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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Stock 2.7 warm starting problem

Hello everyone,

My newly rebuilt 2.7 engine is experiencing a warm start problem. In fact, this problem predated the rebuild, and I guess that I hoped it would be fixed as a consequence of the rebuild.

The engine is a stock 2.7 in a 1975 911s. The only change to the engine itself were the inclusion of Elgin RS-style cams. The engine has had all the usual improvements: oil bypass modification, 3-rib oil pump, Carrera chain tensioners. The fuel system is the stock Bosch CIS, but the ignition is from Permatune. The injectors were not replaced in the rebuild. In fact, none of the fuel delivery components were touched in the rebuild.

The symptoms are:
The car starts very easily when cold.
Once it has reached full operating temperature and been driven for a few minutes, if I stop the engine and restart it up to about a minute or two later, it starts very easily.
If I wait for longer than that, it won't start for love or money. Frequently, it fires the very first time I try to start it, but it won't "catch", and after that no more sign of combustion.

I tried two quick things: I let the fuel pump run for two minutes after stopping the engine, as some have suggested. Also, I let the fuel pump run for a minute before trying to start the engine. Both to no avail.

I replaced the fuel pressure accumulator and there was no improvement in the warm starting problem. I guess there is really no surprise here, but hope springs eternal.

During normal operation, the engine runs very well. It has no dead spots and pulls well from low RPMs, and really well above 3000. Since I am breaking the engine in, I have been do a lot of accelerations and decelerations and everything seems great.

One odd thing: the fuel usage seems high, but again I am being quite aggressive.

I have read several articles on the CIS system, but it is a bit daunting. Any suggestions on how I can solve this mystery? I really need this car to run reliably.

Thanks!

Dan
Old 07-24-2016, 11:51 AM
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Hi, Dan:

I remember seeing your other post in the engine rebuilding forum. If you're not familiar with the CIS system, I recommend starting here:

911 CIS Primer - Index

Read through how the system works before you start troubleshooting. My recommendation is to acquire a CIS pressure test gauge and determine the following before replacing any other components:

- Part numbers for the WUR and fuel distributor currently installed on your motor
- System pressure
- Cold control pressure
- Warm control pressure

Proper function of the WUR is necessary for reliable warm starts. Once you've got this stuff understood, report back and we can help you from there.
Old 07-24-2016, 02:35 PM
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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Dear fanaudical,

Thank you very much! I will follow what you suggest and report back soon.

Dan
Old 07-25-2016, 03:26 AM
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Dan

It could be your fuel pump check valve

Help with CIS Again - Hot start, changed everything...any ideas?
__________________
Tom Butler
1973 RSR Clone
1970 911E Restoration in Progress
Old 07-25-2016, 04:39 AM
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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Thanks Tom! Unfortunately, the fuel pump check valve is located INSIDE the fuel pump, for this model, I believe. (There was a Porsche bulletin on this: Tech Info Gr 2, No. 23, on upgrading the fuel pump to one with an external check valve. Sigh!)

Hi Fanaudical: gauges are on order from our host, due to arrive by the end of the week.

Tom, then I should be able easily to read the residual pressures.

The particulars of the system are:

WUR 0 438 140 009 (This is a little hard to read: the last bit of the number is conveniently obscured by a hose clamp.)

FD 0 438 100 006 (This one is mercifully easy to see.)

Thanks for the link. The material is easy to read and understand.

Best regards,
Dan

Last edited by DanCoffman; 07-28-2016 at 03:35 AM..
Old 07-25-2016, 08:34 AM
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Walt Fricke's Avatar
 
Join Date: May 2004
Location: Boulder, Colorado
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Dan - did the '75 still have the fuel pump located in the back on the torsion tube? That ill advised mounting caused a whole lot of hot start problems because of all the heat which got into the pump and fuel.

One way to overcome it was to pull the air filter, and with key on raise the air measuring plate a bit a couple of times. The notion here was that this let the fuel push the vapor which was locking things out into the cylinders. Don't know if this is, in fact, what happened, but it worked. However, one would think that your method of pressurizing the system ought to do it. I put in a dash push button for just that reason when I was running a pump with no RPV.

The other fix was to install a funnel in the left front of the engine compartment, down which ice water could be poured directly onto the hot fuel pump. Alternatively, a tip in Pano suggested using a Windex spray bottle or the like to reach under things and spray water on the pump.

But if the '74 is the only 2.7 which had this unfortunate mounting, fuggedaboudit.

Gauges are the only way to go with this stuff.
Old 07-30-2016, 09:58 PM
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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Hi Walt,

Thanks for the reply! The car does indeed have the fuel pump mounted in the torsion tube on the driver' side rear.

I am going to try the method of raising the sensor plate next time the problem occurs and see if that gets me going again. Unfortunately, it has been raining heavily here for the last few days, and I haven't taken the car out for long enough to get it hot.

It does really run well, though. I really can't believe that after so many months of languishing in dozens of boxes and bags the engine is running again. So many places I could have made mistakes! I really appreciate all the help I have received from folks here.

I have ordered a gauge set and will report here with the various pressures once it arrives.

Thanks again!

Dan
Old 07-31-2016, 05:18 AM
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Warm start problem.......

Dan,

Check your fuel pressures (residual, control, and system) when you get your pressure gauge kit. Next time you have a warm start problem, check you ignition signal with an inductive timing light. Your engine could easily overcome the residual fuel pressure problem because the FP starts as soon as you turned the ignition switch to ON/RUN position.

I don't believe CIS suffers from vapor lock like those in carb system because gasoline is an imcompressible fluid and would readily displace any gas or vapor present in the system because of the high pressure generated by the system pressure (70 psi. plus).

Your problem seems to be temperature related and I have a strange feeling your CDI could be contributing to this problem. This is the reason I like it tested. Keep us posted.

Tony
Old 07-31-2016, 05:50 AM
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Well, vapor lock or something else, if you run these cars hot enough, like on a race track in the summer's heat, with the fuel pump on the torsion tube subject to all that heat, it sure will act like vapor lock.

I've seen it several times, and conversed with friends who owned and tracked such cars, and for sure no amount of cranking by itself seemed to get it started short of cooling the pump or raising the air measuring flap.

Perhaps if gasoline vaporizes within the fuel pump, the rotary pump simply pushes the vapor around without ejecting any fuel and without drawing any fresh fuel in? The vapor is compressible. I've wondered why 5 bar of fuel pressure won't shove any air along like a squashed bubble. In fact, often after a rebuild I have just cranked the engine over and up it started, without any special bleeding of air from various lines. And the 74 (and so I suppose the '75) doesn't have the fuel pump/air flap interlock that the later CISs have. However raising the flap still always worked despite the fact that cranking causes enough air to flow that the flap raises some.
Old 07-31-2016, 06:01 PM
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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Warm startup: Stock 2.7

Hi Walt,

Thanks for the advice. I am becoming very adept at the "raise the sensor plate" exercise. It is nice to have that as a work around, and it seems to work quite reliably. It just makes me all the more eager to solve the problem fully, especially since otherwise the car runs so well and is a lot of fun to drive.

Gauges should arrive in two days. I'll be able to make the pressure measurements over the weekend, and I'll post the results.

Thanks again, everyone!

Dan
Old 08-02-2016, 03:19 AM
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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Warm startup: Stock 2.7: Pressure measurements

Hello everyone!

The gauges arrived and I have made my first set of measurements.
(I posted the part numbers for the WUR and FD earlier: please see above.)

System pressure: engine cold and not running: 4.15 bar
Control pressure: WUR cold, engine cold and not running: 1.6 bar
Control pressure: WUR warm, engine cold and not running: 2.7 bar

Very interestingly, at first there was no pressure increase in the control pressure when I plugged the WUR connector back in. I checked the voltage on the WUR connector and it was zero! I traced the wire to pin 10 on the large '14 pin' connector and it was continuous. I checked and there were 12 volts on pin 10 with the key in the 'run' position. (This is a 1975 model and there is no fuel pump relay.) I cleaned the connectors and tried again and got a beautiful and very welcome 12 volts at the WUR, followed by a graceful pressure increase -- it took maybe 30 seconds to reach full pressure. I wonder how long this has been going on? I can't wait to put the car back together and see if there is any difference.

The decrease in pressure after turning the key off was virtually instantaneous, falling all the way to zero within a couple of seconds. I followed the instructions in the Porsche trouble shooting guide, as kindly provided at http://jimsbasementworkshop.com/CIS/Troubleshooting Guide. I clamped off the rubber fuel delivery line to the fuel pump with vice grips --- gently! --- and found no change in behavior. So either that was not sufficient to block the fuel delivery line, or the problem is not the check valve in the pump.

The next step in the trouble shooting guide is to block the fuel return line to isolate the pressure regulation valve or the cold start valve as the culprit. Here I am mystified, and hope you can help: the return line is a hard line. How can I block it? Put some kind of valve inline?

In any event, I eagerly await any and all advice.

Dan
Old 08-06-2016, 05:35 PM
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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Warm startup: Stock 2.7: pressure leak and fixed

Hello everyone,

First, thanks to everyone for all the helpful tips. I found the source of the residual pressure leak! The O-ring on the system pressure regulator piston was only a shadow of its former self, shall we say. It had a dandy chunk missing from it. In the process of taking the piston out, I found that the previous owner had chosen to endow the plug with not merely one, but sealing rings. That explained the low system pressure I reported yesterday. I replaced the O-ring and used just one new sealing ring and the new measurements are:

System pressure: 4.6 bar
Control pressure --- cold: 1.6 bar
Control pressure --- warm: 2.7 bar

Residual pressure ( measured as control pressure )
Immediately after turning off fuel pump: 2.2 bar
after 1 minute 1.8 bar
after 3 minutes 1.7 bar

Now all that remains is to put everything back together and go for a drive. Then I will finally know if the warm start problem is fixed.

Do I sound relieved? Thanks again!

Dan
Old 08-07-2016, 01:08 PM
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Just for info, the check valve on the fuel pump is located in the banjo fitting on the side, part of the fuel exit line.
Bruce

911 608 901 00 check valve part number

Last edited by Flat6pac; 08-07-2016 at 01:52 PM..
Old 08-07-2016, 01:49 PM
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1975 911S Targa
 
Join Date: Dec 2012
Location: Connecticut
Posts: 20
Warm startup: Stock 2.7: Epilog

Hello everyone,

The problem seems to be fixed. I just took a nice long drive, brought the car back home, let it sit for 1/2 an hour, and started it up. It has not been that easy to start in an eternity.

Now on to the tie rods...

Bruce, thank you so much for the part number. That just made into my book of things not to forget!

Best regards,
Dan
Old 08-07-2016, 03:43 PM
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Good job, Dan! Am glad it's running better and starting when warm.

For what it's worth, I have a '75 with the fuel pump still mounted in the rear on the torsion tube. I have yet to encounter a warm start issue from vapor lock on this car. Doesn't mean I won't - it just hasn't happened yet. (My old Audi Fox used to do this ALL the time - never did fully solve that one.)

Dan - I think your warm control pressure may be a little low (which should just make it run richer when warm, maybe not a bad thing). Here's a link to the CIS troubleshooting thread I started a few years ago. It looks like your CIS config is perhaps identical to mine. I've got a bunch of tech references in the thread you might find helpful.

Sorry - Yet another CIS troubleshooting thread...
Old 08-07-2016, 09:07 PM
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