Aug 8, 2020
24
28
Montreal
VetteCoins
617
Car
1970 C3 LS5
Province
QC
Just bought the car about two months ago and I’ve been filling up with mostly Ultramar 91 octane, which as I understand contains ethanol.

This week is storage time, I'll be storing the car where it can also be looked at mechanically throughout the winter months. The car is in great shape, the only known issue (up until a couple of days ago) is that the steering needs attention. It gives me a hard time when trying to turn while at a dead stop and this will be addressed while in storage.

Anyway ....

Since I bought the car, I have been experiencing some “dieseling” (engine continues to run, cough and sputter when turned off) on a few occasions. This has happened a total of maybe five times in the relatively short time I've driven the car and this lasts about 5 seconds each time. After doing a bit of research, I decided to start using Shell 91 V-Power two days ago. So I pull into my local Shell and top-off a good half-tank worth of Shell 91 V-power. So now the gas tank is completely full (half Ultramar 91 and half Shell 91).

Took the car for a long drive and after turning the car off the engine keeps going, this time the dieseling is more intense, white smoke, some backfiring. This lasts a good 15 seconds (maybe more). So while this is going on, I turn the ignition on again. The engines resumes but at higher RPM. Can’t get the RPMs to come down. Stopped the engine in ‘drive' gear .... more dieseling.

After letting the car sit for a good 45 minutes, I re-start the car. Normal RPM by the sound of it. The tach is not hooked up because the previous owner had changed (upgraded) the ignition system and the stock tach cannot be hooked-up to the new distributor. This will be addressed during winter storage because it will require the removal of the dash.

My question is are the two issues related (the "dieseling" and the high RPM)?
Is (was) the use of the Ultramar gas the problem? Now that I've changed over to Shell VP 91 will the problem go away next spring?
Or, given the engine is of a higher compression, will this dieseling issue continue no matter what fuel I use (ethanol-free or not) and would octane booster be required going forward?

Could the problem be the ignition system itself? My neighbour tells me it may be too advanced (he drives a '68 Mustang). My mechanic tells me he checked the ignition and made sure it's where it's supposed to be, but he'll consider it again during the winter months while the car is in his care.

Thanks.
 
  • Like
Reactions: 2KforeverC5
I use to own a C3 and loved the thing, it had a 454 which had a few modifications to it. It would constantly diesel, unless it had high octane fuel in it, then the dieseling would stop. Because the build was fresh and it was properly tuned I knew carbon and timing were not an issue. It was the high compression built into the engine causing low octane fuel to ignite after the ignition was turned off.

In your case if your engine has lots of miles on it, it could be due to carbon build up, which can raise the compression ratio. You can check your timing as well to see where it's at as it's not hard to do. Best gas to buy before the winter is "Ethanol Free" and the only one (that I know of) that sells "Ethanol Free" here in Ontario is Shell and its only the premium V power 91. It is written right on the pump.

Dieseling is caused by fuel igniting, so when your ignition is turned off, its not the spark plugs. The only other things left are the fuel that it drawn in when the engine continues to turn over, and what's in the cylinder (steel wall, piston head, valves, and cylinder head)
 
  • Agree
Reactions: vintageracer
1) You have a timing issue. What carb, jets, springs, advance etc is on the engine now? What do your plugs look like?

2) I would take your vette for a long run to get rid of as much of the ethanol gas as possible and fill back up with Shell 91. This will not will not get rid of the dieseling but at least the ethanol won’t gun up your carb.
 
Last edited:
I am no mechanic, but I have had issues with dieseling with an old 1972 Ford 351 Windsor.

First of all, using s**t gas in a carburetor car is a big mistake. These carbs were built LONG before ethanol was ever added to the gas to control moisture in the gas tank and save the producers money.

I use Shell V-Power gas in ANYTHING I own that has a Carburetor; Vette, lawn mower, trimmer, boat motors, If it has a carb, use Shell Ultra, Look at PureGas.org and then check out each claim yourself with the gas companies. Shell was the ONLY one that guaranteed no ethanol in their gas because they use separate tanks to store that fuel, the others could not make the same claim because the store fuel in any tank they had "empty" and empty is not actually 100% true

Now about dieseling. Its carbon build-up on your valves. The longer it is not treated the worse it will get.
Why it happens, not 100% sure, could be fowled plugs (not changing them often enough), wrong plug that does not burn hot enough, poor coil not sending enough voltage thru to the plug, old wires having too much resistance so the power to the plugs are low.
The other thing to do is a compression test on all cylinders, in my case the numbers we all over the place.

When I did a wire/plug replacement on my vette, I measured the resistance on every wire and every plug and joined the plug with the wire so that they were all as close as possible in total resistance to each other. But that's not for everyone.

Now on to the dieseling and how to get rid of it... I was told a long time ago to buy Anti-Dieseling fluid and put it in the gas. I could never find such a fluid since this was before the internet.
My father told be an old farmers trick that I thought was nuts but it worked.

I boiled a cup of water. Then had my brother start the engine and rev it up to just over choke speed and I poured the cup slowly into the carb. I told my brother to keep the revs up so it would not stall the engine.

The CRAP that came out of the tail pipe was unreal.

I did another compression test afterwards and every cylinder was perfect. I don’t remember the numbers but they all matched.

Never had dieseling again.

Like I said in the beginning, I am no mechanic, but this worked for me.
 
This is from Bad Ass Cars site.

"This is a common problem that can be caused by a couple of things. Some people call this "dieseling" or "run-on". What is happening is; when you shut the ignition off, something, somehow is causing a couple of the cylinders to keep igniting, which in turn keeps the engine running, although "sputtering" pretty badly. This can be caused by excessive carbon build-up in the combustion chambers or on the piston tops which when hot it'll act like little glow plugs. This is common on engines that run way too rich or have oil burning problems.

Dieseling is a common problem with high compression / high performance engines, especially in warm weather or when the engine is hot. In this case, it's usually caused by running "cheap" gas with inadequate octane. In this case it can usually be remedied by switching to a higher octane fuel.

In milder street engines, another big cause for engine run-on is having the idle set too high, or the carb is mis-adjusted, which requires the idle speed screw to be screwed-in too far to get the engine to idle. When the idle screw is screwed-in too far, it opens the throttle plates too wide and the engine will actually pull some fuel through the main jets a little, so when you shut the engine off, the inertia of the spinning engine still pulls air (and some fuel) through the carb and into the cylinders. The heat built-up inside the combustion chambers is enough to light a few of the cylinders at random to keep the engine spinning and sputtering. Again, this is especially true with fuel that isn't high enough in octane. The more the octane, the less volatile the fuel. Even though the key is off, the engine won't stop sputtering because the throttle plates are opened up enough to still pass a little fuel and air. This is why even factory cars with carburetors (mostly 80's and early 90's cars) had idle stop solenoids to completely close the throttle plates off when the key is switched-off, which completely shuts the fuel and air off, thus preventing run-on from happening. With an idle stop solenoid, when you turn your key on it gets power and clicks into position which pushes the throttle linkage to the normal "idle" position, but when the key is switched-off, it allows the throttle plates to completely close-off again and won't allow any run-on. its a simple installation and Edelbrock makes one just for this application."
 
Hello Roger. Have you tried Petro Canada's 94 octane to see if that makes any difference. Not sure if they offer it in Quebec, usually in the larger markets. You would still have the ethanol but curious to see if a higher octane might help. Take care and good luck.
 
Thanks for the replies, everyone.

As I mentioned, the car is with the mechanic all winter long during storage and he plans on doing what needs to be done. This he has assured me. He made mention of quality of gas on the market, octane, timing, vacuum, carb adjust, etc. He builds and maintains vintage cars, also races as a hobby locally (Napierville, Que.).

I have noted all your comments and suggestions and will bring these up for friendly discussion with him as I feel awkward telling him what to do.
He'll be working on the car during the course of the coming weeks so I'll pop in and loiter a bit.

@mbp I haven't tried the 94 octane. I'll ask him about it, although I believe it would have been one of the first things he would have suggested. And yes, it is available in Montreal.

Thanks again.
 
I use to own a C3 and loved the thing, it had a 454 which had a few modifications to it. It would constantly diesel, unless it had high octane fuel in it, then the dieseling would stop. Because the build was fresh and it was properly tuned I knew carbon and timing were not an issue. It was the high compression built into the engine causing low octane fuel to ignite after the ignition was turned off.

In your case if your engine has lots of miles on it, it could be due to carbon build up, which can raise the compression ratio. You can check your timing as well to see where it's at as it's not hard to do. Best gas to buy before the winter is "Ethanol Free" and the only one (that I know of) that sells "Ethanol Free" here in Ontario is Shell and its only the premium V power 91. It is written right on the pump.

Dieseling is caused by fuel igniting, so when your ignition is turned off, its not the spark plugs. The only other things left are the fuel that it drawn in when the engine continues to turn over, and what's in the cylinder (steel wall, piston head, valves, and cylinder head)
The car has 99,600 miles (160,000 km) showing on the odometer.

What kind of high octane fuel were you using?
 
So would Shell 91 V-Power be a better choice over any 94 octane regardless of gas station, or is there a 94 octane that would be better than the Shell 91 for this specific higher compression engine?
 
Just bought the car about two months ago and I’ve been filling up with mostly Ultramar 91 octane, which as I understand contains ethanol.

This week is storage time, I'll be storing the car where it can also be looked at mechanically throughout the winter months. The car is in great shape, the only known issue (up until a couple of days ago) is that the steering needs attention. It gives me a hard time when trying to turn while at a dead stop and this will be addressed while in storage.

Anyway ....

Since I bought the car, I have been experiencing some “dieseling” (engine continues to run, cough and sputter when turned off) on a few occasions. This has happened a total of maybe five times in the relatively short time I've driven the car and this lasts about 5 seconds each time. After doing a bit of research, I decided to start using Shell 91 V-Power two days ago. So I pull into my local Shell and top-off a good half-tank worth of Shell 91 V-power. So now the gas tank is completely full (half Ultramar 91 and half Shell 91).

Took the car for a long drive and after turning the car off the engine keeps going, this time the dieseling is more intense, white smoke, some backfiring. This lasts a good 15 seconds (maybe more). So while this is going on, I turn the ignition on again. The engines resumes but at higher RPM. Can’t get the RPMs to come down. Stopped the engine in ‘drive' gear .... more dieseling.

After letting the car sit for a good 45 minutes, I re-start the car. Normal RPM by the sound of it. The tach is not hooked up because the previous owner had changed (upgraded) the ignition system and the stock tach cannot be hooked-up to the new distributor. This will be addressed during winter storage because it will require the removal of the dash.

My question is are the two issues related (the "dieseling" and the high RPM)?
Is (was) the use of the Ultramar gas the problem? Now that I've changed over to Shell VP 91 will the problem go away next spring?
Or, given the engine is of a higher compression, will this dieseling issue continue no matter what fuel I use (ethanol-free or not) and would octane booster be required going forward?

Could the problem be the ignition system itself? My neighbour tells me it may be too advanced (he drives a '68 Mustang). My mechanic tells me he checked the ignition and made sure it's where it's supposed to be, but he'll consider it again during the winter months while the car is in his care.

Thanks.
I use Shell 91 which has no ethanol so I am told. Our 2005 Corvette has run fine for a lot of miles as have all our other vehicles including our lawn tractors. Stay away from the ethanol if at all possible, especially on the older cars. there are little rubber pieces in the carbs that deteriorate over time in the presence of alcohol.
 
  • Like
Reactions: 2KforeverC5
So would Shell 91 V-Power be a better choice over any 94 octane regardless of gas station, or is there a 94 octane that would be better than the Shell 91 for this specific higher compression engine?

In the summer if you are driving a lot you can use 94 from Petro, I do, but when you are not driving much, use 91 shell as its ethanol free. For storage use 91 from Shell. The 94 is not ethanol free if its from Petro Canada and I that's the only place you can get it that I know of.
 
The car has 99,600 miles (160,000 km) showing on the odometer.

What kind of high octane fuel were you using?

I was using Shell 91 and it was fine. I had a family member take it out, fill it up and bring it back, it dieseled after she brought it back, swore she put the 91 from Shell in. When I interrogated her more, she admitted to putting 87 and she thought the octane thing was just a myth. "gas is gas" well its not.

Anyway I used 91 from Shell and it was fine.

If your car is pre catalytic convertor you can use water, transmission fluid, or seafoam to try to clear it out. I would not use anything in the tank as that seems to dilute it too much. You can try but who knows how well it will work. If you are pouring it through the carb, do it slow and keep it about 2500-3500 RPM. When you add it you will slow the engine down, STOP POURING wait for the bit you poured in to clear and repeat. DO NOT DO IF YOU DON'T FEEL COMFORTABLE. Most likely you have a little excess carbon on your intake valve, piston, or cylinder head and it's raising the compression a bit, or its glowing and igniting the fuel after you turn the car off.

After its cleaned you can always put a catch can on, and it will help prevent carbon build up, as it removes the oil vapour from the fuel that enters your cylinder. There are a million opinions on catch cans and some people swear by them, others think they are a product of the devil, but they do (if you get a good one) remove oil from the air/fuel charge entering your cylinder so take it for what it's worth.
 
Nothing wrong with a catch can. I installed one from raceready garage last fall. Just less than 3000 km this season and there was less than 1/4 inch of oil in the can. Every engine will be different though.
 
So would Shell 91 V-Power be a better choice over any 94 octane regardless of gas station, or is there a 94 octane that would be better than the Shell 91 for this specific higher compression engine?

I ONLY buy Shell V-Power gas for every carb I own. No exceptions.

There was one year I had 5 motors out of commission at the cottage because of ethanol. The lake mechanic didn't ask me, he told me I was bringing in my own gas instead of using the stations on the lake because of the mess I had in my motors.
 
  • Like
Reactions: Keith Tedford
Thanks for the replies, everyone.

As I mentioned, the car is with the mechanic all winter long during storage and he plans on doing what needs to be done. This he has assured me. He made mention of quality of gas on the market, octane, timing, vacuum, carb adjust, etc. He builds and maintains vintage cars, also races as a hobby locally (Napierville, Que.).

I have noted all your comments and suggestions and will bring these up for friendly discussion with him as I feel awkward telling him what to do.
He'll be working on the car during the course of the coming weeks so I'll pop in and loiter a bit.

@mbp I haven't tried the 94 octane. I'll ask him about it, although I believe it would have been one of the first things he would have suggested. And yes, it is available in Montreal.

Thanks again.
This is good for Octane but you may want to by some gas that is intended for marine use as it will have no ethanol. I'm not sure if the Petro Can 94 is ethanol free.
 
Hi Roger,
First, congrats on picking up that C3. I hope its easy enough for you to drive out of Montreal in order to really enjoy.
The write up Ruuff Day posted pretty much explains your run-on problem very well.
Regarding the fuel, I guess by now you figured that you should be using Shell V Power whenever possible. If you want to try Petro Can 94, you should do it when you are planning a long drive.

You mentioned in one of your posts that the car now has 1/2 tank of V Power and 1/2 Ultramar with Ethanol. I strongly suggest you don’t leave that mix over the winter. Even 1/2 tank of fuel with Ethanol will cause damage to any cork or rubber gaskets in your carb, and moisture in your fuel tank. The most important time to have your tank full of Ethanol free is over the winter!
 
  • Agree
Reactions: Rruuff Day
“FOOD FOR THOUGHT “
Premium grade fuel burns slower than regular grade fuel.
That’s why a high compression engine of 10.5:1 compression pings using regular gas. The combustion takes place too soon and can damage the engine because the piston, connecting rod and crankshaft are all in one line and the energy is absorbed into these parts instead of turning the crankshaft.
In low compression engines like 8.3:1 compression ratio premium fuel will not burn fast enough and results in unburnt fuel contamination of carbon build up on the heads, pistons and valves. This can also cause sticking valves resulting in backfire and carb cough.
The only way to fix sticking valves is to tear down and ream the valves guides clean.
A worn out high compression engine can lose compression in all or just one cylinder.
That reduced compression will cause carbon build and dieseling/glow plug results when burning the premium fuel.
I expect that’s why some install hotter spark plugs as compensation for carbon build up in bad cylinders low on compression.
Marvel Mystery Oil and Seafoam fuel additives both will help to clear out carbon build up and have a cleaner burn.
But these fuel additives will not free up sticking valves.
 
FYI there is no ethanol in Canadian Tire Premium fuel and that applies to all Canadian Tire gas bars. The Canadian Tire gas bar I use has the statement "Contains no Ethanol" on the pump right under the price of the premium fuel. They are likely sourcing their fuel from Shell. I have been using this fuel without any problems. Avoid ethanol if you can.
 
Hi Roger,
First, congrats on picking up that C3. I hope its easy enough for you to drive out of Montreal in order to really enjoy.
The write up Ruuff Day posted pretty much explains your run-on problem very well.
Regarding the fuel, I guess by now you figured that you should be using Shell V Power whenever possible. If you want to try Petro Can 94, you should do it when you are planning a long drive.

You mentioned in one of your posts that the car now has 1/2 tank of V Power and 1/2 Ultramar with Ethanol. I strongly suggest you don’t leave that mix over the winter. Even 1/2 tank of fuel with Ethanol will cause damage to any cork or rubber gaskets in your carb, and moisture in your fuel tank. The most important time to have your tank full of Ethanol free is over the winter!
Hi Luke, thanks.
I'll be adding STA-BIL to the gas tank. Will this take care of whatever ethanol is in the fuel?

The other option is to drain the tank and fill with Shell 91 for the storage.
 
Old Thread: Hello . There have been no replies in this thread for 100 days.
Content in this thread may no longer be relevant.
Perhaps it would be better to start a new thread instead.

Similar threads

Users who are viewing this thread