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My 87 CABBY will start up and just die after chugging up a lung. I was told it was vapor lock andneeded to make sure the gas cap was on tight. Well I done that and it still happens when it is hot outside. THe car is not overheating( temp gauge works). But I have to wait until it cools off before it will start up again.
It is a stock engine.

Any thoughts or suggestion?
 

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Re: What exactly is "Vapor Lock" (nana)

"Vapor lock" is caused by gas overheating and boiling inside the fuel pump of a hot engine, mostly in engines with the fuel pump in the engine sucking the fuel from the back of the car where the fuel tank is located. Cars nowadays have fuel pumps in the fuel tank pushing the gas rather than sucking it. What is happing is your gas stops flowing. The problem is the engine is too hot. I'd suggest looking at anything that would make your engine run any hotter than normal, check the cooling fan as the plug likes to fall out at times.
cheers. Good luck.
 

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'86 Cabriolet Vapor Lock

For years I was confused, thinking my '86 Cabriolet (195K mi.) was overheating when it would buck, stutter and stall in traffic. Replaced entire cooling system components to no avail. Even added a switch to override radiator's temperature switch to turn on the fan earlier to prevent such "over heating" episodes. My Bentley manual's diagnosis section suggested vapor lock and replacing the fuel pump check valve. I did so for $20 (autohausAZ.com) which appears to have solved the problem. Now, the temp gauge can rise much higher than before with no such bucking. Didn't have to spend needless money on a new fuel pump ass'y which automatically includes a new check valve. Now I have a needless (2nd rear window defroster) auxiliary switch mounted in my dash ;)
 

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...... My Bentley manual's diagnosis section suggested vapor lock and replacing the fuel pump check valve. ........
Help me out here please, I just looked through my Bentley's CIS Troubleshooting Charts, I could find no mention of vapor lock. Please, what page is that vapor lock info on?

My 87 CABBY will start up and just die after chugging up a lung........
Vapor lock can usually only occur on hot summer days due to heat soak after a hot shut down when fuel boils (evaporates) in the fuel loop.
Due to the very high fuel pressures that CIS uses, vapor lock is very rare, maybe impossible, if it were to occur, CIS' very high fuel pressures would clear it very quickly.
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To the root of the car's problem, what does your dwell/duty cycle meter tell you that the car is doing when it acts up?
Have you done a full tune up with quality parts?






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Check the residual fuel pressure. CIS cars have a fuel accumulator that keeps pressure on the injectors when the car is so that when it is hot outside the car will still start. Liquids under pressure take a higher temperature to boil/evaporate. Since the CIS injectors are actuated by fuel pressure, if the fuel is turning into a gas in the line at the hot engine/injector, the injector will not open correctly/at all.


TL;DR
Check residual fuel pressure. Replace fuel accumulator as needed.
 

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Liquids under pressure take a higher temperature to boil/evaporate.
Liquids under pressure boil under a lower temperature. Also, the accumulator is there to remove pulses from the fuel pump and the injectors are completely out of the loop until the airflow plate is not in it's rest position (car running or lifted manually for testing).

Anyhoo, I'm going to suggest checking your in tank pump. Same symptoms as vapor lock, yet an entirely different scenario.
 

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Liquids under pressure boil under a lower temperature. Also, the accumulator is there to remove pulses from the fuel pump and the injectors are completely out of the loop until the airflow plate is not in it's rest position (car running or lifted manually for testing).
http://www.iapws.org/faq1/boil.htm said:
"At standard atmospheric pressure (1 atmosphere = 0.101325 MPa), water boils at approximately 100 degrees Celsius. That is simply another way of saying that the vapor pressure of water at that temperature is 1 atmosphere. At higher pressures (such as the pressure generated in a pressure cooker), the temperature must be higher before the vapor pressure reaches the surrounding pressure, so water under pressure boils at a higher temperature. Similarly, when the surrounding pressure is lower (such as at high altitudes), the vapor pressure reaches that pressure at a lower temperature. For example, in the Denver, Colorado area of the U.S. where the elevation above sea level is approximately one mile (1600 meters), the atmospheric pressure is about 83% of a standard atmosphere, and water boils at approximately 95 degrees Celsius."
......... I paid attention in HS chemistry. Also, have had the exact same issue on a CIS car. Replaced fuel accumulator, fixed problem.
 

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......... I paid attention in HS chemistry. Also, have had the exact same issue on a CIS car. Replaced fuel accumulator, fixed problem.
You are correct, I don't know where that brain fart came from.

As for the accumulator, IIRC, they only put about 50lbs of pressure on the lines, less than that of the actual pumps (I guess it wouldn't function if it was designed to put more pressure than the pumps, right?) It does not hold pressure to the injectors when the vehicle isn't running. A faulty accumulator can cause problems, but you can actually remove them entirely and not notice it's gone. It seems to me the pumps alone are enough to assure more pressure than the accumulator applies. I know I've heard info before about them working against vapor lock, but that logic is lost on me. Not saying it isn't true I guess, I just don't see it as a viable application to stop vapor lock with a good CIS pump.

I've also never seen CIS have vapor lock. Again, not saying it isn't possible, but HIGHLY unlikely.
 

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..... CIS cars have a fuel accumulator that keeps pressure on the injectors ......
This is incorrect and a common misconception.

.....the accumulator is there to remove pulses from the fuel pump and the injectors are completely out of the loop until the airflow plate is not in it's rest position (car running or lifted manually for testing).......
This is correct.

Residual fuel pressure is contained between the fuel pump check valve and the fuel pressure regulator on the fuel distributor. While the accumulator is in that residual pressure loop, the accumulator's spring could break completely and residual pressure should still remain.

All that said, if the accumulator is leaking fuel externally, that would not only be an extremely hazardous condition, it would indeed cause a loss of residual pressure.
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Bentley reference to Check Valve/Vapor Lock

My Bentley Service Manual covers Cabriolet and Scirocco 1985 through 1993. Two sections directed me to suspect check valve/vapor lock:
pg. 5-6 Fuel System--Gasoline, Table a. CIS Troubleshooting Symptom #2 Hot Start..., Probable cause "e. Insufficient residual fuel pressure" = Corrective action e. ...Replace fuel pump check valve...
This also stood out to me:
pg. 5-13 Replacing Fuel Pump Check Valvestates "...This helps to prevent the fuel in the lines from overheating and causing vapor lock. It is also an aid to hot restarting."
I had observed my bucking episodes happening either when ambient temps rose above 100F at highway speeds, summer driving in Sacramento and Sierra Foothills and stop-n-go traffic on freeways. Car would stall out and I would have to pull over and park for about 15 minutes. Therefore, original suspicion of engine coolant system at fault but finally resolved by replacing check valve.
 

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Confirmation - Fuel pump check valve replacement solved problem

It has been four months, I just wanted to confirm that replacing solely the check valve at the fuel pump outlet has resolved past overheating, shuttering and vapor lock issues on my '86 Cabriolet. Warmer weather and idling in traffic has driven coolant temps multiple times beyond past problematic temperatures with no tendency to stutter and stall. Radiator fan automatically responds as it should.
Go VW! Welcome, traffic. Chalk another one up for the Bentley Manual.
 

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It has been four months, I just wanted to confirm that replacing solely the check valve at the fuel pump outlet has resolved past overheating, shuttering and vapor lock issues on my '86 Cabriolet. Warmer weather and idling in traffic has driven coolant temps multiple times beyond past problematic temperatures with no tendency to stutter and stall. Radiator fan automatically responds as it should.
Go VW! Welcome, traffic. Chalk another one up for the Bentley Manual.
Good work! I'm doing my pump (and with it the check valve) within a week or two, and looking forward to similar results hopefully.
 
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