
Dec 16, 2010
Messages
138
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4
Location
Colorado Springs, CO
My Car
71 Mach1 351C 4V
02 Deluxe Convertible
67 Fairlane GT 390
My rebuilt 351C has about 2,000 miles on it. The last 3 days, 3 times after i've shut it off it wanted to diesel on me. It's never done that before. Any ideas? Maybe bad fuel?

 
base timing too far advanced. Engine getting too hot. try stopping it in gear or with the A/C on.

total timing advancing too far. Did you shave the heads, use higher compression pistons, somehow increase the compression ratio of the engine?

 
The heads/block were milled .029, partially to true them (just slightly warped), partially to raise the compression to 9.3, which should run fine on hi-test pump gas. I also run flat-top hypereutectic pistons so they could be running a bit hotter than stock. Around here, it's hard to find pure gas, we end up with a lot of 10% ethanol stuff, which I suspect could be a problem as well. It's been warm around here, in the high 80's, so not really hot, and by the temp gauge (running 180 degree thermostat) I'm not running hot.

I don't have AC, by the way. What would be the point of stopping it in gear?

The main question is: why now after 2k miles? No mods or adjustment to the motor since it was installed with timing & carb adjustments made, with the exception that I raised the idle about 75rpm because it was set too low initially. I believe (I'll have to check) it's timed at 10 degrees BTDC, which works for this altitude (around 6,000 feet). It runs fine otherwise, and has been operated in higher temps than this. I'll check the timing tomorrow if I can, and I'll also check the intake bolts (although I'm not showing any of the classic signs of a vacuum leak caused by a leaking intake gasket).

 
The idle is set with the solenoid. the screw on the carb is set so that it barely idles OUT of gear. Purpose of the solenoid is to get an idle of about 800 RPM in gear, when you cut off the key, it basically shuts off the fuel.

unplug the solenoid to set base idle.

 
I don't have an idle solenoid on my carb (Holley 80475S).
You may have answered part of your own question.You said you "raised" the idle speed. This may be what is going on besides maybe the timeing being to far advanced. That is why it was recomended that you shut the engine off while the car is in gear,to slow the idle speed. Try lowering your idle speed slightly, and play with the timing.

 
higher compression, timing advanced, hot ambient air temp, low octane fuel all lead to dieseling.

one of the tricks the factories do is when you shut off a modern car is to put a load on the engine and pull timing. that is done by the computer in today's cars. also, they cut the fuel injectors off which is the easiest way to stop dieseling. no fuel. no run on.

you may have filled up recently and got a different blend of fuel. it is the right combination for dieseling along with you raising the idle rpm, which feeds fuel to the system.

 
Turning it off in gear is effective because your idle speed is lower and there is a load on the engine.

Are you running vacuum advance on your distributor? What are you using for the source?

 
The idle speed wasn't set to factory specs. I raised it to just about 625, which is where it should be; any slower it the slower idle allows the motor to build heat too quickly. Why would that cause an issue? Also, I still don't understand why shutting the engine off with the car in gear would accomplish? What's the goal behind that?

 
What plugs do you have in it? I would go to a colder plug. I like autolite 24's.

Andy

 
I would double check timing just make sure your distributor didn't walk and advance the timing.

next its Been real hot the last few weeks so increased ambient temperature would be enough if the engine was right on the edge of dieseling before.

check make sure the throttle cable isn't getting hung up on anything or the fast idle isn't sticking that might bump up the RPMS at idle slightly.

usually dieseling is when the timing is too far advanced with the engine hot and the idle rpms too high.

check timing first.

check idle speed with engine warmed up.

check any vacuum motors on the air cleaner aren't working correctly(if you have them).

check vacuum on engine see if you have a vacuum leak that could cause a higher idle rpm.

if you have an idle stop solenoid on the carb throttle arm check that for correct operation as well.

fuel can be an issue you could of got a bad fill up. happened to me once in my daily driver.

 
The idle speed wasn't set to factory specs. I raised it to just about 625, which is where it should be; any slower it the slower idle allows the motor to build heat too quickly. Why would that cause an issue? Also, I still don't understand why shutting the engine off with the car in gear would accomplish? What's the goal behind that?
ok, at a slower idle speed your water pump might not be moving enough water.

The only reason why they are telling you to shut off the engine while in gear is this will almost certainly keep it from Dieselling.

Do you run points or do you have an electronic ignition?

 
To have a motor Diesel you need compression,heat, fuel and air, remove one of the 4 and it ain't happening. Timing shouldn't matter because when you turn the key off the ignition is no longer firing. A blown intake gasket will cause the cylinders drawing the extra air in to go way lean, thus its hot enough to diesel. If the intake gasket is blow usually one sign is the motor suddenly becomes cold natured, but if it is as hot there as it is here you would never see that. Which leads me to my guess, its hotter than usual and you added compression put some octane booster in and see what happens. Thanks for letting me ramble

 
To have a motor Diesel you need compression,heat, fuel and air, remove one of the 4 and it ain't happening. Timing shouldn't matter because when you turn the key off the ignition is no longer firing. A blown intake gasket will cause the cylinders drawing the extra air in to go way lean, thus its hot enough to diesel. If the intake gasket is blow usually one sign is the motor suddenly becomes cold natured, but if it is as hot there as it is here you would never see that. Which leads me to my guess, its hotter than usual and you added compression put some octane booster in and see what happens. Thanks for letting me ramble
I know when I used to mess with the timing on my 73 Grande and had it to high it would do that so I will respectfully disagree with it has no effect.

 
To have a motor Diesel you need compression,heat, fuel and air, remove one of the 4 and it ain't happening. Timing shouldn't matter because when you turn the key off the ignition is no longer firing. A blown intake gasket will cause the cylinders drawing the extra air in to go way lean, thus its hot enough to diesel. If the intake gasket is blow usually one sign is the motor suddenly becomes cold natured, but if it is as hot there as it is here you would never see that. Which leads me to my guess, its hotter than usual and you added compression put some octane booster in and see what happens. Thanks for letting me ramble
yup!

 
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