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High idle on 78SC

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Greetings Gents,

My 78SC(stock motor / valve adjustment 2yrs ago, oil changes every 3-5K, fuel filter's been in for 3yrs, otherwise I haven't touched a thing) has a couple of idle issues:

1. On start up (especially cold) the engine RPM will fluctuate plus/minus 1000RPM - range between 500 to 1500/1800RPM. At times it can sound very rough and even dies. Once it warms up, it's not as bad but, there's still a noticeable cycling up and down with the motor.

2. Approaching a stop as the RPM's come down through 2000RPM, the engine gets stuck between 1500-1800RPM. I tap the throttle and it comes back to the same range. Very rarely will it come down all the way to normal idle (Which is normally 1000-1100RPM). I'm tired of sitting at a red light with my motor revving at 1800-didn't know it was that loud!

The car has been doing this for over a year and between a new baby and military schedule, I haven't put the love required into her. Before taking it to the shop I'd like to hear if anyone can help? Thanks again for your time.

** Haven't driven her much in the past 2yrs (Funny how my son is also two?) my guess is 5000 miles.

Johnny Zayas

Dade City, Fl.

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Check the warm-up regulator first, doubtful its a filter. If the WUR(known trouble maker) checks out then I would start with checking the fuel accumulator, rest pressure, cold start valve, and running fuel pressure. The mixture or the setting on the CIS might be off , but I would start with the basics first. Sorry, I could not be more precise but its gives you a place to start. You might want to refer to the factory repair manual for the diagnostic procedures for the CIS fuel system, ie checking the WUR, fuel pressures, settings. :cheers:

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  • 4 months later...

hey Johnny,

I've owned my 81' SC since 1985 and been driving it since 1995(dad shipped the car from the U.K to Malaysia where i'm from).

Had a similar problem 3 years back.Turns out the air mixture was not right as there was a leak in the 'airbox' and had to have it replaced at a huge cost! But why don't you also take a look at the mixture control unit near the fuel distributor, there could be a leak there too.

Also the idle speed should be 900+/- 50 rpm when the engine is at operating temp and CO2 levels for pre 79' cars are 1.5 to 3.5%. But anyways try to check these out if you already haven't and good luck.


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  • 4 weeks later...
  • 3 years later...

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