[DML] A Short CIS Primer

["content22207" <brobertson@xxxxxxxxxxxx>]

As I composed an eMail Re: CIS, I realized that a short CIS primer
might be useful here.

By Volvo chronology: Prior to 1981, K-Jet equipped PRV's idled via the
brass screws in the fuel/air mixture unit. You set them once, then
left them alone. At that time there were no "shear head" screws --
they were intended to be used. Fast idle was accomplished with an
ambient temperature sensitive valve (the "auxiliary air valve")
somewhat akin to a CIS idle speed motor (it was plumbed in at the same
locations).

Even after 1981, manufacturers such as Renault continued to use
manually set idle on their PRV's rather than CIS.

Provided the brass screws have not been sheared off, a DeLorean owner
can very easily experiment with pre-1981 style manual idle. All you
need do is remove the short piece of hose connecting the idle speed
motor to the cold start tube, plug both barbs with 3/4" vacuum caps,
then set the brass screws. The CIS ECU, idle speed motor, coolant
thermistor, microswitch, etc, etc, etc can all remain active since the
cold start tube is isolated by the vacuum caps. To reverse this
procedure, simply close the first brass screw (other two "balance
screws" can be left with their manual setting since the first screw
isolates them), remove the vacuum caps, and plug the short piece of
hose back in. CIS back in business.

Once the screws are set, an owner can switch back & forth between both
idle methods in less than a minute -- the question isn't why to
experiment, the question is why not?

A procedure to set the balance screws (from Volvo newsgroups) is in
the Files section of the DMCNews Yahoo site.

As another message noted, the brass screws have no provision for fast
idle. As identified above, an aux air valve accomplished that. Aux air
valves are readily obtainable (they even show up on eBay from time to
time with "DeLorean" in the listing, totally confusing owners who have
no idea what they're for). You can also plumb in alternatives. Before
I converted to carburetion, I used a solenoid operated valve from a
BMW, mounted to the pontoon (being from a different engine altogether,
there was no way to mount it to my PRV intake rail). 

Should an owner want to ditch CIS, it *CAN* be done. I'm not arguing
for or against doing so, but I vehemently object to those who claim it
can't be done, whatever the merits may or may not be. Volvo
successfully idled its PRV's without CIS for 5 model years
(1976-1980). Renault did it even longer. 

Bill Robertson
#5939




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