1970 911E MFI High Idle Problem

[6771911esses]

Thermostat clean and hot air hose from heat exchanger connected??

[elflamo]

Hi, have you been able to sort out the problem. I'm very interested as I'm experiencing the EXACT same issue overhere in the Netherlands. (even the same revs!)

[CoreyJ]

An update on the high idle when warm - we removed the distributor to address the advance/retard movement of the springs. We removed one loop from the smaller spring so that it might retract more positively rather than weakly as it was. We really thought this would improve the situation but to no avail - it ran the same.

One point someone brought up was how it acted when cold - and as it happens when you first start the engine cold the distributor advances and retards with a 900 rpm idle. Soon as it warms up - back to the 2000 rpm idle. Per suggestions in the thread, next we will clean the MFI thermostat and make sure the hot air tube from the heat exchanger is properly attached. It does not appear to be running rich at this high idle, no smell of fuel or black smoke. Looks like it is a temperature related issue - what a bear this has been to figure out!

[911skooter]

Mine is doing the same with 100 miles on rebuild now..900 at start up -some black smoke but clears up quickly , then when warmer she is 1500-1800.. everything is hooked up and new..I thought the MFI is out of the loop once it is warm and you only have the throttle stops on each butterfly and the mixture screw to set final idle..am I missing something? (main throttle rod to cab disconnected so nothing is pulling on it)

[boxster03]

Once again is Everyone sure their "throttle bodies" are not worn? I had a good shop dial everything in with the mfi etc. and best I could get warm idle was around 1100rpm because of worn TB.

[elflamo]

How would one find out if the 'throttle bodies' are worn? 'Spray brake cleaner and listen to engine method' - if so, is this really a reliable method?

[tobluforu]

Originally Posted by elflamo

How would one find out if the 'throttle bodies' are worn? 'Spray brake cleaner and listen to engine method' - if so, is this really a reliable method?

Remove them and use the flash light method.

[911skooter]

I would agree with the TB wear, strange that it changes when warm.. i do have some wear there. I also have found if you spray anything around even a real good TB you can pull in some to change idle .but wouldnt it be same when cold?? thats what I dont get..same air leak hot or cold??

[daepp]

I have always thought that one day I would get my TB's restored. But I have the opposite condition:

Cold, it idles around 1100 with no hand throttle; hot (190+) I have to use the hand throttle to keep it from dropping to <500 and getting the Oil light. A little lift on the hand throttle makes this largely a non issue, but I would like to correct it.

[boxster03]

Runs richer when cold thus the lower idle (not affected as much by extra air mix) and when it leans out the air leak on worn TBs increase idle

[elflamo]

Corey, just curious if you solved the issue and we might learn something from it?

[SJBuff]

Bump
Exact same problem on my 75 S. Did you figure it out?

[BURN-BROS]

Your complaint is most likely a Lean mixture issue caused by worn throttle bodies.

Trying to tune out the issue usually bears no fruit.

Here is a pic of a set of 2.2 T-bodies and stacks I rebuilt, moddified and restored.

[elflamo]

That looks great Aaron. But how do you deal with the inner bore (where the butterflies 'eat up' the material over time).

[BURN-BROS]

Originally Posted by elflamo

That looks great Aaron. But how do you deal with the inner bore (where the butterflies 'eat up' the material over time).

You have to bore them out and make new plates. Typically they are enlarged about 1mm. The size is based on how much corrosion is present as that damage is usually more pervasive. Throttle plate wear by itself may only require .5mm overbore.

The set above has been bored out to a massive 45mm throttle plate.