Tandem pump cover swap - minor detail

-- 997 -- said:

Hi all, digging up an old thread here!!

Wanna ask for you guys who knows, what is approx the life expectancy of a Tandem Pump?

Also when it starts to fail how would i know i.e. would the brakes just give way like i wont be able to stop?

Cheers

J

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It gets very rusty looking and eventually becomes porous and starts to weep a mist then a drip of oil.

I did not even know what a tandem pump was till this thread turned up..Duh !

--997--, I doubt Porsche would design a brake system that ENTIRELY depended upon the efficiency of a pump....? t`would seem the pump adds a measure of ASSISTANCE to reduce the effort requiring to be applied to the brake pedal in order to achieve the desired amount of braking at any time...? That being the case..IF.. the pump begins to become less efficient over TIME then to achieve the EXPECTED amount of braking will require more pressure to be applied to the pedal by the driver.

In reality and without hands on knowledge of the Porsche system..... I suspect the pump like any mechanical device will wear over time and that the wear will reduce it`s efficiency slowly, so slowly that over the months/years, the driver will unwittingly be compensating for the wear by pressing the brake pedal a little harder...?

If the unit suddenly failed then the difference would be immediately noticeable given the amount of EXTRA effort required to make up for total failure of the ASSISTANCE aspect of the braking BUT the brakes would still operate..

As for the pump, it is a bit like any pump required to control air, it requires to be machined to VERY close tolerances in order to operate efficiently and it seems in this case the cover is part of the chamber designed to be an essential part of the assembly affecting these tolerances, thus it seems unlikely that the other precisely machined and now worn aspects of the pump may operate efficiently with a replacement cover..? Of course if the cover it`s self had an issue though corrosion having created a pin hole in it, then perhaps a replacement cover might improve things, but by how much..? If the pump had been on the car long enough for corrosion to create a pin hole, then chances may be that the pump would be worn to some degree thus replacement would seem to be a better option...?

If paying a garage to effect repairs it might seem better to replace the pump than pay them to mess around with a new cover that may or may not work out as well as hoped...? I guess it depends on the time required to replace the unit Vs the time to fiddle with the cover in situ ..with a less than guaranteed outcome..?

Replacing the cover alone seems a bit like fitting new a new cylinder head to an engine that has scored bores, and hoping that the oil burning issue is resolved to a workable degree..?

Of course modern engineering is all about replacement units, which in time equates to a different or perhaps lesser level of "purely mechanical" skills, as those replacing them may not be given the opportunity/time to acquire OLD engineering principles of REPAIR... If the computer brings up a code that determines a unit of any type should be replaced... then no point in arguing, TIME is money.. Of course having spent quite some time working with Victorian engineering and also at the leading edge of technological advance, I witnessed a considerable amount of the transitional process and understand that depended on computing to accurately determine the unit to be replaced which is NOT foolproof. Such dependance on computing may end up almost destroying the ability of a skilled enthusiast to repair HIS vehicle when substitution of components becomes the only way to find out where the REAL problem lies, and anyone who has operated in this way well understands that when a number of units are swapped while trying to resolve a fault...the swapping process can introduce further faults elsewhere..... Yeah it can all become a bit of a mine field and potentially a very expensive one too....

Of course I am ever more than happy to be corrected on any aspect of my rambling... on and on and on... :roll:

Just thinking in type.... Luddite OUT

There were no pin holes in mine (photos show the inside) - just exterior rust. I took steps to deal with a potential issue before it became one.
Others have chosen to remove the rust and coat the clean surface with something of their choice but I was not confident of the amount of metal that would remain.
I doubt that any misting would have affected the functionality of the pump as I presume it has a constant and adequate supply of oil :dont know:
Again, I would rate the ACP option as my first choice of repair for this common situation.

Scholester, as I typed I have never even seen a tandem pump though I have had hands-on with vacuum pumps which it seems is that which a tandem pump may be...?

My guess is that the vacuum derived from the pump supplies some sort of servo system that in other cars is supplied by the engine directly from a take off point on the INLET manifold ..?

I have no idea at this time as to why Porsche use such a pump, though I have known diesel engines that also used vacuum pumps to supply the brake servos due to low inlet manifold depression (suck) in their diesel engines. I suspect fuel injection engines may have less suck..? (-:

Given this pump is not supplying a positive pressure it seems perhaps less prone to leaking, ever tried to burst a balloon by sucking the air out...(-:

I am unsure about it leaking oil either as I would not expect there to be much in the way of lube present, other than perhaps a mist lubrication at most on the working faces. If the pump is directly driven from the engine other than via a belt, then I guess in time it`s bearing could wear and allow the passage of oil from the engine into the vanes and cylinder within the pump or a leak between the pump and it`s engine mounting point... Dunno..!

Detecting whether the pump is leaking vacuum has to be difficult by ear given all the noise with the engine running, however it may be possible to use a tube, one end in an ear and moving the other end around the head (cover) of the pump to listen for any suck... though if you allow the end of the tube to touch the pump you will amplify the mechanical sound of it`s operation which will mask any chance of hearing the leak... Of course there are gauges that can measure vacuum which could be "T`d" in to the pipework to check for pump efficiency..? I `m guessing Porsche has a process for testing the vacuum in these pumps..?

Attending to corrosion build up on the pump seems no bad idea. I imagine it would take rather a lot of corrosion before the workings of the pump might be affected...?

I suspect changing the cap is no guarantee of a fix, and even less so if the pump is an old one and the inside of the cap shows no considerable scoring. If scored then the rotor would also be suspect and perhaps unlikely to provide the necessary fit to the new cap in order for it to create the vacuum as required.... Kinda like fitting the old worn pistons and rings to a new cylinder..?

Just using logic here and claiming ZERO Porsche experience of vacuum...err...tandem pumps.

Hope Mr DeMort might have time to jump in and educate us further..or more likely correct my logic... :?:

when mine started to leak oil I also noticed that the brake system did not hold vacuum for as long as it did before, so if you left the car for some time there was no residual vacuum left. This you could feel in the pedal, it would be rock hard.

Timbo, in older design vehicles there is usually a non-return valve built into the elbow which was part of linkage between the flexible rubber vacuum delivery pipe and the servo body. I would be surprised if Porsche do not have a non-return valve positioned somewhere in their system, and more likely at the point of delivery as opposed to the pump end of things...?

In general trying to store vacuum is very difficult and thus over time it seems logical that it will deplete to some degree...? A test is to do as you did, is to press the brake pedal after stopping the engine, and in so doing you are using up the STORED vacuum, after a few pumps the brake pedal will go hard as you use up all the reserve.. Assuming Porsche has a reserve capacity in their servo system...?

As I posted I have no knowledge of the requirement for lubrication and what part it might play in the Porsche tandem pump, though the leak you mentioned may or may not have played a part in the operation of the pump, dependant on the exact source of the leak..?

I notice you have an Elan in your list... On an Elite I once had in which someone had fitted a BOC flashback arrestor (one way gas valve) into the vacuum circuit... It worked well enough... :what:

The vacuum pump supplies vacuum to the brake servo .. this is obviously the same type of servo as from any car but its pump supplied and not a pipe run from the inlet manifold .

The reason is at times there is simply not enough vacuum that can be supplied if it was manifold induced ... it's used pretty much on every car from 997 onwards .

The usual fault is the tin cover of this pump corrodes and as its oil fed you get an oil leak .. you could in theory get a vacuum loss but it's normally an oil leak .

There can be problems with the pipes .. it does after all run from the o/s/r of the car down the n/s and across the front of the car to the servo .

i've seen leaks at the back and the pipe direct onto the pump .. it corrodes and sometimes the plastic part of the pipe that runs up behind the n/s of the air filter housing can spilt .

How long they last is all down to the conditions the car is used in and how close to the coast you are.

we can basically say anything from 5 years up to never .

A coating of wax oil / acf 50 will help prolong the life of this cover and it's something i expect all Indys will apply when fitting a new one or indeed just servicing a car .

OPC wont .

This doesn't affect 987 as the pump is protected more due to the reverse position of the engine .

Many thanks for the heads up Mr deMort.

If it would not take up too much of your time... just one question arising out of timbo`s post, is there a non-return valve built into the system.. :?:

Luddite said:

Timbo, in older design vehicles there is usually a non-return valve built into the elbow which was part of linkage between the flexible rubber vacuum delivery pipe and the servo body. I would be surprised if Porsche do not have a non-return valve positioned somewhere in their system, and more likely at the point of delivery as opposed to the pump end of things...?

In general trying to store vacuum is very difficult and thus over time it seems logical that it will deplete to some degree...? A test is to do as you did, is to press the brake pedal after stopping the engine, and in so doing you are using up the STORED vacuum, after a few pumps the brake pedal will go hard as you use up all the reserve.. Assuming Porsche has a reserve capacity in their servo system...?

As I posted I have no knowledge of the requirement for lubrication and what part it might play in the Porsche tandem pump, though the leak you mentioned may or may not have played a part in the operation of the pump, dependant on the exact source of the leak..?

I notice you have an Elan in your list... On an Elite I once had in which someone had fitted a BOC flashback arrestor (one way gas valve) into the vacuum circuit... It worked well enough... :what:

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well I'm just telling you what I noticed, replaced the pump and vacuum lasted for some time.

Yeah timbo, thanks for the input which helps to increase my understanding of Porsche issues. :thumb:

Ive had my 997 getting for 10 years now and its been through two pumps in that time.

The first one corroded through the cover as per the more usual failure mode, but the second one corroded through the takeoff spigot for the brake servo assist vacuum. This manifested itself by a complete loss of servo assist for the brakes rendering the car with seriously impaired braking capability.

It took me a while to pinpoint the actual cause of the problem as it wasnt obvious were the actual leak was coming from.

monster said:

Ive had my 997 getting for 10 years now and its been through two pumps in that time.

The first one corroded through the cover as per the more usual failure mode, but the second one corroded through the takeoff spigot for the brake servo assist vacuum. This manifested itself by a complete loss of servo assist for the brakes rendering the car with seriously impaired braking capability.

It took me a while to pinpoint the actual cause of the problem as it wasnt obvious were the actual leak was coming from.

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Out of interest is your car garaged and used all year around?

Ok, interesting posts, so heres another question, to change the tandem pump what are the procedures, i.e. how is it tackled to take off? What needs to be stripped before the access the pump?

Cheers..

J

sja360 said:

Out of interest is your car garaged and used all year around?

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Its garaged now but wasn't for the majority of those years and was used all year round.

-- 997 -- said:

Ok, interesting posts, so heres another question, to change the tandem pump what are the procedures, i.e. how is it tackled to take off? What needs to be stripped before the access the pump?

Cheers..

J

Click to expand...

Just raise the rear of the car and its located on the rear of the engine on the offside. I think it's 4 bolts that hold it in place. There is also a length of aluminium tube that it is advisable to replace at the same which needs to come out through the space vacated by the air filter housing. Easy diy, just a bit fiddly to wiggle the aluminium pipe in and out.