ROG100

The usefulness of the in-tank fuel pump on the early and later cars was always suspect and many have exchanged them for the in-tank filter screen with zero issues.

With OEM no longer available or extremely hard to find (there are one or two out there sitting on shelves some where).

Porsche raised the price to $441 from circa $230.

Rest in peace little in-tank pump 8>(

davek9

RIP

Randy V

It was the appendix of the 928.

Only good for causing problems.

drwhosc

Will Just have to wait for mine to fail, then switch out to a filter screen... Wonder why the went so high on these. does not seem to be a tough thing to build. the casing may be the only issue, the pump is not that big of a deal. I replaced the actual pump on my range rover, and it was not that bad of a job

SeanR

This doesn't bother me one bit.

the flyin' scotsman

I replaced my working intank pump.........in the parts bin for when I need to sell for beer $$$$

sstrickstein

On PET it looks like a direct swap. Also says something about the in tank pump only being for AUS? Australia? Could this be because of running the car down under? Or Austria, because of all of the Sound of Music? Who Knows.

hwyengr

In Germany, they came first for the in-tank fuel pumps, but I did not speak up for I did not have an in-tank fuel pump.

Tom in Austin

So why did they do two fuel pumps in the first place if one is sufficient?

hacker-pschorr

Easy answer = German Engineers

To understand what this means, go buy a British or Italian car of the same year.

Petza914

I just went through this on my '79. The failed in-tank pump was causing the main pump to whine badly when trying to pump fuel. I'm replacing it with the in-tank screen and an hoping that resolves the whine or buzzing from the primary pump.

Porsche did it again on the 997.2 Direct Injection engine. 2 fuel pumps - one low pressure supplying the high-pressure pump and had a bunch of failures (I think on the high-pressure pumps). I think all direct injection systems work like this, but no need for the additional complexity on our normal FI systems.

GregBBRD

You are all "sort of" correct.

You don't need an in tank pump....98% of the time. You also don't need that fuel cooler that hangs on the firewall behind the air filter....probably 95%% of the time.

However, if you are going to remove the in tank pump and use just a filter screen, you had better change the main pump to one that was designed to suck fuel and not have fuel delivered to it. That means, if you have a 1989 to 1995 928 and you remove the in tank pump, you need to replace the main pump with one from a 1987-1988.

People that don't drive long distances or run their fuel level low will fair much better with the in tank pump removed than people that drive their cars long distances and run the fuel level low. It's not "rocket science", just first year Chemical Engineering.

The in tank pump is there for a few different scenarios. Hot fuel (long drive, heating up the fuel), low tank level (with hot fuel or on a hot day), higher altitude, or a hot day after the fuel companies have switched to "winter fuel blends".

The main pump creates more "vacuum" (suction) in the tank than the in tank fuel pump. Anytime the vapor pressure of the fuel is reduced significantly (from one of the above or multiples of above) a further reduction of the vapor pressure may allow the fuel to boil....causing stalling and impossible restarting, until all the fuel returns to the liquid state.

People that drive or use their cars hard at higher altitudes are painfully aware of this. Anyone that has ever run their car at a track event in Denver knows all about this problem. They keep their tanks fuller....and never remove their fuel caps after a track session until the fuel has a chance to cool down.

So, if you insist that you are smarter than the Bosch or Porsche engineers, remove that in tank pump. However, keep your fuel tank level high, don't drive long distances, and don't drive at higher altitudes....and never combine any of the above! Make sure that the vapor recovery system is working properly and that you build pressure in your fuel tank....not create a vacuum or vent the tank to atmosphere.

I still build and work on the cars as if people are going to hop into them and drive from LA to Vegas....or to San Francisco. They all get the in tank pump put back in! When Porsche runs out of those....I already have an alternative completely done and ready to go....but until that day, I think it's easier to use the proper part for the proper job.

One quick thought....if you can't afford a $400 in tank fuel pump....a 928 might not be the correct car, for you. These things didn't cost $90,000 in 1989 because they were using really rare and expensive air in the tires!

FredR

Now this is an interesting discussion point- I have often wondered about the wisdom of deleting the in-tank pump and what Greg decribes is pretty much what I would expect given my [professional] knowledge of pumping systems. The systems I typically deal with are 5MW jobs- a little bit bigger than our fuel pumps but the principal is identical.

The problem our fuel system is dealing with is the boiling point of gasoline. Because gasoline is a mix of different hydrocarbons they start boiling at different temperatures and the actual temperature of incipient boiling varies with ambient temperature and what is in the blend. In a cold climates the refineries stick more light ends in gasoline during winter time to help the fuel vapourise [more a problem wth carburettors than fuel injection]. When at altitude the problem is exacerbated because of the lower boiling point at altitude. Over here we do not have a cold season and the refinery has to chuck in too much LPG to achieve octane rating and in summer time the fuel is pretty much at the initial boiling point at atmospheric conditions [44C]. Indeed there is quite a "whoosh" on many vehicles when the gas cap is opened.

Deletion of the in-tank pump shifts the problem to the main fuel pump which receives the fuel at a slightly lower pressure due to the losses in the piping and if the conditions degrade sufficiently cavitation will take place and if that happens the pump impeller can be terminally damaged very quickly. Even when the tank is full of fuel the suction head is negligble when the fuel is at boiling point and this is the problem.

When doing our very modest autocross type track days I tried to opimise my situation by running with minimal fuel to reduce weight. On hard acceleration out of right hand bends the damm thing gave uyp the ghost and that with an in tank fuel pump that was working correctly. I went back to the petrol station and filled up- it immediately ran no problem.

Bottom line pumping a liquid at boiling point is nigh on impossible without adequate head of liquid to offset the suction losses. In my situation I was able to drive to the fuel station but only on light throttle. It will be interesting to know what others have experienced- our conditions here are clearly marginal and omitting the in-tank pump here would concern me.

For Porsche to use the in tank pump, delete it and then put it back in again has been a source of puzzlement for me. I doubt they can specify a pump with a lower NPSHR [nett positive suction head required] so it would be interesting to know what differences the pump without in tank capability has.

I assume the ventilation system has two purposes- one to prevent a vacuum in the fuel tank as the level draws down and the other to prevent overpressure if the fuel heats and starts to partially boil [what most would call evaporation].

Regards

Fred

Bigfoot928

Greg,
If we remove the in tank pump as I have and add the 044 pump with Tim Murphy's higher flow fittings do you anticipate any problems?

GregBBRD

I don't really know....because I have never tried this. I always feed the 044 pump with an in tank pump.

I believe there have been at least a couple of threads about really noisy 044 pumps after running the vehicles long enough to get the fuel hot.