Thread: CIS fuel head
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Alan L Alan L is offline
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Join Date: Apr 2006
Location: New Zealand
Posts: 3,185
I am suspicious of the state of the metering piston. It is the only mechanical defect I can find in the fuel head. I believe, and am happy to be corrected, the system relies on a perfect bore clearance fit between the piston and the metering chamber. The system pressure seems to enter at the waist of the piston, the WUR pressure at the spring loaded head. The only thing that separates this 50 psi gradient is the wall clearance at the piston head. Mine is scored. The only thing that separates the system pressure from bypassing the injectors and pouring straight into your airbox is the clearance at the lower end of the piston. This is my deduction.
I have had no reply yet from the Aus Jetronic people in Spuggys post.
So, posted is my attempt at making a new piston. Not quite finished yet - have left a bit of extra meat on the top and waist to allow me to balance it to the old one for weight. This has probably been my biggest challenge on the lathe - the dimensions are critical, and spent an hour or two polishing down to final size. (The wife doesn't seem to mind - she is used to it, and is beginning to wonder if this car is ever going to go. Me too.).
Hopefully in the pic you can see the scoring on the original piston (LH). What caused this I don't know - but I do believe it will happen if you try to run, or flush your system on alcohol. The fuel heads are not made for it and rely on the fuel for lubrication at this point. The tolerances are less than a thou. Whether the scoring has been caused by the plate spasms, or vice versa I don't know, but only one way to find out, without some specific CIS expertise.
My steel I purchased down the road for $2. Bosch probably spent some time researching theirs, altho it is ferrous. I intend checking the coeff of expansion of the two by putting them in the oven at 50 deg C (a guess at the fuel head working temp), and checking them with micrometer. If this thing seizes in the bore, it could be big trouble - leaness or hydraulicing on fuel. But at the moment it is a beautiful sliding fit in the chamber. The chamber has score marks visible, but nothing to feel - it seems much harder than the piston. You can feel the tears on the piston.
It makes you aware of how critical it is not to have any crap in your fuel - especially at this point. The smallest spec of grit will jam between the wall and hinder the piston.
I can't help thinking the banjo screens were removed to maximise flow. Are their banjo screens on your 930 heads?
Regards
Alan
Old 12-03-2007, 10:11 AM
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