911SC residual pressure/hot start problems

[oregon87]

Hello all-

Before I get started, I feel in incumbent upon me to first state that I have searched this topic heavily as to not be "that lazy forum guy." Having done so, and performed a series of tests, I hope the experts here can help further troubleshoot.

I purchased a 1980 911SC about a year ago. Having checked off the standard maintenance items (oil change, ignition part replacements, valve adjustment, clutch/shifter adjustment, etc), I had a pretty nice running car. With the exception of an occasional hot start issue.

As the issue has become more frequent, I decided to delve into troubleshooting. The first thing I did, prior to purchasing a set of pressure gauges, is buy a new fuel pump check valve, as it was a cheap thing to try. Turns out my car was equipped with an internal check valve. As such, I replaced the pump with a new Bosch unit. Stupid me, I know. Test first dummy, then replace parts!

The pressure gauge arrives and I proceed to check the control, cold system, warm system and residual fuel pressures. Please see pic. The hose closest to the valve is connected directly to the WUR. The vehicle hose disconnected from the WUR is now connected to the other end of the test gauge. The results are as follows, 65 degrees, verified with thermal temp gun:

System pressure: 5.1 Bar
Cold control pressure: 1.8
Warm control pressure having plugged in WUR: 3.7 Bar in roughly 2 mins, steady at 3.7 at 5 mins.
Residual pressure drops off immediately, down to 0 in about 30 seconds, having removed power from the fuel pump. This holds true whether the valve on the test gauge is open or closed. I thought closing the vale while the pressure was falling off would aid in the thought that the issue was not likely accumulator or WUR related.

Having already replaced the fuel pump, I decided to troubleshoot the accumulator further. Removed the lower hose (3 port unit), fuel dripped out. Not gushing, but dripping. From what I've read, this is OK and not a sign of a failed accumulator. Having removed the accumulator, I found fuel in both upper ports, not leaking into the bottom. I emptied the accumulator, plugged one top port and blew into the other top port. No apparent leak through. I sucked on the bottom port as well, no apparent leakage detected. To be a bit more scientific, I put a handheld vacuum gauge on the bottom port and pulled 5" Hg, which remained stable. From my research, the accumulator gets a passing grade. With that, it was reinstalled.

I removed the primary pressure regulator form the head unit and inspected the o-rings. I disassembled the regulator and inspected the internal o-ring, as well. I also thoroughly cleaned the regulator, blew out the head unit with compressed air and reinstalled. Nothing obviously wrong there, but I'm not sure a visual inspection warrants a passing grade.

I'd hate to purchase a new regulator without a bit of input and I'm certainly willing to listen to any of my processes are incorrect. Through a bit of research, I did find a fuel pressure regulator rebuilt kit which is offered by Delorean Industries that I swear I read would work for our cars, but I can't seem to find that post.

I should add that the injector o-rings were replaced by the previous owner in 2018. I understand that it really has nothing to do with a premature loss of residual fuel pressure, but felt it worth mentioning.

Apologies for the long read, but wanted to make sure I didn't miss anything. Current status is unchanged. Rapid drop off of residual pressure, down to 0 within 30 seconds. Any advice would be much appreciated.

[snbush67]

My recent experience with a friends 1978 3.0 ROW SC is that the new fuel pumps check valves do not hold pressure. The car was recently purchased with one of the new style pump installed and was having warm start issues. He purchased another new pump and we installed it and the warm start problem persisted.

We experimented and discovered that both new style pumps allowed back flow from pressure. We checked an old original pump that I had on the shelf and it held up to he highest pressure we had (110 PSI).

So we installed the old original style pump and the warm start problem went away. The car starts quickly, hot or cold.

[oregon87]

I’ve read I can test the pump’s check valve by putting the test gauge between the pump and accumulator, however, the fittings included with the S&G kit don’t allow for that. As such, I’d need to source other fitting. However, when performing the pressure tests and shutting off the valve when turning off the fuel pump, does that not isolate the WUR, accumulator and fuel pump? If pressure is still lost with that valve closed, isn’t the loss somewhere from the distribution head onwards?

[snbush67]

I don’t know, we never got that far. I just assumed that that was the only issue because it’s starting great, cold or hot, and runs good.

Maybe just pull the pump and do a bench test?

[boyt911sc]

Originally Posted by oregon87

I’ve read I can test the pump’s check valve by putting the test gauge between the pump and accumulator, however, the fittings included with the S&G kit don’t allow for that. As such, I’d need to source other fitting. However, when performing the pressure tests and shutting off the valve when turning off the fuel pump, does that not isolate the WUR, accumulator and fuel pump? If pressure is still lost with that valve closed, isn’t the loss somewhere from the distribution head onwards?

With the valve closed at your current set up, you only isolate the WUR. Since you have done a vacuum test with the FA (passed the test), you still have to test 2 likely suspects which are the FP check valve and the PPV (primary pressure valve) in the fuel distributor. Test the FP CV first then proceed downstream. Verify and confirm that the check valve is good and holding fuel pressure. Then proceed to the next suspect.

If both fuel accumulator and fuel pump check valve passed the tests, then the culprit for residual pressure loss is your PPV. The WUR is not a factor for residual pressure loss in an SC unless you are using the wrong fuel distributor. Keep us posted.

Tony

[oregon87]

Hi Tony-

Thanks for the help! As I don’t have the fittings to adapt to the accumulator or filter, assuming I can adapt at the pump itself, under the car? I tried to disconnect the rubber line from the green hard line, but gave up on fear of damaging the hose as it is VERY tight. Orrrrr, I find a M14.5 x 1.5 to M12 x 1.5 adapter? Tie one end of the pressure gauge to the adapted FA and turn the test valve off?

[boyt911sc]

Originally Posted by oregon87

Hi Tony-

Thanks for the help! As I don’t have the fittings to adapt to the accumulator or filter, assuming I can adapt at the pump itself, under the car? I tried to disconnect the rubber line from the green hard line, but gave up on fear of damaging the hose as it is VERY tight. Orrrrr, I find a M14.5 x 1.5 to M12 x 1.5 adapter? Tie one end of the pressure gauge to the adapted FA and turn the test valve off?

Do the testing at the engine bay. Connect your CIS pressure gauge kit in series to the main delivery line to fuel accumulator instead of doing it under the car. You have to remove the skid plate and undo several bolts to access the FP. Keep it simple. Most important is you need to have the right adaptor to do the test correctly. And run the FP just enough to build fuel pressure and quickly stop the FP. The valve must be closed and test for residual pressure.

If the check valve is good and working, you should have at least 1.1 bar/16 psi. or higher after 20 mins. Keep us posted.

Tony

[oregon87]

Will do! Hunting for an adapter currently. ��

[PeteKz]

Way easier method: As soon as you turn off the FP, pinch off the rubber fuel line coming up to the accumulator (use a C-clamp or vise grips or something like that if you don't have the specific tool for this job). That will stop pressure from leaking back through the FP check valve. If it holds residual pressure, then the FP check valve is leaking. If it still loses pressure, then the PPV is leaking.

[oregon87]

Hmm... Having done a bit more research while waiting for parts to ship, I'm fairly confident that the fuel pump I installed does not have an internal check valve (see pic below). The website I ordered it from showed what I'm guessing was an older version with the long neck. The one shipped is exactly as pictured and much different from the advertised pic. Waiting on Bosch to call back to confirm.

So, I guess I either need to see if the check valve I purchased will work, sourcing additional fittings, or find a fuel pump with an internal check valve so I don't have to replace the banjo fittings.

[oregon87]

Confirmed from Bosch, part number 69513 does not have an internal check valve. Ugh... The site I ordered it from pictured the older style, elongated neck which led me to believe it was an internally valved unit. Lesson learned. I guess I can always put a like new fuel pump and never installed check valve in the classifieds section.

[snbush67]

Originally Posted by oregon87

Confirmed from Bosch, part number 69513 does not have an internal check valve. Ugh... The site I ordered it from pictured the older style, elongated neck which led me to believe it was an internally valved unit. Lesson learned. I guess I can always put a like new fuel pump and never installed check valve in the classifieds section.

I think that the pump you received isn’t part # 69513. The part you got is probably 69901.

The pumps that I have with part # 69901 don’t hold back pressure. I think that only the original pumps work correctly.

[oregon87]

Fixed! I was able to use the adapter provided with the Bosch pump with an external check valve, only needing to source a "female" hex cap to replace the male banjo bolt. All is well. Residual pressure is above 1.3 bar after 60 mins. The moral of the story? NEVER begin troubleshooting CIS without putting a set of gauges on the car first. A lesson clearly and often stated on this forum that I stupidly ignored.

[snbush67]

Can I have your old fuel pump?

[oregon87]

Originally Posted by snbush67

Can I have your old fuel pump?

Absolutely! It can be picked up at the following address:

61050 SE 27th St. Bend, OR 97702. 🙂