Rough idle, won't rev above idle

[TeeJayHoward]

Ze throttle, it does nothing!

I replaced my fan and alternator housing.
I drove around.
I parked the car.
Now it has trouble starting.
When it DOES start, it has trouble idling.
When it DOES idle, it won't rev. At all.

If you try to force it to rev by manipulating the rods in the engine bay, there's little pops that can be heard in the air box but the RPM remains constant. If you remove the air filter and press on the CIS plate WHILE manipulating the throttle, you can get it to rev... Barely. It's touchy as all get out and will flat out die if you don't have both arms at JUUUUUUST the right spots.

So... What did I break?

[proporsche]

post a picture of your work ..look at the throttle linkage if it is not touching the metal sheet metal going down to the trans.Just an idea did you try to adjust your fuel mix and left the tool there?Replace fan and shroud do you mean the alternator housing or the engine cover shroud which comes in various colors like green black yellow and red..

Ivan

also ,we can guess but list what year of your 911..

[TeeJayHoward]

Originally Posted by proporsche

post a picture of your work ..look at the throttle linkage if it is not touching the metal sheet metal going down to the trans.Just an idea did you try to adjust your fuel mix and left the tool there?Replace fan and shroud do you mean the alternator housing or the engine cover shroud which comes in various colors like green black yellow and red..

Ivan

also ,we can guess but list what year of your 911..

Oops! The alternator housing, not the engine shroud.
I did not adjust the mixture.
The linkages all work.
It ran fine after the fan swap.
1981 911SC, 3.0L w/ no engine mods except a different muffler.
Pics of the CIS as requested (Removed the heater fan for better access when it refused to rev, will likely reinstall because giant hole in cooling system!)

[proporsche]

Just a thought..is your pup off valve secured in place if you have one--sounds like an air leak...
also this pipe behind the throttle body could disconnect -have seen that..

[TeeJayHoward]

I don’t think I have a pop-off valve installed. At least, it’s not visible in the air box with the air filter removed. My screws aren’t backed out at all, and are phillips instead of the flat-heads I see in the videos online, so I’m not sure if I have a replacement box or not. It’s super clean inside!

Thanks for pointing out what to check. I need a place to start. I was going to install a wideband and see what changing the throttle does, install a fuel pressure gauge and see where that’s at and what changes with the throttle, and maybe pick up a vacuum gauge and replace the brake booster line with it. Then go through every vacuum line on the engine and pull ‘em out and stick my finger on the hole, see if that fixes the issue - I figure that’ll point out the broken hose if there is one.

[mikesarge]

It sounds like a very lean condition. I would check for air leaks before I started messing with the mixture, especially if I didn’t have a wideband installed.

Start by checking vacuum connections, I bet something is all the way disconnected for that kind of change.

If you can’t see anything, then (with a fire extinguisher near by) spray a little carb cleaner at a time in different areas of the engine bay. When the revs change, you found the general area of your leak.

Or, safer, make a smoke tester. I think there’s another thread on that somewhere on the first or second page, as I just read one earlier today.

If no leaks, it’s time to test your CIS system

Good luck

[TeeJayHoward]

At idle, 5 in Hg. Methinks I found my issue - a vacuum leak. Additionally, there's a leaking sound coming from behind the air box. This could also explain why my brakes got "better" but never "good" when I connected the master cylinder after getting it home many moons ago - a leak that's just gotten worse.

I'm taking it all apart. It's got 40 years of filth on it and needs a good scrubbing. Plus, it'll be easier to replace stuff on a bench than it is reaching behind the airbox.

Or maybe I'll call up Patrick Motorsports tomorrow and just buy a set of carbs. Simple, stupid carbs. Aesthetically pleasing, even if it does make my checkbook weep.

[AndrewCologne]

Thanks for sharing the pics so we can see that its a lambda based CIS.
Rough idle and with ugly revs are often caused by a flawed OXS-Relay under the passengers seat.

See here in the link below the part "Rough idle and noticeable jerking at around 2000 rpm and loss of performance when accelerating":

https://nineelevenheaven-wordpress-com.translate.goog/der-911-us-sc-3-0-motor-mit-g-kat/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp
(translated via google) ... there "frequency valve" its wrong named as "clock valve" or "timing valve"

[Dave Kost]

Make sure your Frequency Valve is working properly.

You have the classic symptoms of a no op FV.

[Dave Kost]

Andrew,

Thanks for sharing!!

Everybody with a CIS Porsche should read it.

I lot of great information on the link (yours) that you shared.


https://nineelevenheaven-wordpress-com.translate.goog/der-911-us-sc-3-0-motor-mit-g-kat/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp