Starting problem

[Christien]

Finally warm enough on a Saturday today to get out to the garage and try to diagnose a fuel leak/starting problem with the car. We noticed fuel pooling where the exhaust meets the muffler, so we pulled the muffler off.

When trying to start the car, there's gas coming out the exhaust. I think we might have had a backfire out of one carb, but can't be sure. I was in the car turning the key and my friend was under the engine to watch where the gas was coming out of, but there was a loud noise (although without the muffler on everything's noisy) and a slight burning smell. One thought was maybe a problem with the timing chain.

A little background - car is parked for the winter, hasn't been started in a couple months. Since it last started I've changed the fuel pump, but nothing else. I'm sure the fuel pump lines are hooked up correctly (each line was marked, and triple-checked, and gas is flowing where it should), fuel pump works fine, seems to be plenty of pressure. I've checked all the wires from the distributor to the spark plugs to make sure they're tight. What's really puzzling is that nothing's changed since the last time the car started fine, except the fuel pump.

At first I thought maybe a piece of debris had clogged a fuel line, preventing the gas from entering the carbs, but we pulled the fuel line off the left bank and sure enough lots of gas was coming out there. So it seems the gas is going through the carbs and entering the exhaust without igniting. This is what led to the guess of a timing issue.

Any thoughts?

Thanks,
Chris

[Christien]

Forgot to mention, 72 911T 2.4 with Zenith carbs.

[Jesset100]

Someone might have knocked off the coil wire. you are getting fuel, air, no spark. Check for spark.

[Early_S_Man]

Chris,

If the gas is running out the heat exchangers, then it isn't a case of normal fuel vapor + mist being supplied to the cylinders, but rather excess gas in liquid form.

Suspect stuck float valves in one or both carbs!

Before trying to start again, I suggest pulling an exhaust valve cover, and be prepared to catch a couple of quarts of oil spill ... check for gas-contaminated or diluted oil. If that is what you find ... addressing the carbs leakage and a complete oil change are needed before proceeding with trying to start again!

[Grady Clay]

Chris,

Double check for the correct fuel pump. Is it possibly you installed a MFI fuel pump? That high fuel pressure can overwhelm the needle & seat in the carb and pour gas into the engine. Have you measured the actual fuel pressure?

I agree with Warren, check for gasoline in the oil.

Best,
Grady

[Grady Clay]

To add some more:
Why did you replace the fuel pump? What symptoms?
How is your fuel system plumbed compared to the original MFI?
Is there a fuel pressure regulator?

The Bosch rotary pump for carbs is 911.608.107.00 Bosch # 058096 0009. It is specked at 4.3 PSI with it’s internal pressure relief valve.

The Bosch rotary pump for MFI is 901.608.105.00 Bosch # 058097 0001. If this pump is used in place of the carb version with nothing else done, it will operate on its internal safety relief valve and operate at about 28 PSI – way too high for a carburetor.

Best,
Grady

EDIT for senior moment transposing numbers.

[Christien]

Hey, thanks for all the responses. Finally, something's starting to make sense here.

Ok, start with the simplest first right? Glad I did. Grady, thanks for posting the part numbers for the pumps. That's the problem, I'm sure. For the life of me, I don't know why I didn't check part numbers before - I was assured several times by the guy I bought the pump from (buys wrecks and dismantles them for parts, good reputation), that the pump he sold me came off a 71 with Zenith carbs, so I took his word for it.

I checked the part numbers this morning, and sure enough, it's an MFI pump, Bosch # 580970001. My old pump has part # 0580960009. (Grady, did you by any chance mix up the 6 and 9? You said part # 058069, mine shows 058096).

Anyway, it makes perfect sense that it's simply shooting the gas through the carbs with way too much pressure for any combustion to take place in the cylinders. It didn't make any sense to me at first that there should be any problem with spark, coil, etc. because 1. it started just fine before the pump was swapped and 2. even with the gas pouring out we got a backfire, and it sounded like it was trying to fire - a couple of chugs, etc.

Two questions now:
1. If I replace the pump with the regular one, am I safe to just reattach the muffler and start it up? There shouldn't be any more gas left in the exhaust or heat exchangers - we pulled the muffler off and let everything dry up.
2. There is currently no fuel pressure regulator installed. Can I install one to work with the MFI pump, or should I find and install the right 4.3 psi carb pump? I've pretty much already guessed the answer to this one, but I figure if a regulator is adaptable, it might save some money, as the pumps are mighty expensive, and I'm not sure the guy I bought the pump from will give me a refund, as it was back in January I bought the pump.

Anyway, thanks again for everyone's help - you guys rock.

Chris

[Britwrench]

Pierburg makes a pump that works real well with all early cars and runs the correct pressure. I'm sure if you call Pelican they could supply it.
The part number from our supplier is 912FPMP. I'm sorry I don't have the Pierburg number.

[Christien]

Hey Bitwrench, thanks for that. I'm going to try to exchange it from the place I bought it from, if he's got any in stock. If not, I'll try the Pierburg.

Chris

[Grady Clay]

Aah … Senior Moment transposing numbers.

There is a good solution available.

Your ’72 911 has the return fuel line back to the fuel tank. PMO sells Pressure Control Unit that circulates the fuel back to the tank, controls the pressure, has a gauge, and works with the MFI electric pump.
FAX them at 310-394-6313.

ABSOLUTELY change oil and filter – perhaps again after running.

Best,
Grady

[Christien]

Thanks, Grady. Another option if I can't exchange the pump.

When you say change the oil and filter, do mean before trying to start it again? Can you do that with the oil cold? Or once I get the right pump (or regulator) on, should I start it, warm it up, then change the oil? Ironically, this whole issue arose when I tried to start it to change the oil to put it away for the winter. The fresh oil, filter, gaskets, etc. are sitting, waiting patiently in the corner of the garage... :-)

Chris

[Grady Clay]

Chris,

Yes, change the oil before running. It just takes longer to drain out when cold. There is the possibility of a LOT of gas in the oil. Although the gas should have found its way into the crankcase, it is a good idea to turn the engine (2 turns) with a wrench to insure a cylinder isn’t hydrauliced. When you crank for oil pressure and first start it you want the best lubrication possible. All the parts have to get friendly with each other again.

After several careful drives, I would change it again. Remember you are breaking in an engine that has been resting quietly and anything you can do to make it easy on the machinery is worth it for the long life.

I would keep the MFI electric pump and use the PMO regulator.
Please read this thread:
Weber fuel recirculation- Questions
If you go back to the circulating fuel system as similar to the MFI, you will dramatically reduce the propensity for the dreaded vapor lock.

You might use this as an opportunity to replace the original hoses with new OEM and clean the fuel tank. All worthwhile 30+ year maintenance.

Best,
Grady

[Christien]

Grady, thanks so much. I really appreciate the help. I will change the oil, add the regulator and see how things go.

Just out of curiousity, why do you recommend going with the MFI pump and regulator, rather than the pump intended to work with the carbs? If the answer is in the thread link you included, then just ignore my question - I haven't read it yet (saving it for the morning, it's getting late here).

Anyway, thanks again to everyone - really appreciate the help. While I'm frustrated at my own stupidity for not checking the part #'s in the first place, I'm relieved that it looks like the solution is not only simple, but inexpensive.

Chris

[Christien]

Ok, got the oil changed this weekend, and for my first time doing it, I was really surprised at how easy it was, and how little spill occurred. It'll need to be topped up once the car's been run and the oil has circulated, but so far so good.

So now I need to get a fuel pressure regulator and install it. Are regulators generic? Where can I buy a PMO regulator? The one Pelican sells (PEL-WP31800-063) doesn't indicate the manufacturer. A google search for PMO regulators only turns up carb info, and the closest I could find to an "official" page is pmocarbs.com which is of course all about carbs. I'd rather buy locally, not only because it's way faster, but also because of nasty border surprises.

Thanks,
Chris

[Christien]

Any ideas? I tried to pick up a regulator at Canadian Tire but the guy working there was useless (typically) - he was only able to go by what was in the computer, and obviously they don't list a 72 Porsche in there

thanks.