MFI Fuel Starvation...Out of ideas.....help please

Sean_irl · 05-17-2004, 02:30 AM

Hi All.....man i'm getting a serious baptism of poor luck.

My car won't start and this is really strange. I've spent 16 hours working on it and have hit a dead end.

When connected up to the fuel tank the fuel pump is not pumping fuel out of the pump....so I was thinking it could be a blocked filter in teh fuel tank.....But..when I disconnect the fuel pipe from the tank to the pump...the fuel just pours out with no problem

So I then think that my fuel pump is dodgy...so I get a 2nd hand one that was tested and hook up. I get fuel to the mfi unit for about 30 secs and then the fuel stops flowing. Again I go throught the fault finding....I check the fue out of the tank for bloackages ...but it is pouring ok.

I then disconnect the pump to check it myself and it is pumping fine.

I reconnect the pump to the fuel tank and disconnect the fuel out pipe and nothing happens...the pump is whirring..but no fuel.

So in summary.... I have fuel pouring from the tank to the pump...I have no fuel coming out of the pump.....

Does the pump have to pump fuel at a particular pressure?

Any other thoughts.......again thanks in advance.

I've now had the car on the road for only 6 weeks since I bought it in September and this is hurting hard.

Sean

sundaypunch · 05-17-2004, 03:04 AM

The fact that you get fuel flow for 30 seconds leads me to believe that you might have something that clogs the line. Is it possible that the fuel pump pulls rust particles into the screen clogging it after the 30 sec., but then some particles dislodge after a bit when the pump doesn't have suction on it? I know this doesn't sound logical but I had the exact same symptoms with an old Toyota Land Cruiser. It would run for a bit then starve itself. After sitting a bit it would start and run for a bit again. I ended up pulling the tank to find a couple quarts worth of rust particles in the bottom when I drained it.

Also, someone on the board a few months ago had this problem. It turned out to be improper fuel line which the P.O. had also looped back by the engine. When after the car got hot and ran for a bit the line would soften then kink, starving the engine.

Sean_irl · 05-17-2004, 03:13 AM

Sundaypunch...I was thinking something similar. But I only got fuel for 30 seconds the first time I connected up....any other time and the MFI was dry.

When I disconnect the pipe from the tank and drained about 10 litres of fuel the flow was fine.

***It turned out to be improper fuel line which the P.O. had also looped back by the engine. When after the car got hot and ran for a bit the line would soften then kink, starving the engine.*** ---- This bit does interest me .. before the car failed to start...it was stalling when in traffic. In particular when the heat was builing up. Maybe the fuel line is dodgy. They were replaced by the P.O. not that long ago (less than 2 years).

FAO ..Alan.UK is there anychance this could be the fuel line?

Sean

Alan.UK · 05-17-2004, 04:32 AM

Hi Sean,

All sounds very odd !! I replaced most of the fuel lines when I restored the car, but that does not mean they now have a blockage.

I will call you late Sean, but for now, I am still unsure if the problem is from the tank to pump or from pump to MFI. You can normally here if the pump is dry (no fuel) as the pump note is higher. To confirm have you disconnected the flexi line from pump to the hard feed fuel line which runs to the rear of the car. If that works then the pump is pumping fine. Then hook that back up and disconnect from MFI it reads like it's dry here, so maybe a line is block or the fuel filter is messing up. Have you disconnected it from where it goes into the fuel filter and see what happens there ?

Alan.UK

Alan.UK · 05-17-2004, 04:40 AM

Just in case it helps, disconnect the feed line to the filter (pic) and test in the same way you have been doing. My mind is thinking, the pump has been tested and works fine, the tank is full of fuel and not recently been run dry, lines should be good, but no fuel to MFI --- next place to look is in the filter area.

Alan.UK

Sean_irl · 05-17-2004, 04:46 AM

Hi Alan....when I disconnect the line from the pump to the hard feed.....nothing happens. So you would think it is the pump....but when I tested the pump it was fine.

After work I will need to check the fuel pipe to make sure it is not kinked or blocked...but other than that i'm stuck for ideas.

Sean

Alan.UK · 05-17-2004, 05:00 AM

Hi Sean,

When you say the pump works, have you actually seen it pump fuel or just because it whirrrs.

If there is fuel easily flowing all the way to the pump, but none coming out directly out of the pump, then either the pump is faulty or after the 30 secs gunk in the tank blocks it up, which seems unlikley as fuel is "flowing" out of the pipe which connects to the pump.

Hard to explain, speak to you later

Alan.UK

Luke Hudson · 05-17-2004, 05:22 AM

Sean,

I’ve just replaced all the fuel lines on my 72E with MFi and I have a couple of suggestions, I had some really old hoses that had become very soft, could your pump be sucking the pipe closed? Also just eye-ball the hose routing, anything look like too tight a bend, when the pipes in the engine bay get warmed up they can get malleable and close up (like you mention above).

By the way, the fuel lines at the front of the car loop up over the steering rack, which means that when you disconnect the pipes at the rear (near the engine cross member/mount) fuel will not flood out, you need a pump to suck petrol from the tank, it doesn’t gravity feed all the way out to the back if you see what I mean

Grady Clay · 05-17-2004, 06:10 AM

Sean,

Remove the fuel level sender and inspect the bottom of the fuel tank and the fuel pick-up screen filter that is also the supply fitting for the fuel pump. Get a spare fuel level sender gasket first.
Use a wood dowel (stick) and a cotton cloth to wipe the bottom of the fuel tank and screen with.
Your symptoms certainly sound like contamination in the fuel tank.

A next step would be to drain the tank and remove the filter screen for inspection.

Where is your fuel pump – on the front suspension cross member or at the left rear torsion bar tube? Have you taken compressed air to all the hoses and tubes?

Best,
Grady

Grady Clay · 05-17-2004, 06:33 AM

Useful illustration to facilitate our discussion.
"

"
(C) 1971 Dr. ING. h.c. F. Porsche K.G.

Best,
Grady

jstobo · 05-17-2004, 08:57 AM

Pump Spec is for 1 atm (14.7 psi)discharge pressure. Disconnected it could flow fuel but the pump may not put out enough pressure. Also as the drawing shows one outlet on the pump is for the return line. If you disconnect it and no flow, your filter may be clogged. A "reguler " carburetor pump only puts out 4-5 psi and will not work with the MFI. make sure you have the original Bosch unit. My experience, once they go replace it with a new one. Have not had much luck trying to rebuid them. They have gotten pricey. Good Luck.

Grady Clay · 05-17-2004, 09:17 AM

The correct pump for MFI (14.2 PSI) is:
Porsche # 901.608.105.00
Bosch # 058097 0001

The pump for carbs (4.3 PSI) is:
Porsche # 911.608.107.00
Bosch # 058096 0009

These may up-date to newer numbers.

Best,
Grady

Sean_irl · 05-17-2004, 09:25 AM

Again this board has kept my spirits high!!! Thanks all.

Fuel Pipe: All the pipes to the pump are new....except for the pipe from the tank to the pump. This one is rubber texture and very soft....so following Lukes comment on it closing while being sucked....I will replace it with a new pipe and see what the outcome is.

If this makes no difference i'll check the output on the pump. I have replaced the original pump with another one....but this one is second hand out of an old rally car (it would be bad luck if that was running at bad pressure as well).

And if the pump is at correct pressure i'll take Gradys Advice.

As ever i'll keep you all posted on how I get on.

Thanks BIG TIME!!!!

Sean

Todd Simpson · 05-17-2004, 09:37 AM

I'm not sure this will help, but I had a starvation issue a few years ago after converting my carb'd T to MFI. I used the diagram in the haynes manual to route the fuel lines and it is flat wrong. My car would start but not pull under load...it was easy to diagnose once we pulled the out line off the filter and got virtually nothing, but the in line was strong. Your problem sound different.

Is the pump wired backwards?

pwd72s · 05-17-2004, 09:47 AM

Great thread guys! Grady? The diagram & pump part numbers are now in my "hard copy" notebook. THANKS!

Early_S_Man · 05-17-2004, 11:29 AM

Tod,

I beg to differ ... Fig. 2.39 in the Haynes manual is a direct copy from the factory sevice manual, as are many others ... just HOW could it be 'flat wrong'??? Perhaps you misinterpreted the diagram and created your own problem? Like, maybe, what the three letters on the fuel pump connections represent?

Grady Clay · 05-17-2004, 11:31 AM

Here is the diagram for the ’69-’70 plumbing.
This is one of the few times Porsche actually published something wrong.
All MFI 911s were delivered from the Factory with the fuel lines in the corrected placement,
not as indicated originally in this diagram.

I think the ’69-’70 fuel pump placement is ideal.
I don’t think the ’71-> placement next to the heater
hot air waste is a good idea.
"

"
(C) 1969 Dr. ING. h.c. F. Porsche K.G.

Porsche corrected this diagram in a later update.

Best,
Grady

Sean_irl · 01:57 PM

Quote:

Originally posted by Todsimpson
Is the pump wired backwards?

Tod...the wiring is ok. I actuallly replaced the original pump with a tested one from an ex rally car.

Just anotherbit of background on how this problem materialised. The car had a tendancy to stall while idle in traffic. Usually during rush hour in Dublins busiest streets. Firday 3 weeks ago....it stalled at 18.30 rush hour. It took about 5 mins to restart (same as the other times)......and once I got going I decided to get out of traffic, park it and let the traffic die down. About 30 mins later in a public car park I decided to go home...but this time it wouldn't start. It would turn over and the pump was whirring but not firing. So a €100 lighter and many hours later I arrive home on the back of a trailer......again

So 2 half days off work and most evenings I went step by step through the fuel circuit...starting from the MFI backwards...and here I am

All my brownie points at home are used up..(well carrying the engine upstairs after teh blow out ..probabaly didn't help either

)

Yeh..i'd rather be driving her. since the weather is good. But you've got tot learn sometime...just happens i'm learning more off the road then on...wouldn't mind a balance

Will keep you all posted as usual.
Sean

Todd Simpson · 05:10 PM

Quote:

just HOW could it be 'flat wrong'??? Perhaps you misinterpreted the diagram and created your own problem?

Wow, that's harsh...I posted on this last year, I discovered the problem in 2002 when I first did the swap.

1969 2.0 MFI fuel line routing, to/from filter.

Grady-Do you have the corrected diagram? The difference is the lines into either side of the filter need to be reversed.

ed martin · 05-17-2004, 08:33 PM

I had the same situation, what I did was to exchange the suction line from the gas tank to the pump with the return tube from the pump to the tank. The thinking being that sediment over the years had occluded the suction line just like water pipes in an old house. The return tube on the other hand had filtered fuel passing through it over the years and should be clear. This approach worked like a charm. Also cover the basics, like a clean fuel filter and make sure the order of the lines are correct. Particularly the line order from the pump to the filter. I had mechanic install a new fuel pump with the suction and the return reversed and I couldn't get my car to go over 30 miles an hour. I got the fuel line order schematic and realized the problem.