Run away idle, 78 SC

Johan.s · 09-10-2016, 10:11 AM

Hi,

Had an unpleasant surprise when in started my 1978 911 SC today. Upon start the engine rpm rushed basically to redline before I killed the ignition. Inspected the throttle linkage to ensure it was moving freely before trying again, same result...

I removed rubber hose between the throttle housing and the air box and throttle valve is moving freely. It's not completely shut though when no throttle is applied but I guess that is normal?

Last time I drove the car (a week ago) it started and ran fine. When cold starting the rpm usually fluctuates slightly(maybe 100 rpm up and down) before it settles at 900 rpm after 30 seconds.

Any advice on where to start the trouble shooting?

Thanks!

pmax · pmax's Garage

Check the cruise control cable, the wire connected to that cylindrical alien pod object on the driver's side.

See if it has come loose of the bracket next to the throttle body.

Johan.s · 09-10-2016, 10:28 AM

I don't have cruise control but maybe there is a cruise control cable any way?

Bob Kontak · Bob Kontak's Garage

Check to make sure the throttle linkage returns with spring power. Another thought, pop the arm off the throttle linkage and rubber band it closed.

If it still revs high you have a big air leak that is allowing the engine to rev up only from the cold start valve fuel. Any huge backfires?

I can't picture the engine being able to spin that high with the throttle being closed unless an air leak.

Have you fiddled with anything before today? Like maybe yesterday?

I bet dollars to doughnuts it's something silly.

Johan.s · 09-10-2016, 12:23 PM

Haven't fiddled in that region

I zip tied the throttle linkage. Same thing… Must mean that there is a major air leak somewhere.

Found one odd thing though. One of the hoses on the front of the throttle house (number 13 on this diagram) was disconnected. The hoses on the vacuum limiter is connected in another way than the diagram indicates. It looks like the second picture where the hose from the top of the vacuum limiter is connected to the back of the throttle house.

pmax · pmax's Garage

Quote:

Originally Posted by Johan.s

I don't have cruise control but maybe there is a cruise control cable any way?

Post a pic.

Johan.s · 12:43 AM

I zip tied the throttle linkage and no change unfortunately.

I did find one disconnected vacuum hose from the front of the thottle house (the side facing the rear of the car). Looking at the diagrams available online this hose should be attached to a t-piece that then connects to the thermo valve and the vacuum limiter. Looking at my vacuum limiter there is a tiny hose going from the top of the vacuum limiter in to a t-piece that then connects to the backside of the throttle housing and to the thermo valve.

Is it possible to disconnect the varm up valve and try strating the car without it?

Johan.s · 09-11-2016, 12:41 AM

The upper hose goes to the warm up valve. The one below is disconnected.

Johan.s · 09-11-2016, 12:54 AM

Browsing through pictures online it seems that the hoses to the vacuum limiter is connected correctly. Still no clue where the second hose on the front of the throttle body should go. Can it be related to the air pump that has been removed from my car?

Bob Kontak · Bob Kontak's Garage

Was there an open port on the TB?

How big is the port?

ossiblue · 09-11-2016, 09:01 AM

Quote:

Originally Posted by Bob Kontak

Was there an open port on the TB?

How big is the port?

Agree with this line of thought^^.

This just began suddenly, long after the air pump had been removed so even if the hose should be connected to the air pump, it hasn't been and there wasn't an over rev problem until now.

The air leak theory is the strongest, at this point. You have secured the throttle shut with a zip tie so, as Bob states, the only way for the engine to rev so high is to get enough air from somewhere. Look very closely for air leaks, be it in the throttle body or somewhere else. Rubber cracks and sometimes the leak cannot be easily seen. There are also hoses on the front side of the engine (facing the firewall) that can have leaks but are hard to detect due to their location.

Johan.s · 09-11-2016, 11:09 AM

I believe that the hose that was disconnected is suppose to be connected to a diverter valve.

The air pump hasn't been on the car for 20 years.

I wonder if I could try disconnecting the cold start valve and that way reduce fuel during start up? Would be way easier to find the air leak if I could get it to idle and spray some starter gas around the hoses.

gomezoneill · gomezoneill's Garage

Check the pop off valve in the air box it might have come loose. If it was off completely the car wouldn't start at all. Check the glue and fitment.

ossiblue · 09-11-2016, 11:30 AM

Quote:

Originally Posted by Johan.s

I believe that the hose that was disconnected is suppose to be connected to a diverter valve.

The air pump hasn't been on the car for 20 years.

I wonder if I could try disconnecting the cold start valve and that way reduce fuel during start up? Would be way easier to find the air leak if I could get it to idle and spray some starter gas around the hoses.

Best way to find air leaks is to pressurize the system with air (low pressure, like from a vacuum cleaner exhaust) and spray a soap solution around suspect areas. Many threads on ways to do it.

If you do have a massive air leak, disconnecting the CSV will likely produce a no-start condition.

Pop-off valve is a good suggestion.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by Johan.s

I believe that the hose that was disconnected is suppose to be connected to a diverter valve.

The air pump hasn't been on the car for 20 years.

I wonder if I could try disconnecting the cold start valve and that way reduce fuel during start up? Would be way easier to find the air leak if I could get it to idle and spray some starter gas around the hoses.

Johan,

You don't need a running engine to test the vacuum (air leak) in your engine. All you need is a continuous supply of pressurized air. Even with 5 psi air pressure is sufficient to detect the source of air leaks. Could you post picture of the decel valve located right behind the throttle body. Like to see the vacuum hoses connect to it. Thanks.

Tony

Johan.s · 09-12-2016, 11:04 PM

Thanks for the advice guys. I've been away traveling for work but will do some testing this weekend and report back.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by Johan.s

Thanks for the advice guys. I've been away traveling for work but will do some testing this weekend and report back.

I like unplugging the CSV on the chain cover and see if it will start. If not, you probably have a big leak. This would save you time. There are only so many big leak spots to deal with. I bet your car will not start without CSV function.

My walk of shame.......... #1 The metal tube that comes up from the driver's side back of engine. I think it's related to the brake booster. It popped out of the back of the TB because I did not have the fasteners in place by the intake runners.. It did not rev, just fell out of the hole and the engine just died.

#2 Forgot to hook hose back up to the snoot on the Aux air regulator. High revs then BOOM once shut off. Did it again, yes I did. Boom big enough to get the stink eye from the neighbors.

Not every big air leak has a boom.

Johan.s · 10-02-2016, 09:01 AM

Finally had some time to work on the car. I pressurised the system and sure enough quite a bit of leaks. Two of the injectors was leaking and a lot of leaking at the base of the throttle body.

When I removed the throttle body I discovered that the throttle is not completely flat. Thus the throttle can't close completely. Is it supposed to be flat? The throttle looks like it is home made and deliberately bent. But since it's made from aluminium I wonder if it could have been bent from a backfire?

If it's supposed to be flat I can bend it back or make a new one out of steel and then fit new gaskets.

Bob Kontak · Bob Kontak's Garage

Don't do a thing until you put it back in the way it came out.

I have never had mine out but I do have an 81 in the driveway that I can take the bellows off of to double check so the answer is available if nothing else works.

Here are my thoughts........

- The bend is too straight.
- Re-install and get a set of feeler gauges to check wall clearance. Just a few thousandths.
- Was there a big gap when you took it out?
- You sure it's aluminum?

Johan.s · 10-02-2016, 10:04 PM

Without knowing if it's possible, theoretically if a backfire could bend the throttle I reckon it would be bend very straight since the axle holding it would be the strongest point and it would bend on either side of it.

Yes there was quite a bit of gap as a result of the bent side.

It's definitely made out of aluminum. It's lightweight and not magnetic.