MFI Problems

[Grady Clay]

Terry,

Applying the “one-thing-at-a-time” philosophy, why not drain the fuel and inspect the screen? While I agree that “Dirt in the tank is USUALLY not that predictable.” the symptoms are the same. In any case, you will be better off eliminating that possibility for no cost.

Did you feel the pump? Did you look at it with the IR thermometer? Was it hot? The problem manifested itself both before and after the original pump was replaced with the rebuilt unit and no change in noise leads me to suspect it isn’t the pump, although I certainly won’t rule it out.

I agree that eventually you will want to replace the pump with new. First find the problem without throwing $400 parts at it. I would do the two proposed tests and re-clean the screen before buying a new pump.



Terry, you are right on! There are remarkable benefits to this Pelican Forum. Any DIY guy, shop, or mechanic that doesn’t monitor and record the information here is … well, a fool. I wish this technology had been available when I was in the Porsche business. I still find it hard to believe how hard I had to work to have all the information possible for my mechanics. Today, I would dedicate a substantial part of someone’s day to gather, sort, organize, and disseminate the information available.

I am a firm believer that no one person has “all the answers.” Bruce Anderson, Jerry Woods, Chuck Stoddard and many others and I would spend untold hours on the phone, at Parades and other events discussing this stuff. Now it can be on-line for everyone’s benefit. Interestingly, I don’t think it is any disadvantage to the pros to contribute. “There is no substitute” for those knowledgeable eyeballs, skilled hands, and well organized shops.

Best,
Grady

[Grady Clay]

Jay,

Sorry, I didn’t mean to be coy. I was referring to the screen in the fuel tank.

Now that you said you cleaned the fuel screen I have some other CMA questions. When you cleaned the “horribly clogged” screen, did you clean the fuel tank? Have you tested the fuel flow? Do you have a fuel pressure gauge that you can monitor while driving?

Again, I’m sorry I gave a surely, smart a$$ post.

Best,
Grady

[304065]

Terry,

I agree with Grady, you need to use a systematic approach to fixing this stuff. I've never heard of a failure mode based on heating of the pump, even though it's down near the heat exchanger.

Now, it's your lucky day. IF you agree to check the filter screen as Grady described, then I will send you one of my two spare electric MFI fuel pumps, as used on my 911E. IF you change out the pump for mine and it works, you agree to send me $100 plus the cost of my shipping to you. If it doesn't work, you agree to send me my pump back plus the cost of my shipping to you.

The pump was working when removed and is a much better alternative for testing than $400 for a new pump. No warranty expressed or implied, your mileage may vary, void where prohibited, etc.

If you are up for it send me a PM with your address, I am going to my hangar this weekend and will get the pump and mail it to you.

[Jay Auskin]

No problem Grady, I'm a smart a$$ myself, I wasn't focusing outside of being a hot issue for some other reasons I guess I didn't mention.

The screen was clogged bad by what appeared to be mostly corrosion and such. When I removed the screen, no other dirt really came out. When I looked into the tank during my rebuild, it looked good. Now that I've filled it for the first time in a long time, I should see how this looks. I'll pull the sender, and look for garbage in the tank.

Another reason I see this as a temperature issue is this happened on the exact same route, going to work, only when I was in stop-and-go traffic for a long time (weekdays). I had to go into work twice on the weekend, and I would get on the highway sooner because of less traffic. This could all be coincidence, and if my tank is clean, and replacing the restrictor doesn't help, I'll need to buy a pressure gauge to monitor fuel pressure while driving.

[Grady Clay]

Jay,

This clogging of the screen from rust in the tank is a fairly insidious problem. The corrosion you found on the screen came from the bottom of the tank. If you just cleaned the screen and not the tank, then the same problem will reappear.

Go back and study my recommendations to Terry. Go read the thread I cited. I wouldn’t be surprised if the screen is clogged again.

I won’t dismiss a temperature effect. As I explained above, when the fuel pressure is low and there isn’t much (any) return flow, the mechanical MFI pump pistons can cavitate as the fuel gets warm.

Under normal (proper) operation there are almost two liters per minute of fuel being returned to the fuel tank. With the flow restricted to marginally what the engine is using, the stirring energy of the pump and the heat transfer from the very hot heater waste air to the fuel lines can cause some fuel components to vaporize. MFI absolutely does not like this.

I think this can account for your symptoms although I wouldn’t rule anything out.

Best,
Grady

[thastings]

John,
I have PM'ed you. Thanks for the offer.

I still plan to follow thru on checking the tank screen, installing the meter (even if it says "OIL"), etc.

I don't know if this problem predated the installation of the rebuilt pump. It just plain stopped working while I was installing and adjusting the MFI unit. There were to many other things that were going wrong to tell if I was having this same problem with the original pump.

If I'm lucky it will solve this problem, but I've never been accused of being lucky before, so why start now. Either way, I have learned a lot about my MFI.

Terry

[Jay Auskin]

Well I took the wife for a ride down Lake Shore Drive tonite, thinking 'it's' cool out, it will be a nice leasurely drive...I have a hot fuel issue...right??' well the first half was fun. The second half---my car was bucking like a bronco. I managed to stumble home (car & wife), and then it just died.

The nice thing about an external electric pump like our MFIs, you can hear it run. Well, it sounded like it was sucking some serious air!

Your right Grady, my screen is probably clogged...or, I actually might be almost out of gas. My fuel gage doesn't work properly (I think), and while I thought I filled it up, maybe it just cut out early?? I will feel like an extreme idiot, but that would be nice if it's all it is. I'll look at it tomorrow after work. I only have 200 miles on it since the rebuild, so maybe it's sucking down alot of gas during break-in?

[thastings]

Jay,

Having just brought my 72 911T out of a 10 yr storage with gas in the tank for the whole ten years, here are some of the problems that I experienced on an intermittent basis, that cause problems that I blamed on the MFI:

- left rear rubber brake line was bad. Started off down the road just fine and every time you applied the brake and released it, the brake shoe did not return all the way back. After about 10-15 times of applying the brake the rear wheel was locked up, BUT it felt like the car was gradually running out of gas. Stop on the side of the road for a few minutes and everything was fine again, until you started applying the brakes again. I found it only because one time on the road side, as I was discussing my cars birth right, I leaned against the wheel. Smoke coming out from under an early 911 is not always from oil on the heat exchanger.

- All of the fuel lines were clogged. While they allowed flow at idle and the engine ran ok, at speed it ran out of gas. Clean all the metal lines to the injector nozzles and replace all rubber with new. Reusing one old piece is the same as not changing any. Warning, use Grady's revised diagram for the hoses at the filter. It is correct.

-Check and recheck the fuel pump (see this thread) even if you "know" its good, that includes "new" ones. Short term testing with a pressure gauge will lie to you. I had to run it 30-40 minutes at 4000 rpm, sitting outside my garade, while watching the gauge the WHOLE time to spot problems. Mine would drop for only a few seconds at a time. You already know why I telling you this.

-Replace the fuel filter. It is bad if you have not changed it since you got the car

_Is your idle speed (8mm socket thru the fan thing) actually adjusting? Mine was stuck in full rich position. I would push and turn that screw ( If I remember correctly, this is really only a spring-loaded flat blade screwdriver that you push into a slot on the back of a rod. This is why you have to rotate the screw until it falls into the slot, THEN when you turn it , it will click) all day long waiting for the "click" it was suppose to make. Believe me you will hear and feel the click when it happens. If you don't then its time to repair/replace/clean this screw. This is an on-the-car repair. Very easy to remove. Remember, there is oil behind that cover, so be prepared to lose ounce or two.

Assume Grady and Cramer are right UNTIL you prove them wrong.

I know sometimes you feel like looking for a cliff, but hang in there. The '72 Targa looks to good going down the road to kill it.

Good luck,

Terry

[Jay Auskin]

Yep, most of the above has been done when I rebuilt the engine. I rebuilt the brake calipers, replaced all rubber brake lines, replaced all rubber fuel hose, replaced all ignition tune-up parts, replaced fuel filter, replaced breather hoses, Cleaned my throttle bodies and made sure they moved freely, opened up the MFI pump access covers to make sure everything was free, etc. I basically had every mechanical component on my car disassembled and either cleaned or rebuilt (minues the trans). I haven't made any adjustments to the fuel system yet since I've only put on a bit over 200 miles since I rebuilt the engine, and I don't want to keep adding variables to any problems I may find.

I'm feeling pretty good today since I likely have a clogged tank screen...or no more gas. Both are an easy fix

I'll keep you posted on the root cause, and let you know what else I find as I get this engine rebuild and fuel system sorted out.

[thastings]

John Cramer,
I need to return to pump you lent me. I need your address. Email or call me at 410-215-4732
Terry

[Early_S_Man]

guys,

The restrictor in my system was found at the approximate location circled in red, in the diagram below ...

It was approximately vitamin-pill shaped ... cylindrical, steel, 8.0 mm in diameter, 11.0 mm long, with gently-rounded ends, and had a ~1.5 mm [1/16"] hole drilled in the center, lengthwise. It was a tight friction fit inside the 7 mm ID OEM hose, and the old hose had to be cut away with large diagonal pliers to remove it.

It was actually a miracle that I found it at all ... that only happened because I decided to cut back from the ends of the hose to find out how far the cracks in the rubber part of the hose extended from the ends ... my hose wasn't leaking, but the hose ends were frayed at the cotton braid, and every end had cracks in the rubber where fitted over a barb fitting, including under the swaged crimp rings! I don't remember now if I found one 'consensus' value or not, because I was pretty amazed at finding the restrictor hiding in that hose!

[72doug2,2S]

Warren, thanks for sharing that diagram. I like it, I like it a lot.

???

Anyone know, 1st if it's necessary, 2nd if so, how to bleed the fuel system when you are reassembling the tank, hose, and pump?

[Jay Auskin]

As usual, Grady was on the money. I looked into the tank only to find alot of crap plugging my inlet screen. Now if I can only stop working so much to get that tank out, and have it cleaned and sealed.

There's alot of good info in this thread!

[zotman72]

Jay:
Grady lives in Colorado, I have met him a couple times and he is a great resource. FWIW I just want to point out that on the first page this thread, I offered up what was probably the situation (IMHO) with your corroded tank from my first hand experience with the very same problem on my '72T. It took me three iterations of cleaning my intake screen before I pulled my tank to fix the problem. Rust never sleeps anywhere on an old 911. HTH and this board is amazing source for all of us Porsche buds.

[Grady Clay]

Warren,

The position you indicate in red is in the pressure relief circuit for the pump that returns to the tank. The spec for that pressure relief valve is 2.0 bar. As I understand the system, that pressure relief valve never opens unless there is some abnormal restriction in the main fuel flow circuit. That restriction is not in the circuit that determines the fuel pressure at the MFI pump.

I do not doubt you found a restrictor there. My question is; why is it there?

Best,
Grady

[Early_S_Man]

Grady,

My best SWAG is that it was a cheap, redundant backup to keep pressure high to the filter and plenum 'loop' in case the relief valve in the pump stuck open due to contamination or other problem ... kind of a 'limp' mode, if you will ...

[Early_S_Man]

grady,

Looking back at the Parts Catalog diagram ... i just realized that it doesn't accurately depict the layout and connections to the 'Tee' ... and that I circled the wrong leg in your diagram ... it has only been about 17 years, after all!

Below are the revised diagrams relating to where I found the restrictor:

[thastings]

Warren, Grady,
Has it been determined that this in-line restrictor is a factory item and should be there. I did not find one in my original hose that I replaced (why do P-guys keep "old" parts?). If so, is it needed?
Terry Hastings

[72doug2,2S]

I fixed my 72T MFI with a new intake (gas cap $10.00 Pelican) filter and new tank yesterday. My college buddy helped me with the new repro tank. Once the return banjo fitting finally sealed I put in 5 gallons and my pump is humming happily today.

Warren, your revised diagram matches my pump lines perfectly.

Still three more weeks in the short arm cast, but I'm back in the saddle again!

[thastings]

Update time....
In my most recent bout with my 72T MFI having these "events", I found that I had good gas flow at the tank, 1/2 that flow at the fuel pump intake and output and a simular amount at the filter output to the MFI.The pressure went up to 30 psi when the line was clamped shut.

Well I took Grady's advice and one again went thru the cleaning of my gas tank, tank filter and gas lines up to the fuel pump and a new fuel pump. I found "crap" in the metal lines running thru the body, used compressed air and wire to clean it out. Ended up with 14-16 psi and a liter in 40 seconds at the output to the MFI of the fuel filter. This was a 30-40% increase over the pre-cleaning readings. (how do you tell if the tank filter is clogged, the mesh is so fine I could not see thru it, but air and cleaning fluid flowed thru it ok)

Took the car out and it ran great. Fuel pressure remained at 14 psi and no "events". Once again I was a happy P-car owner
THEN........................
On the way to work, about 20 miles into the 30 mile drive, the "events" started again, and this time the fuel pressure stayed below 10 psi for the remainder of the trip. The car continued to run ok except for the 10 or so 20 second long " events" with lost power and popping in the pipes

The only thing that I have not taken apart and cleaned is the fuel filter housing. Is there something inside this that could allow a blockage to form? Does anyone have a good diagram on taking this apart?

So, am I looking in the wrong place? Is there an electrical item that can affect the pump pressure or flow?
Terry Hastings