Fast idle with MFI after warming up, why?

[brittbolen]

What would cause a car that had been idleing fine when warm (800 rpm or so) to now refuse to fall below about 2050-2100 RPM at rest after warming up?

The car idles fine when it's cold, right around 800.

The car runs great, and it pulls through the gears with no hesitation (finally! 2 coils, points, plugs, and a fuel filter later) once it's warmed up.

I spent a while adjusting the idle cut off microswitch over the weekend. I attached my multi-meter to it, and adjusted the plunger so that it would close consistently when my foot was off the gas (it was not always closing if i came off the gas to easily, now it's nearly impossible to take your foot off without closing things.

The throttle linkage doesn't seem to be the problem, it's fully closed.

I know the fuel injection has not been messed with. I have a good thermostat hose, and it is warm, not hot, to the touch when the car is warmed up and idleing fast at rest. Plus if it were an MFI thermostat issue, that would be a richness problem, not a fast idle problem, right? (i could be wrong here)

The only other thing that has been changed is there is a small "noise suppressor" (thats what chuck said) that is right below the voltage regulator, that is currently disconnected because I was having some problems with the alternator light coming on. Could the lack of that suppressor be causing problems in the idle speed switch?

Anything I can try in the parking lot before I go home would be great, cause with that fast idle, i'm having trouble putting it in first without it grinding (which is odd i might add, the clutch should be disconnecting the transmission from the racing motor, but I think this is not the _cuase_ of my idle problems, just a side-effect.)

thanks,

Britt

[jluetjen]

Originally posted by brittbolen
What would cause a car that had been idleing fine when warm (800 rpm or so) to now refuse to fall below about 2050-2100 RPM at rest after warming up?

The car idles fine when it's cold, right around 800.

The car runs great, and it pulls through the gears with no hesitation (finally! 2 coils, points, plugs, and a fuel filter later) once it's warmed up.

I know the fuel injection has not been messed with. I have a good thermostat hose, and it is warm, not hot, to the touch when the car is warmed up and idleing fast at rest. Plus if it were an MFI thermostat issue, that would be a richness problem, not a fast idle problem, right? (i could be wrong here)

I'm guessing that the thermostat might be gunked up and sticky.

Anything I can try in the parking lot before I go home would be great, cause with that fast idle, i'm having trouble putting it in first without it grinding (which is odd i might add, the clutch should be disconnecting the transmission from the racing motor, but I think this is not the _cuase_ of my idle problems, just a side-effect.)

I'm pretty sure that this is is just a side effect. Even though the clutch may be fully disengaging, the transmission may still be spinning from inertia, thus overwhelming what are most likely somewhat worn synchros.

That's my guess.

[Gasman59]

Hi Britt:

I will let others chime in but I do think it is the thermostat.
On my car (MFI) when I start it cold I pull up the hand throttle to approx 1500 rpm and let it warm up.
It will suddenly jump to over 2500 rpm as it warms.
The only thing I believe comes into play at that point is the thermostat.
I simply lower the hand throttle a bit to regain 1500 until I drive off with the hand throttle fully off.

There are both mixture and idle speed settings at the MFI unit.

Best Wishes,
Ron Y
1970 E

[brittbolen]

Ok, so that is 2 people for the thermostat, but how does the thermostat affect the idle?

Is it as simple as richer == more gas, more gas == more power produced, more power causes the motor to spin faster, thus idle is faster? Is it that simple?

Britt

[304065]

Britt,

It's a combination of the thermostat and either worn or incorrectly set throttle plates. I had the identical problem for two years.

When the engine starts the MFI thermostat pushes the rack extremely rich, like 10 to 1 or so. The engine idles low because the mixture is actually too rich for good running, but it warms up quickly. Then as soon as the discs in the thermostat expand enough to lean the pump out, the mixture goes toward stochiometric and the speed increases. But enough air is leaking past the throttle plates at idle, or is going through the air bypass screws, to raise the RPMs. A slight increase in engine speed results in more ignition advance, which results in a greater increase in speed until equilibrium is reached around 2,500 RPM. Sound familiar?

I finally solved this problem by doing the following:

1) Remove the throttle bodies and soak them overnight in B-12 to remove all accumulated carbon, dried-out grease and crud.

2) Hold them up to a bright light and verify that you can't see any crescent-shaped light shining around the plates. Also, run your fingernail across the inside of the throttle body bore where the plate makes contact. There will probably be a slight groove, caused by the throttle plate slapping against that soft magnesium every time the intake valve closes. If the groove is very deep, the throttle body may need to be bored as part of the overhaul.

3) Next. . . not for the faint of heart. The odds of your throttle plates being exactly the same amount open 37 years later are approximately the same as the odds of my being attacked by a shark in the Central Park Zoo. Which is to say, not very likely! So you need to set the plates the same, by backing off the lock nuts and dialing the throttle stop in or out until the flow is the same. I used my shop vac and the synchrometer to make a rudimentary flow bench in order to verify that the throttles were flowing exactly 3Kg/hr of air with the plates closed and the air bypass screws closed. If you don't feel comfortable with this procedure, send the throttle bodies to one of the rebuilders, have them flowed and have the linkages set and marked with paint.

4) Use a pipe cleaner to clean out the air bypass passages in the throttle bodies.

5) Reassemble and set correlation. Here is how I did it. The MFI Diaries: Correlation at LAST!

If your throttle bodies are pristine or recently rebuilt, you might be able to get away with dialing the air bypass screws in to lower the idle. For a 1970 E you start with the screws open Five one-half turns. If they are WAY open that might be giving you a high idle, but don't dial them in less than the starting value, they are for fine adjustments only.

OK, so the point of that exercise was to make it possible for you to actually throttle the engine back to a 900 RPM idle by limiting the amount of air the engine receives. Again, the reason the RPMS are too high is because the engine is getting too much air when the throttles are closed. The idle mixture adjustment on the pump is then adjusted to provide a good mixture. This must be accomplished with an exhaust gas analyzer like the Innovate LM-1.

It will take several iterations of making sure the throttles are at idle, setting mixture, test, and repeat before you get it perfect. But the engine will reward you with great throttle response, good idle and improved fuel mileage.

Do you have CHECK MEASURE ADJUST? All of these changes MUST be accomplished in the context of following CMA exactly. Only then do you adjust the pump.

Good luck!

[brittbolen]

I did crank the screw on the throttle arm that actuate the idle switch down a bit to ensure the circuit always closed, perhaps that is holding the throttle bodies open? I'm going to remove the switch phsyically (that I way I don't have to readjust it) and see if that helps before I start the large work, especially since this _just_ started happening.

Britt

[69911e]

I would do a quick ignition timing check at idle to see if it is within specs. Your advance mechanism may be sticking.
Otherwise follow the above recommendations....
Ed

[brittbolen]

it's not the idle switch screw, i backed that in an out and i wasn't getting any real change in idle.

mmmmm CMA might need to happen...

B

[eapcpa]

The throttle swith can stick (hard to press) on light throttle release. Remove it as you said, double check your hand throttle inside the car ( i have left mine on a bit before and made adjustments) and check the throttle return spring to make sure you are getting good return tension. Also make sure your carpet at the pedal is clear and the linkages at the bellcranks are free. Tranny and throttle body. Let us know what you find.