"feels like I'm talking to myself here for the last while .. should I start a new thread, or keep posting here ?"

Declam, I'm reading with interest so please keep posting! Have you visited http://www.ddk-online.com/ for possible local help also?

Yes Dr. Dave, I'm a member on ddk also. :)

Got a nice delivery of oil :)

20L of (Fuchs) Silkolene Pro S 10W/50
http://i24.photobucket.com/albums/c2...ro_s_10w50.jpg

This was recommended for my engine by Opie Oils
Delivered from the UK to Ireland, by courier.

Handy when there are no decent oils available locally, and actually cost less than I spent last time on Mobil 1.

Declan,

I had my own fun with MFI recently. See the link here:

http://forums.pelicanparts.com/porsche-911-technical-forum/403260-mfi-help-required-cold-start-microswitch.html

Today I finished putting the stack back in & all the throttle linkages adjusted, refitted all the cam covers with new gaskets, nuts & washers, put some oil in,& tested cranking compression.

Cranking, about 4-5 seconds on each measurement, at WOT :

#1=140
#2=122
#3=135
#4=135
#5=115
#6=137

#5 is the one that has the higher leakage value also.
#2 is also lower than the others

I have no idea what I should have for cranking compression.

I also got delivery of my bigger dial gauge & measured valve lift

Inlet = 11.5mm
Exhaust = 10.6mm


But, thae bad news for today is that I need to fix/replace some components in the module for fuel cut-off on overrun.

I thought I heard a rattle in the module so I took it apart & sure wnough a small transistor was rattling around inside. legs are too small to resolder into place.

I poked some of the other components, and another transisitor broke off very easily.

I'll post the details tomorrow when I take some pics, but I did read post by the late early_s_man about fixing this module, and I remember he mentioned one of the transistors was obsolete .. I think it was the big one, while is still OK in mine.

Or expressed another way, as a percentage LOSS from your strongest compression:

0%
13%
4%
4%
18%
2%


OK, so compression tests (despite what the shade-tree guys say) aren't the best indicator of engine health--the leakdown is (which you have already done) but this confirms that you've got a dead hole in #5. 18% compression loss coupled with the poor leakdown tells you further investigation decoking of #5 is required.

Forget the speed switch for now, I would start the engine and have a helper hold the throttle for 3000 RPM and pour some seafoam, marvel mystery oil or even plain water down #5-- don't pour so much that the engine dies or the cylinder hydrolocks, just a steady trickle. Make sure you have the car outside when you do this, it's going to look like the invasion of St. Lo. Whatever decoking agent you use will vaporize in the cylinder and dump copious quantities of vaporized carbon and garbage out the exhaust. Some guys even fit a spare exhaust for this procedure although I don't think it's necessary if you can drive the car a bit afterward.

This decoking either works or it doesnt, only way is to set up after the decoking (engine WARM, not hot, your hands will thank you) and do the leakdown again.

If THAT doesn't work-- be absolutely certain you have the right valve clearance intake and exhaust. If you a go a long time between valve adjusts the valves get tighter, not looser, and if the valve is slightly off the seat at overlap TDC not only will you get a leak but the valve margin will begin to erode. So double check the valve clearance on that one (forgive me if you have already done so) before committing to a teardown.

Thanks John.

Showing my ignorance of the circuit, can I leave the speed switch module out/disconnected, or should I put it back into place as is ?

For the decoking process with water, (can't locate either seafoam or marvel mystery oil over here so far) should I have the engine cold/warm/hot ?

(I presume hot as it's the superheated effect, but I thought it best to ask !)


Cheers.
Declan

From my compression figures, is there any way to deduce what compression ration I might be running at ?

A few pages back, Grady suggested it may be possible, or do I need some other relevant measurements to get that far ?

Declan,

The speed switch prevents two things:

1) Decreased fuel economy (a BIG deal these days); and

2) Unburned fuel igniting in the exhaust when the throttle is snapped closed at high RPM. As you remember from Warren's speed switch discussion and CMA, when RPM is above 1500 AND the throttle microswitch is CLOSED the solenoid is energized, moving the rack to the cutoff position. If the throttle comes off the stop the switch opens and the rack goes back to normal, or if RPM falls below 1300 the switch cuts power to the solenoid and the rack goes back to normal.

In my own case, I snap off the throttle at about 6800 RPM at the end of straights and stand on the brake hard enough to push the pedal even with the accelerator, which I then blip for the downshift. During the time that the engine is decelerating the throttle is CLOSED but the engine is still turning 6000, 5000, 4500 RPM. With a working speed switch, throttle CLOSED AND RPM above 1500 = FUEL CUTOFF-- but since my speed switch rattles too, I end up with a huge load of unburned fuel- remember that the MFI pump has only two inputs, throttle position and RPM. While throttle position is zero, RPM is still very high so a nonzero (as in rack closed) quantity of fuel gets pumped through the cylinders and out the exhaust where . . .

. . .it ignites into a 12" long column of fire shooting out the tailpipe during deceleration, like a turbo. This is the kind of thing that can damage your muffler, your paintwork or the guy tailigating you.

Anyway, I would get it fixed, but the dead hole #5 is more worthy of focus at this point-- plus I always wanted to shoot fire. But getting to be an expensive show.

You might get in touch with Peter O'Kelly in Dublin, do you know him? Smart electronics guy.

Good explanation John.
I knew the theory of what it was supposed to do but never thought of the reasons for doing it !

I have a fairly good understanding of the electronics & can see how it works. My two broken transistors are BCY58-IX.
I've found both siemens and philips have/had a BCY58-IX, but am still on the search for exact equivalents.
I've found a BCY58-9 & BCY58-10 locally, so may pick up a few of each & see if they work or not. (Looks like only the DC Gain (HFE) may vary between them.)

No, I don't know Peter O'Kelly. is that POKDUB ?

Is there a way to troubleshoot the speed switch as a weekend project? Something like taking a volt/ohm meter to a few leads?

Where is it located and what does it look like? My car is a 1971 911S.

Thanks for any information. I still need to learn a lot about MFI and I hope you all can help as you have in the past.
SmileWavy

As far as I understand, the speed switch usually fails due to the mechnical vibration causing a couple of the components to break off. In my case, as probably John's case, one of the transistors was rattling around inside the case !

A good thread to start with would be http://forums.pelicanparts.com/porsche-911-technical-forum/8074-warning-all-69-mfi-owners-preventable-failure-283-speed-switch-module.html by the late Early_S_Man

The circuit diagram shows D1, T1 & T2 to be the BCY58-IX transistors (D1 simply uses the transistor B-E voltage as a fixed voltage drop)

There are some other good posts/threads on repairing it on here.

EDIT: Also see Post #11 in this thread which explains operation & shows the physical location of the module in the rear electrical panel (the one with the CDI box)

It's also called the "RPM Transducer", so searching pelican is probably better than me trying to link the posts from here.


Good Luck !

Update ...
In the MFI Simplification Thread the circuit digaram has alternative part numbers hand-written on it. I've just ordered some zener diodes and some of the 2N2222 transistors, so we'll see how it goes.

I'm still searching for Seafoam or marvel mystery oil over here in Europe, but no joy so far.

Flieger, the speed switch could not be easier to test.

1) Start engine.

2) Raise RPM above 1500 by moving the crossbar between the stacks that the throttle linkages are connected to.

3) Press the microswitch on the port-side stack. The RPM should immediately blip down to 1300 RPM then go back up to 1500 RPM, then back down to 1300 and oscillate between the two points.

What is happening is that the solenoid is energized when RPM is above 1500 AND the switch is closed, which cuts off the fuel and RPM falls. When RPM gets to 1300, the speed switch cuts power to the solenoid and releases the rack, allowing RPM to rise (you are holding the throttles open so it will) and as soon as RPM goes above 1500 the cycle repeats.

If that doesn't happen either you have a faulty speed switch, bad microswitch, bad wiring or bad solenoid on the rack. The solenoid can be checked with a +12V jumper from the top fuse on the electrical console- it should click. The wiring can be verified with an ohmmeter. The speed switch itself is one of the two rectangular boxes on the electrical console, the other being the rear defroster relay.

Thanks, that helps a lot. :)

I will test it this weekend. I have no defroster (Lexan) and my microswitch works so I should be able to test it pretty easily.

What does the defroster have to do with it?

Just 1 less electric box to wonder about.:)

Well that DOES narrow it down. For everyone else, here is what the box looks like.

http://forums.pelicanparts.com/showthread.php?t=7760&highlight=bump+warning+threa d

Thanks for the detail shots of the inside wiring.

I was about to post some questions on first startup after all my work, but thought I;d do a quick search first.. sure enough, another fantastic post from Grady in this thread from October 2005 (post #4).

What a great resource this forum is. Compliments to both the host and the contributors. You know who you are !