1980 911 SC Possible Check Valve Issue

[landyman96]

Hello fellow air-cooled Porsche enthusiasts. I recently acquired a 1980 911 SC targa US spec. I know this issue has been discussed to death but I'd like to get a second opinion from more knowledgeable people than myself. The vehicle has problems starting both in hot and cold. Runs beautifully after the engine has been started but the fuel pump is loud and seems to run continuously regardless. The previous owner installed a new filter, new fuel pump and new fuel accumulator. I checked the fuel pressure and its a little over 5 bar with the fuel pump running (cold engine) but it goes back down to 0 almost immediately after I shut the fuel pump off. This car has a bosch 0580 254 984 fuel pump which i believe has an integrated check valve. From the fact that the fuel pressure drops back to 0 immediately after i shut the fuel pump off leads me to believe that the integrated check valve is failing on that particular fuel pump. Does that sound like a reasonable deduction? Many thanks.

[macssc911]

I believe you also have a fuel return line to fuel tank on the CIS system so I would think fuel pressure going to 0 would be normal. There is plenty of CIS information on site, just do a search. Sounds like a cold start valve issue
Good Luck

[walt]

If you have your test gauges hooked up correctly the pressure shouldn't drop to zero. Sounds like the check valve is bad, the test calls for residual pressure of 16 psi after 30 min. Are you using a set of CIS fuel pressure test gauges?

[landyman96]

Hi, thank you for your response. I used a regular fuel pressure test gauge, not CIS specific. It has no leaks so I wouldn't think it would give an inaccurate reading. The previous owner also made the fuel pump run when the ignition is in the on position in an attempt to maintain fuel pressure I presume. I hooked up my pressure gauge between the WUR and the injection pump because originally I thought my problem was the WUR given that the fuel pump is brand new along with the fuel accumulator. The car is well taken care of and low miles. Only problem is starting it. It starts on the first try when i lift the cold start valve. P.S. I will fix the botched wiring to the fuel pump once i get it to start correctly, right now it saves me the trouble of bridging the relay to get the fuel pump to run.

[boyt911sc]

Originally Posted by landyman96

Hello fellow air-cooled Porsche enthusiasts. I recently acquired a 1980 911 SC targa US spec. I know this issue has been discussed to death but I'd like to get a second opinion from more knowledgeable people than myself. The vehicle has problems starting both in hot and cold. Runs beautifully after the engine has been started but the fuel pump is loud and seems to run continuously regardless. The previous owner installed a new filter, new fuel pump and new fuel accumulator. I checked the fuel pressure and its a little over 5 bar with the fuel pump running (cold engine) but it goes back down to 0 almost immediately after I shut the fuel pump off. This car has a bosch 0580 254 984 fuel pump which i believe has an integrated check valve. From the fact that the fuel pressure drops back to 0 immediately after i shut the fuel pump off leads me to believe that the integrated check valve is failing on that particular fuel pump. Does that sound like a reasonable deduction? Many thanks.

landyman,

Your starting problem is probably caused by your poor residual pressure. Test and verify. You need a pressure gauge with a shut-off valve to do the pressure test. The three (3) most common culprits for residual pressure for are:

• Defective fuel accumulator.
• Defective fuel pump check valve.
• Defective primary pressure regulator.

To test the integrity of the FP check valve, install the pressure gauge kit between the FP and fuel accumulator. If the FP check valve passed the test, move the gauge after the fuel accumulator. To test the primary pressure valve, install the gauge between WUR and FD. A good residual pressure is 1 bar (14.5 psi.) minimum after 20 mins.

Tony

[landyman96]

Hi Tony, thank you for your very elaborate response. The primary pressure regulator part number for a 1980 911 SC 3.0 is 928 110 920 01 right? As of right now, residual fuel pressure is not held when i install the gauge between the WUR and the the Fuel Distributor. And like I mentioned before the fuel pump and accumulator are brand new (the car came with the old ones and the ones installed are shiny) so I am inclined to think that my problem is not the new fuel pump with the integrated check valve or the new accumulator but the primary pressure regulator. Many thanks.

[boyt911sc]

Originally Posted by landyman96

Hi Tony, thank you for your very elaborate response. The primary pressure regulator part number for a 1980 911 SC 3.0 is 928 110 920 01 right? As of right now, residual fuel pressure is not held when i install the gauge between the WUR and the the Fuel Distributor. And like I mentioned before the fuel pump and accumulator are brand new (the car came with the old ones and the ones installed are shiny) so I am inclined to think that my problem is not the new fuel pump with the integrated check valve or the new accumulator but the primary pressure regulator. Many thanks.

Landyman,

Right now, the residual pressure is not holding and you know that. The question is what is causing it to fail? There are 3 most like culprits causing the residual pressure but we don’t know which one. Without testing and verifying these CIS components we are just guessing. If you are lucky, your guess could be correct. But you can not rely on luck all the time. Some people like to replace parts after parts to correct the problem and that’s an expensive method.

I don’t replace part/s unless it is tested and confirmed defective. Unless you do not have a way or knowledge to test it, replacing it would be a prudent course of action. But these 3 common culprits could be bench tested by any DIYer like you or me. It takes only a few seconds to test one using common tools.

Tony

[landyman96]

Hi, I completely agree with you. The previous owner was replacing parts hoping for the best which obviously didn't work which made him sell it to me. This is my first Porsche so I am not familiar with them(I've owned it a week), I mostly work on land rovers. Can you kindly explain to me how to bench test the primary pressure regulator? I have a strong feeling that's the culprit.

[boyt911sc]

Originally Posted by landyman96

Hi, I completely agree with you. The previous owner was replacing parts hoping for the best which obviously didn't work which made him sell it to me. This is my first Porsche so I am not familiar with them(I've owned it a week), I mostly work on land rovers. Can you kindly explain to me how to bench test the primary pressure regulator? I have a strong feeling that's the culprit.

Landyman,

You have to test them in sequence, you can not test the PPV (primary pressure valve) without confirming that the FP check valve and FA (fuel accumulator) are both good.

If you install the pressure gauge after the fuel accumulator, you could test the following:

• Fuel pump
• Fuel pump check valve
• Fuel Accumulator

If the residual pressure test failed at this point, the culprit could be either the fuel pump check valve or the fuel accumulator or both. However, if it passed the residual pressure test, we could move on to the next.

Install the pressure gauge between WUR and FD with shut-off valve closer to WUR to test the PPV. Keep us posted.

Tony

[targa80]

I agree with Tony’s suggestions. I had a hot start problem when I first purchased my car in 1991. You really need a cis fuel gauge kit with many adapters to properly test the system and fault isolate to the failed part. Since you are having a cold and hot start problem. I would ask is your cold start valve functioning. In my case the car would start cold because the cold start valve gives A straight shot of fuel bypassing the fuel pressure regulator. So not having operating Residual fuel pressure didn’t prevent the car from starting. Once the car was shut off and was still hot the operating residual fuel pressure would drop off quickly and without the cold start valve it would just crank and crank forever. I found that I isolated the accumulator and fuel pump and pressure dropped off. Next, I isolated just the fuel pump and pressure still dropped off. I added an external check valve And retested, the residual fuel pressure stayed Solid, All fixed. The previous owner threw parts at the problem and did not fix the issue. Study the cis primer threads on the forum and test methodically. Your only cost should be the cis gauge kit and the replacement of the actual failed part.

Good luck,

Pat

[landyman96]

Hi Pat, so the way I start the vehicle is by pressing the cold start valve so my engine gets fuel. The last thing I need is to destroy my starter. Does that mean that my cold start valve is working? It moves freely and i can hear it squirt fuel when I press it. Trouble is that my car is stored an hour away from where I live so i can only work on it on the weekends. I should of bought a CIS specific fuel gauge but i was stupid and cheap and bought a generic one. Ill figure out how to make connectors as i have my old fuel pump and accumulator sitting here next to me and the fittings aren't Porsche specific. I will let you guys know what happens as I am a lot less certain that my check valve is bad since my fuel pump is new.

[targa80]

Landyman96,

I think you and I are not talking about the same thing with respect to the cold start valve. The Cold Start Valve looks like this.



The cold start valve is located on the back of the engine down low on the throttle body and cannot be pushed by hand. It injects gasoline directly into the throttle body when the engine is cold and the engine should start immediately if all else is normal.



The below diagram shows each of the components in the fuel system. As you can see that the cold start valve gets fuel directly from the accumulator and the fuel pump and does not require that the fuel distributor residual fuel pressure be at optimum to start the car.

[boyt911sc]

Originally Posted by landyman96

Hi Pat, so the way I start the vehicle is by pressing the cold start valve so my engine gets fuel. The last thing I need is to destroy my starter. Does that mean that my cold start valve is working? It moves freely and i can hear it squirt fuel when I press it. Trouble is that my car is stored an hour away from where I live so i can only work on it on the weekends. I should of bought a CIS specific fuel gauge but i was stupid and cheap and bought a generic one. Ill figure out how to make connectors as i have my old fuel pump and accumulator sitting here next to me and the fittings aren't Porsche specific. I will let you guys know what happens as I am a lot less certain that my check valve is bad since my fuel pump is new.

Landyman,

Familiarize your self with the different CIS components for your engine. You can not press your CSV because it is imbedded at the rear side of your engine. And to add more confusion, the rear side of your engine is actually the front. Were you depressing the gas pedal prior to starting? You don’t need to touch the accelerator to start a fuel injection engine. You do that for carbs.

Do yourself a favor by getting a good set of CIS fuel pressure gauge kit and handheld vacuum gauge too. Why? These 2 tools alone would save you a lot of money if you do your troubleshooting work.

Tony

[landyman96]

I meant sensor plate.

[landyman96]

Hi Tony, last dumb question. On one of your previous replied you mentioned a primary pressure regulator. Did you mean the warm up regulator by that? I am not familiar with a primary pressure regulator.

[boyt911sc]

Originally Posted by landyman96

Hi Tony, last dumb question. On one of your previous replied you mentioned a primary pressure regulator. Did you mean the warm up regulator by that? I am not familiar with a primary pressure regulator.

Landyman,

We all started some where regarding CIS. As long as you are willing to learn, it will be easy. There is no such thing as dumb or stupid question/s, only dumb or stupid answer/s. The PPV or PPR is located inside your FD (fuel Distributor). Your late SC has a PPV or PPR with 2 o-rings. One for the system pressure and the other one for the return line. If any of these o-rings are compromised, damaged or broken, the system or residual pressure would be affected. This is the least common failure but it does occur.

A pressure gauge is a mandatory tool for fuel injection troubleshooting. Without one, you are wasting a lot of your precious time and will lead you to guess-work. You might get away with it if you are lucky. But you can not rely on luck all the time.

Tony

[Walt Fricke]

Landy - I wondered if you meant the sensor plate, because otherwise it makes no sense.

Lack of residual pressure can cause hot start problems - you can get vapor lock in fuel lines on the hot engine. Since higher pressure means a higher boiling point, keeping pressure in the system until it cools is helpful.

One classic short term fix for CIS vapor lock is to raise, manually, the sensor plate. Opens up the fuel flow and gets the vapor out of the system.

However, for a cold start (cold enough that the cold start system will operate) you don't need residual pressure. So if you have both hot and cold start problems you probably have more than one parameter out of whack.

Methodical testing is sure the way to go. I'd never thought about sequential testing of components - probably because the Bosch manual does not mention this? Always something more to learn.

[landyman96]

Hello, air-cooled Porsche enthusiasts. After getting my fittings to work on my fuel pressure gauge i installed it between the fuel pump and accumulator. Turned on the pump, went to a little over 70 psi. Turned the fuel pump off, went back to 0 psi immediately. I think my pressure problem is a defective check valve correct?

[targa80]

if you connected the gauge to the fuel pump line you removed from the accumulator and the pressure dropped the n YES you need to add an external check valve. I have a picture of the unit I bought many years ago but I am having a problem uploading it. I will try again later tonight. I remember when I bought the check valve I also had to buy three (3) crush washers and an end cap. aparently back then they sold the items seperately and not as a complete kit.

[boyt911sc]

Originally Posted by landyman96

Hello, air-cooled Porsche enthusiasts. After getting my fittings to work on my fuel pressure gauge i installed it between the fuel pump and accumulator. Turned on the pump, went to a little over 70 psi. Turned the fuel pump off, went back to 0 psi immediately. I think my pressure problem is a defective check valve correct?

Landyman,

You need to have the valve closed to isolate the fuel pump from the system. Repeat the test with the valve closed and run the FP for 2~3 sec. and immediately shut-off the FP.
NOTE:
The gauge should be located between the FP and shut-off valve.

Tony