MFI thermostat spacers

[Grady Clay]

John,
First I would call Pacific FI and discuss what you want to do and the cost to inspect nozzles. What is the cost of new nozzles? Will he test the new nozzles? Ask about matched sets. Does he have used nozzles available? Discuss pump maintenance.

Next, I would try and find a set of nozzles locally. If you can find a Pelican with a MFI engine not running, ask to borrow his nozzles and try them in your car. You will be doing him a big favor and lets you send yours out for testing without disabling your 911.

I’ll bet somewhere in the Great State of Texas there is a cache of used MFI nozzles. If it is economically feasible, have them all tested and you get the best matched set for your trouble and expense. He gets known good/bad test results.

The process should take some time and effort on your part but shouldn’t cost as much as a new set of nozzles. I am split 50/50 on new vs. verified good used nozzles.


Have you scoured all the local shops?

I searched this for “Porsche + service.” http://yellowpages.superpages.com/listings.jsp?SRC=msn&C=Porsche+service&STYPE=S&MC=1&T=Austin&S=TX&CP=Automotive%5EService+%26+Repair

Austin Foreign Car Center
84 East Avenue, Austin, TX 78701

Austin Vee Dub Inc
1111 Old Bastrop Highway, Austin, TX 78742
(512) 385-2462

Dealership Alternative
8120 Research Boulevard Suite 114, Austin, TX 78758
(512) 323-9100

European Import Car Repair
10622 Burnet Road, Austin, TX 78758
(512) 837-1525
(512) 837-9682 (fax)


How about up in the DFW area? Have you looked down in San Antonio?

See if some old-time Dealer mechanic has a stash. Call the service managers and ask to phone interview their best (only) MFI specialist (if any) as you might bring in your car (some cold day in ….) Actually I shouldn’t say that. One of our Dealers has a retired mechanic who is exceedingly knowledgeable and comes in on call. I think I will chase him down next week and see if I can get him to contribute.

Best,
Grady

[Grady Clay]

OK, so you want to take your MFI nozzles out for inspection.

Remember, none of these engines are younger than 30 years. Too many have been neglected, rusted, hammered on by incompetents, and worse. Be patient. Plan ahead. Be prepared to “do it right” as there is great satisfaction in a properly running MFI 911.

The first issue is getting the nozzle unscrewed safely where you don’t damage the pipes, nozzle, or head. The right tools and solvents are the key. In an ideal world you spray some penetrating oil on the pipe nut at the nozzle, use two “flare nut” wrenches on the pipe nut and nozzle, remove the pipe and move it to the side, and unscrew the nozzle from the head. If everything were only that easy….

The pipe nut seized on the nozzle and pipe is the big issue. Over the years everyone hosed off their engine from above – nice clean (corrosive) environment. Had the pipe nut-to-nozzle connection been on the bottom of the engine and subject to the numerous oil leaks, there wouldn’t be a problem.

If anyone has other suggestions, Please chime in. Links are worthwhile.

Have the engine warm.
Spray penetrating solvent on the nut both from the top and underside.
Lightly tap the nut with a brass hammer to try and get the solvent to penetrate.
Repeat several times.
Only now try the nut with two “flare nut” wrenches. You must apply these against each other, putting no torque on the pipe or nozzle.
Try tightening it first to get it to click breaking loose.
Then try loosening.
If no success, repeat with the solvent and all above.
Remember, you don’t want to apply so much force as to damage anything.
A brass hammer blow to the wrench may help.

Now comes the dangerous part – heat. Remember you have gasoline here.
Heat the nut suddenly – you want differential expansion between the nut and nozzle. Immediately try and unscrew the nut. If no luck, immediately try tightening.
Repeat as necessary when cold. Even dry ice or liquid nitrogen can help with temperature differential.

Now to the grim part – it won’t come loose.
First find access to a set of good used pipes.
Cut your pipe about 1 cm above the nut.
Use a 6-point socket and try to unscrew the nozzle from the head. Follow all the tricks above; solvents, shock, tighten first, and temperature differential.


The order of expendable expensive Porsche parts are:
Nozzle.
Pipes.
Cylinder head.
BIG DEAL – you can see why care is in order.


Assuming everything came apart OK or was fixed; now to reassembly.
Cleanliness is next to godliness is an appropriate admonition. Have everything “squeaky” clean. You want no possibility of junk to get in above the nozzle.

Looking ahead, you want to add protective lubricants to everything. The critical areas are the nozzle-to-head threads, the nut-to-nozzle threads, and the seating surface between the nut and pipe.

When back running, first check for fuel leaks. Remember this is a VERY high pressure system (250 psi.)

Some other important trivia:
Keep the pipes in their clamps until all the pipe nuts are loose.
As soon as you have the nut off the nozzle, cap the nozzle while still in the head to prevent contamination.
Reinstall the nozzle in the head with a cap in place. Leave the nozzle capped until the last connection with the pipe. The nozzle can be contaminated upon installation.
Crank the engine with all the nuts not tight, rack full rich, and ignition disconnected to purge any air. Remember fire danger.

The changing of nozzles should be an “alone” process. That means you should have the engine running reasonably on all six beforehand and expect it to run properly afterwards. If it doesn’t, immediately suspect a contaminated nozzle.

The reason for all the effort about nozzles is the idle, just off idle, and starting off from a stop performance. Hot starting can also be an issue.
The symptoms can be; pop when first opening the throttle, surge while driving at light throttle, too rich to get it to idle properly, and more. Many of these symptoms are also indicative of other MFI situations. Remember, it is an entire SYSTEM and all parts must work properly.

Best,
Grady

[michel richard]

I posted a picture of the insides of my MFI thermostat earlier:



Looking at the discs on the right in the picture, the disc I said was one thick one is actually two discs 0.083 in, or 2.1 mm. The thin discs are 0.005 in (0.125 mm) and 0.01 in (0.25 mm).

I was running very lean when cold. I took one of the thick disks out before starting this morning, and immediately started running pretty rich ! So I made a series of spacers in brass 0.025, 0.040 and 0.060 in. I'll fiddle with the disks over the next few days to find a good combination.

Cheers,

Michel Richard

[speedracer54]

Investigating this topic myself, I have come to read all these posts today, July 29, 2019.

I have owned my '71E since 1978, and have been a student of this wonderful engine with MFI ever since. With an engine that currently runs as designed/expected, I would like to offer my two-cents worth.

As long as I have owned this car (never drive in cold PA weather) I experienced a hesitation during the first few miles of warmup; something that I would think would not be "acceptable" or "normal" by Bosch or Porsche standards... but was this a rich or lean hesitation?

Having the golden opportunity to talk with Mr. Pfister at PFI, he explained how the spacers may be altered to affect warm-up mixture. He also stressed the importance that the T-stat needs to be in the "lean" position by 53 degrees C.

My t-stat was fitted with the standard twenty five pairs of thermal discs, and three spacers; 2.0mm, .8mm, .2mm. Assuming a lean hesitation condition, I chose to remove the two thin spacers to begin my testing. Upon my first cold test run, that nasty hesitation, which plagued me forever, was gone. I did further experimenting with shims (I made several sizes in my lathe using a special mandrel that I made) only to find that the one shim of 2.0mm in size worked best for me. I plan to be certain (essential) that my thermostat is in full lean position at 53 degrees C.

In my humble opinion, I would think there is no "one size fits all" rule regarding spacer quantity or ultimate combined thickness. I would like to believe that each pump was assembled and treated individually, requiring different sized spacers for precise initial setup. Hence the existence of the .2mm, and .8mm spacers found in my thermostat, I would bet that spacers found in other thermostats will be of varying sizes.

Last but not least, I do have my fuel:air mixtures for idle, cruise, and full throttle dialed in very well, something essential to this bit of history called MFI.

I hope that my uneducated, seat-of-the-pants insight may be of help to others.

Eric

[manbridge 74]

Well done.

Will you use an infrared temp gun to check the 53C figure?

Hard to find those shims as I recall. You should make some up....

[30westrob]

Does anyone know of any correlation spec. for temp. to thermostat rod extension. I imagine when Bosch made these thermostats they used a spec. to determine shim thickness. Rob

[manbridge 74]

Interesting that in 72-73 Porsche went to the stove flap set up that other auto makers had been using for some time, at least mid-60s. This was to provide quicker warm up with less hesitation complaints from customers and more consistent intake temps. As I am not running with flap set-up I try and set A/F mix so cold running has just a hint of stumble but certainly not bucking, and it’s all over within a mile, usually less. The idea being to avoid well known cylinder fuel wash with imprecise MFI. Been awhile but seem to recall cold running mix at 12:1 at outside air temp approx 50F.

One more thing: I’ve learned with my wide band that high summer temps can cause car to lean out fast enough to make me think it’s too lean when it’s just quicker t-stat action. Then in winter here in CO it’s the opposite.

[Rick's 911E]

This was a fascinating read. Thanks for all who contributed.
This is exactly what I'm experiencing right now.
The car is starting well cold and runs fine over 80 degrees C (oil temp), but between 50 and 80 is very difficult. Bucking, hesitating etc.
Years previous my mechanic removed 2 spacers, both around .2mm thick from the thermostat.
There is 1 remaining, .8mm thick.
I tried removing the .8mm and reinstalling the 2 thin ones, no help.
I then removed the 2 thin ones.
Now she runs perfect.

I will keep my eye on the oil level, and continue to check for a gas smell in the oil.

Thanks again for all who contributed.

[Rick's 911E]

here she is....

[SFE]

Using this thread to ask to MFI experts another question about that thermostat...car is a 69's 2.0E.

My thermostat has 25 pairs of thermal discs, 2 thick spacers and 2 thin spacer.


My cold start is like that :
- pull the hand throttle about half way up.
- engine starts right away.
- I adjust the hand throttle to obtain about 1200 rpm.
- after a few minutes rpm rises to 2000 rpm so I push the hand throttle all the way down.
- idle rpm at this time is a little bit low (700/800 ?) and when the engine is above 80°C idle rpm is stabilized around 900.

I assume all this behavior is as per the book and then the car is running great all the way to the red line with, I think, a very acceptable milage (around 11 liters per 100 km).

I tried to add a 2mm spacer in the thermostat --> no problem to start the engine but difficult to rev up with popopops in the intakes, I assume the engine was too lean.

In my understanding, by adding a spacer, the thermostat was able to push a little more towards the lean position.
But then the partial load was to lean and I would have to richen up the pump with the partial load screw.
My conclusion is my thermostat is not in the full lean position even with the engine warmed up.

So my question is does the thermostat rod had to push all the way to the lean position with the engine hot or not necessarily ?

[356RS]

To answer your question on the thermostat rod, Yes the rod is pushed, via the thermo disc's expanding, all the way in once the engine temps are up to normal. You must have the correct 2 piece hose going to the MFI thermostat for this to work properly.

[SFE]

Although this is not of interest to many people, I am posting the results of my investigations.

Here is the position of the thermostat axis when it is cold :



And then when it is hot :



When hot it is impossible to push more in the lean position so I assume my spacer setting is fine.


For the record, this is what it looks like when you remove the internal spring constraint...

[356RS]

The thick & thin spacers/shims are for adjusting the amount of fuel being delivered at cold start and cold idle. Once the engine is warmed up the thick & thin shims/spacers are not in play anymore. The thermo disc's have expanded and the thermostat rod is keeping the engine in the normal operating condition. Adding a thicker shim at this point will not cause a leaner condition when the engine is warmed up. A thicker shim during a cold start condition will lean out the amount of fuel being delivered during the cold start and cold idle.

[SFE]

Originally Posted by 356RS

Adding a thicker shim at this point will not cause a leaner condition when the engine is warmed up.

I agree but for some reason, when I added a thicker shim, the engine (which was fully warmed up) was hesitating during accelaration.

My theory is the system is very sensitive and that extra shim was probably pushing just a very little bit more on the lean side and the engine response was affected.