There seems to be a general misunderstanding of what I meant by the pump “parking itself”. The dynamics of it are really very basic school stuff. My own analysis comes from eight years as a Development Engineer in Perkins Engines R & D Test Shop and over 40 years in the F/I business, not out of the FSM or Google.

The facts are:

1) The piston is 70mmØ. That is 6in².
2) The stroke of the piston is 10mm.
3) The strength of the spring determines the level of vacuum up to what is practically achievable.

An absolute vacuum (hypothetical) would exert a forward pull of about 90lbs on the piston rod. This determines the max. strength for the spring, 90lbs to compress 10mm(length of stroke), and achieve equilibrium (i.e. park it) at a hypothetical 29.92”hg. A stronger spring would achieve nothing, just overload the mechanism and bearings and cause the pump to stroke vainly and continuously. It would be idiocy to design a pump to run continuously that is only required to work for around 5% of the time the engine is running.

A pump can be set to park at any chosen level of vacuum by adjustment of the spring strength. For example - a spring set to 45lbs to compress 10mm would park at about 15”hg. The pump starts pumping at full stroke and as the vacuum builds, the stroke progressively shortens until the vacuum pull from the piston equals 45lbs (+/-15’’hg) and equilibrium prevents the spring from pulling the piston back. There it will park and be held fully forward until vacuum falls below 15"hg allowing the spring to lower the rocker gently into the cam again. To park at 25" the spring strength is calculated to balance the vacuum "pull" at that level. It’s not rocket science!

If you drain the vacuum system, start the engine and listen with a stethoscope to the pump you should clearly hear the thump- thump of the pump on full stroke change to a tick-tick over 30 sec or so as the rocker lifts out of the cam and the roller just nicks the cam crests.

My own car still topped out at 25” recently and I am at over 2000ft here. I had changed the plate valves and bearings twice since I bought the car.

This is not correct and a guess I made off the top of my head just to illustrate a point. 35kg would have been closer to the mark – Apologies.

Well that’s it folks – I’m outa here. Once again - Adios Amigos!

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Beagle

Thanks Beagle. That explains a lot in my mind. I see what you mean and how it works more clearly now.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi

I am modeling this, and will test it on an engine shortly.

On a new or low mile vacuum pump attached to a good sealed vacuum system, this sounds correct.

Sadly many systems leak, and the vacuum pump is tired.


.

May I suggest:

1. Start with a brand new vac pump, ideally a piston type.

2. Drill a hole on the casing such that a borescope can be inserted, focusing on the cam and cam follower and shoot a video of the test.

Seeing is believing!

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

NO!!!

I have a new old stock, and grave bodily harm will happen to anyone cutting holes in it..


.

3/8" hole is all that's needed. Won't weaken it and can be easily plugged.

Wanna send me a rebuild kit for my spare pump? I'll drill a hole, shoot and post a video of the test.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked