mfi idle on max, still lean on idle?

[Grady Clay]

The thermostat pushes when hot.

It is unfortunately too common to find a MFI 911 running with the
thermostat on for a variety of reasons. Most common is the left
heat exchanger has rust so not enough hot air gets to the
thermostat. Other reasons include the plastic/paper/alu foil
concentric hoses have become restricted internally, a mouse
nest in the hose, etc. The thermostat elements need to be
nice and clean to function properly.

In normal operation you want the thermostat to turn off as soon
as possible – even at the expense of perfect running during
warm-up. A MFI 911 likes rich – it feels great. It just dilutes the
oil with gasoline and washes the lubrication from the rings and
cylinders.

When the thermostat is not functioning properly (in your case
not there at all) then the engine runs VERY rich. There is enough
adjustment range in the main mixture screw to get the engine
leaned enough to run. The difficulty is (other than too rich) that
the relation between the mixture at WOT, part throttle and idle
is now improper. You may get the part throttle close but the
other extremes will be way off and not in the adjustment range.

This diagram is so everyone understands what goes on.
NEVER GET TINKERING INSIDE THE PUMP.
You can turn a rebuildable pump into something
that the Bosch repair facilities will refuse to work
on. Very expensive proposition.

This view if from the right side of the pump looking to the left.
The thermostat rod is in red.
The pivot is in green.
The part the pivot acts on is in blue.
The spring (below the blue) is not as strong as the spring in the
thermostat. As the bi-metal discs expand from the heat, this
spring in the pump is compressed to where the mechanism hits
a stop. At this point the “thermostat function” is off. As the bi-
metal discs continue heating, the spring in the thermostat is
compressed. This prevents the forces from becoming too great.



When you make a mechanical screw to replace the thermostat,
the end of the screw that contacts the pivot should be rounded
and nicely finished. You will find that the screw will need
refinishing periodically because the pivot is much harder steel
than the screw.

You will find the last ˝ turn or so of the screw won’t change the
mixture. Always set the screw where it isn’t firmly holding the
spring assembly against the stop.

Remember:
NEVER TINKER INSIDE THE PUMP

Best,
Grady

[svandamme]

thanks Grady,

btw, Peterfrans pointed me out to something else
my loud pedal had been bugging me as wel lately
i just thought it was simply my pedal cluster, but i couldn't figure out what it was

but Peter guessed that i did not have full WOT and finally my little brain figured it out.

for some reason , the linkage from my pedal , to the control rack , is to short, and because of that, my pedal is not "high" enough when not pushed in.... so the travel from out to wot is to short... and i never get full wot when i floor it, so that's not helping either

it's strange , because i never adjusted anything during the rebuild , i think the rod between the 915 and the pivot on the left TB is bent slightly... not sure but i will check this next saturday

pretty annoying not to have the car near my house , can't check things during the week , only in weekends, it's so frustrating...

[beepbeep]

There is another thing to consider:

How is your WB02 sensor attached? If using test pipe, LM1 will indicate lean AFR on idle beacuse of "backwash"-effect of pulsating exhaust gases. With other words: fresh air is drawn inside the exhaust pipe between the puffs and LM1 would indicate leaner condition that it actually is.

It only happends at idle or near idle, using test-pipe. Full-accelleration AFR is usually spot-on even if you use test pipe.

If sensor is welded on the header and you still get lean AFR's on idle, check for minor exhaust leaks... if no leaks, you are too rich.

[svandamme]

the sensor is mounted in the tailpipe with a bung



would the backwash pull fresh air that far in the pipe?

i do think it's reading accurate on the lm1, because there is audible popping when it does read low afr

[Grady Clay]

Here is a likely candidate for your accelerator pedal position.
This “Accelerator cushion” 911.423.223.00 is at the rear of the
tunnel, where the throttle linkage comes out of the tunnel and
just prior to the bell-crank linkage at the transmission.


It can come apart internally permitting the linkage to be longer.

Throttle Cable Snapped....on my '68 911L....i think

Another potential problem place can be the plastic bushings in
the accelerator linkage bell-cranks (one behind the pedal, one
alongside the transmission and one between the MFI pump and
#3 intake.) If a pair of these bushings disintegrate, it can cause
this problem.

Is the hinge in the accelerator pedal in good condition?
Is the bell crank behind the accelerator pedal not interfering
with the tunnel sheet metal?


Start getting back together all the parts for the MFI overrun.
That will be your next project after you get the thermostat
issue resolved. You are going to need to reinstall the “Speed
switch,” the “Micro switch” and the “Shut-off solenoid.” Is all
the associated wiring still in place?

Best,
Grady

[svandamme]

tomorrow another day of testing, saving this thread to my laptop
and try to get some focussed testing done... not easy , attention span of a chipmunk here...i'll post updates tomorrow

[Peterfrans]

we need updates

[svandamme]

nobody at the garage this morning , turns out he was off to some shop and cell phone not charged up... so went back home... got call later on .. most likely nobody there tomorrow either

it's a solid PITA to not have a garage in your house, or not to have anything within a decent range either

i'll try and get a day off at work this week, but it's real busy at the moment, so most likely it's going to be next weekend

[Grady Clay]

Stijn,

Let’s use the time to make a plan.

Can you make the thermostat replacement screw piece this weekend? If so, do you have the little pieces necessary? You will need:
1) A piece of steel about 5 cm X 3 cm X 2 mm.
2) Two M5 nuts (10 mm hex)
3) A M5x30 hex-head bolt (10 mm hex)
4) Ability to drill holes in the steel
5) Ability to braze (preferably) or weld a nut to the steel piece.
6) Ability to grind the end of the bolt into a hemisphere.
7) Some craft paper to make a gasket.

I assume you have the two screws and lock washers that hold the thermostat body to the MFI pump.

You might want to make a couple of spare adjusting bolts. If you forget to tighten the lock nut, the bolt will fall out and disappear under the MFI pump. Ask me how I know.

An easy way to make a template for drilling is to lay it out on a sheet of paper at 10X scale. You can then photocopy reduce it to the proper dimensions. You can also photocopy on craft paper for the gasket.


If you can’t deal with the thermostat replacement screw this weekend, on to Plan B:
Can you collect all the pieces to reinstall the overrun system? What was removed? What is still in place?
You will need the RPM transducer, the connector and wiring in the electric panel, the wiring in the main engine harness, the Micro-Switch & associated hardware and the shut-off solenoid.

If you have all these parts, let’s inventory and bench test them. We will be able to lead you through the process. When you install everything, you want to be sure it works as intended upon installation and not have to spend any time trouble shooting that issue.


If that isn’t possible, on to plan C:
Let’s address the accelerator linkage. Can you safely get under the 911 at home? I don’t mean it teetering on the original jack. I mean safe jack stands and wheel chocks.

The first issue is to look at the accelerator cushion. Push the rubber gummi bellows back. See if there is any appreciable play between the accelerator rod coming out of the tunnel and the linkage piece to the bell-crank on the side of the transmission. (look at the thread I posted above.) The failure mode for this part is the rubber cushion separates from the steel end piece and adds about 5 mm to the travel of the accelerator rod. It is held captive by the tin cover and that limits the failure to only part throttle.

If this is the problem, see if you can order up the part today or first thing Monday.

This problem is usually caused by old age rubber. It can also be because the throttle stop on the floorboard is improperly adjusted or the plywood floorboard has deteriorated and flexes too much. When you mash on the loud pedal, the stop should prevent the linkage at the engine from going to its full-throttle stop. You want it limited at the accelerator pedal. The last 5% of throttle doesn’t mean much in terms of performance.


Stijn, it would be useful to post more pictures and every little detail of this project. There may be other significant issues that need to be addressed. You have a nice, proper rebuild. You can damage it with improper running during this break-in period. We want you to be successful in having a really good running ‘73S – one of the best of the best.

Best,
Grady

[Peterfrans]

Stijn,

If needed I can make you the bolt on piece this weekend. Stuck home with parental duties while the wife is at work. Let me know.

[svandamme]

Grady , i frankly can't do a thing if i can't get to the garage

i live in an appartement, there isn't even a closed parking space
my car is located about 30 miles from my house, out in the country...since in R'dam , there's no chance in hell to even find something affordable, safe or practical to store my car in this area, let alone finding a place to work on it...

so any shop time, is purely limited to weekends... and sometimes the shop just ain't open ... bummer

thx for the offer Peter, but it's no use rushing this , i've missed the summer deadline allready , i'll get to it when i get to it... got all winter to prep for springtime...

[Grady Clay]

Peter,

If you can make this part this weekend, please do so. Not only does Stijn need this but one for yourself will help with your MFI. This is a good diagnostic tool. Let your 4-year old help, he is the next generation for Porsche. My son (now 20 in college) could weld at age six with supervision.

Stijn, What are the things that you CAN do this weekend? The winter just makes things more difficult. Prep starts now.

Best,
Grady

EDIT to correct my lame spelling

[svandamme]

no car, no tools, no parts... i can hoover my living room and do my dishes... that's not going to improve my mfi though

[brokke]

What is wrong with the standard Porsche setup with the bi-metal disc's and hot-air tube between the heat exchanger and the injection pump. Works really perfect for me. Sometimes standard solutions are better than home-made one's

Another point is that most likely the speed switch is broken because the transistors legs are rusted through. Mine just fall out of the speed switch housing. There are a couple of very useful threads on this BB which can help you out in replacing the transistors with new one's.

Peter

[Grady Clay]

Peter,

I agree; the original thermostat is much preferable. Apparently Stijn doesn’t have a functioning heat exchanger on the left side. He is currently running without a thermostat at all. This left the engine way too rich. He got it to run by leaning the main circuit but both idle and WOT are still off scale. In order to get it in the adjustable range he needs something to turn “off” the cold running enrichment. The screw adjust will accomplish that.



Here are some links for the MFI RPM transducer speed switch.

speed switch repair...


posted the scanned pictures, here is the link:
http://www.picturetrail.com/gallery...4565&uid=778009
Keith (porschedude)
Unfortunately this link has expired. This was the Bosch wiring diagram.


Early_S_man posted this list a couple of years ago. I haven’t tested the links yet.
“Several people posted multiple threads on one problem, and that unecessarily complicated keeping track of the various threads!
http://forums.pelicanparts.com/show...ght=mfi+warning
http://forums.pelicanparts.com/show...fi+speed+switch
http://forums.pelicanparts.com/show...=rpm+transducer
http://forums.pelicanparts.com/show...&threadid=78685
http://forums.pelicanparts.com/show...mfi+speed+relay
http://forums.pelicanparts.com/show...tch+replacement
Good luck! Warren Hall”

Best,
Grady

[svandamme]

here it is ! the long overdue update

in short , things i did today

- balanced my front wheels, needed 25g on the left front tire, the shaking is gone now
- changed the length of my throttle pedal linkage
the pedal felt better, but the drivability was worse
- fixed the thermostate workaround...
no improvement in drivability
- changed my TB linkage with new ones
and noticed that with pedal out , the TB's were not at their stop
so i disconnected all linkage from the control rack
and then replaced with the new ones (old ones were not adjusting well due to corrosion and old age)

continued setting up the linkage, so everything was at the stops
then put my laptop on my targa bar, connected to my LM1
got the synchrometer in there, and started fidling with the settings

at first a pain in the ass, wouldn't idle, so used the shutoff switch, as a new stop (my pedal was still not connected so the linkage would be unaffected by any problems in there)
so i set that shut off switch, screwed it in so the control rack was slightly open... then adjusted the pump (thermostat workaround)and TB's till the car idled again with everything on the stops...

day was over, so went for a drive, to evaluate

what a difference !!!
still a couple of flat spots, but that's definately tunable now
the car has good WOT, and at 6500RPM ... it kicks like a mule, savage power

all things considered, i was just looking at the wrong place
my biggest issue was the linkage, which was ordered some time ago, and now is properly adjustable... getting the thermostat worked around, i had this previously , but it was poorly made, so it fell off... then the pedal linkage...
so multiple problem troubleshoot... probably typical for MFI , maybe you can comment on that Grady? is that typical?

anyway , tomorrow another day , looking forward to get some finer adjustments made...

one setback though ... stupid mistake on my behalf
went for mornign drive , still foggy , with the targa top on
but the latches weren't locked, right when i noticed, the roof went airborne.... slightly dinged, but still attaches fine... i think it's the cars way of telling me it likes beeing topless what a girl !!

[svandamme]

picture of my thermostat workaround



the front plate fabrication didn't work out , didn't have the right gear to hard solder or braze a nut on the plate

so went the other way , the top plate was thick enough for a hole with thread... and it was very easy to control the linkage with a screw directly on the adjuster ( instead of using the built in lever thingie)

drilled a hole, pulled thread on it, in with the M5 screw
done...

it's slightly less accessible than the front plate, but i don't have dokworkers hands, so it works for me...

[svandamme]

did some more tuning today , get's better everytime
pedal is a lot better, but i'm sure the bushings and what not can use some refreshing, the stuff you mentioned Grady

at the moment, no major flat spots, zero hesitation. but still a tad on the rich side and a lumpy idle, but it's just a matter of refining things a little bit more
i can notice that at 6800-7000 it suddenly finds a great deal more power( the sorts that makes your face deform) and it keeps it up till max rpm


if you wanna see/hear how it behaves right now :

http://members.chello.nl/s.vandamme1/takeoff.mpg

looks a tad slow ,but that's the angle the camera has to the road, and the poor framerate, that was actually a reasonable launch, albeit not flatout bouncing of the revlimiter.. it wasn't fully warmed up yet...

so next time i'll be working on the loud pedal linkage
and i'll try to lean it out a tad more, while at the same time improving the idle....

[Grady Clay]

Stijn,

I hope you are keeping track of all these “little” adjustments. Yes it is common to have multiple problems – many self induced.

Good to see some progress.

Best,
Grady

[svandamme]

got the shutoff solenoid back on today , works and all that
but the engine is still a bit rough and lean down low, so the shutoff kills
it

i'll redo my procedure once more on saturday , every time i go through it , i end up with improvement


i noticed that with the main gas pedal linkage attached, i'm having a heard time getting a steady rpm , it flutters a bit , which makes it impossible to measure airflow over the individual stacks

so i disconnect that one
use the shuttoff switch stop screw, to keep the revs up and steady
then i measure
close the stops a bit
synchronise them
use the lm1 to see if my mixture richens up
as it richens up i can unscrew the shutoff stop a bit, and my revs drop
then keep doing that untill the engine idles with everything on the stops(pump and TB's) and the shutoff stop is no longer used

once that's done, i can lean out the pump a bit more
time for a drive and evaluation

if necessary, repeat the above steps again


it's the only way to do it , because if i do it to fast, eg , close all the TB's and then run the engine
i end up without idle, or the change is to big and i get weird symptoms
this way i can keep an idle , and have the ability to measure airflow , so i can synchronise the TB's

but so far it works fine, done it 2 times now, every time i get major improvement
WOT is very good now, i think one more time will get me a decent idle and midrange AFR