Big Cadillac Keeps Running After Shutdown!

[Camu Mahubah]

And yes, I know I had better go catch it!

1970 Cadillac with orginal 472. Can anyone tell me what type of fuel additive this big cat needs. I have to shut her down in drive to keep her from detonating. I just got the car with a full tank of gas.

Should I:

A. Install octane boost

B. Continue shutting the beast off in drive.

C. Tune-up

D. Lead additive

E. Fill with Premium after this tank of gas.

F. None of the above

Thanks for recommendations. And please don't tell me to head to the local junkyard. This car has done wonders for my ego. And she only gets driven on weekends in good weather. A lot cheaper for me to own this car than go out and buy a Harley or a boat. Alas, that is if wifey lets me keep her. Which is kinda like asking the wifey if I can get a leopard to keep inlaws at bay.

 

[Shannow]

Had quite a few cars that would keep running after the ignition system was shut off.

Usually, it was because I made them a little high on compression, and low on camshaft. Just stalled them, as you do.

Does the carb have an "anti dieseling solenoid" ?

These can either add air when the ignition is off, or cut air off when the ignition is off (those two work on the carb internal passages), or drop the throttle position after the ignitions is off (that's a fat solenoid with a plunger that holds the throttle open at idle).

I've seen the internal systems fail.

I've seen the throttle positioning solenoid fail, and people have resorted to the choke speed adjustment to hold idle speed, with run-on resulting.

Can you post pics of the carb, in the area of all the linkages ?

I've also seen people use the choke speed sdjustment rather than

 

[XPR]

Get a tune-up (or do one yourself) and add a bottle of Amsoil PI to the tank.

 

[oilboy123]

I think Shannow is on the right track. Between ignition timing, shut off solenoid setting and idle adjustment screw setting to high.
Too much air at idle lets it keep pulling air and fuel thru the carb. That along with hot spots in your cylinders lets it keep on running.

 

[Camu Mahubah]

Do I need Lead Substitute?

 

[Chris142]

That 472 has a 10-1 compression ratio. That combined with a huge bore and carbon deposits will make it run on after being shut off.

What octain gas are you using? In 1970 we had gas near 100 octain and that engine needs it. Plus lead as an additive will help stop knocking and run on. Don't have leaded gas any more.

About all you can do is to make sure that the timing is perfect, run premium gas mixed with some race gas and when you turn it off have the shifter in drive instead of park. Put it in Park after it's off.

 

[oilyriser]

Did the previous owner use the car only as a Sunday churchmobile, and never drive hard enough to loosen the carbon? Hopefully some deposit control add. like techron will help, if you're not into checking what Shannow suggested. (this is just my best uninformed guess)

 

[Big Jim]

As Shannow posted, the biggest reason I experienced for this was from the idle being set incorrectly. If it has an anti-dieseling solenoid it may be adjusted improperly, or the hard stop idle may be adjusted too high. If it doesn't have such a device, it may simply have the idle adjusted too high.

You could also have a vacuum leak.

You could also have a very tired engine that needs the idle speed turned up to keep the engine from stalling.

With as little as it has been driven for the last several years I would catch up on any needed maintenance, and I would especially (for this problem) make sure you chemically clean the carbon from the combustion chambers.

 

[Big Jim]

After posting I remembered that I have a 1973 Chilton Manual and looked up idle adjustment information for a 1970 Cadillac. It shows that the idle speed should be 600 RPM in drive with the A/C off, the air cleaner off, and the distributor vacuum like disconnected and plugged. You must also make sure the A/C speed-up device is not engaged. An anti-dieseling solenoid was not used by Cadillac until 1971, that is not part of the equation. Although not mentioned, the idea of the A/C speed-up device is to compensate for the idle speed drop off when the A/C compressor is engaged. If memory serves me correctly, the A/C compressor nearly always runs on those old Cadillacs, so it may be necessary to actually disconnect the wiring connector at the A/C clutch. With the A/C running, the idle speed-up device should bring the idle back up to 600 (or maybe even slightly higher, maybe 625).

It may be necessary to spray some carb cleaner into the throat of the carb before you can fully adjust things.

Also a possibility at this advanced age is that the carb throttle shafts are worn, causing a vacuum leak and slop in the linkage limiting the ability to set the idle with any precision.

I'm betting that something is just out of adjustment.

 

[mechtech2]

Use higher octane if she spark knocks when driving, not to cure a running on condition.
Like the others have said, a high idle is the culprit, and this was a common problem on older carbureted cars.
The idle solenoid is faulty or maladjusted, or the idle is otherwise too high [sticking plate or screw adjustment].

 

[Camu Mahubah]

Thanks guys! If I get to keep the car I will run Premium gas and have the carb rebuilt over the Winter. Now I have lots of good ideas on how to stop dieseling if a carb rebuild doesn't fix the problem. I am gonna try and tune the timing by ear first, which will be a learning experience in itself.

 

[Shannow]

Get yourself a timing light, and shade tree manual from the early 70s.

Tuning early 70s stuff is a great past-time. (specially if you've got idle mixture screws)

 

[deepsquat]

well as it has no anti disel valve ( I was with sannow all the way on that ) then check idle speed and set timing ...until then ..when you shut the car off just make sure it is in gear ..then once off put it in park ..shouldn't get any run on with it stopped in gear

 

[another Todd]

Lower the idle.

 

[mechtech2]

Setting the timing by ear can be OK, if you are an expert.
Otherwise, even if there is no spark knock, it may be retarded.
You may not be able to set the timing to factory specs, depending on what the compression ratio is and what fuel was originally required.

 

[Master ACiD]

i had a porsche that would diesel backwards if i had the ignition timing set too high.
when i had this car i was a cheapskate. the engine was speced for 93, but i was cheap and ran 87 in it. i thought if i just retarded the timing a few degrees i would be ok, and i was! but whenever i shut the motor off, it would stop, then immidiately pop and start running backwards for 5-10 seconds. so whenever i shut it off i had to shut it off by leaving it in 3rd and letting the clutch out to kill the motor. the reason i knew it would run backwards was because i could put it in reverse, shut it off, and when the motor would start back up i could let the clutch out and start moving foreward, even though i was in reverse! also it would pump air and smoke out the carbs filters.

it was pretty wild. i bet that was really hard on the motor. think of the oil pump going in reverse, starving the bearings as i drive in circles laughing my head off.

i eventually wised up and started running 93 in the car, and set the timing back to factory spec (actually a little advanced from that) and she ran great. no more reverse running!

on youre cadilac, you might consider setting the ignition timing to factory spec, and then run the appropriate octane fuel for youre engine. i bet the problem gets cured then.

 

[Bobert]

Pour in a full bottle of Redline SI-1, and for that tank of gas run your car hard. Do a couple of Italian tuneups by flooring the car and taking the car to whatever speed you feel comfortable with. Another way of achieving Italian tuneup is if your on the highway, drop the car to the next lowest gear and run 4 or 5 miles at higher than normal rpm. Repeat if necessary.

Make sure your oil is on full mark, and radiator coolant is up to snuff. You don't want overheat your engine.

 

[Chris142]

Quote:

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Pour in a full bottle of Redline SI-1, and for that tank of gas run your car hard. Do a couple of Italian tuneups by flooring the car and taking the car to whatever speed you feel comfortable with. Another way of achieving Italian tuneup is if your on the highway, drop the car to the next lowest gear and run 4 or 5 miles at higher than normal rpm. Repeat if necessary.

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Probly not a good idea with a 37 yr old plastic coated timing gear.

 

[Camu Mahubah]

I would go bankrupt rather quickly doing Italian tune-ups. Did I not fail to mention this a 472 with four-barrels? Indeed, she could probably handle Italian tune-ups all day long. A fuel-tender would be needed and that gets expensive.

Isn't there a chap on this site in the Air Force who refuels jets from the sky? Seriously, I don't wanna tear her up because I don't wanna break what don't need fixin'. Plastic coated stuff included.

 

[Bobert]

One tank to go bankrupt? A highway run of ten minutes in a lower gear will sometimes be enough.

Actually I had a Ford 460 which was enlarged to 472 many years ago, it wasn't that bad though at around 12mpg. Glad I don't have it anymore though with $3 dollar gas.