Help with my 911 problem

[EA911]

Ok - I've searched the Tech forum for a similar problem as mine with no results so hopefully someone can help diagnose the issue.

Symptom:

- Starts fine
- drive for a few miles and then the engine starts loosing power
- Tach does not suddenly drop to zero but a gradual decine in power
- As i step on the gas pedal car does not want to go and sputters a bit
- pull off the side and place in neutral and engine does not hold idle and dies
- attempt to start, it turns and starts but does not hold idle and dies
- I wait for a minute and try again and it starts and holds idle and i proceed to merge onto the freeway
- I feel a sputter as I accelerate like its deciding whether to loose power or not
- car drives find for another 5 miles and then starts to loose power again so I repeat the starting sequence again.
- there are times when the car starts and I keep revving since it won't idle and the engine is running rough

Car specs:

1969 911T, original fuel tank never removed or internally cleaned, webers newly rebuilt, original Bosch CDI box, new plugs and wires, rebuilt Bosch distributor, new points/cap/rotors, all visible fuses are good, both batteries are new and grounded well.

My amateur diagnosis:

- fuel starvation from a going bad fuel pump?
- going bad Bosch CDI box?
- rust/dirt from fuel tank?
- bad relays?
- bad tach?

Thanks in advance

[carrera turbo]

hello

and good diagnostic on the fuel delivery problem.

i would check fuel pressure first and probably remove fuel filter and check its flow IE blow through it. you can also go to a auto shop and see if they will cut apart your fuel filter with a oil filter cutter. this will let you know if there is any sort of rust from your tank or even the inside of your rubber lines clogging the filter. if this isnt the case maybe your float level is off on your new rebuilds on the carbs.


your off to a good start

cheers ed

[Wilhelm]

Did this problem come on suddenly? Or did it gradually get worse?

I am no expert, but sounds to me like a fuel flow problem. Fuel filter, Fuel pump, Carb problems . I am sure there will be plenty of help for you from the "Pelicanites". There are a lot of good, knowledgable people on here.

[french911]

Sounds like a clogged fuel filter somewhere. After leaving for a while the crud floats away and alows fuel to flow again for a few more miles. Check filter in the tank as well as the one in the fuel lines and carbs.

Barry

[ntrylvl]

I had a similar problem with my 72T after I first bought it last year. It would run great for a few miles, then stall. By the time I had it towed back to the shop, it would start again because the crud had settled. The previous owner had not driven the car much, and the original fuel tank probably had never been cleaned. So it turned out that I needed to remove/clean/reinstall my fuel tank (professionally done), then replace the fuel filter and the fuel pump, both of which had crud stuck in them.

Chris

[EA911]

Thanks All for your help. I had a strange feeling it had something to do with the gas tank/fuel flow. I try not to run the tank low because of this and come to think of it I did notice that this would sometimes happen after filling my tank.

I have a fun drive this morning that I really want to attend this morning. Do you think I can buy some more time if I do a quick fix by changing out the fuel filter? If so - will any auto parts filter do?

Thanks again for all your help!

[ntrylvl]

Originally Posted by EA911

Thanks All for your help. I had a strange feeling it had something to do with the gas tank/fuel flow. I try not to run the tank low because of this and come to think of it I did notice that this would sometimes happen after filling my tank.

I have a fun drive this morning that I really want to attend this morning. Do you think I can buy some more time if I do a quick fix by changing out the fuel filter? If so - will any auto parts filter do?

Thanks again for all your help!

Edric,

Sorry I didn't respond soon enough to help you make your decision about this morning's drive. From my experience, I'd say don't try a quick fix. In my case, I tried changing the fuel filter and fuel pump BEFORE I got to the tank, and not only did they not prevent the car from stalling out (I got stuck between LA and NorCal) but both actually ended up being wasted and needed to be replaced again because they were overwhelmed by crud from my tank. This is not to say that your problem is absolutely the tank, but you would be taking a risk unless you hit the problem right on the head with the new filter.

BTW, I have a couple of extra fuel filters in my garage if you need one. Just let me know.

Chris

[Eddyvdw]

Not knowing the weather where you live but was it close to zero celsius?
I had exactly the same as you had but with a clean tank and filter. Turned out to be the missing air regulator (to heat up the air intake) which 'froze' the engine. Was driving in freezing conditions when it happened though....
Good luck,
Ed

[EA911]

OK - so I went ahead took out the original fuel filter and there was quiet a bit of dirt/rust/dark particles coming out of it and replaced it with a fram brand to test. I figure it would a least tell me how badly the fuel pump is contaminated. It ran ok for the first couple of miles then it started to stall out on me again. I ran it for another 10 miles or so with no problems and then as soon as I hit the twisty roads it started to act up frequently. Talking to Chris (ntrylvl) who happened to have the same problems a few months ago - the tank needs flushing and resealing as well as cleaning out the fuel lines, replacing the fuel pump and fuel filter.

I'll start there and see what happens.

Now does anyone know of a good place to have my gas tank cleaned up in the San Francisco bay area?

Cheers!!

[Grady Clay]

Here is the common problem and why.

While earlier cars suffer this indignation, the ’69 was the first to really suffer. The reason is the circulating fuel system and fine rust particles in the gas tank.

The outlet fitting in the gas tank is actually a very fine mesh brass screen filter. It is located horizontally a few cm above the lowest point in the tank. The lowest point is where the drain plug is located.

With the circulating fuel system, Porsche cleverly routed the return to the tank to a banjo fitting at the drain plug. This allowed the continued use of the prior gas tank and not to engineer a new part. All is good.

Over time, fine rust particles wind up in the tank. Some comes in with the fuel. Some is from corrosion of the inside of the tank.

Very fine particles pass the screen filter and are retained by the fuel filter. These are discarded with the used filter. Even finer particles pass through the engine with no damage. The largest particles remain in the tank. Some stay stuck to the screen but most fall off when the fuel flow stops and fall to the bottom of the tank. As they should, they gravitate to the lowest point – at the drain.

Here is the rub.
With the circulating system, the return at the drain plug stirs up the sediment and suspends it in all the fuel. Over time and many miles of continuous driving at first, the suspended sediment covers the screen filter. Eventually this reduces the fuel flow to the engine and effects performance, i.e. it stops.

Frustrated, you pull off and when it won’t restart, you turn off the fuel pump. With no flow, the particles on the screen fall off and drop back down as sediment. Towing the 911 home or to the shop sloshes the fuel causing more cleaning of the screen. Checking fuel pressure shows normal pressure. Replacing fuel filters does not help.

As the screen becomes increasingly restricted, the time/distance until no-run becomes shorter. I suspect sitting there with the ignition off and swearing while flailing arms jostles the fuel enough to get the car to run for a bit more.


Diagnosis:
The first thing to do is measure fuel flow. This is best done with a long fuel line from after the filter in the rear. Return the fuel to the gas filler. You can let this run for an hour or more, taking flow measurements regularly. If the flow decreases over time, you have the problem. Do this waaay outdoors. Be careful.

Another useful test is to Tee-in a pressure gauge at the engine and use a long fuel hoes to a gauge in the cockpit. You can then monitor the pressure while the flow is restricted. It will drop to zero or close. This has the danger of a fuel leak. Be careful.


Maintenance:
You can siphon almost all of the fuel out of the tank via the level sender opening. Remember to replace the sender gasket. Many have use-once crush washers under the five scews.

Cleaning the fuel tank at the drain is part of regular maintenance. Since you need to remove the front pan (4 bolts) to get to the drain, it is not much more effort to remove the screen filter fitting and clean it.

The brass mesh is so fine that the rust particles can almost totally clog the flow yet the wire screen pattern is still visible. Many an expert mechanic hasn’t recognized that the screen was clogged because he could see the screen.

These screen filter fittings are still available new if the screen is damaged. There is a plastic 914 version of the screen filter fitting but I don’t recommend it in a 911.


Improvement:
I replaced my screen filter fitting with new and installed the good-used original in the gas heater supply port. This way the return fuel is dispersed and doesn’t disturb any sediment. It just sits at the bottom until I drain and clean the tank.


This problem is not as much an issue with CIS. I think this has to do with the ‘swirl pot’ design, the vertical position of the screen, the screen size and the significantly larger size of the filter. It still needs regular cleaning.

Best,
Grady

[EA911]

Thanks Grady for your explanation and education about the fuel system. Although I like wrenching on my cars I really don't like working directly with gas so I'll most likely bypass the test and just take off the tank and have it flushed and cleaned/sealed. I'm even considering buying a new repo tank if I can find one since I do have a rather large bottom dent + a bit of surface rust. Problem is I can't seem to find them for sale again anywhere else.

I'm also going to replace the fuel pump and filter so I have a fresh start and perform regular maintanence if needed.

Cheers!!

[BoxsterGT]

Do what Grady recommends about cleaning or replacing the fine mesh filter first.

I will bet you find it clogged w fine rust particles.

Here in the rust belt it happens much more often than one would expect.

Len

[regency]

While doing the restoration of my 73 T, I sent my tank ( it sat for 25 years ) to a raidator shop in Stockton, Ca. They had it about 1 week and did a great job inside & out. Will have to go back through my receipts to get their name & #. It cost about $400.00

Steve

73 T

[kens_74_911s]

Been there... I agree to do as mentioned remove the screen and clean it. Flush some gas through the tank (you can leave it in the car) and you will likely see the particles exit. you can probably go a few months just doing that at least until you have an opportunity to pull the tank and seal it.
I need to seal mine... When I get >1/2 full the seam seem seems to have a few pin hole leaks. I have an early tank in my 74 so I can run a normal size spare and have capacity to hold the flat up front if necessary.
I was just looking at these videos just yesterday:
Videos - Demonstrating How To Use KBS Coatings - Stop Rust
It may give you an Idea of what the process is. The KBS product recommended not using the tank for a week? I think other brands kick off faster.

[JP911]

I picked up a repo tank in the Bay Area at Prts Heaven.

[Eddyvdw]

I have exactly the symptoms as mentioned in the original post although only after 10-15 miles. Removed the tank, cleaned it but didn't remove the screen (looked okay). That was a wrong call, wasn't it?
Ed
1972 911E

[Eddyvdw]

Grady, I read in your reply that you installed the good-used original filter in the gas heater supply port. Did you use this filter for the return fuel line?

I'm in the process of cleaning the tank because I had the problems as described before. This is how the tank looked...

[RWebb]

Grady - thoughts on how often the mesh filter should be replaced?

[RWebb]

I see these items in the Pelican catalog for 1974-on:


911-201-141-00-OEM

Fuel Tank Screen, 911/911 Turbo (1974-89)
Brand: Genuine Porsche - $40.00

911-201-143-00-OEM

Fuel Tank Screen Sealing Ring, 911/911 Turbo (1974-89)
Brand: Genuine Porsche - $4.00

I do NOT see what one should use for a 1973.5 (CIS tank) car; or for even earlier ones...

[Grady Clay]

I think the screen filter should be removed and cleaned periodically. Unless it is damaged, it should last longer than any car.

Cleaning the filter lets you assess the debris condition of the tank. If there is gunk in the bottom, clean it out. Once the gas is drained, you can remove the plugs, screen filter and fuel level sender. This lets you use a garden hose and dish soap.

I have seen screen filters that appeared good (able to pass gasoline) but were plugged. The screen can be covered with fine rust particles where the weave of the screen is still very prominent. Aerosol carb cleaner from the inside works well.

The screen filter fitting comes with two different size pipes to fit either 7 mm hose or 9 mm hose. I’ll see if I can find the part numbers.

When I put the old screen filter in the gas heater supply port, it dispersed the return fuel, not stirring up the sediment in the tank bottom. The Factory plumbing returns the fuel to a banjo and banjo bolt that replaces the drain plug. This stirs up any debris and water that collected at the lowest point – by the drain plug / fuel return port. I think the rationale was to stir the fuel letting the debris go to the fuel filter and the water to the engine. This prevents water filled sludge build-up on the bottom of the tank.

To not confuse things, we are talking about ONLY cars 1973 and earlier (not ’73.5 CIS). The 1973.5 and later cars use a different system with a different filter screen.

Best,
Grady