Hot day, my wife's car is vapor locking. Thinking about upgrading to the higher volume prepump.

Of course, IPD sells this prepump (7845K) as an upgrade which includes pump/screen/hose/clamps ($125):

http://www.ipdusa.com/products/5799/7845k-in-tank-fuel-pump-upgrade-kit

I did find this article about using a prepump (3517845) from a 740/940 turbo:

http://people.physics.anu.edu.au/~amh110/Fuel_pumps/Fuel_swirlpot.htm

Seems that FCP sells this pump/filter (no hose/clamps)for $61:

http://www.fcpgroton.com/category-exec/category_id/151/sub_top_menu_item/by_make-by_model-by_year/by_make/78/by_model/1127/by_year/42

But the article states that it needs the mounting bracket (1312390) which is not available from FCP.

Anybody have any suggestions on either?

Paul

I had a similar problem a few years ago w/my '87 and came to the bb for the answer. Exact same symptoms and conditions. My trouble was the fuel pump relay having a cracked board and split solder. When the car was hot, the board would expand and stretch the solder apart, limiting the voltage being sent to the pump. When the tank got low and the recirculating fuel got hot, it would overheat the main pump and I would lose pressure and stall. I replaced the relay and never had the problem again. The pre and main pumps both survived the strain and are going strong 3 years later. Bonus: cheap part and easy to change.

The fuel pump relay has been resoldered and still looks good.

Paul

I would try another relay, I have had mixed results with soldering circuit boards. Besides the relays can also fail due to age and use.
Dan

I threw in a another fuel relay and I have a jumper wire to bypass the relay if it does it again.

I did run the prepump and it does run. How well? Can't really judge it listening through the filler neck.

Paul

If the pre-pump is working and pumping fuel everything is fine, it's only job is to keep fuel in the line feeding to the main pump. The main pump does not require any extra pressure or volume from the pre-pump to do it's job.
Dan

Hi!


Yeah! That‘s exactly right! But there is of course the possibility that the in-tank hose has deteriorated. Organic solvents are hard on elastomerics, and one thing that is worse then gasoline is HOT gasoline. The fuel temperature may exceed 150F because of convection from the hot pavement. The plastic nylon fuel line is a great insulator from dissipating the heat from the fuel!! Instead it constantly flows through the metal fuel rail mounted to the cylinder head. How hot is that!!!

The loss of fuel pressure could of course be caused by an electrical problem, and the pumps and lines themselves may be in perfect condition. It would be best to wait until the problem occurs and use a jumper on the fuses to verify the condition before replacing parts.


Goatman

The hose may have disintegrated. No way of knowing by listening in at the filler neck. Looks like I need to pull the sending unit and check it out.

I'll just replace the prepump when I pull it. Can't really have my wife breaking down and having to pull it again. I'll gather up all the parts before pulling it.

Paul

You can remove the fuel line from the intake side of the main pump to see if the in tank pump is working.
Dan

Malloy,


The 940 Turbo pump works very well. Because it reduces the lift of the main pump, the life should be extended quite significantly.


Goatman

Symptom: Loss of fuel pressure on hot day (100* F), sitting in heavy traffic, less than a quarter tank of fuel. I did not check if the main pump was cavitating before the car stalled out, or if the prepump was running (still haven't checked if the prepump is running or not).

Car started right up after sitting for a half hour. Filled up tank, no problem the next day at 100* F traffic conditions.

Sounds like vapor locking to me?

So, if I need to replace the prepump, just use the stock model.

Paul

That has been my experience. More importantly, do the WHOLE job correctly including:

Replacing the small hose between the pump and "pipe" with the correct submersible rated hose.

Testing the new pump before installing.

Possibly replacing the o-ring on the access panel, ground wire, etc.

In short, it is an easy, if messy, job that you only want to do once every 150K.

Onkel Udo

Are you certain it is vapor lock? Vapor lock is uncommon in cars with fuel injected engines. I haven't heard of it before in a 240 series with LH fuel injection.
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john

Hi John,

I'm sure the daffynition of "vapor lock" is at risk here. In a carbed car, the fuel boiling occurs in the float chamber. In a defective FI car, that same boiling might occur in the rail or, as I think in Paul's perception, in the tank-to-main pump line. A good indication of the latter would be the cavitation noise in the main, roller pump. Low pressure combined with heat -- same trouble, same effect.

None of us are really sure what our cars are suffering from, if we need to post for help on the board. We can relate symptoms to previous experiences.
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Art Benstein near Baltimore

If speed counted, rabbits would rule the world.

I need your email address. The one at flametrap.com doesn't work. Send me a message johnsargent dot one at gmail dot com.
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john

I've experienced "vapor lock" caused by the fuel vaporizing in the fuel line prior to the mechanical fuel pump on the engine. The result is the same, the engine won't run until the operating conditions are cooler.

Fuel injected cars usually run high enough system pressures to keep the fuel from vaporizing in the fuel line. However a metal fuel line adjacent to hot exhaust may get the fuel hot enough to be vaporized. Also, fuel injected cars generally have a constant flow of fuel through the lines, which will lower the temperature of the fuel.

If the intank pump isn't making enough pressure to exceed the vapor pressure of the fuel (at that temperature), there will be vapor lock. For sure, the stock system works well, and the T-brick mod for a high pressure pump doesn't seem like a good idea to me.
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john

Agreed. I think the distinction - carb vs FI - is that a carbed car may exhibit vapor lock symptoms even though the fuel system is in good repair, where an FI system in good repair will not.
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Art Benstein near Baltimore

Thought for the day: Never be afraid to try something new. Remember that a lone amateur built the Ark. A large group of professionals built the Titanic.

Sweet write-up Art. I just sent it over to my overflowing Save-it-file. Thanks.
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'86 sedan with 459,000 miles on the original engine and automatic transmission

Hi Paul,

Mark me down as advising against this mod the TB folks call an "upgrade."

The 86 240 just needs a working pre-pump to keep the main pump supplied with fuel, not a high pressure/high volume upgrade. Some of those TBers are bursting their tank-to-main pump hoses putting Wallbro 2-stage pumps in their tanks.

Surely there are those who love to out-think Volvo and spend money on "better ideas" who will disagree with me, but I say just diagnose it thoroughly and fix it.

In The Tank
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Art Benstein near Baltimore

Do not argue with an idiot. He will drag you down to his level and beat you with experience.

Art,


I think the real question is "what really is meant by 'vapor locking'", and what is the solution for Malloy1's car? The question is not about "what the folks on Turbobricks are doing".

I really believe that while the poster is motivated to fix the problem, he is not on the track of fixing the problem. Because "vapor lock" is something that happens exclusively to carbuerators, we are already on the wrong track.

What exactly is meant by the use of the term "vapor lock"? Please explain Malloy1.


Goatman

By the way Art, the celebration of the complacency of mediocrity is not something to be proud of. We would still be using kerosine lanterns had those who experimented not had been driven by the concepts of betterment.

While it is true that without people pushing the envelope, we would probably be walking everywhere...or still riding horses. However better is the enemy of good. An enormous amount of engineering effort goes in to designing and producing our cars. Replacing parts because someone says they are "better" without thinking about what requirement you are trying to meet is a waste of time and money.

That said, I would agree that you must first figure out what the problem is. While "vapor lock" is most commonly used to describe what happens to carbs, similar events can happen with fuel injected cars.

Charles
Houston, TX

Yup, there are a couple of posts here on the board today where a Brickster is asking for our help deciding on a course of action to fix his spouse's car. All I can do is advise from my own point of view, which is as a so-far successful maintainer of safe vehicles for my family-- not a racer or a racer's consultant. Those who disagree about making their wives' cars into testbeds for experimentation can write their own helpful replies.
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Art Benstein near Baltimore

Grandchildren are God's reward for not killing your own children.

what about 242GTs?(& I assume other B21, B23 240 & 244s of late seventies. When you have only a few litres left and standing on a hill, pump struggles an dcar will stall. I often find myself coasting back onto flat road or pushing in order to get some gas back into the feed line from the tank. Maybe its a K-jet issue or just the pump and the reason why TB'ers upgrape on early 240s?...keen to get other views