It started as an occasional problem, but it seems to be getting more frequent and more severe. After I turn the key off, the engine seems to keep slowly turning over. There is a BANG, BANG, BANG...that coincides with the car shaking somewhat. After this bang a few times (like 10 or 12 last time which is the longest it has done it), there is a sound like a squeaky wheel turning somewhat fast and slowing down to a stop after a second or so. Sometimes there is a rotten egg type smell when I get out of the car.

Please keep in mind that I know little about working on a car and am trying to learn as I go. Thanks in advance!

The car is "dieseling" things that can cause this are: idle speed too high, fuel too low octane, carbon deposits in combustion chamber.

The rotten eggs smell is probably the unburnt fuel passing through the catalytic converter.

Run a can of Seafoam through your engine, and check your idle speed.

Run a can of Seafoam through your engine, and check your idle speed.

What is Seafoam and what does it do?


The car is "dieseling" things that can cause this are: idle speed too high, fuel too low octane, carbon deposits in combustion chamber.

The rotten eggs smell is probably the unburnt fuel passing through the catalytic converter.


On this note, what octane am I supposed to be using? No tach but the idle speed seems to be what I would consider normal. What is the procedure for checking idle speed w/o a tach?



Thanks to both of you for your helpful responses!:thumbsup: Glad to see there is still some good help/support out there even for a 34 y/o car!

My 1980 coupe deville was doing the same thing,I had the carb rebuilt.its runs beautifly now. The problem with mine was more the linkeage than the carb itself,the linkeage was sloppy and when the key was turned off,the linkeage was not turning off the flow of gas to the carb and so it dieseled.
I'm not a mechanic either ,thats the best I can do to help you.

My 1980 coupe deville was doing the same thing,I had the carb rebuilt.its runs beautifly now. The problem with mine was more the linkeage than the carb itself,the linkeage was sloppy and when the key was turned off,the linkeage was not turning off the flow of gas to the carb and so it dieseled.
I'm not a mechanic either ,thats the best I can do to help you.


Still helpful. Seems like a more drastic option, so I'll go that route if the other more simple ideas do not resolve the problem. Thanks for the reply! What did the rebuild cost you? If I get there, I am thinking of maybe attempting it myself unless a rebuild is pretty cheap.

Still helpful. Seems like a more drastic option, so I'll go that route if the other more simple ideas do not resolve the problem. Thanks for the reply! What did the rebuild cost you? If I get there, I am thinking of maybe attempting it myself unless a rebuild is pretty cheap.

It cost me $325.00 canadian,but like I said it was more the linkeage than the carb,so it required new linkeage.Now we will get responses that I spent too much,it was worth it to me, the place I had it done does their work to show car quality,every moveable part ,pins, hinges ,valves etc is new. so it's a lot more than just a cleaning.

What is Seafoam and what does it do?





On this note, what octane am I supposed to be using? No tach but the idle speed seems to be what I would consider normal. What is the procedure for checking idle speed w/o a tach?



Thanks to both of you for your helpful responses!:thumbsup: Glad to see there is still some good help/support out there even for a 34 y/o car!

Seeing as thats a '73 regular 87 octane should be fine, that being said try putting in premium and see if the problem goes away. Seafoam cleans out carbon from the engine. The proper method to set the idle speed is go get a tachometer and hook it up to your ignition system and adjust the idle to approximately 650 rpm I think you need to unhook the automatic timing advance cannister too. I suggest you go on eBay and buy a factory service manual for your year Cadillac. They tell you everything you can imagine about your car and how to fix it. Invaluable source of information. One trick you can try is to shut the car off in drive instead of in park (while stopped of course) and see if it stops your problem, if it does then your idle is too high.

The engine is all carboned up from granny driving. Carbon hot spots are working as igniters to keep the engine runnng with the key off. Get a plastic spray bottle and fill it with tap water. Take the air filter loose so you can run the engine at a fairly fast idle and spray water into the intake. You'll get clouds of black carbon out the exhaust. Take the car out and give it a couple of WOT blasts to finish the job.

It is probably running backwards as well.

Ok, so I have a couple cans of sea foam. Just pour one in the gas tank and one in the crank case and forget about it? I saw something online that said it's a good idea to change your oil a few hundred miles after using this because it will have all kinds of nasty crap in it.

I'm a novice too, but idle speed is the main reason I have heard for dieseling. To set it, you need to buy a tachometer (for diagnostics, i.e. you are not buying one to install in your dash) and hook it up IAW the directions that come with it. My timing light reads rpm.
My '69 doesn't have one, but do you know if your car has an anti-dieseling solenoid? Maybe someone who knows will chime in. If you are supposed to have one and you don't, or if you have one and it's faulty, that would be a big clue. I think it would be on the driver's side of your carburetor and would have an electrical connection.

I have the dieseling problem occasionally and I think it is because I only have 1 of 2 throttle return springs correctly hooked up. I think the throttle valve is supposed to basically slam shut when you kill the engine. Anything that holds throttle valve open can cause dieseling. The linkages the previous poster are an example. My throttle linkage is what I think is causing mine. I guess it could also be the linkage on the passenger side of the carb that opens the secondary throttle.

I got a factory shop manual on eBay for $42 from Bishko Books. It is a reproduction. I was very happy with it:
http://stores.ebay.com/Bishko-Books-Better-Buys

Setting your idle speed sounds like your first check.

I'm a novice too, but idle speed is the main reason I have heard for dieseling. To set it, you need to buy a tachometer (for diagnostics, i.e. you are not buying one to install in your dash) and hook it up IAW the directions that come with it. My timing light reads rpm.
My '69 doesn't have one, but do you know if your car has an anti-dieseling solenoid? Maybe someone who knows will chime in. If you are supposed to have one and you don't, or if you have one and it's faulty, that would be a big clue. I think it would be on the driver's side of your carburetor and would have an electrical connection.

I have the dieseling problem occasionally and I think it is because I only have 1 of 2 throttle return springs correctly hooked up. I think the throttle valve is supposed to basically slam shut when you kill the engine. Anything that holds throttle valve open can cause dieseling. The linkages the previous poster are an example. My throttle linkage is what I think is causing mine. I guess it could also be the linkage on the passenger side of the carb that opens the secondary throttle.

I got a factory shop manual on eBay for $42 from Bishko Books. It is a reproduction. I was very happy with it:
http://stores.ebay.com/Bishko-Books-Better-Buys

Setting your idle speed sounds like your first check.
The engine will not diesel unless there is a source of ignition such as a carbon hot spot or a glowing exhaust valve. The compression in a gasoline engine is not nearly high enough to produce a real compression ignition diesel effect. Lose the Sea Foam. Get a plastic spray bottle. Put tap water in it. Take the intake loose at the aircleaner and get the engine running at a about 1500-2000 RPM. Spray water in the intake and watch the carbon fly out the exhaust. I've been doing this for the last fifty years with success every time. There's no rocket science here, just good old fashioned mechanics horse sense.

Setting your idle speed sounds like your first check.

Sounds about right. Maybe the fast idle cam (hooked to choke linkage) is holding the idle too high.

Sounds about right. Maybe the fast idle cam (hooked to choke linkage) is holding the idle too high.
If there's no source of ignition such as a hot spot you can hold the throttle wide open and there'll be no running on. It takes three things to make an engine run; compression, spark(ignition source), and fuel. Remove any one and the engine quits. Turn the key off. If there's no other source of ignition the engine quits. It's as simple as that. If there's an untimed source of ignition such as a carbon hot spot you can do serious damage to the engine. You better deal with it.

Well, I will be starting with seafoam, then looking at timing/idle and then carb. I will go through all of these even if the seafoam eliminates the problem. This car is 34 years old and I have no idea what it's past is, so it could very well be some of all of it.

Ok, so I have a couple cans of sea foam. Just pour one in the gas tank and one in the crank case and forget about it? I saw something online that said it's a good idea to change your oil a few hundred miles after using this because it will have all kinds of nasty crap in it.

What dkozloski (http://www.cadillacforums.com/forums/member.php?u=13228) is talking about with the water is the same idea with the Seafoam. You slowly pour the whole can into the carburetor while running, or better yet, find a big vacuum source like the brake booster, and let the engine suck the Seafoam out of the can. The engine is gonna struggle like hell to stay running, but keep revving it to keep it going. Pour the whole bottle in and wait a few minutes. Start it up and drive the hell out of it! :thumbsup:

What dkozloski (http://www.cadillacforums.com/forums/member.php?u=13228) is talking about with the water is the same idea with the Seafoam. You slowly pour the whole can into the carburetor while running, or better yet, find a big vacuum source like the brake booster, and let the engine suck the Seafoam out of the can. The engine is gonna struggle like hell to stay running, but keep revving it to keep it going. Pour the whole bottle in and wait a few minutes. Start it up and drive the hell out of it! :thumbsup:

The brake booster line is the bigger rubber one coming out from near the back of the brake reservoir, right? Does it just pull loose?

This reminds me of something that happened to me once before a long time back that I was boosting my 1972 Caddy off once with my 1971 Mark III. I had the Mark III running when I hooked up the booster cables... apparently they didn't like each other... sparks... burned cables on the Caddy and it caused the Mark III to diesel for a long time. Normally when I had one diesel on me it would eventually shut off. Not the case here the Mark stated dieseling I thought well it would shut off like before so I went in the house got something to drink and the damn things was still dieseling when I can back outside. I ended up having to drive up the street and back then it actually shut off.

This reminds me of something that happened to me once before a long time back that I was boosting my 1972 Caddy off once with my 1971 Mark III. I had the Mark III running when I hooked up the booster cables... apparently they didn't like each other... sparks... burned cables on the Caddy and it caused the Mark III to diesel for a long time. Normally when I had one diesel on me it would eventually shut off. Not the case here the Mark stated dieseling I thought well it would shut off like before so I went in the house got something to drink and the damn things was still dieseling when I can back outside. I ended up having to drive up the street and back then it actually shut off.

Ok, that is kinda scary. I have used some seafoam, and have been hutting the car off with it in gear and it has only happened twice since those changes, but last time it went for a good 10 seconds.

EDIT: And maybe related, I had to replace my positive battery cable because the insulation had disintegrated and it was grounding on the inside of the wheel well. I wonder if some how something electrical is making a difference? I wouldn't have any idea how though.

How high is your idle speed????

How high is your idle speed????

Can't tell you since I don't have a tach. It doesn't seem to be faster than I would expect, but then again that all depends on how good of a judge of it I am. I need to get a decent timing light somewhere, preferably one of the ones that has a tach built in

SFX, Did you ever try the water trick? And BTW, Advance Auto Parts and Autozone both had the same three timing lights when I bought mine. They offer a timing light, a nice timing light and a really-nice timing light, I guess you'd say. I got the really nice one for about $100. It has the tach. I don't think the two cheaper versions had tachs. But if you don't want to spend that much right now, you could get a cheap tachometer from those same places for much cheaper. Later on you could invest in a timing light with or without a tachometer. I don't know how nice they are, but I like the looks of the one at this link because it's battery powered: http://www.performancedistributors.com/accessories.htm

I have to connect mine to positive battery terminal, ground, and of course clamp it on the no.1 spark plug wire. That's three wires under the hood. I am sure one day I will snag one of those wires in my fan... So anyway I don't know how well the battery powered ones work, but they seem pretty luxurious to me.

SFX, Did you ever try the water trick? And BTW, Advance Auto Parts and Autozone both had the same three timing lights when I bought mine. They offer a timing light, a nice timing light and a really-nice timing light, I guess you'd say. I got the really nice one for about $100. It has the tach. I don't think the two cheaper versions had tachs. But if you don't want to spend that much right now, you could get a cheap tachometer from those same places for much cheaper. Later on you could invest in a timing light with or without a tachometer. I don't know how nice they are, but I like the looks of the one at this link because it's battery powered: http://www.performancedistributors.com/accessories.htm

I have to connect mine to positive battery terminal, ground, and of course clamp it on the no.1 spark plug wire. That's three wires under the hood. I am sure one day I will snag one of those wires in my fan... So anyway I don't know how well the battery powered ones work, but they seem pretty luxurious to me.

Haven't tried teh water yet. Maybe tomorrow. A guy at work actually recommended the same trick, but using transmission fluid.

thanks for the tach info. Didn't really want to spend $100, but I guess if that what it costs to get what I want...

I'd love to have a battery powered on with tach!

I'm sure it's just idle speed or timing. It sounds like it hasn't had a tune up in a while. Back in the day I was lucky to get a year out of a set of points. Maybe I was fussy but for $4 or $5 it wasn't worth adjusting them and messing with them. I refuse to mess with them now and convert any old vehicle to electronic ignition.

If you are shutting it off in drive and it turns off normally then the idle is too high. Caddys can idle pretty low.

well seafoam down the carb didn't help so next weekend I guess it's time to set my timing and idle speed.

Years ago there was a process available where they blasted the combustion chambers of the engine with walnut shells through the sparkplug holes to cure problems just like yours.

Find the vacuum advance can on the bottom of the distributor. Follow the vac line to the carb. Now remove it from the carb and begin sucking air through it. It should tighten up where you can no longer pull air. If it allows you to continuously draw air through it then your vac can is bad. Cheap part at auto store. Also have you opened the dist. to see if the mechanical advance is sticking? May need a little oil to loosen things up and get them working. Springs may be bad also. If your vac advance is not working the car will detonate( ping/run on, not explode). Also when running won't have much power. Take a look. Easy fix and cheap. Worked for me. Do all this while the engine is off by the way. Good luck.

Find the vacuum advance can on the bottom of the distributor. Follow the vac line to the carb. Now remove it from the carb and begin sucking air through it. It should tighten up where you can no longer pull air. If it allows you to continuously draw air through it then your vac can is bad. Cheap part at auto store. Also have you opened the dist. to see if the mechanical advance is sticking? May need a little oil to loosen things up and get them working. Springs may be bad also. If your vac advance is not working the car will detonate( ping/run on, not explode). Also when running won't have much power. Take a look. Easy fix and cheap. Worked for me. Do all this while the engine is off by the way. Good luck.


thanks for all teh ideas. I'll try them out next weekend when I am home again.

87 Octane will cause that car to diesel.

1) Try High test

2) Water through the carb is good, trans fluid is better. This will clean some carbon build up off the valves.


If these work and the car runs perfect consider having the car tuned. If it doesn't work or if it works a bit have the car tuned up. Could be a timing issue, but the tune will cure that. A decent mechanic will pick up any vacuum issue related to the vacuum advance. You could even have the timing retarded so you can run 87.

Sounds like you're running crappy gas to me.

My '73 Eldo diesels & backfires if I run the cheap stuff so I don't run the cheap stuff. I'm going to do the trans fluid through the intake and in the crank case next week, the lifters are getting sticky.

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