Question for the MFI people out there...

[bbthomson]

Hello All,

I am going through the CMA process and was wondering if what my engine is experiencing is common or not. I am currently at the timing stage and before I dive in (I am new to all of this…) I thought I would bounce this off of some of you that are far more experienced…

Regardless of what I have done to date, my car seems to rev inconsistently. This is a freshly rebuilt 2.4S spec motor. It runs moderately OK when it is cold but does not run well once it gets warm or when it gets up in the RPM’s. Once the motor is warm it will not consistently run properly through out any gear.

I can hold the throttle at 3000RPM and it will cut in and out of power. When it is cold it does the same but just not as severe.

I am going to continue to go through the CMA but thought I would check in and make sure that this is not something obvious.

Thanks,

Brent
_______
70’ - 911

[HawgRyder]

I sort of have the same problem.
In my case the pump was slightly out of cal...and the distributor was sticking on the advance mechanism.
It's a difficult problem to get rid of completely....just try everything in moderation and remember where the original settings were.
Check your linkage to the pump and from the pedal to the back...make sure there are no hang ups or drag.
Good luck
Bob

[Early_S_Man]

Brent,

I strongly recommend that you start a logbook dedicated to maintenance, and that you write down everything that you do to the car in it, in sequence as done, and spare no details! In addition, you should build a personal reference manual/binder of documents pertaining to all systems ... MFI, electrical system, CDI, etc.

Since you are at the ignition timing point ... here is a thread I recommend you read and give your distributor a clean & lube job! Check the advance curve with an 'advance dial' timing light!

distributor lube...

Here is a thread that should answer any questions about the Bosch CDI unit:

History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ...

Good luck, and have fun!

[superhighperf]

is the fuel pump working? the car will run with a broken fuel pump. just not well and that is an under statement. check the big stuff first
i had a similar problem and it was the points and condencer. it seemed like i could only tune the car for good top end or good mid range power. it was frustrating but i found an easy solution. optical ignition box
i have driven the car for 20 years and only needed to touch the injection pump when the belt broke. i am just now starting to adjust the idle setting and checking the linckage. i would check the injection pump belt and pulleys for alignment and wear then concentrate on ignition.

[Grady Clay]

CMA was written when the MFI 911s were new. Unless some incompetent had been there adjusting first, the usual problem was easy to diagnose and correct. The newest MFI is now more than 30 years old and some approaching 40. It is not uncommon to have multiple things out of spec and giving conflicting diagnostic symptoms. The degree of difficulty in correctly diagnosing a running issue goes up exponentially with the number of problems. What to do?

First, don’t expect to solve everything with one pass through CMA. Many issues take several iterations to finally resolve the problem.

There is much more to CMA than Porsche published. CMA assumes you to be a very skilled mechanic and diagnostician. It also assumes you have been through the Bosch MFI school. Most significant it assumes the components are all close to new. Do a Pelican search on my name and “CMA2” and you will get an idea of the differences.

Almost every component is 30+ years old. There can be issues Porsche never envisioned when CMA was written.

There are some basics that every engine (carb, CIS, DME and MFI) must have. The basics of the engine must be in good-working form – compression, cylinder leak, cam timing, valve adjust, clean oil, etc.) The ignition system must be in perfect operating condition and some components (high voltage mainly) replaced regularly to always be “like new.” I am a proponent of independently testing and setting the distributor on a test machine. The fuel supply must work properly; clean filter and screen, good electric pump, proper circulation and pressure, etc.

The usual admonition is to not touch the MFI adjustments until all these other sub-systems are known correct. The first places to start with MFI are the cold start sub-systems. The cold start enrichment should work properly and completely turn off; no runs, no drips, no errors. The cold running thermostat should be clean, work properly and completely turn off asap. The nozzles should be in good working order. And more…. Much more.

As warren said, keep track of everything. You may have to adjust one way and then return after another issue is resolved.


For example, let’s take your symptoms and just attack the MFI based on that. The symptoms are: runs OK cold, runs poor warm. Inconsistent running when warm.

Clyde Badfmechanic decides the mixture is too lean and richens the main rack to get it running properly hot. He installs new sparkplugs for good measure. 911 runs much better.

In fact the MFI was already too rich because the thermostat wasn’t turning off and the better running was solely replacing the fowled sparkplugs. Add to this the distributor sticking in advance (as Warren duly noted), bad nozzles, dirty air filter, gas boiling out of the oil when hot and going into the air cleaner, the MFI pump timing continuously changing as the belt skips teeth, and more and more. It is truly amazing that these engines will run at all even with multiple problems.


You are going about the CMA correctly. Get every sub-system KNOWN properly functioning. Check and Measure first; only then Adjust. There isn’t any detail that is so insignificant that it can be ignored.

When you get close, you should do some extended driving and then immediately repeat everything.

Best,
Grady

[Paul Thomas]

If the motor was recently apart, i would check my pump timing one more time.

Paul

[bkreigsr]

I'm with Grady, sounds like the thermostat is stuck, or it is not getting enough forced hot air from the heat exchanger.
You are not running headers, are you? that set up will not work unless you have a manual, cable type adjustment for the rack.
Bill K

[bbthomson]

Hey Guys,

Thank you for the input and feedback...all of this is great info.

Warren: I have started my log book and have been documenting everything. I also started a binder that can be easily referenced. I was reading about your post regarding distributor maintenance...Great Stuff - Thanks!!

Grady: As always great input. Although these parts are 30+ years old it can be deceiving as everything on my engine looks brand new and was rebuilt...that said, they are still 30+ year old parts...

I will review CMA2 as suggested.

Paul: The engine & pump are fresh rebuilds.

Bill K: I checked the thermostat and it appears to be fuctioning correctly. With respect to your comment about the heat exchangers...They are not blocked off and there is no heat in this car (No connection between heat exchanger and the cabin) Also, I have recently disconnected the air hoses to the heat exchangers as I read that the hot air can vaporise the gas from the pump to the cylinders. The only air hose I have connected is the thermostat to heat Exchanger hose. The other air hoses are just pointing at the ground. I am awaiting the delivery of my heat block off plates. I am sure that some of this is adding to my problem and I am quickly realising that I probably have multiple issues at the moment.

A sincere thank you to all - this is a great forum!

Brent
_______
70' - 911

[Early_S_Man]

Brent,

OK, that info on your air hose situation is critical!!! The left heat exchanger needs pressurized air from the fan to push hot air to the MFI thermostat ... just having the hose present without airflow won't ever have the thermostat warm up and lean out as expected! I suspect this situation without heated air to your thermostat is the root problem you are currently experiencing with driveability!!!

[bkreigsr]

I think the thermostat must have forced hot air to operate properly.
re-connect your fresh air ducts/blower just to the left side and cap the cabin outlet on the exchanger if you dont want heated air in there
your symptoms sounded a lot like mine when I was running headers, with no air to thermostat. I even bolting a little stove on top of the left side, then the hose up to the pump. no luck

keep at it, the system is well worth the effort to get it right. once within specs, look forward to thousands of trouble free miles.

[bbthomson]

Hey Warren,

The reason I disconnected it in the first place was because of the problem...I will reconnect but I am not sure that this will solve the problem.

Thanks,

Brent

[bbthomson]

Hey Bill,

What do I use to cap the heat Exchanger? I believe this can easily be reached from underneath the car...correct?

Thanks - Brent.

[304065]

Brent, if you do not have hot air to the thermostat from the port heat exchanger, your mixture will be around 10:1, enough to make the tailpipes black and your eyes hurt when you stand behind the car.

If you then lean the mixture to compensate, when the engine gets REALLY hot, the thermostat will be erratic, making tuning impossible.

Correctly adjusted the thermostat is VERY precise. With the enigne running at high idle with the hand throttle engaged, at the exact point that the discs reach the proper temperature, the mixture changes and the idle drops down. If you shut off the engine for 30 seconds and start it again, things have cooled enough so it's re-engaged, and you can restart the engine, let it run for a minute, and hear it drop down again. (OK, I confess I've done this a few times in a row just for the sheer joy of knowing I've mastered that part of the system)

Now, Warren posted the distributor thread, I read his and added a little information of my own that was relevant to SWB ignitions, but the principles are the same. The ignition advance mechanism must be operating perfectly and the curve must be a close approximation of the factory curve for the engine to rev up correctly. The distributor curve for your particular distributor is found in the shop manual or in the old up-fixen.

Before there was BHKZ: SWB engine electrics

Good luck!

[Early_S_Man]

Brent,

Here is an addendum to CMA2:

Step 0: Test hot air outlet on MFI Thermostat for Hot Airflow & temperature with thermocouple probe! It should be at least 240°F after five - ten minute warmup cycle.

[Grady Clay]

Brent,

Woha haus,

Don’t completely block off the outlet of the heat exchangers.
Neither should you leave the outlet completely open!

With the heat exchanger outlet blocked, the exhaust pipes
inside will get too hot. This can possibly start a fire and will
send too hot air to the thermostat, possibly damaging it.
The concentric hoses to the thermostat will almost certainly
melt.

With the outlets completely open (not connected to heat
control valves) the engine can overheat because so much
cooling air is passing through the heat exchangers. The
thermostat won’t get hot air either. Porsche specs a
restrictor with a 30 mm diameter opening when not
connected to the heat control valves (see p. E120 in the
early 911 Factory Workshop Manual).


For you to be able to diagnose the running of the engine
you must KNOW that the thermostat is completely
off. I posted a technique where you remove the little
2-screw cover plate above the thermostat on top of the
MFI pump (yellow arrow). This is also where a mechanical
thermostat replacement can be a good diagnostic tool.

Warren’s “Step 0” is right on (CMA2 has it as #12). You
should eventually know the exact exhaust air temperature
and conditions your thermostat turns off. This will vary
somewhat with the setting of the thermostat and the
condition of the left heat exchanger. Ideally you want the
thermostat to turn off slightly “too soon” and put up with
a few minutes of lean running while still cold. This helps
prevent oil dilution.

What is the condition of the left heat exchanger? Is the
original large rubber hose for the thermostat in place
between the heat exchanger and left tin? Are there
two concentric hoses from the left tin to the thermostat
(blue arrow)?



What other have you done while chasing this problem?
Every little detail is important.

Best,
Grady


BTW, great looking engine! A few decals and you are done.


A few “alsos”
Did you inspect, clean & preserve the double panel at the
rear of the air filter assembly where the breather connects?
This can rust internally from the combustion moisture in
the breather fumes and shed rust particles into the intakes.

I like the originality of non-Carrera tensioners. Are they
Turbo tensioners with the JWE stops and the up-dated
idler arms?

You can use the crankshaft pulley from a ’78-’79 911SC
and that turns the fan 1.63:1 in place of 1.3:1. Of course
you can change the fan also to get 1.82:1 ratio (in your
case you now have a removable fan pulley where you
can find another from a 5-blade fan.)

In cool weather (whenever you have the heat on) the
engine will like the warm air from the right frame heat
connection. This will let it tolerate the thermostat
turning off sooner.

It appears you retained your ’70 case. Make sure the
early distributor clamp can’t let the distributor turn and
change the timing. When you remove your distributor
for testing, don’t forget to set the engine at TDC #1
cylinder.

Is your cold start enrichment hooked up? On a 2.0 and
2.2 MFI, that can start a fire. The engine should start
and run cold with only the cold running thermostat in
most cases.

G.

[bkreigsr]

Grady, thanks for straightening him/me out on this. Thats the same thing B&B said to me when I told them I had no air coming in. (Spent last weekend hooking everything back up.)
I posted a question thread as to how hot the boxes actually get, but got no response. Do you have an idea of what surface temps are like on the boxes with air hooked up? thanks
sorry Brent

[superhighperf]

how about gas? my car never ran good on regular, it needs supreme to run right.
i am just starting to realise how lucky i am. i've had the car for 20 years with no thermostat hoses or any cold start enrichment and it runs great. every time i thought the injection needed to be adjusted it turned out to be somethig else.
good luck

[bbthomson]

Hey Guys: This is great – I no longer feel like I am in the dark…I can’t say I understand everything that you are telling me but I know that I can figure it out.

Bill: No issues – I was just heading out to the garage to block off the H/E’s and I thought I would check in to see if there was an update on this post….

John: Yes – my car is running rich and the tail pipe is black as you suggested it would be…

Warren: I will need to get a thermocouple probe. Where does someone normally find something like this…?

Grady: As always great info! I don’t have the early 911 workshop manual…I found a link to download a shop manual but it does not go earlier than 72 CIS cars…? Does anyone know where I can get a set ?(Hard or soft copy is fine)

I still have a question on blocking off the top of the H/E…Do I basically do what Bill was suggesting, just making sure that the hole is no greater than 30mm??

I will review the technique to verify if the thermostat is working properly.

The left heat exchanger is in good shape. No visible rust. I am not sure if the concentric hoses are original but they are in good shape and in place.

As far as what ‘I have done’ to diagnose the problem…; Very little other than reading and getting/ordering the tools I need to do CMA. I brought it to a local P-shop and quickly realized that they were not going to be able to get the car tuned. So, I am unsure what they did…I know they adjusted the idle and air/fuel mixture but I could not tell you to what extent? I decided that there was no local resource that I can trust so I will dive in and figure it out.

To answer the rest of your questions:

Air filter housing – I will inspect. There is no visible rust.

Yes they are Turbo chain tensioners with chain savers.

Yes the 70 case has been retained. (not sure how you can tell…amazing?!!!!) The distributor clam is very tight at the moment…

The cold start was rigged up with a switch in the drivers compartment…I hit the switch for a couple of seconds and the car starts. From what I can tell the gas is not spraying anywhere other than in the stacks. (i.e. the air filter is clean) It also cannot be activated unless I press the button.

One other note that may have something to do with back pressure is that I am running an M&K muffler. 911R styled...the sound is awesome...I have capped one side as it was just a little too loud. Now it is good but still sounds phenominal once you begin to raise the RPM's. I know that people have these mufflers running on MFI cars...but if you think that could just be adding to the issue then I can switch back to stock.

Once again – thank you all for your feedback!!

Brent
______
70’-911

[Grady Clay]

Brent,

Here is what Porsche has to say (Volume I, p. E120) in the
911 Workshop Manual.

“A certain amount of dynamic pressure must be present
in the heating system so that efficient cooling of the engine
is not impaired. This is provided by the heating pipes and
flap boxes, when the engine is installed.

“It is therefore essential that the heater output is
blocked during the output testing of the engines which
have been removed from vehicles.

“The correct dynamic pressure is provided on engines
which have been removed from the vehicle by an extension
table on the heating air outlet opening on both heat
exchangers. The extension tubes must be secured to
the heat exchanger with a hose clip or tube clip.”

They are talking about testing on an engine dyno here but
the situation is exactly the same as using heat exchangers
(not headers) without the outlet connected to the heat
control (flap) valve.

I have a pair of these for engine dyno testing. They have
an extension “U” pipe that directs the hot air down under
the engine into the exhaust air duct. I would recommend
this if you fabricate a pair of these for use in a 911.

Here is the Porsche diagram for fabricating this pair of
restrictors:

“
"
© Dr. Ing. h.c. F. Porsche A.G.


“…reading and getting/ordering the tools I
need to do CMA. I brought it to a local P-shop and quickly
realized that they were not going to be able to get the car
tuned. So, I am unsure what they did…I know they
adjusted the idle and air/fuel mixture but I could not tell
you to what extent? I decided that there was no local
resource that I can trust so I will dive in and figure it out.”
GOOD FOR YOU!
Becoming your own well equipped best expert will really
pay off. Follow the admonition to make yourself a personal
manual/scrap-book/archive with everything you can find
and what you do. Try this: “Personal Car Specific Manual”


No, I don’t know what a “normal” temperature of
heat exchangers is. All 911s have things to do
with temperature.
Being able to measure temperatures accurately is
important. The two tools you should consider are an
infrared non-contact “temperature gun.” The other is
a Type K thermocouple reading instrument.
HERE and HERE.
It is easy to make your own thermocouples with some
Type K thermocouple wire and connectors.
You can make a $50 probe for under $5
Omega.
Look for this stuff as industrial salvage also.




“The cold start was rigged up with a switch
in the drivers compartment…I hit the switch for a couple
of seconds and the car starts. From what I can tell the
gas is not spraying anywhere other than in the stacks.
(i.e. the air filter is clean) It also cannot be activated
unless I press the button.”
Excellent! You might check for the “aim” of the cold start
spray in the air filter assembly. You want all the fuel to
go in the stacks and none around the tops of the intakes.
Check this when cold, outside and with BIG CO2 fire
extinguisher handy.

Another issue with the cold start enrichment is the solenoid
valve can leak or not shut off crisply. This causes the
mixture to be rich at idle and on some cylinders with no
effect from the normal settings. When testing, disconnect
the hose at the solenoid valve and seal with a plugged
hose piece. Use compressed air to blow out any residual
fuel in the spray system plumbing.

If you ever find the opportunity to get a ’69 MFI injection
pump governor cover; buy it. It has a second solenoid
for fuel enrichment and can do away with the fire hazard
’70-’71 system.


“One other note that may have something to
do with back pressure is that I am running an M&K muffler.
911R styled...the sound is awesome...I have capped one
side as it was just a little too loud. Now it is good but still
sounds phenomenal once you begin to raise the RPM's. I
know that people have these mufflers running on MFI
cars...but if you think that could just be adding to the issue
then I can switch back to stock.”
You are correct. I thought of that and didn’t say anything.
It would be worth the minor effort to reinstall the stock
OE muffler during this break-in, testing and adjusting
phase. This eliminates one possible variable. This will
also tell you if exhaust is an issue when you install the
“sport” muffler.

Pelican superhighperf is correct, your choice of fuel
is important. What is your compression ratio?

Speaking about fuel, did you clean your fuel tank and
screen filter at the tank outlet fitting? Do you have a
new fuel filter? Have you measured the flow rate and
pressure? I notice you have new fuel lines at the filter
console. Are those screw-type clamps or the original
Norma Schellen (N-S) type? The screw-type can damage
the hose and lead to fuel leaks. Did you replace the
hoses at the tank and pump and under the rear? The
return hose connections didn’t originally have clamps
and will benefit from N-S clamps. I assume your ‘70 911
still has the electric fuel pump located on the front
crossmember.

Do you carry a fire extinguisher?

Best,
Grady

[bbthomson]

Hey Grady,

I am in the process of building the manual as this latest leap in has forced me to get more organised as I typically need reference material while I am working on the car.

I am going to reroute the air from the H/E's as specified...seems simple enough. Basically, as I understand it, I have a pipe pointing downward coming off of the heat exchanger with a 30mm opening...Correct?

I will check the aim of the cold start spray to make sure it is going directly into the stack.

Thanks for info on the thermocouplers - I will build/source one asap.

Yes - I am going to switch mufflers but will be eager to switch back as soon as the engine is running properly.

I am using supreme gas and I had the gas tank reconditioned and both the fuel filter and fuel screen at the gas tank newly replaced. The fuel lines are new as well. I will check if they are screw type but I don't think so.

The fuel was measured at 15PSI. Yes it is located in the front of the car. My car previously had a low pressure pump as it was orginally carburated. I replaced with new pump at the time that we put in the engine.

I don't have the compression numbers...it is a fresh rebuild...the shop indicated that all was fine when they checked.

I have 3 fire extinguishers that are readily available. One mounted on the wall in front of the car, one mounted on the wall at the back of the car and another in the car...call me paranoid.

Thanks for the input Grady!...it is comforting to know there are guys out there willing to share their vast knowledge!

Brent