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transmssion heat causing vapor lock
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Greg in GA
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PostPosted: Sat Aug 26, 2017 10:31 am    Post subject: transmssion heat causing vapor lock Reply with quote

So I have a new issue after a transmission/engine rebuild.

I started getting vapor lock (yes, it can happen on FI) and gurgling in the tank after a longer drive (1-2 hours). I replaced the fuel pump and filter thinking that was the problem. It wasn't.

I'm running FI that was retro fitted previously to my '74 bus.
So, I'm thinking the trans is heating the fuel pump from the outside in. (The fuel pump is located directly above the trans.)

Some more background.. this setup has been working fine for years before the trans rebuild. I can move the fuel pump, but I'd rather not do that unless I have to. any thoughts?
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Tcash
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PostPosted: Sat Aug 26, 2017 11:31 am    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Trans temps are not hot enough to cause vapor lock. Running the fuel line to close to the Exhaust is usually the problem.

Gas tank Gurgling is not normal.
Remove the gas cap and test. Ok: check your vapor lines are clear and not kinked.
Not ok: test fuel pump pressure and capacity.
Image may have been reduced in size. Click image to view fullscreen.

Not ok: check gas tank for obstruction in outlet or fuel hose.

Good luck
Tcash
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raygreenwood
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PostPosted: Sat Aug 26, 2017 11:37 am    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Greg in GA wrote:
So I have a new issue after a transmission/engine rebuild.

I started getting vapor lock (yes, it can happen on FI) and gurgling in the tank after a longer drive (1-2 hours). I replaced the fuel pump and filter thinking that was the problem. It wasn't.

I'm running FI that was retro fitted previously to my '74 bus.
So, I'm thinking the trans is heating the fuel pump from the outside in. (The fuel pump is located directly above the trans.)

Some more background.. this setup has been working fine for years before the trans rebuild. I can move the fuel pump, but I'd rather not do that unless I have to. any thoughts?


Its very possible...the transmission gets fairly hot while driving. Not "engine" hot...but hot.

But first make sure that your line pressure is not bleeding down rapidly after shut down due to either the pump check valve or the fuel pressure regulator.

These two check valves should allow keeping fuel pressure on the line in the 18-23 psi range for a while.

The purpose of this is exactly to prevent rapid fuel vaporization in the lines. The higher static pressure raises the boiling point.

Vapor lock can occur when the actual pressure of the fuel falls below its vapor pressure and there is enough heat to cause evaporation.

If the check valves can keep fuel pressure up until the line temperature and fuel temperature inside drop....you get no vapor in the line. Ray
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SGKent Premium Member
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PostPosted: Sat Aug 26, 2017 12:26 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

so it is a 1974 bus. Did you install another fuel tank nipple, a FI tank, or make an adapter to recirculate the fuel?
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Greg in GA
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PostPosted: Sat Aug 26, 2017 12:43 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

SGKent wrote:
so it is a 1974 bus. Did you install another fuel tank nipple, a FI tank, or make an adapter to recirculate the fuel?


It has the correct FI tank plumbed correctly. (Except for the fuel pump placement)

On testing the leak down of the fuel pressure regulator, that's a good idea, that's next.
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PostPosted: Sat Aug 26, 2017 5:12 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

since you have the right tank I doubt it is boiling in the lines unless the FPR is bad, or you are missing lots of tin / foam seal etc. The pump pushes the fuel into the side opposite the FPR where the pressure should be held in the 30's. That is 2 atmospheres or like 66 feet of salt water pressure - double that of a pressure cooker in the kitchen. The tank gets heat from the heater valves if they are the early style. The late style dump heat behind the bus. If those valves are missing or the fuel pump is near them that may be your real issue.

That said - heat is a nasty thing. It has been really bad in the western USA this year.
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Wildthings
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PostPosted: Sat Aug 26, 2017 5:31 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

If you have the access hole in your rear deck to get to the fuel sending unit, remove the unit and see what is floating around in your tank.

If you haven't done so already check your fuel pressure, too high or too low the engine will not run right.
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Greg in GA
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PostPosted: Sat Aug 26, 2017 5:59 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Wildthings wrote:
.

If you haven't done so already check your fuel pressure, too high or too low the engine will not run right.


That's the weird thing, the bus runs great up to the point it vapor locks or whatever it's doing. The tank gurgling is always present when the it dies.

I haven't had a chance to the check the fpr but wold that cause a hot only stall?

We just got back from a trip to tenn (500 mile round trip) and the bus only died twice. Once on the way, once back and it's only after driving for a good long while on a hot day.

I forgot to mention that the new fuel pump (just like the old pump) gets too hot to hold when it stalls.
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tootype2crazy
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PostPosted: Sat Aug 26, 2017 7:14 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

What is happening to your heat from the heater boxes when the heat is turned off to the dash? In later buses they actually installed tubes that vented that unused heat down and away from under the tank. I have actually had an FI pump fail with the heater box heat blasting onto it on a hot summer day. Perhaps something similar could be happening with yours?
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raygreenwood
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PostPosted: Sat Aug 26, 2017 7:46 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

tootype2crazy wrote:
What is happening to your heat from the heater boxes when the heat is turned off to the dash? In later buses they actually installed tubes that vented that unused heat down and away from under the tank. I have actually had an FI pump fail with the heater box heat blasting onto it on a hot summer day. Perhaps something similar could be happening with yours?


Tank gurgling?.....aside from ushing air through the system.....have you che ked you vent lines? You may be pulling a suction on the tank. As suction builds up it will slowly cause the fuel pump to starve until it cavitates....and that will run it hot. Ray
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PostPosted: Sun Aug 27, 2017 12:31 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

if the vent system is not working right so that the tank can't breathe, the lower pressure in the tank might cause the warmer fuel to boil. When this happens take the gas cap off and see if the gurgling stops.
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raygreenwood
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PostPosted: Sun Aug 27, 2017 12:50 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

SGKent wrote:
if the vent system is not working right so that the tank can't breathe, the lower pressure in the tank might cause the warmer fuel to boil. When this happens take the gas cap off and see if the gurgling stops.



Not boil.....at atmospheric unleaded boils at a little above 180F. Even with a vacuum...it would still have to be roughly 150F + to actually boil.

But vaporize.....sure. At about 95F and above it will vaporize.....But I think you are still in the right train of thought. Somethings up in the tank...either vapor or suction.

The other thing to look for....is that he noted his pump gets hot when this happens.

That is usually caused almost strictly by CAVITATION. This can happen from a clogged strainer, a clogged filter, a kinked line, a reversed filter (depending on type)...suction in the fuel tank preventing free flow of fuel to the pump....

And when the pump cavitates it shears vapor out of the fuel and in the early stages of cavitation before total loss of prime...and pushes that vapor into the main fuel loop.

It causes gurgling in the tank as it comes down the return line...just like air in the lines does.

Oh...also....what fuel pump is this?

If by chance someone installed a really bad-ass roller cell pump like say....a Bosch 044....that one will be more than you ever need ...until the tank volume gets down past a certain level on some applications....and then the pump will begin to cavitate and run red hot...because that pump in at least 50% of applications requires a feeder pump to stop this from happening.

Ray
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Greg in GA
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PostPosted: Sun Aug 27, 2017 2:23 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Just put a gauge on the fuel system. When I start the engine the fuel pressure shoots up past the 60 psi on the gauge. When the engine is shut off, the pressure goes to 35 PSI.

I can get a video of this but it doesn't make any sense.

SGKENT: the gurgling happens long after the cap is off and the engine has been shut down.

Ray: it's the "acceptable" FI pump from Bus Depot for $99ish

tootype2: the HEs have the mushroom on top that diffuses the exiting heat. anything's possible though.
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Tcash
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PostPosted: Sun Aug 27, 2017 3:21 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

60 psi is way high.
Go down to pg. 20 and run fuel pump test.
https://www.thesamba.com/vw/archives/manuals/afc_f...Manual.pdf

Good luck
Tcash
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Wildthings
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PostPosted: Sun Aug 27, 2017 3:55 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Greg in GA wrote:
Just put a gauge on the fuel system. When I start the engine the fuel pressure shoots up past the 60 psi on the gauge. When the engine is shut off, the pressure goes to 35 PSI.


Sounds like there is a restriction in the return line, this would cause a high pressure when the fuel pump was running but wouldn't effect the pressure after the pump was shut off.

By almost doubling the pressure you would also be almost doubling the heat being added to the fuel by the fuel pump, which in turn could cause the fuel to boil.
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Greg in GA
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PostPosted: Sun Aug 27, 2017 4:30 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Tcash wrote:
60 psi is way high.
Go down to pg. 20 and run fuel pump test.
https://www.thesamba.com/vw/archives/manuals/afc_f...Manual.pdf

Good luck
Tcash


will do, the only part I'm confused about is which hose do I disconnect? the hose going into the PR or the one coming out? (going back to the tank)
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Wildthings
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PostPosted: Sun Aug 27, 2017 4:40 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

You hook a hose to the return (forward) side of the FPR and collect the fuel coming out.
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raygreenwood
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PostPosted: Sun Aug 27, 2017 5:50 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Greg in GA wrote:
Just put a gauge on the fuel system. When I start the engine the fuel pressure shoots up past the 60 psi on the gauge. When the engine is shut off, the pressure goes to 35 PSI.

I can get a video of this but it doesn't make any sense.

SGKENT: the gurgling happens long after the cap is off and the engine has been shut down.

Ray: it's the "acceptable" FI pump from Bus Depot for $99ish

tootype2: the HEs have the mushroom on top that diffuses the exiting heat. anything's possible though.


That data point...60 psi at start up.....says a lot.

Either Wildthings is spot on and you have a restriction in the line.....could be anywhere downstream of the test nipple....even in the regulator itself.....or even between the regulator and tank.

But... If it were a restriction.....it should stay restricted.....not rise and then fall. Unless.....its keeping the regulator metering plate wide open.....and that higher spring pressure is the extra back load on the pump. Ok....that could fit.

All fuel pressure regulators work off of spring pressure. Every spring has an "unseating" pressure. It takes a few extra pounds of pressure to actually get the spring to unseat and start moving.

On a vacuum indexed FPR like the bus has.....you will see generally a "few" psi higher than stock idle setting when using an accurate gauge....when you first start up ....cor two reasons....
1. The spring unseating pressure

2. until vacuum is generated you do not get any pressure reduction for idle vacuum. This spike in fuel pressure should just be at most about 10 psi and last for maybe a second or so until the engine catches.

So....check to make sure you have vacuum at idle. Also use a hand pump to make sure the regulator holds vacuum.

Also.....make sure you have the right regulator. There are a few out there that look the same, work the same but are for systems that use higher volume.

Check for restrictions. Ray
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Tcash
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PostPosted: Sun Aug 27, 2017 6:43 pm    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Greg in GA wrote:
Tcash wrote:
60 psi is way high.
Go down to pg. 20 and run fuel pump test.
https://www.thesamba.com/vw/archives/manuals/afc_f...Manual.pdf

Good luck
Tcash


will do, the only part I'm confused about is which hose do I disconnect? the hose going into the PR or the one coming out? (going back to the tank)


For the Fuel pressure test. You hook the gauge to the fitting on the 3-4 Injector rail (line).

Testing FI Fuel Pressure FAQ
https://www.thesamba.com/vw/forum/viewtopic.php?t=175058
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Greg in GA
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PostPosted: Wed Aug 30, 2017 10:51 am    Post subject: Re: transmssion heat causing vapor lock Reply with quote

Update: I ran the fuel pump volume test and it passed. Output was 1/2 a quart in 30 seconds. My next step is to drain the tank and check the line from the FPR to the tank. I would have bet money the FPR was the problem.
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