vapor lock

[Cape Cod Fleetwood]

And this 'is' the first summer/hot humid day I've driven The Ark.
Slow easy ride to the gas station, V-Power instead of Sunoco Ultra, only put in 3 gallons.
Up the street to get dog cookies.
Car WILL NOT START trying to depart.
Did my flood protocol, nothing. Finally it started but very violently then died again.
Fuel pump?
What was the last thing I did, got fuel. So I took off the gas cap.
Started right up, ran fine all day for 3 more errands.
Home to detail the engine for the show next week, put the gas cap back on.
Started up the car to return him to his lair, running like a lawn mower. Tried to go around
the block, all it did was spit and buck.
Stopped, took off the gas cap, and its a rocket again.

Has my gas cap failed? Did I have it on too tight? What else? It has just under a half tank of fuel.
If its the gas cap, do I replace with a modern one or a replica?

All answers and opinions are, as always, greatly appreciated.

\m/
Laurie

PS you should see the engine.... video on my FB page.
https://www.facebook.com/laurie.kraynick/videos/1832874253494382/

[35-709]

Sounds like your fuel tank vent system is plugged somewhere.  Your fuel cap does not "breathe".

[Jeff Rosansky CLC #28373]

The tank vents to atmosphere by a tube by the back axle. Mud dauber or something probably got in there. That is a lay on your back kind of fix. You could probably get a vented cap for now to get you to the next show. If you do then be sure to drive it around a bit to be sure.
While you are under there I would suggest replacing the rubber lines. Not hard while you are under there. Good preventative maintenance however with the tank being under vacuum those lines would have collapsed. You could now have debris on the lines because those old rubber lines would have deteriorated and pieces would have broken off internally. When done change your fuel filter too.
Jeff
Good job taking the cap off by the way.

[pmhowe]

Also, of course, do the easy thing first: Check to see if the cap itself is plugged. (I have had that problem, on a much older car.)
Phil

[The Tassie Devil(le)]

Here are pictures of the vent for a '69, and I would nearly hazard a guess that the 70 would be very similar.

That is unless Cadillac changed the fuel system to meet with anti-pollution laws, and installed the Charcoal Canister up front and made everything a semi-sealed system.

Then it could simply be a compromised cap.   The caps using the '69 and earlier systems were sealed.

Bruce.

[Cape Cod Fleetwood]

Thanks guys...

No charcoal canister on this car Bruce.

I remember when I was YOUNG and my dad's old Caddy's, him having the same problem. He just took off the gas cap when he was driving and put it back on when it was parked.

Could the summer fuel blend be the issue...

For $6 I can get an 'OEM' gas cap at Autozone, I'll start there.

Tuesday at 0900 The Ark will be on Scott's lift getting the new EB cables installed.
I can screw around with the fuel tank vent while he's busy.

Thanks so much, as always.

BTW, I detail cleaned the engine today, holy cow. Its smells like Mr. Clean! Sponge bath, "light" hosing off. Towel dry. Mothers VLR on all the wires and hoses, WD40 on everything else, it just sparkles.
One thing that absolutely pinned my OCD from the moment I saw the car in December was the reservoir for the washer fluid. A previous turd spray painted the wheel well and spray painted half the reservoir black in the process. That came off today, scrubbed with gas, washed thoroughly, now its pearly white again.
Wash and polish the outside tomorrow, wash and polish the inside on Sunday.
The Ark is in a 'judged' show a week from today, stock class 1968-1970. And I know the winner will be a club
member, and I'm not a member of that local club. But I want to give The Ark every advantage I can. That's fair to the car.

Thanks again guys.

\m/
Laurie

[Mike Josephic CLC #3877]

As others have stated, your system has a non vented
cap. Therefore, the venting system must be plugged up
somewhere if it ran OK once you removed the cap and
allowed air to enter.

If you want to get a temporary "fix" you'll need to get
a "vented" cap until you can get it properly fixed.

Mike

[Cape Cod Fleetwood]

As others have stated, your system has a non vented
cap. Therefore, the venting system must be plugged up
somewhere if it ran OK once you removed the cap and
allowed air to enter.

If you want to get a temporary "fix" you'll need to get
a "vented" cap until you can get it properly fixed.

Mike

Agreed. A temporary fix is not using a gas cap at all while driving.
For $6 I take the gas cap completely out of the equation, its an aviation thing, LOL!
From the looks of it, the current cap is original.

\m/
Laurie

[Mike Josephic CLC #3877]

Driving without a gas cap on is not really a safe idea.
Depending on how full your tank is, gas can splash out
when the car corners, especially at higher speeds on
sharp bends. 

I would invest the $5 on a vented cap.

Mike

[Scot Minesinger]

Laurie,

Let me tell you how to start a 1970 Cadillac (probably applies to 1967 - 75 or later) after it has been warmed up real well during hot weather, and shut off for 15 minutes.

What happens is that the new gas with ethanol boils out of a hot engine because the engine picks up temperature when shut off.  I installed a temperature gauge, and while during hot weather it operates around 205'F, if I check the temp gauge 15 minutes after a hot start up and just before restart, it is 230'F.  Upon start up it immediately cools down.

You have a fuel return line and it is extremely unlikely your Cadillac suffers vapor lock, and if it is running OK, you should be fine.  Next time car has been running a while and warmed up shut it off during hot weather say to fill the tank, it should start up normally after five minutes because all the gas has not boiled out. 

To start it after it has been shut off for 15 to 90 minutes during hot weather after a thorough warm up, depress the gas pedal about 1/2", or so in that when it starts it will be in fast idle speed.  Do not pump the gas pedal, just depress it slightly.  Turn the key and it will crank a while, but should start.  When it does start you need to keep it at a fast idle speed for 20 seconds or so to get some cool fuel up into the carb. 

This is the way it is, unless you install a complicated post shut off cooling fan with time delay shut off - many modern cars use this feature.

In this circumstance the Cadillac is the opposite of flooded, it has no gas, so the flooded starting procedure is of no value.

Keep cool and enjoy your Cadillac!

[chrisntam]

snip... unless you install a complicated post shut off cooling fan with time delay shut off - many modern cars use this feature.

I thought about doing this, but don't you need an electrically driven water pump to circulate the hot water?    Fans running after shut off only cool the radiator which (I think) really won't help the heat soak of the engine.  So, using the thought just noted, I quickly abandoned any attempt of after engine off cooling.

Thanks for the advice on the hot restart, I'll give it a try.  Overall, mine does pretty well, though every once in a while it gets a bit testy...

["Cadillac Kid" Greg Surfas 15364]

Laurie,
It seems like everything up to and including rebuilding your flux capacitor has been suggested.  I do not believe you will find ANY vented gas cap that will fit, I've tried for different reasons.  When you run a tank below 1/2 full it does not take much to pull a vacuum on the tank IF THE TANK VENT IS PLUGGED.
You have spent innumerable hours making your car look "pretty". It also should be capable of running well as intended. CHECK AND CLEAR THE TANK VENT(please).

Scott,
As an engineer you must know that the addition of (typically) 8% Ethanol to modern gasoline blends will lower the boiling point of the mix a phenomenal 3/10 of a degree F.  Ethanol seems to get blamed for anything we can't fix, but aside from it's solvent effect on certain older materials and the 3.2% decrease  in fuel value (and of course the ridiculous idea of using food for fuel stock) there  are no operational issues to its use.
Greg Surfas

[Scot Minesinger]

Chris,

Another member of the club had a 1973 SDV and he used an additional electric fan on a timer so that he claimed would cool the engine enough to prevent this long cranking time start up after a hot run and 20 minute shut -off.  This can be verified, although I did not try it with a simple box fan.  Run car for half hour in hot weather, shut it off, open hood, aim box fa at engine bay, and see how it starts in 20 minutes.  To me this was just not worth it.

Greg,

Yes, but undeniably the gas is different in 1970 then today including no lead, less octane and etc.  If it is not the gas, how is this condition repaired without adding an auxillary means of cooling to prevent engine from getting really hot after shut-off on hot day?  This does not happen to me much under 70'F.  It is not a huge thing for me now, but if there is a reasonable fix, like to know what it is?

[G Pennington]

A quick and cheap fix for the carburetor heat soak problem is to add a phenolic insulator/spacer between the carb and the intake manifold.  Stick with a 1/4" to 1/2" thick spacer and you should not have any issues with linkages & fuel lines, or hood with clearance.  Might need longer mounting studs.  Another thing you can do is pull the intake and block off the exhaust crossover passage.  Only disadvantage would be longer warm-up times.

[Scot Minesinger]

Greg,

The automatic choke is bolted to intake manifold and linked to carb.  A spacer of a 1/4" would make a problem that possibly could be overcome.  Not a bad idea.

["Cadillac Kid" Greg Surfas 15364]

Scott,
First of all I assume your last post was intended for Garey.
Secondly, this thread has gone far afield from Lauries question regarding tank venting.
However, in regards to the discussion of heat soak and hard hot starts the reasons for heat soak are built into the design of the motors, and are compensated for by the captive overflow cooling system and the use of high torque (read that long armature) starters.
These motors were high output, high thermal efficiency in design and were intended to run somewhere around 220 degrees.  That said combustion temperatures and thus cylinder head metal temperatures are near 1000 degrees (+), so it does not take much for that heat to transfer to the stagnant coolant and raise its temperature to about 260 degrees.
The difference in temperature is not enough to appreciably effect thermal expansion of the steel/iron assembly and again, the correct starter will provide sufficient torque to spin the hot motor freely.
Fuel "boil-off" from a heat soaked 472/500 is absolutely minimal unless there is something wrong such as a leaking carb plug of possible faulty fuel check valve (in the fuel filter).
Part of the issue, I believe is comparing the operational characteristics of these cars with the new cars.  They were born in different eras and probably the difference in "real" cost between them is an indication of the sophistication of new cars.
Lets leave this thread for Laurie's "dilemma" and if there is enough interest in dredging up this well beaten issue of hot soaks, perhaps start a new thread.
Greg Surfas

[Cape Cod Fleetwood]

Laurie,
It seems like everything up to and including rebuilding your flux capacitor has been suggested.

PMSL!

You have spent innumerable hours making your car look "pretty". It also should be capable of running well as intended. CHECK AND CLEAR THE TANK VENT(please).

And well over a thousand on mntx and repairs too Greg. Car heads to the shop tomorrow morning, I'll brief Scott on the tank vent, I'm sure its the first place he'll look. Thank you!

I don't have my shop manual handy, its in the Ark's trunk, part of the 'display'. Can I assume the tank vent is above the tank meaning the tank has to be dropped?

\m/
Laurie

[Scot Minesinger]

You do not have to drop the tank to get to the vent.  It could also be that the screen at fuel pick up is clogged and or needs to be replaced (often referred to as the sock).  This is easy to check with some compressed air.  The tank is easy to drop too, have done it several times - bring it to him with not much gas in the tank.  The tank is 26 gallons, and on "E" there will be five gallons or so (40lbs or so of fuel) plus tank weight.

[Cape Cod Fleetwood]

Thanks Scot, it goes in tomorrow with a half tank of fuel.
Where is this 'sock'?
Tank might be easy to drop but I'm still paying an hourly labor rate...and there's much to do tomorrow.

\m/
Laurie

["Cadillac Kid" Greg Surfas 15364]

Laurie,
The "sock" is a metalic filter that slides on the end of the fuel pick-up tube in the tank.  Blowing back through the fuel line is a sure way to clear any debris that may have the sock plugged.  It does that by simply blowing the sock off the end of the pick-up and you are left with no filter in the tank to screen out the debris.
The symptoms you described have nothing to do with the fuel pick-up filter. TANK VENT, TANK VENT, TANK VENT.
Greg Surfas