idle question

[onebad911]

relative newbie here, i own an 84 carrera car runs great but every few weeks i develop an idle surg at a stop. it fluctuates between 300-1200 and actually almost stalls. my question is what could be causing this? Is it something going bad. like my dme relay? If it was a vacuum leak wouldn't this be a constant problem? it's very frustrating because i can go weeks without a problem than all of the sudden it appears...i've had it checked by my mechanic and he adjusted the mixture that seemed to help but it didn't eliminate it..as usual it didn't do it while it was in the shop
My car has an autothority chip in it and no o2 sensor.
any help would be greatly appreciated..thanks phil

[svandamme]

could be an intermittend fuel pressure or clogged injectors problem, a vacuum leak of some sorts, or a problem with the idle stabilizer

i'de check those things first, in that order

[DonMo]

Once it does it, is it always warmed up and does it until it cools off?

[onebad911]

Don, it can go for weeks at a time without doing it..but when it does it it has done it right after start-up or sometimes it won't do it till after she's warmed up.........i'm stumped

[svandamme]

i have the Bosch enginge management and injection book here

comes with a troubleshooting table

3. Engine hesitates or idles roughly (cold or war)

a Vacuum leak
b idle system faulty
c temp sensor faulty
d airflow meter faulty
c cold start system faulty
f fuel injectors clogged or faulty
g fuel pressure or delivery insufficient

[svandamme]

the intermittend part makes me think injectors or delivery of the fuel...

how much miles on the filter?
what kind of fuel pressure?

you could get a mechanical fuel pressure sensor, and screw it on the rail.. then next time it happens, check the reading

[DonMo]

Weird ones like that are often cold solder joints, have you opened up your Motronic box and used a manifying glass to look for cracks on the solder. Do a search on cold solder, there is plenty of pictures to show what to look for. I would rule out the vaccum leak since it comes and goes, same for ICV and CHT. When my 84 Motronic started to fail from heat related component failures, it was much like you describe. On and off, then progressively worse, finally dead.

[stlrj]

A bad ground if you still have the single wire CHT.