SC - Rough Idle & other Newbie Questions

[hgray]

I am new to my '81 SC. It is a Petrol Blue Targa, with 90K, rebuilt top end (broken studs) by one of the primo shops in town (if you have to ask how much this will cost, then....). Good service history, all the right updates and drives the best of any of the 15 or so I drove looking for 'the right one'.

All is good, except for one thing - it has a slight miss at idle, between starting cold, and being fully warmed. When completely cold or completely warmed up, it idles great - the concern is just in the 'in between'. The weather has been in the high 30's/low 40's. The car does not seem to warm up fully until it has run more than 20 minutes (still no heat in the oil cooler line to the front). The car was just tuned.

The miss is annoying, and feels like a fouled plug. It is strong enough to cause the car to rock. No drivability problems at all, at any point. The engine is willing and pulls strongly throughout the rev band at all points in the warm up. When it is missing, the idle speed is 850RPM and when fully warm, runs fine and idles at a steady 900RPM.

Having done a search, it seems that a typical problem is the O2 sensor. This was last replaced 25,000 miles ago. Is replacing the O2 sensor a logical starting point in getting it to run better?

Is it 'normal' to take longer than 20 minutes to get warm oil to the front oil cooler?

Is this a problem I should get used to, or is an SC able to idle smoothly as it warms up??

[aigel]

Mine (81 SC) needs one or two revs to 3k rpm to get it going when I start it and it idles smooth from there on. In cool weather, the thermostat for the front may not open for 20 minutes, that's not uncommon. I never checked, but on cool days, I think I make it the 15 minutes to work without using the front oil cooler. As you know, there is also an engine mounted oil cooler.

George

[Jdub]

Sure, your SC should start fine from cold, be smooth, and idle around 1600rpm or so. Then it warms up by about three minutes max and settles to 900rpm or so.

First off, have you followed up the top end job with a valve job? Valves settle in after a top end and it is much like retorquing a head after the heat cycling.

Next, plugs and wires: what shape? See any tracking at night across any cables after misting with water?

Also, how is the CO%? It should be set at 3.5%, hot. I'd also have a look at all your vaccum lines.

Check also that there is no oil in your airbox, on the sensor plate, and in the tube that runs from the oil filler neck back to the backside of the throttle body. When you clean oil, it helps to remove that fat bladder (the plenum) that stretches from the throttle body to the sensor plate/airbox.

Is the air filter new? They often appear clean but spreading the sheaves shows dirt at the base. Cheap to replace. How is the gas filter? Any oxygenated gasoline at your stations this time of year?

Finally, and worst, would be a broken valve spring. If the valve spring breaks, there is not enough umph to push the valve back up due to friction from the valve guide and heavy (read: not warmed up) oil. What happens is the oil finally heats up, is less friction, and allows the valve to work via the single, working, spring. Folks test this theory by going, temporarily, to a straight 30 and seeing if the miss disappears "more quickly" than normal.

All of the above, except the valve spring, are things you would regularly do and are not a waste of money. Do not start poking around until you have set your car up properly, within specs.

Best of luck,
John