Idle speed follies

[RDM]

My car runs great. Good throttle response, extremely flexible, will pull right up to redline (and a bit beyond if I'm not careful) starts warm or cold, summer or winter, with minimal effort. Two flies in the ointment: my idle speed is about 1200 RPM warm, with the idle screw all the way closed, and I get fuel "economy" of about 15 mpg. Driving is mostly village to village and I usually don't drive like a teenager.

So knowing the questions that Tony and Bob and Tim and Kevin and [your name goes here if I've missed you] would ask, I set out to figure out why.

1. Throttle plate visual looks OK.

2. Air metering plate looks OK



3. System pressure 5.0 bar



4. Cold control pressure 1.8 bar (slightly low for an ambient temp of 18 C)

5. Warm control pressure 4.2 bar after 8 minutes (too high- spec 3.0-3.8 bar)

6. Residual pressure 1.5 bar after 10 minutes, 1.35 bar after 30.

7. AAV almost entirely closed after about 6 minutes (picture taken at about 3 minutes). The Bentley says to pinch off the line going in to the air box, but they are all metal and would only pinch once. I did this instead.



8. Engine nearly dies when oil tank is opened

So high idle with the idle screw closed indicates air leaks, which the oil tank test suggests not. Gaskets / sleeves / o-rings / boots / etc were replaced 3 years ago at engine rebuild (and I've had this problem ever since).

Would the high warm control pressure cause a high idle (seems like it would work the other way and reduce fuel), or is it time to look at timing / lambda dwell / mixture setting?

Or is it time to go for the Chevy Monza with the blown 406 on BaT; more power, less money, and about the same gas mileage?

[rwest]

I'm by no means as good at this as the experts on the board, but I would think that by plugging the port on the back of the boot and the hose attached to it, you would be able to figure out if the AAV or AAR are bleeding extra air into the system after it is warmed up.

[boyt911sc]

Dru,

Did you make any adjustment to the WCP in the past? The WCP is out of spec and should be adjusted or calibrated first. I will be coming back home this week after being away for a month. Send the WUR back to me and will re-calibrate it to spec. Since you have an APO, shipping from Germany to USA should do it. Include the AAR in the package so I could test it too. PM me if you are interested. I will have them tested and shipped 3 to 4 days upon receipt. BTW, I suspect you have unmetered air going into the system.

Tony

[RDM]

I do have a spare WUR. I'll try that this afternoon. I also bought a can of starter fluid last night so I can hunt air leaks. We'll see what turns up.

[RDM]

Spare WUR is out of spec the other way (cold pressure too low), but in spec on the warm side. I installed it and took a drive but didn't make a difference with idle speed.

Following Bob's example, I have a can of starter fluid to hunt air leaks, and I shouldn't be afraid to use it.

I still have the question as to whether some combination of timing and mixture could create this condition. Any ideas?

Tony, I will box up one set of adjustable parts and get them on their way to you.

As I learned, but I'm sure Tony already knew, is that there are two versions of the 072 1980-only WUR, one with a vestigal vacuum port, one without. Who would expect the French to produce a more efficient part than the Germans?



Neither comes with a golden retriever; those have to be purchased separately.

[boyt911sc]

Dru,

WUR-072 is non-vacuum assisted unit so it is not critical whether if it has a vent port or not. WUR-072 with a vent is normal and the one without vent is an oddity. I also have a couple of these oddities in my collection. They both work well as long they are calibrated and the bi-metallic springs and heaters work. What are your fuel pressure readings for these WUR's? Do you still have my home address? Keep us posted.

Tony

[RDM]

OK, so starting fluid killed my engine. That is, when spraying on the driver's side of the airbox, some effect of that spraying (ice from the humid air?) managed to choke off the extra unmetered air causing the engine to die. So, part of this winter's work will be to pull the airbox and hunt down the leak.

Tony,

I have the parts boxed and taped, but no address for you anymore. I went through all my emails and messages back three years and didn't find it, so if you could PM or email it to me, I'd appreciate it. And did you do anything with LED tail lights / fog lights?

[WJL911]

Have you checked the peanut over good?

[Jonny H]

See the screw to the right of your 'toilet seat'? If it won't tighten, then the box is split underneath it.

[boyt911sc]

Dru,

Sent you a PM with my shipping address. Please confirm.

Tony

[T77911S]

you asked if hi CP would cause hi idle. no it wont. without adjusting anything else, if you set the CP to spec and set the mixture to 14.7, then set the CP out of spec and readjust the mixture for 14.7, the idles should be the same.
if your mixture was adjusted with the CP in spec, then the CP was adjusted out of spec and the mixture left alone, the idle probably went down.

you have to be careful so that the starter fluid does not get pulled into the normal intake.

[pmax]

No expert here, but I would check the "toilet seat" for leaks.

The WUR is not the cause, as you have confirmed with your experiment and as T77 said.

Now, if I can only understand the need for tearing down a CIS system which starts all the time warm or cold summer or winter and has great throttle response...

[Tippy]

Did you check for vacuum leaks?

This will cause an increase in RPM.

[picinkoff]

Dru,

It appears that you removed one of the oil breather hoses from the top of the oil tank; I can see the hose clamp in your first picture.

Did you ever change/replace that hose? I'm referring to the hose that plugs into the tank just under the filler neck.

The reason I ask is that, if you forgot to install the cone shaped check valve/restrictor inside the end of that hose (which then plugs into your oil tank), you will see the behavior you described. ie, higher idle, and engine dies when you remove the oil cap.

This is the part number for your '80 SC.
930 107 289 00

Here's a link to a picture of what it looks like installed.
https://rennlist.com/forums/attachments/911-forum/553489d1311258719-89-carerra-3-2-stalls-when-engine-oil-cap-is-removed-0060-motor-check-valve.jpg


Paul

[Dave Kost]

Originally Posted by Tippy

Did you check for vacuum leaks?

This will cause an increase in RPM.

This^^^^^^!!!

before you do anything! (smoke test)

Then set the CO to the correct specification.

Its obvious you are getting unmetered air under the throttle valve.

[RDM]

Originally Posted by picinkoff

Dru,

It appears that you removed one of the oil breather hoses from the top of the oil tank; I can see the hose clamp in your first picture.

Did you ever change/replace that hose? I'm referring to the hose that plugs into the tank just under the filler neck.

The reason I ask is that, if you forgot to install the cone shaped check valve/restrictor inside the end of that hose (which then plugs into your oil tank), you will see the behavior you described. ie, higher idle, and engine dies when you remove the oil cap.

This is the part number for your '80 SC.
930 107 289 00

Here's a link to a picture of what it looks like installed.
https://rennlist.com/forums/attachments/911-forum/553489d1311258719-89-carerra-3-2-stalls-when-engine-oil-cap-is-removed-0060-motor-check-valve.jpg


Paul

Paul,

There are two hoses from the oil tank. The top one is just straight through. The one below it, with the two hose clamps on it, has an adaptor for linking the two hoses together, but no valve on the inside. The PET seems to indicate that it's for '81 and later; is it recommended for my year as well?

Originally Posted by Jonny H

See the screw to the right of your 'toilet seat'? If it won't tighten, then the box is split underneath it.

Jonny, how tight is tightened? It resists my turning it, but it's plastic and if I wanted to keep turning it I could with a medium level of effort.

Originally Posted by pmax

No expert here, but I would check the "toilet seat" for leaks.

The WUR is not the cause, as you have confirmed with your experiment and as T77 said.

Now, if I can only understand the need for tearing down a CIS system which starts all the time warm or cold summer or winter and has great throttle response...

High idle, poor fuel economy, and high temperatures at high engine speeds, even with a brass cooler, which makes me suspect a lean mixture at high RPMs. Unfortunately I don't have a way to measure the richness of the mixture.

Plus, it's a hobby car. If there's nothing to mess with, the hobby isn't as interesting.

[Flat6pac]

The restrictor was used in all from SC. Install in the hose that installs in the filler neck.
Bruce

[RDM]

Originally Posted by Flat6pac

The restrictor was used in all from SC. Install in the hose that installs in the filler neck.
Bruce

Does it go right on the filler neck, or partway down the hose somehow? My new hose that goes there is one continuous piece.

[T77911S]

my 77s had a restrictor in the hose to the oil tank.

[picinkoff]

Originally Posted by RDM

Does it go right on the filler neck, or partway down the hose somehow? My new hose that goes there is one continuous piece.

Dru,

The simplest thing to do will be to insert the restrictor in the rubber hose only a small bit. Make sure the cone is facing outwards towards the oil tank.
Then place a small bit of dish soap on the oil tank nipple/tube to make it easier to slide the hose on.
Then push the hose/restrictor assembly onto the oil tank nipple/tube. Push the hose all the way in. This will cause the restrictor to sit against the oil tank nipple.
You'll notice that the restrictor is designed to sit on the oil tank tube opening, but not go in.

Paul