81 FI Fast Idle 1600RPM out of ideas

[sclebo05]

Greetings all. I've had the car a few years, it's always run pretty good, just went through replacing all belts and hoses successfully thanks to this forum. I'm moving on to one of the last items on my hit list - the car has always had a fast idle. Like 1500-1600rpm fast.

I spent some time reading posts, and have verified the following:

- Verified main plenum is in good order, no cracks
- Verified any vacuum or intake connectors that I've found
- Tried silly things with propane and carb cleaner, haven't found any leaks.
- Pinched off brake assist unit vacuum line, idle does not change
- Plugged charcoal cannister completely with my finger, idle does not change. Slight suction when I remove my finger.
- Pinching the Auxiliary air intake line does affect the idle at first startup, but does nothing once warm. As expected I think.
- Auxiliary air intake lead is sending 12v while the car is running.
- Fiddled with throttle set screw and the screw on the other side is all the way in. If I back it out, engine REALLY ramps up.
- Throttle cable is not taught at rest
- TPS switch tests OK and I've rotated until I hear a click, and set it there. It 'click's when the throttle cable comes off adjuster screw.

Car runs well, I just have a tendency to chirp the tires when pulling out I have to be missing something obvious. Help!

[RRoller123]

That is an odd one, sounds like you have done all the usual stuff. Usually the most obscure culprit is the TPS holding the throttle plate open, even with the cable stop loose and the air bypass screw all the way in. Sure that it is set so that it does not effect the throttle plate? Maybe take the main air hose off and look inside, the throttle plate should be almost completely closed, just the tiniest bit open. Loosen the TPS and see if it has any effect on the plate itself.

[ORFORD2004]

Bent throttle plate?

[RRoller123]

good idea, maybe the 2 mounting screws are loose and it is letting too much air in?

[RRoller123]

Check the air inlet for the distributor advance, which is just under the larger crankcase bypass hose at the throttle plate.

[sclebo05]

OK Guys, I have a few more datapoints.

- I replaced the hoses for the AAV and the brake booster
- Sealed off or used pliers to stop any 'extra' vacuum hoses, like distributer, brake booster, charcoal filter, and AAV (once warm)
- Disconnected and pinched off my cold start injector power and fuel
- Messed with TPS switch again, I'm rotating to the right until it clicks. If I rotate left, it simulates throttle cable moving, and engine revs up.
- looked at my throttle plate. It doesn't appear bent and makes a good seal.


I recruited my wife to help with a series of tests that may have led me down another path.

I completely removed the plenum, and with the engine warm, blocked the AAV and manipulated the DISCONNECTED air flow meter. By using my fingers to swing the flap inside, I was able to get the engine down to 500rpm (a little shaky) and all the way up to 2000rpm (could have maybe gone more. I could hit my mark of 800-900rpm quite easily.

So my attention is now on this device. I'm curious how to test the AFM electronics, and how to tell if the gear inside has been altered in any way. Is there a an ideal angle during idle that the door SHOULD swing? Any ideas on where to start? Is this a sign of some other electronic issue?

[ORFORD2004]

By moving the AFM, you only run more rich. so rich that you hit 500 RPM and probably smoking black at the tailpipe. Get the hell out of the AFM except if the track is worn out. Then you move up or down on the track. The only way to really adjust the AFM is having a wideband O2 sensor with a gauge or if your old school the color of the plugs. Too rich and you loose power. Too lean you can destroy your engine
I suspect AAV but usually you can compensate with the throttle stop screw.
TPS closed circuit between pin 2 and 18. Circuit open as soon as you hit the gas.

https://www.mirafiori.com/faq/fiatFI_may2002.pdf

[RRoller123]

Just for giggles, trying disconnecting the throttle cable completely from the throttle body, and loosen the TPS so that it has no effect at all on the throttle plate. (Don't remove it, as I think it is required to make the air seal at this end of the throttle shaft.) Then start it up and see if it is still running too fast. Also, with the throttle cable and TPS disconnected as above, and the engine not running, rotate the throttle body and see how smoothly it rotates through its range of motion, and verify that it very smoothly returns to a fully closed position. Pull the air hose so you can see the throttle plate. It should be very smooth not have a catching spot anywhere.

One other idea is that it is possible that the throttle plate axis is worn and that air is leaking in there. I am referring to the opposite end of the throttle shaft, at the large coil spring, where there is no particular seal. You could use an unlit propane torch at that point while the engine is running to see if there is any change of speed, that would indicate point of leakage. (Of course be careful with propane, it is heavier than air and will collect)

Another possibility is the air coming up from the crankcase vent hose, the large hose that goes down the side of the engine to the oil separator under the intake manifold. That thing is generally out of site, and the small 90 degree hose at the bottom takes a lot of heat and stress and may be disconnected or broken.

Good luck, a solution will be found!

[RRoller123]

One more thought: Is the small round seal on the top of the AFM, where the idle mixture screw is, still intact? Or missing? That would give you a pretty good indication of whether the AFM has been monkeyed with in the past.

[sclebo05]

OK a few more test results.

- Throttle plate looks to seal very well to me (checked again)
- Tried propane around the throttle return spring. No leak there.
- Loosened the TPS, didn't seem to affect the TPS returning.
- Worth noting for orford2004 - when I did move the flap and hit 500rpm, it did NOT seem to run too rich. How can I check for a bent AFM flap?
- Mixture screw on top of the engine is completely snug. Throttle cable disconnected.
- Pinched off the EGR hose, no change.
- my AFM has DEFINITELY been opened, the glue job gives that away. Picture of the inside here, maybe something amiss?


One final comment, as I've been looking at other things. I checked my timing with an old timing light, and unless I'm mistaken, my timing is WAY early. I tested on Cylinder 1 and 4, and the timing is way before any of the 3 notches on the gauge next to the crank pulley. Would a distributer/timing issue cause a fast idle? My car has always preferred premium, could that be another clue?

[RRoller123]

When you say "way before", you mean that the indicated light mark is closer to the fender wall than the three timing marks? That would be very much advanced, too far, which would raise the idle. 10-12 degrees BTDC (just a tiny bit beyond the third point, or the one closest to the fender wall, is fine. This could be the answer right here, and the 93 octane falls in line with this possibility. Re-Time the engine with a light and report back, yes? Clockwise rotation of the disty retards the timing, which is what you would want. Doesn't take much. I put a small white dab of "white out" in a suitable location to act as a simple reference point in case I move it too far and forget where it started from.

FWIW: Mine is in the block position, behind the Alternator, where you have to be very careful reaching in to turn the disty when the engine is running. That is pretty close to the moving belts, etc. and a loose shirtsleeve could prove tragic. Also be careful with the wrench on the disty nut. If the Alternator output stud is not covered properly by the rubber boot, the 13mm short wrench could short (no pun intended) with a nice fireworks show resulting. (I just did time mine, I think it is 13mm, if I remember correctly. Might be a 10mm, hmmmm...).

[sclebo05]

Yeah.. so if the crankshaft pulley rotates clockwise, and I'm using the timing light correctly, I'd be at 18-20 degrees. I was under the impression this wouldn't work at all! I noticed that regular pump gas sometimes 'pinged' and the high octane ran much better.

Also, my distributor is on the passenger side, and alternator on the driver's side, as mine is an '81

The timing is worth a check, haven't touched the dizzy yet. I have sorted many many things since I purchased this car. I can say I better understand many components after this process, so it hasn't been a total waste of time. I'll check timing in the morning, I'm off to do some reading. Thanks!

[sclebo05]

OK, I adjusted the ignition timing and walked it back to 10 degrees BTBC. This reduced the idle RPMs by 300-400. So now I'm at 1100 - 1200 rpms at idle. Good enough?

While changing my timing belt, I also replaced the hoses on the oil vapor separator. I had taken it off, and it's ... completely empty. Should this have a screen in it? Any kind of gasket between the separator and the block? I've sealed the hose between here and the intake so I'm not worried about leaks. More of a side question.

Definite progress but I'll keep looking. Thanks guys.

[RRoller123]

Regarding the separator, I think it may have varied through the years, but some have a brass mesh in there, sort of a brass screening, scrunched up in a ball shape. It is a flame arrestor only. Would be a good idea to have it. It is good that it is dry in there. Only a very fine mist works its way up the tube. Check inside the throttle body where that large hose resolves itself into a very small diameter (maybe only .02") inside, and you should feel a slight film of oil working its way back to the throttle plate.

Try adjusting the idle now with the throttle cable and the idle screw to get the idle down below 1000. Loosen TPS completely, close the idle screw, adjust cable for about 800, then open screw for about 900. Then adjust TPS. Then you should be very close.

As an alternative (although this idea gets the feathers flying around here, which is both fine and interesting ) you can adjust the timing for highest STEADY manifold vacuum. That usually gets you close and gives the most amount of advance that is safe. I did this for the first 115,000 on my X1/9 and it worked fine.

I have Mark's cams installed and can't get a good idle below 1000, exhibits the typical "lumpy" street cam idle. Anyway I am not sure the dashboard tachometer is accurate enough to discern a couple of hundred RPM down there, so I bought a cheap multi-meter with a tachometer feature that reads a spark wire through induction just like the timing light does, and indicates ~actual RPM. Since all it has to do is read pulses and divide by time, it should be very accurate. I haven't used it yet, but this has got me interested to find out. I will try it and report back.

Glad you are making progress!

[sclebo05]

I've learned a lot on this thread, thanks for the posts.

A few interesting points:

- my mixture screw is ALWAYS completely in. Backing it out at all revs my engine, so I believe the seal is good. I've tried sealing it completely, no change.
- Throttle cable is always slack.
- My throttle set screw is always all the way out.
- Rotating my TPS to the left revs the engine
- New info: I'm told my engine was rebuilt ~50,000 miles ago. This may explain some of the odd findings here and there. I'm wondering if I have stock cams, etc. Adds some complexity to troubleshooting at times when I find something installed incorrectly, etc.
- Idle is around 1000-1100rpm, went for a drive and when engine was warmed up I started moving towards 1500 again. Car runs great.
- Yesterday I tried to FORCE my throttle plate closed by counter rotating the throttle plate mechanism by hand. I can drop maybe 100rpm at most. The spring isn't exactly stiff, but seems to return the plate 99% of the way.

I'm also suspecting the tachometer, so I'm going to try discerning my RPMs via one of my multimeters or even a phone app that uses acoustics and looks for patterns.