CIS idle drop and missfire when braking
 

CIS just keeps getting weirder and weirder! 80SC at 950 idle, engine warm. Press brake pedal and rpm drops to 900 with noticeable missing of idle. I'm running with O2 unplugged, CO at 3 to 3.5%. Have SSI's, Monty, MSD, magnecors, new cap and rotor. All new injectors. New plugs gapped to .45. Have had a tiny miss at idle for awhile, but generally idled steady, smooth power at cruise. All pressures are in spec. I'm thinking it's electrical but with CIS who knows? Any thoughts, similar experiences, suggestions. Appreciate any help!

Sounds to me like a vacuum leak within the brake booster system - just my guess.

Check for a leak in the line that runs from the vacuum port at the CIS to the brake booster vacuum port. Cracked or split hose, loose connection etc.

You could possibly temporarily plug the vacuum port for the brake booster and then see what happens when you step on the brake.

Thanks a lot guys. Didn't think about the booster line. So does the line run from the CIS to the brake vacumn thru the tunnel? Where does the line typically go bad? The only other thing I thought of was an electrical drain somewhere. My optima battery went dead a week ago (left interior lights on for two days), but it took a full charge and no reactions from my new alternator.

+1
you can get a hand vacuum pump and pull a vacuum on the line and booster to check the entire system.
it is a pain to fix. search for JW's thread on the brake booster line.

The SEARCH function is a Pelicanite's best friend!! Lots of threads on the booster line/booster problems. Just got through re-setting timing, CO, and idle. Checked all my new magnecors. Everything to spec. Still lose rpm's and break-up/missing when brake pedal is pressed at idle. Traced the booster vac line from the engine to booster. Visually it looked fine. Did check with engine off and foot on pedal. Started engine and pedal went in about 1/4 inch. I'll pull off the booster line next to the MC, plug and report the engine response.

pull the line at the engine. the line from the engine to the booster is what goes bad.

you should really pull a vacuum on the entire brake system.
this could also be an indication of an air leak somewhere else.

It looks like the booster line branches at a metal Y pipe then the two lines go into the engine on my 80SC. One line goes into the lower driver's side of the air-box. The other line goes around behind the throttle body. The Y-line is metal. Going to my mech. on Tuesday I'll let him run the vacumn leak trace. Told him I'm close to junking the CIS for PMO's. He said if I do that I'll have to find a new mechanic!! My guy's an old-school Porsche mech., been at it for 40+ years. Guess I better listen to him Huh!

then he should be able to do carbs too.
just buy a pump from advance, about $40. i never thought i would use one as much as i do.

T77911S... I see what you did there (with your sig).

CharlieB, Hopefully he'll find the leak in the rubber line, not the hard line that goes to the front.

Oh yeah, he can most certainly do carbs too! He's got a shop full of hot P-cars. Just likes to keep things the way they originally came out of Stuttgart. Unless of course your taste runs to AX, DE or Daytona. Then he'll fix you up!!

do i have to pay you for copyright infringements?

i think its a bit much...no?:confused:

how do you like my added touch?

UPDATE: I pulled the booster vacumn line off the fitting that comes into the trunk and plugged it. Started engine, hit the brake pedal and no change in idle. I suppose this confirms no vac leaks in the line from engine to front. So the leak is either in the short vac line to the booster, the check valve or the booster itself. Would a faulty check valve cause my symptoms, ie: idle drop and missing when brake applied at idle?? Going to the shop on Tuesday!

Updated Update: Got the car to the shop on Tuesday. Booster was bad. Everything else held vacuum. Sourced a good '84 booster that tested fine. Now idle is perfect with no drop or missing when brakes applied. Interesting how overlooked vacuum leaks in the booster system affect engine performance. Thanks for all the input guys!