Erratic idle for 30 seconds then good??

[Bravo]

I am having a small problem with my 78SC with CIS. On startup hot or cold the car hunts for idle reving up almost dying then reving up again. After about 30 seconds it smooths right out. This happens warm or cold. I did a search and could not find a problem similar. Most seem to be a warm or a cold problem mine is both. So I'm perplexed someone help. Thanks

[BlueSkyJaunte]

Unplug your OXS (oxygen sensor) and see what happens.

[ruf-porsche]

Check your WUR.

[ChrisBennet]

Originally posted by BlueSkyJaunte
Unplug your OXS (oxygen sensor) and see what happens.

'78 SC doesn't have an oxygen sensor.
-Chris

[BlueSkyJaunte]

D'oh! Thanks for the correction. WUR or fuel accumulator then. My '81 had the exact same symptom(s), was fixed by unplugging the OXS.

Just going down the list of things I played with to fix my warm up issues, cheapest to most expensive:

1) unplug OXS (as Chris indicated, not on '78s!)
2) Fuel accumulator (for me caused a hard start when warm, though, may not apply to you)
3) WUR

I also checked for vac leaks (or intake leaks).

[Bruno Lavion]

Same thing happens to my 87 Carerra - should I unplug the ox sensor?
will it affect anything such as smog, performance etc?

Bruno

[ChrisBennet]

Originally posted by Bruno Lavion
Same thing happens to my 87 Carerra - should I unplug the ox sensor?
will it affect anything such as smog, performance etc?

Bruno

If your car is idling funny because it is running lean from intake leaks, disconnecting the 02 sensor will cause the car to run richer and mask the problem. It won't fix it.
-Chris

[Bravo]

I suspected the WUR and will check the feul accumulator Thanks!

[Elombard]

My 78 does the exact same thing. Let us know if the WUR fixes it. What do those cost $150??

[laddsc]

How about $380 from our host. Pretty costly. I would make sure there are no intake leaks first. I suspected my WUR was malfunctioning but found a cracked hose that resolved the problem.

[Bravo]

Is there any way to test the WUR?

[tsuter]

Yes- you check the control fuel presssure while the engine is running and cold - like 30F (easy around here this time of year). It should be around 15-20 PSI. When hot and running it should be about 50PSI for your 78. Don't disconnect any vacuum lines to test.

[Bravo]

well I have checked lots o different things and I am going to try the fuel pump check valve Now. It could be the problem if it is letting just a little of the pressure go down and when it starts it takes it a bit to catch up. Well see

[fintstone]

cold start valve!

[ubiquity0]

The cold start valve just sprays an extra dose of fuel into the intake manifolds for the period the starter motor is engaged. I can't see this causing the erratic idle. This erratic idle is a VERY common problem w/ CIS cars. Have you checked for vacuum leaks?

[Bravo]

yah I have and the funny thing is this happens warm or cold?? I would think that a vacum leak would not go away like it does. after it goes away the car runs great. By the way it does not start hard at all warm or cold-- fires right off-- really perplexing

[tsuter]

So did the WUR check out OK?

[keagy]

Bravo, I have the same problem on my 79SC. I think this actually a common problem. I hold the idle at 1100 rpm for a minute rather than letting the idle rev up and down. It forces me to properly warm the car before driving which is a good thing.

[fintstone]

It seems to me that the cold start valve is the only thing that happens every time you start and then goes away. Vacuum leaks would not go away after 30 sec. A too rich or too lean condition upon starting will both give an erratic starting idle. Fuel Mixture set too lean would do the same. I have a 78 SC topend and fuel injection without this problem.