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Starting a hot engine


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I have answered this question several times in the last week, both via telephone, and on this and other automotive forums.

 

The issue:

 

Hot engine is anywhere from very difficult to impossible to start from maybe 5 minutes after shutdown, until the engine cools.

 

The cause:

 

Once the engine is shut down, the inlet check valve in the fuel pump, if a fuel pump is installed, closes, preventing fuel from draining out of the fuel line back to the fuel tank. The engine heat now heats the fuel in the fuel line, and because of the volatility of modern fuel, creates a pressure in the line that the float and float valve in the carburetor are unable to withstand. The result, depending on the diameter of the fuel line, and the distance from the pump to the carburetor, somewhere between a teaspoon and maybe 3~4 tablespoons of fuel will dump into the carburetor. This raises the fuel level in the carburetor bowl(s) above the main discharge nozzle(s). The fuel runs out the main discharge nozzle(s) onto the throttle plates(s). If the throttle plate(s) are cracked open, fuel runs down into the intake manifold where it is vaporized. If the throttle plate(s) are closed, fuel runs out the throttle body beside the throttle shaft and drips onto the intake manifold, as well as vaporizes into the air cleaner. At the same time, engine heat is causing the fuel left in the bowl(s) to vaporize, and escape the bowl(s) through the bowl vent(s) into the air cleaner. The air in the air cleaner, as well as that in the venturi area of the carburetor and the intake manifold is now so dense with fuel that the mixture will not burn; thus the engine will not start.

 

The operator:

 

Most of us are old enough that we were taught to floor the footfeed, activating the "unloader" circuit in the carburetor, allowing the engine (an air pump) to pump the overrich mixture out of the tailpipe. This worked well before fuel volatility became so high. But the unloader circuit opens the throttle valve(s) wide open, and today's fuel will simple continue to fill the intake, and the engine still will not start. One may picture the operator, with both hands grasping the steering wheel, pulling as hard as possible on the wheel, and very red in the face ;) This before the air in the cabin turns blue ;)

 

A possible solution:

 

We have found that GENERALLY (no guarantee, but then no charge!) if one forgets one's lifelong teaching in this area, the engine can be started. The method is to enter the driver's seat, and don't even look at the footfeed, let alone touch it! Now crank the engine from 3~7 seconds. Each engine has a "sweet spot" which one may find by trial and error. WHILE STILL CRANKING THE ENGINE, GENTLY press the footfeed down about 1/3 of the way (gently so as to not activate the accelerator pump making things worse). DO NOT STOP CRANKING BEFORE PRESSING THE FOOTFEED! The engine should start, and run rough. Run the engine in neutral at a high idle for 20~25 seconds. This should be sufficient time for the systems to stabilize, and the engine to run normally.

 

If you have the issue, try the solution (it costs nothing) ;) and it just might work for you.

 

Jon.

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Good advise. The owners' manual for my '40 Packard 110 says that with a hot engine, one should crank over the engine while slowly depressing the accelerator pedal. This always works ,for this application anyways.

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For postwar GM cars that is two questions and the service manual usually has a section for "cranks but does not run". The other side is when a hot engine does not crank at all. This is most usually found in federal (68 and later) V8s without fuel injection).

 

What happens is that on a hot day after a few minutes a smog engine usually "soaks up" 20F-30F and the solenoid gets too hot to work. Can leave the hood up for 20-30 minutes and should crank again.

 

Back in the day  there was a Corvette big block heat shield that would work  on a small block. Have solenoids improved in the last thutty yar ?

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2 hours ago, J.H.Boland said:

Good advise. The owners' manual for my '40 Packard 110 says that with a hot engine, one should crank over the engine while slowly depressing the accelerator pedal. This always works ,for this application anyways.

Yep, always best to trust the engineers! I wonder though, do different types of gas make the problem worse than others? Especially ethanol vs non ethanol. Lots of people place the blame for issues like this on the ethanol but is that an actual cause or is it the way the petroleum part is made these days?

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Another issue with the described heat soak, is that when the fuel runs down into the intake and into the cylinders it can dilute the motor oil.

Fuel dilution can be a a very bad thing for oil's protection abilities and longevity.

 

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Ethanol raises the Reid Vapor Pressure (RVP) approximately 1 psi; so it does add to the problem. But even that fuel that does not contain ethanol has a higher RVP than gasoline of yesteryear.

 

We don't have a significant lobby to change the fuel, but we can adapt.

 

Jon

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Carbking - your explanation lines up exactly with my findings over the years with my '38 Buick Special.  (fitted with the Stromberg AAV-167 - factory recommended service replacement carb).

 

Many people will chime in with the quick answer - "Vapor Lock".  However, this is not the case.  Fuel percolation / expansion, exactly as you describe, is the culprit.  The starting technique is just like you describe as well.

 

But, here is what stumps me... my local old car buddies ('39 Packard Six, '50 Hudson Commodore, '37 Dodge, '40 Packard Super Eight) have no such issues!  Must be just the difference in the way the carb and fuel lines tend to heat soak on one car vs. another.  But, you would think at least one of these guys has the same frustration!

 

Thanks again for your explanation,

Jeff

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Carburetors take the heat for a lot of starting problems. (Yeah, a punishing statement. All puns included). But don't forget a lot of these 60 to 80 year old cars have spent time sitting. The distributor points plate may have dried lubricant on the cork pad or mechanical pivot points. When they start cold the plate has had a chance to creep back to its normal start position. On a warm start it may be slightly advanced. That will give you the hot start grunt and no start that points some to the starter, the 12v conversion, the welding cable battery cables, and a whole host of cures that overlook the initial timing.

 

So when the proper hot start procedure still doesn't do it check the action of that breaker plate. Especially if it grunts.

 

A couple of decades ago John Utz was at the shop with an old Rolls-Royce car. He richened the mixture a bit just before shutting the car off. A little while later he said "Watch this" and gave the spark lever a pull. The engine started right up nicely without the starter. "That's how easily they should start" he told me with a big smile. A little fuel in the cylinder and a snap of spark it all it takes.

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" gave the spark lever a pull." used to be known as "on the spark". My 88 Buick usually starts on the first tap, 3800s are like that. Suspect some engine/carb combos were better at it than others.

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Yep. This is why engines with down draft carburetors did not last as long as modern fuel injected engines. Gasoline back washing the rings. When I was young a lot of cars were considered worn out at 100,000 miles. Now we have cars that will go 300,000 plus with the same engine. Here in the Not so great North East the bodies rot well before the engines die today. Dandy Dave!  

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1 hour ago, Dandy Dave said:

Yep. This is why engines with down draft carburetors did not last as long as modern fuel injected engines. Gasoline back washing the rings. When I was young a lot of cars were considered worn out at 100,000 miles. Now we have cars that will go 300,000 plus with the same engine. Here in the Not so great North East the bodies rot well before the engines die today. Dandy Dave!  

 

Cars rot, but no where near as fast as they used to.   I can clearly remember 10 year old cars in the 70s being rot boxes.  These days a 10 year old car might have zero rust.

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On 5/5/2021 at 5:15 PM, padgett said:

For postwar GM cars that is two questions and the service manual usually has a section for "cranks but does not run". The other side is when a hot engine does not crank at all. This is most usually found in federal (68 and later) V8s without fuel injection).

 

What happens is that on a hot day after a few minutes a smog engine usually "soaks up" 20F-30F and the solenoid gets too hot to work. Can leave the hood up for 20-30 minutes and should crank again.

 

Back in the day  there was a Corvette big block heat shield that would work  on a small block. Have solenoids improved in the last thutty yar ?

This was a common problem with the GTOs. To make things even worse, the exhaust manifolds wrapped around very close to that solonoid. Pontiac also had a heat shield that helped-sometines.

Terry

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14 hours ago, Dandy Dave said:

Yep. This is why engines with down draft carburetors did not last as long as modern fuel injected engines. Gasoline back washing the rings. When I was young a lot of cars were considered worn out at 100,000 miles. Now we have cars that will go 300,000 plus with the same engine. Here in the Not so great North East the bodies rot well before the engines die today. Dandy Dave!  

If I get 440,000 miles out of my modern electronic whizbang, like I did my carbureted Econoline (before the salt and cinders we have to have in the winters in Missouri to keep poor drivers on the road) I will be happy. After the Econoline rusted out the third time, I gave up! Oh, and the cylinder head was never removed from the engine, nor any ring jobs.

 

Jon.

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On 5/9/2021 at 10:43 PM, carbking said:

If I get 440,000 miles out of my modern electronic whizbang, like I did my carbureted Econoline (before the salt and cinders we have to have in the winters in Missouri to keep poor drivers on the road) I will be happy. After the Econoline rusted out the third time, I gave up! Oh, and the cylinder head was never removed from the engine, nor any ring jobs.

 

Jon.

Was that a 300 CID 6 Clyinder? Also, did it get a lot of road miles between shut offs. That would be the difference between a vehicle that was run short distances and shut off constantly. Had a buddy here that had a 70's Chevy van with around 500,000 on the engine( 6 Cylinder 250 I believe.) before it was no longer worth fixing the other stuff. He was in the plumbing, electrical, and HVAC business. That van rarely got cold after it was started in the morning and always had a lot of road miles on it picking up new furnaces and whatever supplies to keep the help working. Those were tough motors given they had regular maintenance. A lot of cars around my area where it was a farming community got run short distances and shut off. A couple of miles to town and get a bag of feed, back to the farm. 5 Miles to the hardware store. Back to town. 1.5 miles to church on Sunday. 

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Shop truck, and yes, 300 CID.

 

Long distances to swap meets.

 

1/2 mile to parts store; just whatever was required.

 

So far the best I have done with one of the electronic whizbangs is 221,000. This is the one that replaced the other; and this one required a rebuilt engine at 140,000! Trans-axle blew at 221,000. Same driving cycle. Current shop truck (another electronic whizbang) required rebuilt engine at 106,000. So far, no other major expenses. I am preparing a 1968 (with 2 carburetors) to replace the current one when it gives up!

 

Maybe some folks are getting better longevity with the electronic whizbangs, but I am not.

 

To be fair, the carbureted vehicle did have a factory electronic ignition. In 440,000 miles, I replaced it 13 times! Had two new ones (spares) in the glove box when I took it to the salvage yard. 

 

I don't think electronics like the humidity in the summer and the salt in the winter in Missouri.

 

Jon.

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