1975 911 – Will Not Start, Fuel Pump Not Receiving Power

[Louisieg55]

Figured I’d start a new thread, since my original post title doesn’t really reflect the issue anymore.
Car: 1975 Porsche 911 Carrera

The car was running great, but now it won’t start.
(Original thread: https://forums.pelicanparts.com/porsche-911-technical-forum/1182315-starts-1-2-seconds-after-lowering-had-adjust-fuel-line.html)

At first, I thought it had to do with undoing a gas line — maybe air in the system, a safety float or safety feature I was not aware of, something simple I was overlooking. But after digging deeper, I don’t think the issues are related. Just bad timing?

Current problem:
Car will not start. I believe I’ve narrowed it down to fuel, but not sure if it’s electrical or mechanical.

Symptoms / checks so far:

When the key is in “on” or while cranking, the fuel pump is not turning on. It will burp for 1 second and 2-3 second (jump 2-3k) if I let the starter kind of push the engine if that makes since. (Not the best I know for the engine)

From what I’ve researched, a ’75 doesn’t have a dedicated fuel pump relay or fuse — power comes from the keyed ignition? Can any one confirm this?

If bypassed this by applying 12V directly to the fuel pump. It turns on and hums (so I assume it’s pumping), but the car still won’t start. I will check the pressure some time this weekend, see what I get but I don't think the fuel pump went out + have some type of electrical issue? As I cant get any 12v to the pump.

I tried jumping every relay just to confirm (30/87a, etc.) but got nothing.

Even with direct 12V to the pump while cranking, still will not start.

So now I’m wondering if I actually have two separate issues:

Fuel pump not coming on with the ignition/cranking.

Even when I force the pump to run, the engine still doesn’t fire.

Looking for ideas on what to troubleshoot next — both electrical and mechanical possibilities.

Thanks
L

[PeteKz]

Do you know for certain whether you have spark or fuel? I assume the starter cranks the engine fine--correct?

Simple stuff first: Spray starting fluid in the intake, then try to start it. If it fires briefly, then you are lacking fuel. If it doesn't fire at all, then check for spark.

[fanaudical]

I have a '75 with relatively stock wiring. The fuel pump runs whenever the key is in the ON position. Has the wiring in your car been modified at all?

The CDI box should also make a whining noise indicating it has power (assuming the wiring has not been modified).

[Louisieg55]

Update:

Spark: Confirmed. I tested all six wires with an inline spark tester and I have good spark.

Fuel pump power: With ignition on/ACC, I’m not getting power to the fuel pump. This seems odd since it should basically be a direct feed.

I tried running a fused/switch 12V directly to the fuel pump to bypass the original wiring. The pump runs, but the car still will not start.

Fuel pressure: Hoping to test today. I know I’m getting some fuel (cracking the line at the WUR gave fuel), but I don’t yet know how much or if it’s correct. Planning to follow this video for pressure testing: https://www.youtube.com/watch?v=Q7qjmfJHoLI

Wiring check: I found this thread helpful: https://forums.pelicanparts.com/porsche-911-technical-forum/750815-fuel-pump-power-qs-75-s.html. Checked the wires up front and they look okay, but I’ll dig deeper and double-check.

Background:
This car has been in the family since new. My father-in-law bought it and gave it to my wife and me as a wedding present 10+ years ago (not bad, I know ��). It had some 70s aftermarket stuff—radio, alarm, etc. My wife remembers that when they were kids the car would sometimes stumble if the A/C and radio were both on, which makes me think it was a voltage issue. But since then, I’ve removed the A/C and alarm (not functioning anyway), and installed a new radio with direct fused 12V.

The car has been running great for the past few months with all this setup, so I don’t think those old issues are related to the current no-start problem.

Today. I am going to try and complete the pressure test with that video and see what feedback I get.

[Louisieg55]

Updated/Question.

With the ign. switch turned on the CDI should hum correct? I pulled the (3) pin plug and my test light all (3) did not light up with the ign. turn on. There should be power going to this plug correct? The middle red wire I assume?

Ign. switch seems to control a lot of this?

I am only getting power to the CDI when I am cranking. Should it be there on the on position and cranking or just cranking?

I confirm I had spark again, 1 about every second +/- with the test light. The CDI is not humming at all thought with ign on or when cranking (well cant tell when cranking)

[zuch]

Sounds like a bad ignition switch.

[PeteKz]

I know you checked for spark with a spark tester, but shoot starting spray in the intake anyway an make sure it fires. It should start and run with the pump hot wired to 12v. That's why you should also do the starting spray test.


If it fires, Then you're back to finding out why no power to the pump.

[Louisieg55]

Originally Posted by PeteKz

I know you checked for spark with a spark tester, but shoot starting spray in the intake anyway an make sure it fires. It should start and run with the pump hot wired to 12v. That's why you should also do the starting spray test.


If it fires, Then you're back to finding out why no power to the pump.

Yea, I just tried that with my wife.

I can get it going for a second or two but only with the starter engaged (basically pushing the engine) as soon as I let it go. It wont stay running once I let the key go back to the one position (not starting position) but even then its not really running.

[Louisieg55]

Update:

I finally checked the fuel pressure following Joe Engineer’s video.

System pressure is around 4.5–4.6 bar (about 65 psi).

When I open the valve, cold control pressure only drops about 0.5 bar to around 4.0. On Joe’s video, his dropped down to 1 bar and then built back up to 4.5 after a few minutes.

Fuel pump seems to be working, but I’m not sure why the pressure isn’t dropping when I open the valve for cold control pressure.

I also ordered a new ignition switch, which should be here tomorrow. I might toss that in to see if it makes a difference with the fuel pump not kicking on.

[PeteKz]

Your SP is within specification (barely). Resetting the SP higher won't fix your no-start problem, but put that on your list of things to do soon.

When cold, the cold control pressure (CCP) should be much lower. Depending on the part number on your WUR, somewhere around 1-2 bar. When warm (WCP), it should rise to about 3-3.5 bar (without vacuum applied).

Find out why your CCP is so high, as that is the probable cause of your failure to start and keep running. Also check your residual pressure (RP). If you put the pressure gauge/valve tester between the FD and WUR properly, that result indicates a faulty WUR. I would then remove, inspect the inlet screen of the WUR, and make sure the return line from the WUR to the fuel return lines is clear, and that the return lines to the tank are clear (blow through them with the gas cap off).

[kach22i]

Originally Posted by Louisieg55

Update:...........

I also ordered a new ignition switch, which should be here tomorrow. I might toss that in to see if it makes a difference with the fuel pump not kicking on.

That was going to be my suggestion.

[Louisieg55]

Update: I ordered a rebuild kit for the WUR and a new fuel pump. When I tested again this morning, system pressure dropped to 4 bar ( dropping about 0.5 bar on control pressure), which makes me think the pump is on its way out.

Got the ignition switch in — what a pain to remove and reinstall. I actually put the URO switch in wrong at first since it has a cross instead of just a slot in the back, but got it flipped and seated correctly.

At this point, the fuel pump still doesn’t come on with the key in the “On” position. Once my wife gets home, I’ll check again while turning the key to see if it engages when starting?

For now, I’m not sure if the 12V feed to the fuel pump is working correctly or not. I plan to pull the WUR later tonight to see if there’s anything obvious. If not, I’ll just wait for the rebuild kit and new pump to arrive later this week.

[boyt911sc]

Louis,

Based from your information, you have two (2) major problems namely:

• Your FP should be running with the ignition switch turned to ON/RUN position (not START).
• The cold control fuel pressure is too high at 4.0 bar.

Fix these problems first to get your motor to run.

Tony

[Louisieg55]

Originally Posted by boyt911sc

Louis,

Based from your information, you have two (2) major problems namely:

• Your FP should be running with the ignition switch turned to ON/RUN position (not START).
• The cold control fuel pressure is too high at 4.0 bar.

Fix these problems first to get your motor to run.

Tony

Tony,

I will be sending you my WUR to hopefully rebuild/rule that out for why the control pressure is high. Honestly hoping its a clog WUR and fixes that problem.

Update on the Fuel Pump.... I am getting power when its cranking but not on the "on" position.

I replaced the ignition switch.

What else can it be? The wire works as I am getting power when cranking just not on the "on" position.




I am thinking maybe the bigger portion of the ign. switch. I replaced the electrical side?

EDIT>Question

If I disconnect the return line from the WUR. Can I find a bolt/banjo same size run to a jug and start the car this way? Would that tell me if the fuel line is clogged vs the WUR?

[fanaudical]

It sounds like you need to get out either a test light or a multimeter, start tracing the wire from the fuel pump to the ignition switch, and verify you have continuity along the whole path...

[Louisieg55]

Update>

I’m still chasing down why I get power to the fuel pump when cranking but not in the “on” position. That will be my project for next week.

I ordered a new fuel accumulator, fuel pump, and fuel filter. Just arrived this week.

I’m also in contact with Leon Cummings for new tunnel lines and fuel lines for the pump (still TBD if I’ll re-locate it).

I’ve sent my WUR and fuel distributor to Tony for rebuild.

At this point, while carbs or EFI would be nice, the cost of $6–8k plus supporting upgrades (cams, etc.) just isn’t worth it right now. I’d rather get the CIS sorted out and drive the car for a while.

With the parts above, plus the rebuilt FD and WUR, are there any other components I should inspect or replace while I’m at it? I was thinking Injector test? Replacement? All the hoses seam okay for now. I also can really find any replacements?

I know this car will never be 100% reliable, but honestly I’ve towed it home more times than I’ve driven it. I’d love to start taking it up the California coast for weekend getaways with my wife, but right now I don’t have much trust in it. Money is always a factor, but I’d rather spend another $500–1000 now than get stranded again.

Any advice on additional parts to rebuild/replace for a general CIS refresh would be much appreciated.

Also, any thoughts on why the fuel pump only receives power when cranking? I was thinking maybe the ignition switch (just replaced), but is there a separate pin for cranking vs. “on”? If that the case, maybe the wire came loose or something but back in the engine bay its just (2) wires.

[PeteKz]

CIS is very reliable when you get it sorted, so don't throw in the towel yet. It will be much more expensive to switch to carbs or EFI than to keep the CIS, and those have their own sets of problems.

As fanaudical said, you will have to spend some time troubleshooting the starting circuit with a test light or meter, from the ignition switch to the starter solenoid and from the ignition switch to the fuel pump. I know you are frustrated, but you are overlooking something, probably something simple. Since you don't have the WUR, you don't need to be in a rush. Take it easy and slow and document what you have done in order. You will find it.

After you get the WUR back from Tony, then jump the fuel pump to make it run, and do a fuel delivery test and adjust the FD screws to get the same delivery to all injectors. As you do the delivery test, you will see if any of the injector patterns are bad.

Last, do NOT throw parts at the problem. That will complicate your troubleshooting by introducing new variables and unknowns. Troubleshoot first! Trust me on this, BTDT.

[Louisieg55]

Petekz,

Thanks again, I really appreciate the advice. I’ll slow down, document everything, and work through the troubleshooting step by step instead of throwing more parts at it. Once I get the WUR back from Tony, I’ll jump the pump and run the delivery test as you suggested. Hopefully that will point me in the right direction.

On a side note, I found this thread that seems to describe a similar issue:
https://forums.pelicanparts.com/porsche-911-technical-forum/750815-fuel-pump-power-qs-75-s.html

Based on that, I did some checking. The red/black wire at Fuse 10 (hot side) shows power with the ignition on, but not off. I ran a temporary jumper from that hot side to the fuel pump, and the pump comes on with the ignition. According to the diagram, that red/black wire from Fuse 10 should be feeding the fuel pump.



My question is this: would there be any issue with simply running a new wire and splicing it into the pump circuit at the rear of the car? Somewhere between the front and back I’m losing continuity, and tracing it through the loom looks like a big job if I can even find the break? This seems like the quickest and most straightforward solution, but I wanted to see if you had any objections before I commit. I was thinking of adding a fuse/relay inline up front for some extra protection. If I do this and eliminate the factory wiring?



You can see my temp wire to get this working this way and worked fine to turn the pump on and off with ign.

On CIS, the fuel pump is meant to run continuously once the engine is running correct?

Is there any safety concerns. It seams on a 75 the pump is either on or off. No real safety features? If I get into an accident is the pump supposed to shut off? Or if ignition is on a 75 is just on pump still runs?

Thanks for all the help.
L

[PeteKz]

Definitely run the wire as you did to make the pump run when the ignition switch is ON. On a 1975, the pump runs as long as the ignition is ON. The safety switch was installed in 76 or 77, and all the SC's. So by routing another wire around the chassis harness, you aren't making it any less safe than it was.

Do not eliminate the factory wiring. I suspect that the broken circuit is at one of the connectors in the engine bay. When you get around to sleuthing it, check the connector to the pump for power, and for ground. If you have power there, then check for power and ground at the pump.

[Louisieg55]

Pete,

Thanks for the reply. I now have the fuel pump working off the keyed ignition with a fuse and relay in place.

At this point I’m just waiting on Tony to finish rebuilding my FD and WUR, then I’ll move forward from there.

On a related note, does anyone have a better way to reach Len Cummings? I originally contacted him by email and he replied, but it’s been about a week since I last heard back. I’d like to get the tunnel lines and other fuel lines handled while I have some downtime if possible.

Are there any other kits or recommendations out there? Could I just make my own lines using AN hose? I’ve got a good amount of AN-6 and AN-8 left over from other projects, which is close to 3/8" fuel line size.