MFI advice - About to toss the towel in

[Mhead]

I have a 72 911T with MFI, about 104K on the ticker...Seems to be running rich, as plugs(Bosch copper WR5DC) are continually fouling with carbon. I re-gapped each to 60mm and it ran well for a bit, afterward fouled again. Have had one of the best guys in the NE Check, Measure and Adjust(where needed) until the cows come home but still runs like crap. Seems like its running on 4 or 5 cylinders.

Any advice from you guys out there would be very welcome? Should I pull the MFI pump and send it off to Pacific?

or...should I take it to the bay for a Kennedy Car Wash...

[silverc4s]

Do you have CD Ignition? If not, get one. Have your injectors checked to make sure not clogged. Discover which cylinders are dead or missing. Check fuel pressures. Check cylinder compression. CMA is the last thing to do.

[Mhead]

PO put a Crane Fireball XP700 unit in the car...seems to be working...but no knowlege of how it does with the 911.

Injectors are new...replaced them with Bosch. Fuel pressure is at 11.5...new tank, new filter...Shell 93 octane. I pulled all plugs and each was fouled equally...At least its a consistent mess!! Compression on all cylinders is good.

[Joe Bob]

When was the mechanical pump last looked at? Lobe could be worn.

[Mhead]

Not sure if it ever was...Although there is some mention of work done about 25K miles ago. It has been difficult to dial the tune in...seems like it too rich at the low and high end...and somewhat lean in the mid range.

Would that point to a worn lobe?

[Joe Bob]

Try contacting Henry at Supertec, he's the guru.

[matthewb0051]

My .02:

Get a Bosch CDI and OE coil.

Get an LM2 so you can know what your AFR really is doing.

Also, check your plug wires and timing.

[BURN-BROS]

Another tool required for a proper tune is a STE Synchrometer Flow Meter.

What is your readings @ Idle?

What are your readings @ 2500 rpm

[David E. Clark]

Assuming the car is timed properly, the points and plugs are properly gapped. your ignition system is working correctly and your fuel flow is within specs, I would start by checking or getting my mechanic to check and set he CO2 levels. Follow the CMA manual, "measuring exhaust emissions" section pages 24-33, and factory manual, SF 35-39, closely.

a. Check at part load (2500 rpm in second gear with engine at normal operating temperature). This is the most critical adjustment.
b. Check at idle (900 rpm with engine at normal operating temperature)
c. CO2 values are at CMA page 47
- 5-8% at idle for best running (9-11% at 2000 rpm)
- 2-3% according to manufacturer specifications

If you need to adjust the settings do so very carefully (small adjustments):

Idle screws - lean is left and rich is right

Main rack (be VERY VERY careful adjusting the rack - never more than two clicks without testing the CO2 levels) - opposite of idle screws - lean is right and rich is left.

Finally (or maybe first since it's easy), if the CO2 level is too rich and won't decrease, check to see if the warmup thermostat and/or hose is clogged up. The hose can also sometimes get squeezed together which will cause the car to run rich. A blocked hose or gummed up thermostat is quite common and can cause the car to run very rich. Good luck!

[Mhead]

David, I have had the mechanic perform steps a-c and that's where we are having an issue. he can get the CO2 levels in spec at the low end ...mid range is off a bit. It seems that the margin of error here is nil...

When I got the car I had him check the system. He found that the MFI was timed of the #4 cyl vs #1....so we started from scratch...and nothing has improved. Our latest is that we pull the pump and get it redone.

For what its worth, I just pulled, cleaned and re-gaped each plug...now it runs worse...won't even idle and backfires a ton. I beginning to feel vanquished...feeling like this is a bad car...

[BURN-BROS]

Originally Posted by Mhead

David, I have had the mechanic perform steps a-c and that's where we are having an issue. he can get the CO2 levels in spec at the low end ...mid range is off a bit. It seems that the margin of error here is nil...

When I got the car I had him check the system. He found that the MFI was timed of the #4 cyl vs #1....so we started from scratch...and nothing has improved. Our latest is that we pull the pump and get it redone.

For what its worth, I just pulled, cleaned and re-gaped each plug...now it runs worse...won't even idle and backfires a ton. I beginning to feel vanquished...feeling like this is a bad car...

You have to verify the air -sync before any adjustment of the pump in performed.

[Mhead]

Can you help me with the the air-sync? Is that the idle adjustment screw on each intake?

[BURN-BROS]

It's a bit more involved than just the bypass screw.

First step is

Air @ idle

Air @ 2500 rpm

The Sync reads in Kilograms per hour. Post those numbers.

one poorly adjusted throttle body will affect mixture so making mixture changes is done last.

Also;

Verify that the Rod from the crossbar to the injection pump is at the correct distance. 114mm center to center.

Verify the the injection pump throttle arm @ idle is resting on it's stop.

report back your findings.

[David E. Clark]

It sounds like you are at wits end! Sometimes it helps to take a step back and review the basics. When dealing with MFI there is a well defined sequence of tests that must be followed in order:

CHECK IN SEQUENCE
1. Air Cleaner Cartridge
2. Compression Loss
3. Spark Plugs (Spark Plug Connectors)
4. Dwell Angle
5. Ignition Timing
6. Fuel Pressure and Flow
7. Injection Nozzles
8. Injection Timing
9. Correlation
10. Exhaust Emission Test
at part-load
at idling speed

NEVER DEVIATE FROM THIS SEQUENCE

No tune-up or synchronization will make up for an engine that has low compression!

If the engine is sound and nothing else works, don't forget to check the thermostat and the feed hose. The thermostat receives its heat signals from a double walled feed hose connected to the driver's side heat exchanger. When the engine is cold, the control rack is set to fully enrich the fuel mixture for easier starting. As the engine warms up, hot air is feed through the hose to the thermostat. This causes the discs in the thermostat to expand moving a lever in the injection pump which alters the control rack to lean out the mixture. If the thermostat or hose are gummed up or blocked it will not operate as designed and the car will run very rich and foul the plugs very quickly resulting in very poor running.

While you are following Burns-Bros instructions for getting the air synchronization correct you should probably make sure the air correction passages are not gummed up.

1. Remove and clean air-correction screws and passages with strong aerosol fuel injector cleaner and pipe cleaners.


2. Reinstall the air correction screws all the way closed then open each 3 half turns and retest with synchrometer at 3000 rpms at engine operating temperature.

3. Add the values for each cylinder and divide by 6. Adjust air synchronization to the average value with the air correction screws.

When adjusting be sure that the air-correction screws are not opened more than 8 half turns. If the air control screws are opened more than 8 half turns with little change on the synchrometer, the air channels are still carboned up. Thoroughly clean both the screws and passages with powerful fuel injection cleaner and pipe cleaners.

* Remember: Right = less air / Left = more air.

Keep us informed about your progress.

[stownsen914]

Lots of good advice above. You checked your fuel pressure and found it to be 11.5 psi. This is NOT enough for MFI. It needs 1 atm (14.7 psi). At 11.5, it might run OK, it might not. I wasted an entire weekend at the track years ago chasing a misfiring, crappy running engine. Fixed fuel pressure, and voila! Ran beautifully after that. I am not sure if low fuel pressure can cause a rich running condition like you describe, but it make your engine run like crap for sure.

Adjusting fuel pressure isn't simple though. The MFI fuel console has a check valve in the fitting where the fuel retun line slides on, and it's not adjustable. The spring in the check valve gets weak over time and can't hold 15 psi. I don't know if they make new check valves. I just purchased an afttermarket fuel pressure regulator and put that in line after the check valve in the retun line to get 15 psi.

[David E. Clark]

Testing your fuel flow and pressure again is an excellent idea because low fuel pressure and/or low fuel flow will definitely cause an MFI car to run lousy. Your original test found 11.5 psi which is within specification. The specification Porsche uses is (11.8 ± 3.0 psi). You also need to test fuel flow because having adequate pressure does not guarantee adequate flow in these cars. Always remember the basics: fuel pressure and flow issues are always addressed prior to messing with air flow or fuel mixtures. Here's how I would recommend you proceed when checking fuel pressure and fuel flow.

Install the fuel pressure gauge at the filter console where the MFI pump return line (left side of MFI pump) is connected to the upper side (closest to the rear of the car, easiest one to get to) of the fuel console (has an arrow pointing inward).

Fuel pressure is specified as 0.8 ± 0.2 bar (11.8 ± 3.0 psi) measured in the hose supplying fuel from the filter console to the MFI pump; the console fitting toward the rear of the car. Pressure is regulated by a device in the center fitting – the fitting that points toward the engine and returns fuel to the gas tank (P/N 901.110.915.00). If fuel pressure is low, replace the fuel filter and re-measure fuel pressure and flow. Low fuel pressure reading can indicate a restricted fuel filter. The flow test is also performed at the fitting on the fuel filter console that is to the rear of the car. This is the same fitting where you test fuel pressure. With the engine off, but with the ignition switch on the specified minimum flow is 900 to 1000 cc in 30 seconds. The reason you test fuel pressure at the return from the MFI pump is if the pressure after the pump is within spec, you know the MFI mechanical piston pumps are seeing proper inlet pressure.

The reason for maintaining proper fuel pressure in the MFI mechanical pump is to prevent intake cavitation when the mechanical piston draws in fuel. In that situation, low pressure and hot fuel can flash into vapor and cause a lean situation and poor running.

While the MFI system pressure is regulated to 11.8 ± 3.0 psi by the fitting in the outlet at the fuel filter console, the electric pump ‘safety pressure relief valve’ never opens during normal operation. Porsche specifies it operating at “approx. 2 atm (28 psi)”.

An alternate method of testing fuel flow is to extend the hose forward to the fuel filler for the gas tank. Put a large funnel in the filler and allow the fuel to circulate back to the tank for a while. All it takes is a calibrated container and a stop watch to check the fuel flow.

You get better measurements using a large container and longer times. It is easy to take several measurements and dump the fuel back in the tank. Always plug the open port on the fuel filter console because there is usually some residual fuel that can spill.

The low flow limit is 1.8 Liters per minute or 900 to 1000 mL (cc) per 30 seconds. If low, the first place to look is the screen filter in the fuel tank supply fitting. If screen is OK then the fuel pump is suspect.

This is also a good time to inspect your fuel hoses. These hoses should be “like new” all the time. They are inexpensive and easy to replace. Always use the Factory hose clamps, not the worm-screw type.


WARNINGS!

Always disconnect the CDI ignition box prior to opening the fuel system.

Do all tests where gasoline fumes are exposed out of doors. Definitely not in a garage where the fumes are exposed to an open flame from a hot water heater!

Have a BIG fire extinguisher close at hand. Gasoline fumes are VERY volatile!

Good luck and please report back with your progress.

[Mhead]

Thanks all...I am planning to set aside some time this weekend to approach the car with David's advice/sequence (see above). I'll report back with the outcome...

If you don't hear...that means I've taken it to the car wash...Only kidding.

[al lkosmal]

I would contact Mark Jung (356RS). He is an expert with these pumps and has a pump dyno to check current state and can rebuild to factory specs, if required. I had my 71E pump rebuilt by Mark a while back and it turned out perfect. I recommend him highly. Here is a link to his website: MFI Werks

regards,
al

[dicklague]

Originally Posted by matthewb0051

My .02:

Get a Bosch CDI and OE coil.

Get an LM2 so you can know what your AFR really is doing.

Also, check your plug wires and timing.

Don't get a Bosch CDI the Crane is much better than the old Bosch. Make sure you have a good coil on the Crane and a good points signal. The Crane optical sensor is great. Pertronix system works well to replace the point too.

Get a good AFR gauge with a wide band sensor. I like the Daytona-Sensors that our host sells. I have used one with my MFI for 8 years.

the LM2 is just OK and nothing special. You want data acquisition so you can really set up the MFI.

I went through the same journey with the MFI, and you need to make sure the engine is in good shape. Good compression and valve adjustments.

Next make sure the ignition is very good. I replaced my Bosch with a Daytona-Sensors CD 1 kit which includes a great coil.

A good powerful modern CDI like the Crane is important with MFI.

The most important thing is to know the AFR. With a good instrument properly installed you can really know what is going on and tune the pump.

[Mhead]

I have used Crane before...on other cars, not Porsche, so I know the works well in place of the old point and condenser setup ... a little less finicky...especially on the coast with the moist air I have to contend with in the summer.

The valves are done and I know the compression is good in all cylinders(wonder if the crap performance will screw that up though) so we can rule that out for the moment...Things are pointing to the pump...

What pisses me off is every time I touch the plugs...nothing else...it gets worse.

Never in my days under a hood have I experienced such temperament...Oh well, its a learning event for the old dog.