Porsche Repair: 911 stutter, bosch fuel injection, bosch fuel injection systems

Question
Hi Dave,

Thanks for the reply, I went out re-worked all the grounds (including the connectors) but to no avail. Based on some non-scietific investigation, it appears that the problem occurs only above an idle and is proportional to the temperature of the air being sucked into the air filter. Is there an air temperature sensor or other gadget that could be misbehaving when the aspirated air becomes very warm? Any insight would be appreciated!

Thanks, Brian.
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Followup To

Question -
Hi Dave,
I have an '87 911 3.2 that intermittently looses spark for several seconds at a time but only when completely warmed up. I have checked the grounds, changed the plugs, wires, cap, rotor and coil. Any ideas? Thanks, Brian

Answer -
Hi Brian,

I don't have a secret spot to check for but I've learned through a ton of trial and error lessons that "intermittent" virtually always involves loose grounds.  Up til recently I would not have made this statement but I've given in...it just seems to be true invariably.

In your case I realize you've already checked the grounds but let me suggest one more go around.  Most of the Bosch fuel injection systems are complemented by an electronic igniton control box.  This black box (often silver) has several coil-related wires entering/exiting it and one (usually a .75 mm brown wire) needs to be grounded and grounded securely. A bad ground here could cause  a stutter or an intermittent no-start.

Good luck finding your gremlin.

Dave

Answer
Hi Brian,

Well...as a matter of fact there is a "thermoswitch" which could conceivably cause your stutter.  The normal complaint is engine stumbling on warmup...but maybe you have a "special" thermoswitch.

I can't tell you exactly where it's located but it'll be in the oil circulation path as it is on or off depending on oil temperature.

To test it, remove the wires (an '85 diagram shows Blue/white and Blue/red) and remove the switch so you can see its markings.  It'll indicate at what temperature it is activated.  If you use an ohmmeter with your probes at each end, you should get continuity below the switchover temp and no continuity above it.  Try that.

Dave