Bosch MFI excessively high idle

[McClellr]

Folks,

My first entry and am in need of help.
I have a '69 911E with mechanical fuel injection.
It starts readily and runs great at speed, but after initial warm-up, the idle speed goes to 2000 and wont come down. I must admit I have fiddled with everything and had a Porsche mechanic (not familiar with MFI) look at it and we are stumped.
What I think I know is that the RPM sensor that powers the fuel shut-off solenoid by way of the micro switch, doesn't appear to be doing its job. I can hot wire the solenoid at the microswitch and the engine shuts down immediately. Is this enough to be my headache? Does the engine actually need the shut-off solenoid to get out of the mid-range and down to the idle control. The thermo - switch seems to be OK, at least it is clean and moved and has plenty of heat coming out the exit side.
The RPM sensor was working at one point because I got into a loop once with a high idle speed. The rpm sensor just played ping-pong between its two settings. I blew the fuse that powers the rpm sensor and after that nothing (clue).
I think I have two problems: (1) the rpm sensor and (2) tough time adjusting idle speed.

All help appreciated

McClellr

[Flieger]

Go through CMA and then remember "I Lean Left". You will likely need to tweak the idle screw to make the idle mixture right for the rpms you want.

Resources:

Ultimate MFI resources thread

MFI Pump - Open Heart Surgery

http://www.early911sregistry.org/forum/index.php

[BK911]

A quick way to tell if you are lean or rich at idle is to disconnect the throttle linkage between the pump and throttle bodies. Slowly twist the throttle cross bar to open the throttle bodies. If the engine dies right away, you are running too lean. If the idle speeds up, you are too rich. Ideally you want the idle to increase slightly then start dying. That means you are barely rich.

That is a quick way to tell. You really need an exhaust gas analyzer. One of the links Flieger posted is the CMA for MFI. Print it out. Follow the adjustment procedure. That will optimize your MFI.

Good luck!

[Steve@Rennsport]

Hi,

I would also carefully check the distributor to make sure the advance weights are not sticking; that makes the idle hang higher as well.

Those poor things are oftentimes totally ignored.

[356RS]

The RPM sensor/fuel shut-off solenoid has nothing to do with a high idle problem. When activated, it lifts the styles off the space cam pulling back the main rack which shuts off fuel. I think Steve's suggestion about the distributer advance system hanging up is a simple and easy thing to check first. Also the CMA has almost all the information and check lists needed to help sort out MFI problems. Let us know what you find out.

[pszemia]

i have been in your situation, 2 years ago. I ended buying the gas exhaust analyzer, MFI tools, and lot's of new gaskets, reason? The previous owner was a porsche owner, vw mantain.

The first thing you have to do is CMA, there are a lot of help on this forum.

But first, buy the things you need for CMA, and do it yourself, it's easy, have to learn some things, but you will save the mechanic costs, and learn a lot of things.

Also, don't try to low the idle avoiding the CMA process, with the throttles screw.

as i learned here, the MFI doesn't have sensors, so you have to put everything in it's place, and when you finish, it will run like never did.

[911st]

To much air or to much timing advance. Has to be one or the other.

Running a bit rich would not do it. Running a lean might bump it a little but probably not that much.

First check timing at idle.

Might check that the throttle shafts are not shot and that they are comming back to there stops. Check the vac take off from the throttle bodies is not leaking.

[boxster03]

Worn throttle bodies

[Zeke]

Originally Posted by boxster03

Worn throttle bodies

While definitely an item to consider, I don't think 2000 RPM can be blamed on worn shafts or throats. I had that problem and it made for closer to 1200 RPM before I tamed it down. Never achieved the 950 PRM though.

[boxster03]

Agreed MiltCENTER]A[/CENTER]

[McClellr]

Wow, thanks for the interest and the suggestions.
The distributor swing weights are free.
Took a good look at throttle body shafts. Five feel and look very tight, one definately worn. They return to the stops just fine.
Found this:
The car will idle (quite low with all my fiddling) until I rev it a bit. It then settles at 2000. I took the allen screw out and inserted a screw driver and gently pushed on the shaft. It immediately settled to low idle. I did this several times. If I push far enough, it shuts down. Have I a sticking shaft?
I did the same thing at the front of the pump behind the rubber cover and that shaft seems fine.

[356RS]

I assume your talking about the allen head screw that covers the main rack adjustment on the MFI pump.

[BURN-BROS]

Originally Posted by McClellr

Wow, thanks for the interest and the suggestions.
The distributor swing weights are free.

Yes, but does the advance come on too soon?

Originally Posted by McClellr

Took a good look at throttle body shafts. Five feel and look very tight, one definately worn. They return to the stops just fine.

Most of the time if one is worn there are more. the middle (2/5) shafts get more wear as this is where the droplinks connect from the crossbar to the t-bodies.

Originally Posted by McClellr

Found this: The car will idle (quite low with all my fiddling) until I rev it a bit. It then settles at 2000. I took the allen screw out and inserted a screw driver and gently pushed on the shaft. It immediately settled to low idle. I did this several times. If I push far enough, it shuts down. Have I a sticking shaft?
I did the same thing at the front of the pump behind the rubber cover and that shaft seems fine.

A couple of things.
There are a couple of things that can stop the return to idle with the stock setup.

The micro switch and the flapper valve(72-73) combined with old return springs can prevent full return.

worn shafts and excessive axial movement will result in the throttle plate making contact with the bore, preventing full return.



disconnect the rods that come from the crossbar to the t-bodies to isolate them in order to see if the wear is a problem...same with the crossbar/ microswitch..

[dicklague]

A few months ago I started serious learning about my MFI system and started tuning. As i learned more and the car started running better and better, I had a stage where the idle was creeping up like yours.

I checked loads of things and went back through the CMA. It was then that I discovered the linkage from the throttle assembly to the pump was too short. I believe it has to be 114mm [check this out in the CMA] And this was making things too lean at idle. I also found out the Tach was about 200 RPM high in its readings.

You really need a good AFR gauge to troubleshoot and adjust the pump. The new generation are excellent and and give very accurate readings. I have one permanently installed in my 2.7 MFI 1973 and it has been a revelation. Our hosts sell them.

[McClellr]

I removed the rubber seal on the shaft at the front of the pump and pushed the shaft in several times as the engine sat at 2000 rpm. As I pushed it in, the rack closed and the engine speed reduced; to shut off if I went far enough. Each time I released it, it sprang back without any sign of sticking, but to the 2000 rpm position. Seems like it is just set to rest there at "idle". I have run it with the linkage off and no change.
My one bad throttle body is middle left as predicted. I have run it with all the linkage rods disconnected.
When adjusted to 114mm, the rod opens the rack slightly.

[BURN-BROS]

Originally Posted by McClellr

When adjusted to 114mm, the rod opens the rack slightly.

All the throttle shafts need to rest on their respective stops. The rods should not influence any of the throttles, once they are on. With the drop lings removed from the crossbar, does each t-body return to their respective idle positions?

[304065]

Too much advance, distributor is sticking. I had the EXACT problem you are describing for YEARS until I had the distributor rebuilt. Call Ed Fall at Vintage Werks Homepage or Barry Hershon at IAE in Detroit, they can help.

[Zeke]

Originally Posted by BURN-BROS

All the throttle shafts need to rest on their respective stops. The rods should not influence any of the throttles, once they are on. With the drop lings removed from the crossbar, does each t-body return to their respective idle positions?

I had the micro switch problem. That was not an easy fix with old ball joints, etc. And a heavy axillary spring just makes throttle pedal pressure annoying.

But to address BROS statement, in the CMA it tells you to tune the idle with all of the rods disconnected using a sychrometer. This by far was the most productive for me and found other problems to fix.

Doing the disconnect thing showed me one butterfly that was actually clicking as the pulses and stand off sent it rattling inside the bore. This one stack needed the air bypass screw to be pretty much closed. It was leaking by the butter fly that much. I finally found around 1000 RPM on a pretty old and worn system.

Still, one needs to start at the beginning and go thru the whole Check, Measure, and Adjust right down to the spark plug gaps. Don't go messing with the pump adjustment screws until all else is done. I did that and lost my place ending up taking a whole day to make it right.

[Flieger]

The rod from the main pump must be 114mm. Period. Adjust air screws and such to achieve good idle with the rod at 114mm.

[Zeke]

Originally Posted by Flieger

The rod from the main pump must be 114mm. Period. Adjust air screws and such to achieve good idle with the rod at 114mm.

CMA says 114 +- 2mm. I found that the bolt holes on the pump will allow the pump to be mounted a little forward or a little aft. Then the stacks can move a fraction as well. Put one aat one extreme and the other........ This is where the correlation tools become useful.