My AAR has low OHM reading.

[brianhiggi]

I'm trying to figure out why my '78 SC isn't running right when it gets to temperature. The idle starts to go up and down but only after it is warmed up.
I tested the OHM resistance on the AAR and it showed 17 OHM and I believe it should be at 33. What effect would that have?

I took off the AAR when cold and the passage was half open and when I looked at it after a 15 minute drive it was almost completely closed.

[boyt911sc]

Brian,

Do you know the number on the AAR? Post the number. Do you have a -209 or -218 or something else? Keep us posted.

Tony

[brianhiggi]

Hi Tony,

The AAR is a -209.
I have noticed that when the car not yet at running temperature and I remove the oil cap the rpms drop considerably but when the car is at running termperature and the oil cap is removed there is not drop in rpms at all. Could there be false air problem only when an engine is warmed up?

Thanks,

Brian

[boyt911sc]

Brian,

Your auxiliary air regulator (AAR) has the correct heating resistance of 17 Ohms. And it seems to be working fine based from your description. If I were in your shoes, I'll check the following:
1). Vacuum/air leak test..........you have to test and confirm the absence of unmetered air going into the system.
2). Check air fuel mixture with a gas analyzer when engine is completely warmed up.

Have you tinkered or made some adjustment on the fuel mixture screw? It seems to be on the rich mixture range but a gas analyzer would confirm where it is now. Keep us posted.

Tony

[brianhiggi]

The local shop I took it to last year did a smoke test and didn't find any air leaks. My car was missing the fuel mixture screw so the shop put in a new one and adjusted it. After the adjustment the car still had a surging idle when warm, and was now the car is almost impossible to start when cold. Maybe the shop adjusted the mixture to be correct when warm but when cold it's too lean or too rich.
Even though the shop did a smoke test could their still be an air leak?

[Scott R]

Mixture and timing cause the surging. Advance the time a bit while it's surging see if you can get it to stop.

[brianhiggi]

Advancing the timing does get the surging to stop. If the timing is advanced enough to make the surging go away then it's near impossible to start when cold. What does that mean?

[Scott R]

So you have a piece of the puzzle now. Couple of things here, by advancing the timing you're likely raising the idle a bit and overcoming a low vacuum level at idle. If you had an 80+ it could be different issue, but a 79' makes this a bit easier.

When the vacuum is low at idle the sensor plate bounces a bit and then the surging happens. You can see this by removing the air box cover and observing the sensor plate arm while it's surging.

So why is the vacuum low at idle? Leaks as mentioned above since this is a < 80 CIS system. That's the likely issue. The majority of times I've corrected this the leaks seem to be at the intake runners, either the boots or the rubber couplers.

Do the tests for leaks as mentioned above, fix them then reset the mixture and timing and you should be good to go.

You don't have bigger cams do you? If you do there are some other things we can do to even this out.

[brianhiggi]

Scott,
I told you wrong, actually If I retard the timing the surging goes away, NOT advancing it like I told you. The warmer the engine gets the higher the idle gets, and when the idle gets to a certain rpm the surging begins. When I retard the timing the idle drops and the surging goes away.

Your theory about a vacuum leak is probably still the culprit.

It looks like replaceing the intake runner boots and couplers would be a big job. Any guesses as to how much it would cost to have a shop do it? Or is it something that an inexperienced but determined guy like me could do?

[NoYouRelax]

Brian... did you correct this issue? I have similar issues and this thread is of interest,

[Paulporsche]

Have you checked your control pressures?

Sounds like your cold pressure may be too high (lean) and/or your warm is too low (rich).

This may be why a correct mix setting for warm means the engine won't start when cold, or a correct setting to start results in a too rich condition when warm.

[brianhiggi]

I have not corrected this issue yet. When I do I'll let you know.

Paul, what you say about the correct mix setting makes sense to me. I have not checked my control pressures. What causes cold pressure to be too high?

[86911tlcab]

my guess is the WUR is not functioning correctly...

[brianhiggi]

boyt911sc tested my WUR on his car and said that it operated fine.
Could it possibly be that a vacuum leak is casing an insufficient amount of vacuum to operate the WUR correctly?
I still suspect a leak(s) somewhere in the system.

[aoncurly]

My 81SC was hunting like that at idle and the cause was a faulty O2 sensor. Disconnect the O2 sensor and see if the surge disappears. If so, replace your O2 sensor.

[brianhiggi]

I don't think that a '78 has an O2 sensor. Am I right?

[Scott R]

Originally Posted by brianhiggi

I don't think that a '78 has an O2 sensor. Am I right?

Correct, not until 80' and on the US cars.

[aoncurly]

I think you are right - looks like O2 sensors started on model year 1980.

[boyt911sc]

Brian,

Check that you don't have unmetered air going into the system. Your WUR was tested on my engine and did run. Start the basic tests like:
1). Measure the fuel pressures ( control and system).
2). Confirm the absence of air/vac leak and have the exhaust CO level checked using a gas analyzer.
3). Avoid tinkering the mixture setting until step #2 is completed.

And keep us posted.

Tony

[brianhiggi]

Thanks guys.
I've been avoiding doing the fuel pressure testing as I'm unexperienced but I think I just need to man-up and do it. I'll keep you posted.