GerritD

hello fellow 928-friends,

is there any kjet specialist who can give me an answer to the following questions:
1. When testing fuel delivery via output FD when bridging pump, I only get 500 cc / 30 sec instead of at least 1120cc
=> is this related to my FD or WUR ?
( pump, filter, accumulator are all new and original, FD and WUR are completely overhauled by professional shop)
2. when testing the input of the fuel delivery, directly from accumulator to FD, I get 1800 cc / 30 sec
=> is this a normal delivery ( I would say yes because my pump Bosch 0580 254 053 can deliver 175 L / hour )

the reason why I am asking this is because I have a hard start warm and cold

thanks in advance

Cymadesign

Have you considered a non-CIS cause for your starting issue? Low compression or a fault in the circuit that bypasses one of the ignition resistors during cranking?

GerritD

I found the root cause : bad WUR.
This is really frustrating because it is an overhauled WUR from a professional shop. And it is already the 2nd overhauled WUR from this shop since the first one also gave problems after 2 months
What I find strange is that the CCP = 1.2 bar and WCP = 3.9 bar which seem to be in range, yet the WUR caused a bad start.

So currently I have put a used WUR of the same type ( 0438 140 086) and the car started instantly.

No idea what exactly failed in the WUR even with correct pressures.

karl ruiter

Could your WUR have a vacuum problem?

GerritD

How can I test this?
it has a port at the left side on which vacuum is connected, and there is also a port on top of the WUR which connects it to the air metering unit

Guy

WUR likely getting blocked with sediment. I would flush the fuel system and change the filter. I have seen MANY cheap fuel filters allow sediment thru.

a4sfed928

I fixed an issue like this before and it turned out that the WUR was getting knocked out of adjustment from a casting boss on the water bridge.
Look at the bottom of your WUR and the water bridge it bolts to and see if there is a wear spot on either part. There is a casting boss slightly above the two screw holes on the water bridge that can hit the bottom of the WUR where the metal plug is driven in the WUR. This happens when the small spacer plate is missing. The car would run great for a few weeks after the WUR was replaced and then come back running poor.
This is a long shot but definitely worth looking at.

Brian

belgiumbarry

Gerrit , you must write a book about that CIS junk
sorry , can't resist it.
But it keeps you buzzy , that's all about our hobby.

GerritD

Apparently it is not a WUR issue since the 2nd WUR I used did not bring a solution for the cold start.
So I reinstalled my original refurbished WUR
So issue is not solved

But I am not a quitter...so the story continues ....
The main question here : what can cause the return flow to the tank to become only 500cc / 30 sec instead of 1120cc , even with all fuel pressures (incl system pressure) within range ??? Return flow directly measured at the outlet of the FD.

gbgastowers

I haven’t read each of the threads in their entirely but here is what I would do at this point. After it’s fixed it stays solid and reliable if you run the car regularly. I would make sure all the part #s are exactly as original on all CIS components. Make sure all these parts are new or rebuilt/calibrated. By these parts I mean in order of importance: fuel distributor, WUR, injectors, fuel pump, fuel pressure regulator in RR wheel opening. That’s it. Do every item and it will be solid. Gunar

belgiumbarry

yes, but by the time you have all those parts new , you have doubled the value of your 928

wopfe

@GerritD I was just browsing the rennlist, and I just noticed this, I admire your persistence! I thought I had the determination to get it done when I was struggling with the system (and that WAS the wur not being done correctly, remember). Did you ever consider sending it to another rebuilder (maybe even the one in Belgium that fixxed it for me?) Or did you already try that? Like for a second opinion? All the best!

GerritD

Hi Arie, I don’t think the WUR is the culprit since I tried another WUR and it gave same result.
I think there is an issue with the delivery itself but no clue where it blocks since I replaced everything . Perhaps I have a leaky injector, although these are new.
Here some test movies :

https://youtube.com/shorts/tG0aCqyc78Q?feature=share

https://youtube.com/shorts/IO37j1wmc1A?feature=share

https://youtube.com/shorts/0mCIKnjWZEs?feature=share

https://youtube.com/shorts/FdrkrdRL4MY

Alan 91 C2

are you measuring the WCP and CCP against the pressures/temperatures listed in the service manual? these thing are very fiddly. And you must fall in range for both conditions WCP and CCP., at actual ambient temp CCP. The vacuum port on the WUR, does lean out the fuel pressure, raises output pressure with normal vacuum. Do note CCP pressures too high, will prevent cold start, whereas low WCP will yield slightly rich operation, but will run.
I have been struggling with my 79 car, but think i am close to solution after changing two fuel injectors.
As an aside, in a moment of frustration, i did buy 4 duel throat Weber knockoffs 40IDF, but realized these are good above 3K rpm, and not good for daily driver rpms. Cool look though, and could work with vacuum controlled throttle position, past 1/3 throttle..(Throttle linkage is not trivial, but solvable) Recognizing i would still have hot start flooding, typical with my 61 Corvette dual quad setup. May still go there, as the adapter for the carbs, is a reasonably simple 3/4" thick aluminum adapter plate, to match carb spacing with intake port spacing. Hood spacing close, but workable. The positive side to the Weber solution, is it can work for all years, as fuel can be decoupled from spark. I did consider a fuel recovery solution, basically pumping down the fuel from the carbs back to the tank, and refill on hot start, 5 second delay, with elect fuel pump. So Weber-ish solution not dead, just posponed for now.

GerritD

Hello Alan,
I measured the CCP and WCP as mentioned in my service manual of the car, so in my case it was at 18 degrees Celsius instead of 20 , CCP = 1 bar without vacuum
CCP = 1,5 bar with vacuum, , WCP = 3,9 bar with vacuum and 3,3 bar without vacuum
So CCP is in range ( service manual indicates 1,1 and 1,6 bar), but my WCP is rather high since service manual indicates max 3,8 bar

My biggest concern is the electrical fuel pump delivery rate test : it should be at least 1120 cc / 30 sec while I barely have 500 cc
No idea where the issue lies : WUR or FD although they have been both overhauled.