Author Topic: Why does it vapor lock - nothing has changed?  (Read 4442 times)

Offline haroldwca

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Why does it vapor lock - nothing has changed?
« on: June 03, 2018, 10:14:41 PM »
I have a 1985 Silverado with an engine swapped in from a 1992 Silverado about five years ago.  I retained the 1985 truck's original Q-jet intake and added a non-electrical connection Q-jet from a 1977 truck.  I run stock (from the 1985 truck) exhaust manifolds with stock exhaust in stock location.  I still have the original breather and intake tube out to the core support.  The original steel fuel line is in place, with only the in-carburetor fuel filter.  The filter is still functional, and the fuel pump is new (3 line, as original).  The engine has been in the truck for five years.  Only recently has the truck been vapor locking after driving for a while, stopping (which lets it heat soak), then after a hot re-start it vapor locks in about a minute at the first stop sign/light.  The only warning sign is that the engine begins to progressively run rougher, then stumble, and then in dies.  It takes less than a minute for it to die.  If I can get it out onto the open road, it doesn't die, and eventually smooths out. 

The last time this happened was yesterday.  After it died at an intersection, I was able to pour about an ounce or two of gasoline into the bowl vent tube.  Then, the truck started immediately, with no further problem.

I understand this is the common scenario for vapor lock, but why only now?  I have read a multitude of info on the internet, and many solutions (such as wrapping the fuel line with insulating material) will probably work.  But what is the cause that wasn't present from the factory, or even five years ago when I introduced this engine combination. 

The last carbureted vehicle left the factory decades ago, but as I recall, they didn't vapor lock from the factory.  Why does time, alone, cause it to begin?  What do I need to fix, replace, re-adjust, etc. to make this problem go away?  I don't like the idea of wrapping the fuel line to keep the truck from vapor locking.  It didn't need it for over thirty years.  I would like to be able to correct the issue without introducing a band-aid solution.

Online bd

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Re: Why does it vapor lock - nothing has changed?
« Reply #1 on: June 03, 2018, 11:26:24 PM »
Make sure the fuel tank vent line to the charcoal canister, the fuel supply line and the return line between the tank and fuel pump aren't kinked or restricted then perform a routine draw, volume and pressure test on the fuel pump.  Don't assume the pump is functioning correctly because it is "new." 
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Henry

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Re: Why does it vapor lock - nothing has changed?
« Reply #2 on: June 04, 2018, 12:42:02 PM »
Hi Harold:
I would not jump to the conclusion that the problem is vapor lock at this time. It could be something else that is going wrong when the carb transitions from cold start mode to fully warmed up mode. Do the checks as BD has advised and also go through some other basics like:
check all your vacuum lines and connections
Check your linkages for free action
What kind of choke do you have?...divorced?...make sure it is operating ok.
How many miles do you have on the carb since you last opened the air horn for inspection/cleaning/adjustment of float?
What kind of filter do you have?...long, short, or sintered?...if it has a check valve, what is the condition of the valve?
What does your water temp gauge read when you are getting the problems?
Is that metal line between the carb and the pump resting on the engine anywhere?
Regards,
Henry

Offline haroldwca

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Re: Why does it vapor lock - nothing has changed?
« Reply #3 on: June 04, 2018, 09:25:23 PM »
Thanks to both of you for your replies.  I will make the suggested line/hose checks, but what do you mean by "draw"?  BD, I understand what you are suggesting when you say don't assume the fuel pump isn't the problem just because it is new. "New" parts can be defective.  I will say that I am less likely to suspect it since the exact symptoms have persisted from the previous fuel pump to the new one. 

Henry, to answer your questions:
The choke is electric and operating as it should.
Mileage since rebuild on carb - probably about 15-20,000 mi.
Stock Q-jet filter.  Inspected for blockage. No check valve.
Water temp gauge reads normal, which is to say less than the halfway mark, which is normal for this vehicle.
Metal fuel line does not rest on engine, but stock routing has it passing close to the engine, behind the alternator.

I'm not sure if we're on the same page in this statement.
"It could be something else that is going wrong when the carb transitions from cold start mode to fully warmed up mode."  The problem demonstrates itself only when the vehicle is at operational temperature AND is allowed to heat soak, i.e. sitting briefly with engine off, or idling for a few minutes.  This is the critical factor, and in my judgment points to vapor lock.  If I'm missing something, please tell me what you think.

Offline blazer74

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Why does it vapor lock - nothing has changed?
« Reply #4 on: June 04, 2018, 10:30:53 PM »
Been down this road. Basically the same symptoms except stumbling and bucking briefly then would clear up. Would never stall.

Stop at Sonics for 15 minutes, back on the road and within a half mile or less  it would happen among other times also.

Tried non ethanol gas and helped but never completely solved.

Replaced every thing rubber from the tank to the carb with fuel injection hose. Filters, pumps including a edelbrock high volume pump to no avail.

Tried leaving the gas cap off.

My 74K5 did not have a return line system installed from the factory.

Finally used the vapor line from the carb to the tank as a return line for testing,  added a fuel filter with a 3rd return line connected to said vapor line up close to the carb. Vented the gas cap other wise the tank will pressurize from the returning fuel.

Seems to have corrected the problem, for the time I drove the Blazer after installing.

Currently down for Rear end work and replacing the fuel sender for one with a return port as well as a 31 gal tank.

Don’t know if yours has a factory return system at the fuel pump or not. Have heard some still have the problem with the factory return system at the pump.

I’ve owned my Blazer since 1988 and never had a problem either then all of a sudden some time ago.

Also swapped distributors at one time.
« Last Edit: June 04, 2018, 10:38:01 PM by blazer74 »

Online bd

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Re: Why does it vapor lock - nothing has changed?
« Reply #5 on: June 04, 2018, 10:50:43 PM »
"Draw" is synonymous with pump suction.  Fuel pump draw should measure >20"Hg while cranking, fuel transfer volume should measure >1 qt in 30 seconds of cranking (typically 1 qt in ~20 seconds), and developed pressure should measure 5-7 PSI but can be as great as 9 PSI.

I wrote,
Don't assume the pump is functioning correctly because it is "new." 

The key word is "functioning."  Don't forget the fuel pump is actuated by a pushrod, which reciprocates by following an eccentric lobe on the camshaft.  Although rare, issues can develop with the pushrod and/or camshaft, typically due to inadequate lubrication.  In select cases, the pushrod will score and partially seize in its bore, limiting the fuel pump lever travel.  Did you switch fuel brands recently or begin using higher alcohol content fuel?

Another possibility is weak spark.  But, since you feel the symptoms are fuel related, perform the suggested checks to eliminate them as unknown variables. 
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline roadrunnerkitten

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Re: Why does it vapor lock - nothing has changed?
« Reply #6 on: June 05, 2018, 07:31:27 AM »
"Draw" is synonymous with pump suction.  Fuel pump draw should measure >20"Hg while cranking, fuel transfer volume should measure >1 qt in 30 seconds of cranking (typically 1 qt in ~20 seconds), and developed pressure should measure 5-7 PSI but can be as great as 9 PSI.

I wrote,
Don't assume the pump is functioning correctly because it is "new." 

The key word is "functioning."  Don't forget the fuel pump is actuated by a pushrod, which reciprocates by following an eccentric lobe on the camshaft.  Although rare, issues can develop with the pushrod and/or camshaft, typically due to inadequate lubrication.  In select cases, the pushrod will score and partially seize in its bore, limiting the fuel pump lever travel.  Did you switch fuel brands recently or begin using higher alcohol content fuel?

Another possibility is weak spark.  But, since you feel the symptoms are fuel related, perform the suggested checks to eliminate them as unknown variables.
I went thru similar issues. More than 1 resolution lol. Weak spark from burnt plug wire. Then fuel issue: elec pump mounted n motor compartment, duh, shoulda been near tank. So I switched to mechanical pump, using elec as primer, then found out tank line kinked. Runnin strong for now...


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Offline Henry

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Re: Why does it vapor lock - nothing has changed?
« Reply #7 on: June 05, 2018, 01:45:59 PM »
Hi Harold:
Concerning my comment about the problem showing itself when the carb transitions from cold start mode to fully warmed up mode, I am suggesting that when the carb gets fully heat soaked from idling or shut down there can be something else going on than just vapor lock...something in the carb may be affected by the high temps when the float chamber is full of fuel and the fuel in the chamber is getting hotter than normal running (because it is not circulating out from mid or high range operation). Possibly this scenario is causing gunk in the float chamber to dissolve and move to block the primary jets...or the float pontoon gets stuck in the up position when hot...or the stuffer is degrading and interfering with the float when it gets real hot...I am suggesting that something new has changed/broken/degrading to cause this problem...and since you should have seen vapor lock a long time ago this may not be what is happening (after you made no more changes to engine).

When you say you have a new fuel pump, do you mean it is new as of now...or you mean it was new when you made the engine swap 5 years ago?

What is the configuration of your ECS (or EVAP) system?...do you have a single charcoal cannister or two?...typically the designs on later models is for the cannister to purge when the engine is only above idle...purging at idle can cause rough running...so in your vacuum hose inspection you should take a look at where these ECS hoses are terminated at...there are solenoids (mechanical valves; vacuum and temp) either on the cannister or intake manifold or carb that controls the cannister purging...possibly one of these solenoids has gone south. On later models the hose routing on ECS became spaghetti so you really have to make a diagram of your config to see what is going on.
Regards,
Henry

Offline haroldwca

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Re: Why does it vapor lock - nothing has changed?
« Reply #8 on: June 06, 2018, 07:59:37 PM »
Chalk one up for BD - never assume a new fuel pump isn't the problem.  This morning, I filled my fuel tank and went out to duplicate the circumstance of stalling.  Achieving that, I attempted the cheapest guess that I could - I slit a piece of heater hose lengthwise and zip tied it around the fuel line to insulate it.  Repeating the test proved that I had solved nothing.  After the truck cooled I could start it again.  Returning home, I removed the fuel line from the carb and cranked the engine.  Nothing came out of the open end.  I had eliminated the carburetor from the equation.  Next I removed the fuel supply line to the fuel pump, and attached a fuel hose to the open inlet on the pump.  I put the other end of the hose in a Coke bottle filled with gasoline sitting below the pump.  Again, I cranked the engine.  Again, nothing came out of the fuel line.  I had eliminated everything but the fuel pump.  After replacing the pump, I ran the test route one more time.  No more stalling!  I have no idea as to why a four month old fuel pump failed, and I REALLY have no idea why it failed only when it had heat soaked, but now my truck runs reliably once again.  Thanks to all who chimed in with advice ! 

Offline Henry

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Re: Why does it vapor lock - nothing has changed?
« Reply #9 on: June 10, 2018, 12:56:03 PM »
Hi Harold:
So from your testing would you conclude that your fuel pump started to not work when cold as well as not work when it was hot? Either way seems like you should get your money back on that fuel pump....where did you buy it and what was the brand of the fuel pump?...do you know what the pressure of the pump was specd as?...stock?
Regards,
Henry

Offline Cheyenne1010

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Re: Why does it vapor lock - nothing has changed?
« Reply #10 on: September 21, 2021, 12:44:24 PM »
The gas we buy now has ethanol in it and ethanol has a very low boiling point.what I did to my truck is,i put that fuel line sleeve on anywhere the fuel line comes close to a hot spot.then I add a electric fuel pump,the kind that you bolt to the inside of your frame close to your fuel tank