CIS idle problem

[bigchillcar]

Car has been running great...idle has been perfect; always set it at 950 at 180 degrees per book specs...

today? whether hot or cold, the idle is very weak...down at 500 or below, but it isn't dying...just barely idling.

i'll start looking for vacuum leaks later, but none around the airbox from just a quick check at work...hmmm...any ideas? the fact that it doesn't improve when at operating temp should be provding clues...
ryan

[john walker's workshop]

see if the point gap closed up from rubbing block wear. that would retard the timing and slow the idle speed. is the fuel filter clear?

[bigchillcar]

thanks jw,
i'll check...i recently replaced these and set the gap...always a very fine line between too much and too little gap with those gauges. i have wondered if i used enough lube on the post...could indeed be wearing. also noticed a slight buck this am when driving off cold...often been a sign of points closing. i'll bet you're right.

fuel filter is over a year old maybe year and a half. ran techron a month or so ago.
ryan

[john walker's workshop]

a jumpy tach is another sign of points barely opening.

[sammyg2]

Pull your oil cap and see if it dies or if the idle drops significantly. if it doesn't, major vacuum leak.
Also try and lift the air sensor plate slightly at idle. if the idle goes up, you're running lean (vacuum leak).
if it dies or stumbles the problem prolly isn't mixture related and ignition should be the likely cause.

[bigchillcar]

jw,
i checked my points gap and found it to be too close...needs to be .012 and was more like .010...reset it at spec to .012, but that didn't solve the problem. beginning to wonder about the vacuum retard mechanism on the distributor. i haven't disassembled and cleaned this particular unit ever, so it's easily gone over three years without maintenence.

so, idle still drops down to 500 or less cold or at operating temp...what surprises me is that it won't stall...not used to it being able to keep running at so low an idle.

sammy,
i'll go check the oil filler cap...i forgot to do that. i can tell you that jiggling my airbox cover some caused the idle to go up and down though...
ryan

[bigchillcar]

while i'm there, i think i'm gonna check my timing again...i had last set it at 5 degrees btdc with vacuum connected, but had trouble getting total advance checked right without a partner to hold it steady for me at 6k...gonna dredge up a volunteer...may even disconnect vacuum retard. could it be malfunction and retard more than the 10 degrees it's meant to?
ryan

[fred cook]

I recently had a similar problem with my '80 SC. It drove and ran normally on the way home from work but after supper, I needed to run back into town and suddenly it idled and ran poorly. Lack of a high pitched sound from the CDI box led me to believe the problem was a slowly dying Bosch CDI box. After considering the options (rebuilt Bosch unit $350 or MSD/coil $190), I elected to convert over to an MSD system and matching coil. Once everything was mounted and plugged in, my car went back to running properly. I took a look inside the Bosch CDI and found that a lead from the voltage step up transformer was lying against the case and had slowly burned the insulation off the wire and shorted to the cast aluminum case.

Good luck with your repairs!

Fred Cook
'80 911SC coupe

[bigchillcar]

fred,
i can still hear mine whining...

been busy tonight...guess it will be tomorrow before i can resume troubleshooting...
ryan

[bigchillcar]

could the vacuum retard be retarding too much? is that possible? could removing and cleaning solve that?
ryan

[bigchillcar]

sammy,
hope you're still tuning in...

ok, opened the oil filler cap when running...no change at all. even think i can hear a vacuum leak, but not certain. tried spraying carb cleaner around the intake and airbox and think i detected some idle fluctuations, but none dramatic.

lifting up or pulling down on the air sensor plate will definitely cause a stumble. pushing up the first time, it immediately died.

as i mentioned yesterday, i reset my points gap back to .012, but haven't checked timing. all plug wires seem connected. plugs are only a month old, wires are year old magnecore. disconnecting the vacuum retard at distributor advances everything, but has no effect in correcting the idle problem.

help!
ryan

[sammyg2]

Sent you an e-mail, what was the idle speed when you pulled the cap? If it was around 500 still that may be too slow for it to pull much vacuum, just guessing on that one, haven't BTDT.


Pulling the cap should drop the idle 100 rpm or so and make it idle rough unless something else is really off. I hope it's ignition related and we haven't ruled that out at all but now I'm leaning towards big vacuum leak.
try to raise the idle up to a reasonable speed with the air bleed screw on the side of the throttle body and try the cap again, and this time carefully put your hand over the hole to see if it gets pulled down some. Do it slow, don't let it pull you down hard enough to hurt yourself.
if it doesn't pull, you don't have vacuum on the tank and that is not supposed to happen with CIS unless you disconnected the hose because you installed a turbo

getting the idle speed up closer to normal will also help when you try to verify timing and dwell.

[bigchillcar]

sammy,
you have an e-mail...

could the vacuum leak be from one of the blow-by hoses? one of my black hoses leading over to the body side developed a hole about a year ago...i removed it and wrapped it in duct tape (i know i know...) and forgot about replacing. this past weekend i used a commercial grade of degreaser in the engine compartment (very quick to remove as it likes paint a whole lot)...wonder if it has a hole in it? think i saw a little oil underneath those hoses...
ryan

[sammyg2]

Yup, that may very well be the culprit. the crankcase breather hose goes to the oil tank, and then the oil tank breather hose goes to the intake between the throttle body and the air meter.

cvome to think of it, i have to correct a previous statement: there won't be much vacuum on the tank, just what is caused by the restriction on the metering plate. But a leak there will still allow unmetered air into the intake without fuel to match.

[bigchillcar]

thanks, sammy
i'm gonna go remove it and take a look...could clean and re-patch for the time being if it looks like the culprit. it's been really odd to not experience the car trying to die at such low idle...ordinarily, i would expect it to quit running. it sounds smooth, just verrrry low idle. also, otherwise when driving it drives normally as far as i can see...no noticeable loss of performance that i ought to detect if my timing was severely retarded or advanced...so guessing ignition is ok.
ryan

[bigchillcar]

sammy,
removed that oil hose which runs from the engine to the side of the body...what a mess! that old duct tape repair turned into a helluva sticky mess to remove! anyway, got it off and replaced with new 1" heater hose. haven't checked tonight to see if this solves the vacuum leak or not...i'll find out in the morning. hoping that it didn't take much of a leak to cause my symptoms.
ryan

[bigchillcar]

bump for more ideas...

[bigchillcar]

replacing the hose may have fixed it...hard to tell. i still had to adjust the idle back to normal at operating temp...but it seems to be running as normal now. hope i still don't have a vacuum leak...
ryan

[}{arlequin]

does any one have pics of the items discussed? The hose, the plate, idle adjustment screw? I have a stumble occasionally, but no cold/hot start problems. But when I tried the oil filler cap removal "test", there was absolutely no difference in the idle. Does that automatically mean I have a leak? I would like to perform the above tests, just not sure where to begin. How much of the intake do I have to remove (if any)?

a jumpy tach is another sign of points barely opening.

what kind of jumpiness are we talking about? Does it move around erraticaly at speed? Or is it something else? My tach needle oscillates with each gear change. It goes above the current rpm, then below, then above again but a bit less, then below, and then it settles on the current rpm. While cruising it stays in place without jumping. Is that a normal occurence?

[bigchillcar]

dave,
your tach jumpiness sounds normal, mine acts like yours when it's working right. the kind of jumpiness we're talking about you'd notice in a hurry as unusual...just sitting at idle it can go all over the place like it's possesed. that's usually a sign of points closing in a car like mine still equipped with them.
ryan