83 911SC – Fuel Pump Check Valve Question

[logdudeslc]

I’m new here and this looks like a great Porsche site with a ton of information.

I just pulled my 83 911 SC out of the shop after (7 months) and a top end and head rebuild. It ran rough for about 4 or 5 blocks and died and will not start or fire.

During the rebuild my mechanic informed me that the electric fuel pump was bad and not running. I ordered a new (rebuilt) electric fuel pump for him to install. My existing fuel pump has a brass check valve on the discharge. We talked about checking the existing check valve and swapping it over to the new fuel pump during installation. When he returned my fuel pump core the brass check valve was still on it and he informed me that the new fuel pump has a check valve built into it. They both looked identical to me with the exception of the brass check valve on the existing pump.

Could installing the pump without the check valve be the problem?

Thanks
Dave

[Flat6pac]

The 80 to 83Bosch has an external check valve, the 78/79 has internal. The job of the check valve is to keep pressure in the fuel lines for start up.
Lift the airflow sensor on the engine with the key in the run position and it will turn on the pump and pressurize the system and add fuel to the cylinder to start.
If the check valve is the problem, it should start,
Bruce

[Joe Bob]

The FP check valve is there as an assist/help for hot starts. Cold starts have other systems to assist in fuel pressure. CIS is all about consistent fuel pressure.

[boyt911sc]

Originally Posted by logdudeslc

I’m new here and this looks like a great Porsche site with a ton of information.

I just pulled my 83 911 SC out of the shop after (7 months) and a top end and head rebuild. It ran rough for about 4 or 5 blocks and died and will not start or fire.

During the rebuild my mechanic informed me that the electric fuel pump was bad and not running. I ordered a new (rebuilt) electric fuel pump for him to install. My existing fuel pump has a brass check valve on the discharge. We talked about checking the existing check valve and swapping it over to the new fuel pump during installation. When he returned my fuel pump core the brass check valve was still on it and he informed me that the new fuel pump has a check valve built into it. They both looked identical to me with the exception of the brass check valve on the existing pump.

Could installing the pump without the check valve be the problem?

Thanks
Dave

Dave,

The first problem you'll have with a FP without a check valve or a defective check valve would be the residual pressure. You'll encounter starting problem both cold and warm. But you could start the engine if you are familiar how to manually manipulate the FP's operation. The frequency valve (FV) might not be working but that should not prevent the engine from starting.

Ask the mechanic to check for the fuel pressures (control, system, and residual). That will give you an idea which is the culprit for your starting problem. Do you know the part # of the FP installed? Is the car still in the shop? He should not have allowed you to take the car out in that condition.

If you are inclined to do the troubleshooting yourself, it is easy and simple. Just ask and you'll get plenty of help from these guys. Keep us posted.

Tony

[Gunter]

Originally Posted by logdudeslc

During the rebuild my mechanic informed me that the electric fuel pump was bad and not running. I ordered a new (rebuilt) electric fuel pump for him to install. My existing fuel pump has a brass check valve on the discharge. We talked about checking the existing check valve and swapping it over to the new fuel pump during installation. When he returned my fuel pump core the brass check valve was still on it and he informed me that the new fuel pump has a check valve built into it. They both looked identical to me with the exception of the brass check valve on the existing pump.

Could installing the pump without the check valve be the problem?

Thanks
Dave

Quote: I ordered a new (rebuilt) electric fuel pump for him to install.

What did you order? Bosch or Pierburg? Or? Either one, what is the number on it?
If you post the type number, I may be able to tell you if it has an internal check valve.

Any SC '78 - '83 pump can have external or internal check valves and both types work fine. The important issue is working pressure. For the Bosch K-Jetronic, 5 bar or about 70 PSI system pressure is needed and that is controlled by the WUR hot and cold.

I don't like rebuilt pumps but the Mechanic can check if it has the right pressures.

What brand "rebuilt" did you order? What type-number is on it?

[logdudeslc]

The car is back in the shop. The fuel pump is from Performance Parts No. P906817 – Pump 911 80-94

He is convinced that it has an internal check valve.

He is having trouble developing and maintaining 3.2 bars of pressure – what ever that equates too in PSI. I’ll have to check into it.

Thanks for the help.

[logdudeslc]

It looks like I need 35 to 50 PSI. He is also seeing a lot of AIR in the fuel discharge.

[Gunter]

3.2 bar is not enough for the K-Jetronic.

I recommend you get the Bentley SC Repair manual which is about the best for any DIY.

Page 240-12 tells us that the system specs are 4.5 - 5.2 bar (65-75 PSI) In other words, you need the right pump for your SC.

I would be very skeptical of a generic rebuilt pump without getting the specs for it before buying.
What brand is it? Rebuilt from what?
A good Bosch or Pierburg pump with the right specs for an SC will last easily 20 plus years.
Yes, they cost more than questionable "rebuilt" units.

Without the right pump, the Bosch CIS cannot work and is more costly in the end requiring more trips to the Mechanic and a possible hassle returning the pump.

Bosch 0580254984 or Pierburg 7.21659.70.0 is right for your SC. Both have internal check valves and are compatible with the banjo fitting on the line from the tank.

[Gunter]

Originally Posted by logdudeslc

It looks like I need 35 to 50 PSI. He is also seeing a lot of AIR in the fuel discharge.

Where do you get that info from?

Bosch K-Jetronic needs a pump with check valve delivering 65-75 PSI.
In other words: High Pressure.

Seeing air in what fuel discharge? From where?

Normal procedure to bleed air out of the CIS is: Ignition ON, (Not start!), remove air filter/cover, carefully lift the air sensor plate for 3 seconds, Injectors will squeal and bleed out air.

A Porsche Mechanic would know that.

Get the Bentley SC Repair Manual.

[logdudeslc]

We are replacing the new fuel pump which is bad and you are correct that the pressure should be 65 to 75 psi.

He is also telling me that I have a bad Frequency Valve. I'm in the process of searching the function of the valve and the cost involved.

Thanks for all the help

[Gunter]

Get the Bentley!

It tells you exactly what the duty-cycle is for your year and how to check it.

Before changing the FV, you have to ensure power to the relay Lambda brain (ECU) under the passenger seat as follows:
Fuse 18 5A supplies power to the relay. If you have no interior light or glove box light, the fuse may be blown. If the fuse is good, start the engine, let it idle a little, then pull the relay off the ECU. The idle should get noticeably rough. Replace the relay, idle should get smooth again. If nothing happens when you pull the relay, the relay may be bad or the pins may be corroded or it doesn't get power due to wiring or fuse-issues.

I suspect that your Mechanic may not be too familiar with the Bosch CIS and may not know how to check the duty-cycle for the FV?? The FV seldom fails.

If he learns on your vehicle how to service the K-Jetronic, it may become expensive. Are you confident about the shop?

Get the Bentley and learn about your car; you'll love it.

[boyt911sc]

Originally Posted by logdudeslc

We are replacing the new fuel pump which is bad and you are correct that the pressure should be 65 to 75 psi.

He is also telling me that I have a bad Frequency Valve. I'm in the process of searching the function of the valve and the cost involved.

Thanks for all the help

logdude,

The FV (frequency valve) could be checked easily by running the FP (fuel pump). First, you need a FP that could deliver a fuel pressure between 95 - 100 psi (min.). The system pressure of 70-75 psi is controlled by the FD (fuel distributor) and the control pressure is controlled by the WUR (warm-up regulator).

BTW, if you need some used CIS components, send me a PM. In the course of my experiments, I have accumulated boxes of assorted CIS components stored in my basement. Is your mechanic familiar with CIS? The reason I asked is because I personally know many young talented and good mechanics that are not familiar with this old (antiquated) CIS.

Tony

[T77911S]

that was my thought too, does he know porsche's and CIS.

what gunter said in his last post. there are other things to check before replacing the FV.

is it out of gas?

was it running when he got it?

[KNS]

I've got a fuel pump question for Gunter (hope I'm not hijacking this thread). I just replaced the fuel pump on my '80 SC. The new fuel pump looked like the earlier 77-79 fuel pump (with the internal check valve) and the elongated neck. I don't recall the part number on the actual pump itself but the box for the new one says: Bosch 69513. On the Bosch website this part number is for 1980-83 911SC.

I unscrewed the check valve off the old pump (with the shorter neck) and screwed it onto the new 69513 fuel pump with the longer neck. Does my new fuel pump now have two check valves (internal and external)? Do the new fuel pumps come this way with a longer neck whether the check valve is internal or not?
I thought I would contact Bosch and ask them about the physical differences.

The car starts and runs fine.

Sorry if I'm hijacking, this may be related to the original poster's problem..?

[KNS]

Well, to answer my own question (and perhaps help the original poster), I talked to a Bosch rep. I basically have the correct fuel pump for my car (69513). According to Bosch, the 1980-83 replacement pump has an internal check valve - contrary to what Pelican's catalog shows. I screwed my old check valve onto the new pump and at this moment have two check valves... fun stuff.

He said if the car is running correctly there is nothing really wrong with the current set up but suggested I remove the extra check valve and get the correct fitting for that pump. My only concern is that I might have some restriction in flow but he didn't seem to think so. I wish the box had some info regarding the replacement pump having a check valve built in.

I gave the old pump part number to the rep and it turns out that that pump is for certain Audi and VW applications but not my Porsche. It was replaced sometime long before I owned the car by a shop here in Phoenix.

logdudeslc, it appears as though the pump with an internal check valve has an elongated neck and a short neck for the pump with an external, brass check valve. Hope this helps...