Fuel pouring out of exhaust

darkstar · 07-26-2007, 02:25 AM

Hi everyone, I've just recently bought a porsche 911 76 model with a 2.7 litre.
I've replaced the alternator with a new one that has a built in reg, so i've bypassed the reg with no issues.

My problem now is, once i have the ignition on, petrol comes pouring out of my exhaust, i drove the car before i replaces the alternator and didnt have any issues with excess fuel.

What should I check for??

Im pretty sure the 2.7 is mechanical injected.

ossiblue · 07-26-2007, 06:56 AM

Sounds like a possible stuck injector. The CIS system is under constant pressure and if an injector is stuck open, it would probably result in what you describe. Then, again, there are parts of the system that I know nothing about that may cause your problem. Be patient, others who are experts on the system will respond soon.

foamy911 · foamy911's Garage

the 76 is CIS not mechanical, if your diaphram is somehow stuck open, yes the injectors would be spraying continuously. Pop off your air filter cover and look at the diaphram plate and see if there is something in there or it's stuck open.

Superman · 07-26-2007, 07:32 AM

Yes, the '76 CIS system is pure mechanical. No, it is not MFI, but it is purely mechanical in that there are no electronics. No computer. The position of the sensor plate pushes the plunger up into the fuel distributor and you get gas. Yes, if the air flow sensor plate is sufficiently misadjusted, then there could be fuel getting sent to the engine when the sensor plate is at rest. But the other thing you'd need is fuel pressure. With just the key on, I believe the fuel pump should not be running. And so, the amount of fuel in question would be quite low. System pressure would bleed quickly. And also there is the cold start system. It is a seventh injector located probably in the back of the air box. It is an injector with a square electrical plug attached to it. It sprays temporarily while you're starting the car. It requires less fuel pressure than the six CIS injectors.

Set the air flow sensor plate rest position according to the specs in the book, or at least verify that it is set correctly. If it is not centered, a slip of paper around the plate while the bolt is loose will sensor it. Also make sure your fuel pump does not just run because the key is on. That's a safety problem.

azasadny · azasadny's Garage

Yep! I had this exact same thing happen on my '74 in May and when the car was being put on the flatbed trailer, gasoline started to pour out of the exhaust. My car has Weber carbs and an aftermarket fuel pump and it turned out to be a faulty fuel pump, which was replaced and the oil was changed because of fuel dilution and all is well. We still can't figure out how a failing fuel pump put fuel into the exhaust, but that's my story and I'm sticking to it...

foamy911 · foamy911's Garage

Ok technically it's not electronic, but it is pressurized from the electric fuel pump rather than machanical. But with the key on (at least on mine) the fuel pump does pressurize the system. So do check the plate and also the warm up regulator.

vash · vash's Garage

i know what it is!

your air sensor plunger is stuck "up". open the airbox, take off the air cleaner, and have someone flip the ignition on. quickly lift the air sensor bar. if it feels perfectly balanced, like there is zero weight...that is your problem. you should have some resistance. on the early CIS cars, the fuel pump comes on with ignition. i think it changed in 81.

CliffBrown · 07-26-2007, 10:40 AM

Could your re-wiring of the alternator circuit cause the cold-start valve to be energized along with the fuel pump? This would allow fuel to flow and some cylinders could be full of fuel.

No matter what the cause of the problem, you should not engage the starter until the problem is solved, could result in a bent rod, fuel will not compress.

darkstar · 07-26-2007, 01:36 PM

when i changed my alternator, i had an auto electricitian help with disconnecting the external regulator cause my new alternator had one built in, other than that, nothing else has changed.

i've only owned the car for a week,

Superman · 07-26-2007, 02:30 PM

Quote:

Originally Posted by CliffBrown

Could your re-wiring of the alternator circuit cause the cold-start valve to be energized along with the fuel pump? This would allow fuel to flow and some cylinders could be full of fuel.

No matter what the cause of the problem, you should not engage the starter until the problem is solved, could result in a bent rod, fuel will not compress.

Someone suggested that the fuel distributor plunger is getting 'stuck' inside the fuel distributor bore. That is very possibly PART of what's happening. The plunger can be removed, but be very careful. DON'T drop it, here's why: The plunger and the bore are a PERFECT fit. So perfect in fact that fuel, at about 60 psi at the inside end of the plunger, does not leak out. No kidding. There is no o-ring. The fit between the plunger and the bore is so perfect that fuel under 60 psi does not leak out. If you remove it, clean it carefully and the bore and reassemble. Indeed, a few bottles of Techron in the tank might stop this problem. Get your tank down to 1/4 full and add an entire bottle. Drive it until you really need to gas up. At least one night of soaking with the Techron in the 'system'. Then fill the tank up and add another bottle. When you'r edone runnign Techron through it, then do an oil change. The Techron does not burn up. It gets in your oil and dilutes it.

Now, for the part that's even more important. There wouldn't be much fuel leaking out if your fuel pump were not running at the wrong time. It should ONLY run 1) when you are starting the engine and 2) when the engine is running. You do NOT want to have a fuel pump running in the case of a collision. For this reason, CIS systems are designed to stop the fuel pump when the engine is neither running nor being started. If someone has defeated this important safety feature, please do yourself a favor and return it to proper operation.

JP911 · JP911's Garage

The fuel pump runs on older CIS cars as soon as the key is turned to the "on" position. Worked that way on my 76 and it works that way on my 74. But, the injectors will not spray until the plunger is lifted.

Superman · 07-26-2007, 03:45 PM

That surprises me a little. The early CIS Rabbits I've had were wired so the fuel pump would not run in the event of a collision.

Good thing our engines are of the Horizontal Opposed variety. If our cylinders faced upright, those flooded cylinders would be hydrolocked and the result might very well be bent rods.

CliffBrown · 07-26-2007, 04:01 PM

Quote:

Originally Posted by Superman

Now, for the part that's even more important. There wouldn't be much fuel leaking out if your fuel pump were not running at the wrong time. It should ONLY run 1) when you are starting the engine and 2) when the engine is running. You do NOT want to have a fuel pump running in the case of a collision. For this reason, CIS systems are designed to stop the fuel pump when the engine is neither running nor being started. If someone has defeated this important safety feature, please do yourself a favor and return it to proper operation.

I agree with you but it's been my experience that many of the CIS cars have been altered over the years and the fuel pump will often energize when the key is placed in the on position. Many of the cars have not had the sensor plate switch maintained or have the plug disconnected.

I have four SC in the garage right now and only one of them operates the fuel pump as the factory originally wired them.

dougcl · 07-26-2007, 04:12 PM

Hi, simple test. Take off the air filter, reach in and gently pull down on the air sensor plate. If this solves the problem, then your air sensor plate rest position is too high. On some cars this is held by an extremely flakey paper clip thingy that will break with some regularity.

Doug

AshEvan · 07-26-2007, 04:26 PM

My 74 is the same way. Once the key is turned to theon position the fuel pump is activated, prior to turning the ignition to start. Same way with my friends 75, don't know about the 76 model year.

Peace

Superman · 07-26-2007, 04:55 PM

Sometimes CIS fuel pumps will run as soon as the key is turned to the "on" position without activating the starter, but those systems usually allow the fuel pump to run for just a few seconds. Again, manufacturers generally designed these systems so that in the event of a collision and a ruptured fuel line, the fuel pump will not continuously pour fuel onto the exterior of a hot engine, exhaust, etc. The fuel delivery rate for those pumps is somewhere in the neighborhood of a liter or two per minute. I believe that many cars' safety systems have been defeated by previous owners, but I'm just not sure I'd like to drive one.

2.7RACER · 08:56 PM

Lots of guessing and just bad advice here.
First, when the ignition key is turned on the fuel pump is not supposed to run. The reason it doesn't run is there is a switch in the air metering circuit that is closed when the air sensor plate is in the down position, (no air flow).
For those that doubt what I say, refer to a '76 schematic. It's all there.
If the fuel pump is running as soon as the ignition switch is in the "on" position, one of two things is happening. Either the air flow sensing plate is not returning fully to the down position opening the air flow switch. Or the wires to the air flow switch have been disconnected.
The air flow switch when down, is closed energizing the fuel pump relay.
When the fuel pump relay is energized, power from the ignition switch is blocked except when cranking the starter.
Once the engine begins to pull air the air sensor plate lifts, opening the air switch, and providing power to the fuel pump through the now de-energized fuel pump relay.
Yes, when properly running the fuel pump relay is de-energized. Backward to what you would first expect.
It is possible to turn the mixture screw far enough clockwise to trip the air flow switch, which de-energizes the fuel pump relay and runs the fuel pump with the ignition switch on.
The wires to the air switch is in the back of the CIS air circuit. You may need a mirror to see if the wires are present and connected. Just above the sensor plate in back.
Again the fuel pump is not supposed to run unless the engine is cranking or running.
If the engine stalls, the air flow switch closes and energizes the fuel pump relay blocking power to the fuel pump.
If this all sounds like jibberish, have someone familar with this engine look at it.
Good luck,

azasadny · azasadny's Garage

Thanks for the explaination, but what would have caused fuel to pour out of the exhaust on my car with Webers when the fuel pump failed?

Ken911 · 11:01 PM

Quote:

Originally Posted by azasadny

Thanks for the explaination, but what would have caused fuel to pour out of the exhaust on my car with Webers when the fuel pump failed?

the float getting stuck by the odd angle and who knows how your wiring is set up. might have started the fuel pump

dark star
As far as what 2.7 racer said its the green plug on the back next to the fuel head a lot of times it gets unplugged during trouble shooting and not plugged back in. If it's not plugged in it will do exatly what he said will happen. my turbo is set up the same way. if you look in the post titled a little fuel line help there is a good picture of the plug mentioned

vash · vash's Garage

2.7 is probably right about the 76.

but my money is still on the plunger being stuck in the up position. i had the exact same thing happen to my car.