Erratic Idle /'78 SC

wright77 · 10-30-2003, 10:45 AM

On my '78 SC, at times after warmup the idle is very rough.
Hardly wants to run. Than other times, its fine runs at 900rpm.
Than if the weather is hot, the idle will be over 1000 rpm.
Is there an idle control valve on the 3 liter engines? Could this be the problem?

Thanks! Bob

TerryBPP · 10-30-2003, 10:49 AM

Could be a vaccum leak, AAR, WUR. I'd check for vaccum leaks first. Take a can of carb cleaner and mist the engine with it. If the idle changes then theres a leak. Use directed spary to narrow down the location of the leak. Good luck.

joecrum · 10-30-2003, 12:40 PM

If the engine is up to temp when you're spraying carb cleaner, do you need to worry about igniting the cleaner?

DanTheCat · 10-30-2003, 03:09 PM

I know the Warm Up Regulator, but what's an AAR?

I am also having idle issues with my '78. What kind of issues with the WUR cause bad idle after warmup? I haven't gotten the repair manuals yet, so I am unsure of how the WUR functions. It runs fine when it's cold, except for an occasional backfire when cranking the first time.

Dan

pbs911 · pbs911's Garage

Before you spend a bunch of money replacing parts, check for vacuum leaks and check the fuel mixture. With CIS if the fuel mixture is too rich the idle will hunt when the motor warms up. The temp plays a part in hunting. From what you describe I place my money on the fuel mixture being too rich. Set the timing to 35 degrees before at full advance and 3.5 on the fuel mixture.

wright77 · 10-30-2003, 08:43 PM

Dr Sopp, Where is the fuel mixture screw? Thank you!
Bob

Hetmann · 10-31-2003, 06:38 AM

I had a similar problem which turned out to be a hairline crack in my airbox. Apparently, it would open and close with changes in engine temp. It took my mechanic (and me) 2 years to track it down. I could not see it until everything was apart and off the car. Replaced airbox and problem went away. Hope you find a cheaper fix than mine.

Jdub · 10-31-2003, 06:55 AM

I would first do those things that ought to be done on any SC: the valves, change the oil, and then adjust the CO. A new air filter and gas filter make sense as well. You can do all of this yourself, though if you don't have the time or skill, have the valves done elsewhere.

With the car set to correct specifications and no problems from a dirty air or gas filter, or dirty oil, you are in a much better situation to analyze what is going on. It may be a simple air leak, but you would do well to NOT chase AAR, WUR, etc. until you KNOW the car is to spec. Plus, all of what I have suggested are standard, must-do items that
are money well spent.

Best of luck,
John

EDIT: BTW, the airbox is a good place to look for problems. If you can get a fingernail under any of the screws holding the airbox down you probably have a blown airbox (you do have a popoff valve in the airbox?).

wright77 · 10-31-2003, 08:18 AM

Thanks to everyone for the suggestions! Yes it has a pop off valve. I just had the oil changed, new air filter. I will check the air box

wright77 · 11-01-2003, 12:17 PM

Maybe someone will read this! I was checking the oil level, and the car was running rough. As soon as I put the oil cap back on, the idle smoothed out! I did this 3 times! Any ideas??? thanks

rlane · 11-01-2003, 01:58 PM

The idle does always get a bit rough when you take the oil cap off..???Vacumn. Also if the weather has cooled off where you are I have noticed my 78 always idles..."odd" for a few days..drove it yesterday and it wanted to idle at 1100 instead of the usual 900.

wright77 · 11-01-2003, 10:10 PM

Thanks Richard. Yeah I think the weather has something to do with the change in idle. I drove it tonite and it ran rough again. . Cant figure out why it runs good sometimes and not other times.

rlane · 11-02-2003, 03:38 PM

Drove my SC again today..steady as a rock at 900 rpm..but its been mid to high 70's with low humidity..It is supposed to rain and get cooler end of the week and I bet it will idle differently. I think it would be a good idea to check over all the vacumn connections (the vacumn advance onmy dizzy is frequently getting caught by tools) and check timing again.

wright77 · 11-03-2003, 01:02 PM

Today the SC ran pefect. I didnt do anything different. The barometric pressure is raising today, wonder if that could have an affect on valves, etc. I didnt think '78 SCs had a MAP sensor.

MotoSook · 11-03-2003, 01:14 PM

There are a few things on the early SC's that are temperature dependent. So when the weather changes, our CIS tend to behave strangely. The electric heaters in the AAR, TTS, TTV and WUR will take longer to warm up. Add to that that the air density changes from season to season. My car takes longer for the idle to settle to normal. I just accept it. As long as the engine idles well when the oil temp is in the normal operating range, everything should be fine. If not, adjust the mixture a little (at normal operating range!).

As for the idle with the oil cap off, the oil tank vent is plumbed into the CIS boot downstream of the air sensor plate. Removing the cap with the engine running allows more air into the intake, but not more fuel to compensate. A rough idle with the oil cap off in an SC is normal.

CIS cars like to be driven. If I let the car sit tooo long, it'll be a few spirited miles before everythign seems normal again.

wright77 · 11-03-2003, 03:28 PM

Thanks Souk for the Advise! Maybe I need to drive it a little more!

Chris79SC · Chris79SC's Garage

Forgive my ignorance, I know a WUR is the Warm Up Regulator, but what exactly is are AAR, TTS, & TTV????

MotoSook · 11-04-2003, 03:24 AM

TTS: Thermo-time switch
TTV:Therm-time valve
AAR: Aux. air regulator