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New MFI idles high after 20 minutes

Hi all,

Iím hoping to get some ideas why my í70 911 E idles at 1600 rpms after driving about 20 minutes or so.

My car had carbs but I purchased an MFI and had the engine and MFI rebuilt. Unfortunately due to family situations, my engine sat for a little over a year. I only mention that in case that has some bearing on my problem. Last time out, I drove the car and it idle at 1000 Ė 1050 rpms. Iíve checked linkages, microswitch and that the lowest I can get the car to idle. After about 15 minutes, the oil temp was about 190 degrees or so and the car idle fine, about 1050 rpms. By the time I returned to the house, the engine was hunting (surging). I disconnected the microswitch and the idle rose to 1600. I double checked the linkages and everything seemed to be fine. Nothing appeared to be sticking. If I shut the car off and restart it, the car would idle again about 1000 rmps. It would only raise back up to 1600 after I would blip the throttle.

Iím assuming that the problem is not with the air/fuel mixture or air correction screws since the engine initially idles fine when the car is at operating temperature. Time seems to have an affect on the idle. Not sure if other components on the engine are heating up cause a change.

Any thoughts or ideas?

Thanks

Chris

Old 08-28-2020, 05:10 AM
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Was the stack of belview washers/springs reassembled the same as they came from the factory?

These move the adjustment rack by expanding/contracting with temperature change. The amount of change can be adjusted by changing the direction that each washer faces. If they face the same direction, the rack will only move by the growth of one washer. If the washers are aligned facing each other, then the growth in the length will be equal to the total growth of the stack. I doubt that they all faced in the same direction when they left the factory (doing so would negate the adjustment aspect of having them there). Unless the person who rebuilt the FI pump was careful to not change the alignment of the individual washers in the stack, they could have been reinstalled incorrectly. Also sometimes a washer or two is lost which will also mess the whole thing up.

You'll need to do some research because I'm not aware of the details of the alignment and the amount of length added by each washer. But if you know the temperature gain and the additional length per washer, you should be able to calculate the amount that you want the rack to shift, and then flip the washers accordingly.
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Last edited by jluetjen; 08-28-2020 at 05:38 AM..
Old 08-28-2020, 05:35 AM
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It would be good to isolate the problem between the MFI Thermostat and the RPM transducer fuel shut off circuit. I ended up replacing my Thermostat temporarily with a plate and screw kit so I could get the mixture correct without the Thermostat moving it around and also so I could measure how far I needed the thermostat rod to be at both cold and hot settings before I re-installed the Thermostat. You can also use a heat gun to speed up the movement in the driveway. There are threads that show the proper orientation of the discs and how to clean and assemble the Thermostat.


The car should run fine with the RPM Transducer solenoid disconnected except you may run the risk of a backfire on deceleration. You can test it by raising the idle above 1300 and then depressing the micro switch and the car should hunt between the upper and lower settings, Usually 1300-1600 rpm. The car would hunt and surge if that circuit was activated at any time other than deceleration.
Old 08-28-2020, 06:32 AM
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Quote:
Originally Posted by jluetjen View Post
The amount of change can be adjusted by changing the direction that each washer faces. If they face the same direction, the rack will only move by the growth of one washer. If the washers are aligned facing each other, then the growth in the length will be equal to the total growth of the stack. I doubt that they all faced in the same direction when they left the factory (doing so would negate the adjustment aspect of having them there)..
No. The thermo disc's are not altered or changed in their direction, some facing each other or some not facing each other. The only correct way to assemble the 25 pairs of disc's are to install them with the "concave surface facing each other as a pair.

Correct way:




Incorrect way:



Adjustments are made after every rebuild to make sure the "f" setting is correct so the correct amount of fuel is added for cold start and the warm up period. Shims are added to lean out the amount and removed to richen up the amount. Shims are sized from .10mm to .80mm, then some larger ones are 1mm, 2mm and a 3mm.



All this is to make sure that the fuel on cold starts and the warm up period starts out
with 30% more fuel. Things work great as long as the thermostat is receiving good warm air coming off the heat exchangers.

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Old 08-28-2020, 07:32 AM
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I thought about the thermostat but doesn't that lean the rack out once hot? Seems like I would have a low idle, not high.

Chris
Old 08-28-2020, 09:45 AM
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Quote:
Originally Posted by 356RS View Post
....Things work great as long as the thermostat is receiving good warm air coming off the heat exchangers....
We are all assuming that you have the double-walled hose from the heat exchanger port to the thermostat connected????
Your description of the problem sounds like you skipped this step.
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Old 08-28-2020, 10:44 AM
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Yes. I do have those two hoses connected. They are brand new.
Old 08-28-2020, 12:27 PM
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Quote:
Originally Posted by chris belyea View Post
I thought about the thermostat but doesn't that lean the rack out once hot? Seems like I would have a low idle, not high.

Chris
Chris, I was responding to the post about thermostat discs, not your high idle problem. Sorry.
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Old 08-29-2020, 05:41 AM
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If the distributor advance mechanism is sticky it can cause an inconsistent idle in exactly the way you describe.

Having an AFR gauge would help to diagnose the MFI system and allow you to monitor the mixture. I would not own an MFI car without one.
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Old 08-29-2020, 06:08 AM
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Regarding the distributor. This may be normal but I don't know because I don't have anything to compare it to. With the cap removed, I rotate the rotor clockwise and release. The rotor returns without sticking. BUT....if I try to rotate the rotor counter clockwise it does move a little. Maybe a degree or two. I rotate back clockwise and the same thing again. I can rotate counterclockwise a degree or two.

That may be normal. Don't know.

Chris

Old 08-29-2020, 07:15 AM
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