Is this a vapor lock issue with the CIS?

[Sunroof]

I took my 1973.5T with CIS out for a long ride yesterday given the Atlanta weather was in the 70's and a perfect day to be out. The car started well (hand throttle for warm up, warmed up well and I had a great ride for about 40 miles via highway and lots of stop and go and traffic lights. On the very last two miles before home I came to a light and the car just DIED!! No startup, not even a cough.

I managed to coast down a hill and stopped in a median strip thinking this is a good time to finally blow this thing up yelling, NOW WHAT?! I even had the company of the local police who adored the car help with traffic but did not share in my frustration. I did call my favorite tow company for a ride home.

After 15-20 minutes the car came back to life and started up. I took the tow anyways. The only thing I can think of was VAPOR LOCK. This first year for CIS in the 911 has the fuel pump mounted in the rear just inside and out of sight of the rear left wheel well. The fuel pump (Pierur less than five years old) has to pull fuel from the fuel tank then push it to the fuel head. When you think of "Hot soak" when at a stop, how vapor lock happens and the fuel pump action I have no idea what else it could be. I just saw a video that recommends that you open the gas cap and listen to the fuel. You can hear it either hissing as it returns to the tank or boiling, which is a good indicator you have vapor build up. Afterall, most gas sold has a 10% ethanol additive which boils at a lower boiling point. This is not the same gas from the 1970's. The fuel pump has an IN and OUT line. The fuel pumps in the 70's American cars that suffered from overheating and vapor lock had a third line to remove the vapor and send it back to the tank.

It has been mentioned that the throttle unit can be the culprit as pressures change or the WUR is not doing its job. Would'nt that be a relief?

So my question to the community is this diagnosis correct and just how do you correct it? Carrying a bucket of dry ice or a few cans of compressed air to cool down the fuel lines is not what I am looking for although it has been suggested over the years on this board. I installed the 914 hot start kit many years ago, my fuel filter is new, coil is new, CDI unit rebuilt, new spark plugs and wires, etc, etc.

Thanks
Bob

[356911930]

Hi Bob,

Vapor lock is very rare, although I hear many that think they have it, but probably not. I think your issue is with your CDI unit. Your car will probably start now and run until the cdi warms up and shuts down, twenty or thirty minutes later it should start again.

There is a member who rebuilds the cdi boxes, I don’t recall his name but search should turn him up .

Best of luck,

Rahl

[Sunroof]

Rahl
I had the same issues with the CDI awhile back whereby the CDI heated up and suddenly shut down the system. As a result I had the CDI unit rebuilt, but most importantly removed and replaced the wire harness that goes from the CDI unit to the coil and distributor. The original wire harness at that location lost conductance due to age and heat which accelerated the shutdown. So for now I do not believe its the CDI unit. Its something going on with the fuel control valve or in or somewhere in CIS but has all the symptoms of vapor lock. I am going to install a new fuel accumulator and fuel filter I have on the shelf to see if that does anything to help.

[Sunroof]

It just might be time for a CIS overhaul! Can the entire CIS unit be removed by lowering the engine?

[356911930]

Hi Bob,

I think you can remove the cis by lowering the engine but not 100% sure. You should check for spark and fuel before going to extremes. Remove a spark plug hook it up to the spark plug wire and ground to the engine. Do you have a spare coil you could use to see if that is the problem? The coil would be the next component to check. There I also a test method for the coil described in the workshop manual using a volt ohm meter.

Rahl

[Charles Freeborn]

Yes, you can pull CIS with a "partial engine drop" But I've never heard of vapor lock with these cars. The fuel lines don't run near enough to the exposed (not heater boxes) manifolds. I've run mine at track events well into the 90's and flat out (had to come off track because of oil temps -and I've got carrera cooler) and never experienced what you did.
I agree - CDI boxes fail with higher temps. I run a MSD 6L.

[fanaudical]

My opinion - A failed fuel accumulator or a faulty fuel pump check valve are both sometimes mistaken for vapor lock. You can test for both of these with a CIS fuel pressure test.

[boyt911sc]

Originally Posted by Charles Freeborn

Yes, you can pull CIS with a "partial engine drop" But I've never heard of vapor lock with these cars. The fuel lines don't run near enough to the exposed (not heater boxes) manifolds. I've run mine at track events well into the 90's and flat out (had to come off track because of oil temps -and I've got carrera cooler) and never experienced what you did.
I agree - CDI boxes fail with higher temps. I run a MSD 6L.

I would be happy to give an incentive to anyone who could demonstrate a vapor lock condition in a CIS. I’ve done some testing and experiments to create a vapor lock condition heating metal fuel lines and the heated fuel lines never showed any fuel flow restriction at all. Be aware that the test involves unsafe and hazardous procedures to vaporize pressurized fuel.

I don’t believe that a vapor lock if any in a CIS could prevent the fuel to flow with 70 psi. System Pressure (930 has 95~100 psi.). This is a myth. A vapor lock could occur in a low pressure system like carb (2~3 psi.). Not in a system with very high system pressure such as CIS.

Tony

[356911930]

Hi Bob

Your issue is probably not the fuel accumulator or the fuel pump check valve. It could possibly the fuel pump itself but I would think that would cause poor running condition and not an instant shutoff as you described. You will need to get a set fuel pressure gauges to confirm, very important and necessary tool. My money is still on electrical, you need to check the coil now to confirm good or bad.

Rahl

[kenzinger]

My previous problem related to intermittent shut off was a bad coil and/or CDI - it was very loosely correlated to temperatures in that when I would sit in traffic for an extended period the engine bay would heat up and make the problem worse... 20 minutes to cool off and I would be able to start it up again.

[HarryD]

A quick test to see if you have an overheating CDI is to put some ice on it after it cuts out. If it goes again, you got your culprit.

I had a Permatune that had this failure mode. I spent several days driving around with an ice chest to verify.

[DaytonaCoupe66]

Originally Posted by 356911930

Hi Bob

Your issue is probably not the fuel accumulator or the fuel pump check valve. It could possibly the fuel pump itself but I would think that would cause poor running condition and not an instant shutoff as you described. You will need to get a set fuel pressure gauges to confirm, very important and necessary tool. My money is still on electrical, you need to check the coil now to confirm good or bad.

Rahl

My 930 had a similar problem. Two fuel pumps so a bit different but it turned out to be fuel pump relay/fuse wiring related. Discovered the electrical glitch after I had already replaced one pump and the CDI to no avail. It wouldn't act up when pressure testing in the garage, only when it got hot and I was nowhere near home.

[E Sully]

One of the first things I do when checking any engine is to remove the spark plugs to see what they look like. This is a simple way to get an idea of what is going on inside the engine.
If it dies again, you could keep a spare spark plug in the car and remove one of the spark plug leads and put in the spare spark plug to see if there is spark when cranking. This will give you a quick check of the ignition.
My '73.5 never had a problem with vapor lock, and it has run fine in 90 degree temperatures with oil temps extremely high. This was with the original fuel pump. There is no relay or fuse on the power to the fuel pump this year, so that can be ruled out.
It could be possible that fuel supply is an issue. Is the fuel tank free of corrosion and has the screen in the bottom of the CIS tank ever been checked? Is the evaporative system working properly, as this vents the fuel tank to prevent vacuum from forming in the tank restricting flow. A fuel pump can overheat if there is insufficient fuel flow, as the gasoline flow is what cools it. Since it ran well till you came to a stop, it seems less likely. As suggested in prior post, checking pressures with a proper CIS gauge setup is always a good idea.

[Walt Fricke]

I don't know about vapor lock per se, but the 911s with the CIS fuel pump in the rear mounted on the torsion tube, left side, were notorious here in Colorado for having trouble restarting in the summer at track events. There were two fixes: install a plastic funnel sort of by the shock absorber, in that left front corner, and pour ice water into it. so the water came out on the fuel pump.

Or pull the air filter, and raise the air flow plate.

Both approaches resulted in the car promptly starting.

There is plenty of heat in that area under the car - for instance, air is forced through the heat exchangers whether you have called for heat or not by the engine's cooling fan. With the heat set to off, the heat valves close off the flow to the cockpit. But they vent that air into the area inside the fenderwell - which is just where the pump was located on those cars. For sure that's where it was on the '74s.

Everyone called it vapor lock. On US carbureted cars, I could amaze tourists in the mountain park where I was a ranger who couldn't get their cars to start by opening the hood and opening the choke while they cranked.

Some mechanism is at work here. If there is air entrained in a fuel line, is it not so that the higher the pressure in the line, the more the air stays in solution, so to speak? The hotter the fuel, the less the air wants to stay dissolved or whatnot? Car stops, pressure down, air forms bubbles?

I admit I always wondered just why a fuel pump, especially at, say, 5 Bar, wouldn't just pump through that. You sort of do that by pumping when you have boiled your brake fluid.

On the other hand, if the pump itself has a big bubble of air, might it not spin its moving parts, but not seal well enough against air to shove the air out of the way, especially against some resistance (as in a pressure regulator)?

Maybe Ed didn't run at 5,000 or 7,500 feet elevation on a track? As one can see, I can't be said to understand fluid dynamics in the way an engineer would.

People who had this problem have also solved it by moving the fuel pump back to the front. I've always wondered why Porsche ever moved it to the rear, though by '75 or '6 they moved it back to the front.

[MichaelSJackson]

I've had a fuel pump relay that would heat up, due to a poor socket connection. This in turn led to starvation, until things cooled down.

I used to keep a short wire on hand, to jumper the fuel pump when this would happen.

Eventually, someone showed me how to expand the relay's prongs, just enough to get a better fit. Problem has never reoccurred since. (Knock on wood.)

[theiceman]

Originally Posted by HarryD

A quick test to see if you have an overheating CDI is to put some ice on it after it cuts out. If it goes again, you got your culprit.

I had a Permatune that had this failure mode. I spent several days driving around with an ice chest to verify.

They have Modern technology called freeze sprays now Harry .. spend the 8 bucks and save the ice chest for the beer...

incidentally my sons Audi would intermittently die ... i gave him a can of this and told him when it dies spray the crank position sensor ( they are known to die of heat ) .
he sprayed it , car started immediately and got him home, we put on a new sensor and 3 years later he is still motoring.

[E Sully]

It's always fun to be an armchair quarterback.
Could it have been from the fuel pump getting too hot, sure. Some have mentioned relay or fuse issues, but as mentioned earlier, the '73.5 fuel pump is direct power with no protection unlike later cars. Possible the connector from the chassis harness to the fuel pump needs attention.
Walt,
My track experience was with air cooled, but 2 wheels and a knee on the ground around turns. Elevations were only in hundreds of feet. It is true that E10 has a lower boiling point than straight gas, and altitude lowers it more. The restarting issue could have been vapor lock, as you said there was a problem with restarting the 2.7's. Difference is on the track, cars were run a lot harder and usually with minimal fuel in the tank to cool the pump, and then shut off allowing heat to build up.
Bob was on a pleasant days drive, and it died at a stop. I can't see vapor lock being the cause of the car stalling as the fuel pump is still flowing gas at 70psi, keeping the pump cool enough to prevent vapor lock while running. On a 1973.5 the pump runs as long as the key is on, unlike late 2.7 engines that had a switch added to the air flow sensor. It could be his Decel valve is not working well and letting the rpm drop too rapidly causing the stall. It also may just need a little adjustment of idle speed or mixture.
One of the interesting things in the 73.5 manual is regarding hot starts. For hot starts, Porsche recommend pushing the gas pedal all the way down when starting. Cold and warm starts were just lifting the hand lever up. Now for the 1973.5, this would also activate the cold start valve regardless of engine temperature. This would seem to me to be similar to what Walt said earlier to raising the air flow plate, as it would push a little extra gas into the engine. The 2.7 engines used TTS which would shut off the cold start valve when warm. I would think if the fuel pump was vapor locked, this would not move fuel, just spin the vapor.
Bob, if it happens again, you could try lifting a air plate and seeing if it works as Walt said for the 2.7's at the track. I would also test to see if the micro switch on the throttle body and your cold start valve are working.

[HarryD]

Originally Posted by theiceman

They have Modern technology called freeze sprays now Harry .. spend the 8 bucks and save the ice chest for the beer...

incidentally my sons Audi would intermittently die ... i gave him a can of this and told him when it dies spray the crank position sensor ( they are known to die of heat ) .
he sprayed it , car started immediately and got him home, we put on a new sensor and 3 years later he is still motoring.

True. But ice is easily accessible if you are stuck somewhere and cheep.

[tperazzo]

Ethanol blended fuels combined with a rear mounted fuel pump can be a problem at high temperatures.

The inlet side of the pump is low pressure. Actually negative pressure in some cases because the pump has to overcome the fuel tank screen pressure drop and the friction in the supply line. The lower the fuel pressure the higher the likelihood of bubbles forming in the line. Too many bubbles and the pump can't pump air and you got vapor lock.
This could happen if all the stars align. Fuel tank less than 1/4 full, inlet screen dirty, E10 fuel, parked on hot asphalt for 30 minutes with a hot engine, rear mounted pump.
Walt said the same thing I am. I'm just adding my spin to it.
Please do tell us what you find out and if you're able to reproduce the problem.

[eastbay]

Probably not your issue, but I'll throw this out there.

Have you checked your fuel pump suction line in the back? I have seen the rear pump supply line kinked in a few mid year rear pump cars.

The rubber supply line back there can get jammed into the corner of the chassis and kinks easily. You need to be careful when working the fuel pump or line to not kink it.

I took a photo of the last one I fixed, rock hard, been that way for years. Maybe that way from the factory: