Vacuum Test

[Scurry]

I'm looking at doing a vacuum test with the engine on to see if I have problems. I don't have any that I know of more edification on my part. Looks like you can tell a lot with this type of test.
I was thinking the connection from the throttle body to just before the Venturi tube. Also thought from brake booster to where it connects to the Venturi tube.

Thanks
David
1986.5

[dr bob]

Hi David,

The easiest place to test engine manifold vacuum is at the hose connected to the front fuel pressure damper. The engine will run fine with that damper disconnected for the testing.

For testing client system integrity, use a MitiVac or similar at each of the hoses to the little cross manifold near the firewall. There's a stub there from the brake booster fitting to a small check valve, then the plastic cross. HVAC, Cruise and vacuum accumulator, and on S4+ cars there's a resonance flap actuator.

There's 'ported' vacuum for the fuel tank vent solenoid too.


On my car there's a vacuum system diagram under the hood itself, with a diagram that will appear reversed left-to-right. Fair warning. It shows the whole system and components. There are some other diagrams found in manuals and some aftermarket guides. I can scare those up for you if you can't do with my narrative.

[Scurry]

Dr. Bob

I created a T for doing it at the damper up front. Since I got the car back small fire at the U line, it had been shifting hard.
I was mucking around with trans most of the time when I decided I wanted to measure the vacuum while it was running.
Got 0 lbs of pressure. Tore into 7 way spider and tested all the mechanical devices and the control valve was leaking so replaced.

Still didn't get 19 lbs on the pressure gage though. So I was following the main feed from the throttle body and found that was disconnected and torn due to tech that had installed it. Fixed had vacuum all over now.

I will need to back under and readjust my Tranny back up to 4.4bar for everything in there to be working correctly.

Thanks for your response Dr. Bob
David

[dr bob]

"Small fire at the U line" got my attention.

For those following along at home --

The vacuum system on the engine itself is EPDM elbows with hard plastic (nylon?) flex tubing between them. 40 years of heat and engine fumes definitely take their toll on the elbows. The tubing is a bulk commodity. Elbows too. Takes maybe 30 mins to replace all the elbows and tubing. Easiest with the intake removed, otherwise easy to use the old tubing to pull new into place. There are enough bits of rubber idles air control valve and oil vapor recovery hoses under the intake that they should be inspected/replaced at the same time.

There's a plastic cross fitting back by the throttle and MAF that offers direct manifold vacuum to the auto gearbox modulator, fuel pressure regulator and dampers. As you know, this is very close to the U hose in the fuel system rear of engine, so a "small fire" would easily do damage to that cross fitting. The same part is common to Mercedes V8 cars of the same era, should you need a replacement in a hurry. Cheaper at the Mercedes parts counter too, generally.

There are two close-together nipples from the intake plenum below and forward of the throttle; one is full manifold vacuum, the other 'ported'. Don't get the two connections mixed up. That's often what owners are testing for with a vacuum gauge, as mixing them up affects fuel mixture and auto gearbox shifting pretty significantly.

More for those playing along at home --
The fuel system includes a U-shaped hose to connect the fuel pressure regulator and the rear fuel pressure damper. It ties the rear of the two fuel rails together, and allows for block expansion and contraction as block temperatures change. The original hose was not particularly robust, and with time and heat they do fail. The result is usually a fog of raw fuel that finds it's way to hot exhaust, and spontaneous combustion brings a fire that's a chore to put out at best. Many 928's have carbequed , and the telltale from pictures is the hot spot on hood paint near the windshield and on the left (driver's) side where this U-shaped hose lives.

Several attempts have been made to adapt modern lined hose to the duty, but the tight bend radius of the original shape is too tight for the modern lined hose. Options include a bigger loop rather than the horse-shoe shaped original one, but the larger loop must be supported in the middle to keep it out of the moving throttle linkages and the tight electrical elements routing.

Braided-covered hose is not an answer, since it's the plastic core that kinks and fractures when the hose is bent too tight. Plus the fittings the hose slides over are sharp barbs, not ideal for hose clamping.

I've looked at various metal tubing options, but the access and space limitations pretty much doom us to using hose there. So every dozen years or so, the various rubber fuel hose sections get replaced with new. Including that U hose. Maybe especially that U hose. What looks at first like a very simple hose connection quickly gets more complex with the limitations of the hose itself in the space we have. The hose has to be installed to the FPR and rear damper first, then they are carefully placed and fitted mechanically. Else risk kinking the hose during installation. Once the section is kinked just once, it will eventually fracture inside at the old kink. So extra work and care needed. Nobody wants to watch the car burn.

You can inspect the hose and service it after removing the bottom part of the air cleaner housing. Easier with the MAF out of the way too.

[Scurry]

Dr. Bob

What really chaffs me is I replaced this U tube and all other fuel lines with new in 2020 when I was rebuilding the engine out of car. I have driven it about 9k from then and this hose failed with pin holes(pic below). I had Garrity at SLC Motorsports do the work on this for it failed from Las Vegas going to Colorado to visit family about 200 miles south of SLC.
The hose was from a 928 vendor that was having them made by a hydraulic fluid vendor, of which he told me they stopped selling them a year after I bought mine. So I don't how many more of those type of U hoses are out there right now. Would be nice for them to check their sales and send a notification to the people.

I had him replace all fuel hoses with Greg Brown type. I paid more but they are rated for different types of fuel.

As far as the vacuum tube from the throttle body to the 7 way spider I will be replacing it with the factory one. Ordered from 928srus this week.

Here is the failure I encountered this spring:

[PSU_Crash]

Wow ...

[dr bob]

Looks like you dodged a flaming bullet there. Pinholes in the jacket from a fractured liner. I worked my way through a couple of hose versions Roger was promoting but struggled to keep the min bend radius during installation. I ended up with a non-Roger specialty hose piece that I'll need to look at to remember exactly what it is. It was before Greg started selling the version you have now. There was a lot of local/internal discussion on what could be safely fit there. My initial solution was to use a longer piece of hose in a bigger loop, avoiding the tight bends. But as you know it's busy in that area already, and the combination of the lack of support options for the bigger loop and other local abrasion concerns that forced my final selection.

I was 'shopping' in hoses in a gas turbine maintenance shop in a power plant, and had choices from the high-pressure liquid fuel systems. So far, the duty in my car has not been even close to the 1500º-plus combustor can-adjacent conditions the lines are spec'd for. Way overkill is just barely enough. Not nearly as racy-looky as Greg's pieces. And a very serious chore to install and make leak-tight without any kinking during the process; took a few tries.

Were I making the same decisions today, the GB hose is really the only realistic choice. As much as I enjoyed Roger's enthusiasm, he's more marketing and enthusiasm than engineering mindset. The fuel system stuff is all risk really.


Happy for your outcome!