Porsche warm start woes MY1978 911SC

[ZV8888]

Hi

I have a warm start problem on my MY1978 911sc. After leaving the car for 30 min to 3 hours it may not start. If it does start, it'll start first turn of the key and either run just fine or die after a few seconds. Of course it may just not start at all. Outside these times it always starts and runs fine.

I have done a full set of CIS pressure tests and the pressures between the WUR and FD are correct for my 045 WUR :

System Pressure: 4.8 bar

Cold and not running: 17C / 1.7 bar

Cold and running: 17C / 2.0 bar
Warm and running without vacuum: 2.8 bar after 4 mins
Warm and running with vacuum: 3.5 bar after 10 mins

Residual pressures tested with and without gauge valve closed and were exactly the same:
10 mins: 2 bar
20 mins: 1.6 bar
30 mins: 1.4 bar
40 mins: 1.3 bar
60 mins: 1.2 bar

I have pulled an injector when the problem happens and I see there is no fuel coming out. So I have to I remove the air filter and raise the plate for a few seconds with the ignition switched on to get the fuel into the system. Then I can start the car.

I can't think of anything else I can check to solve this. (Fuel pump / FP Check valve / FP relay all good too) Any ideas?

[ahh911]

The system is still pressurized after 30min to 1 hour. It squirts out enough fuel to sometimes fire up the car right away, then it dies or it won't fire at all, the injector sprays only after the air plate is raised for several seconds under these conditions.

At 1.4 bar the injectors won't open, they need 2.5bar min assuming they're not leaking. But I think the cold start injector will work at much lower pressures. So perhaps the cold start injector is providing enough fuel from residual pressure to cause a fire and die. Then a few seconds later the pressure builds up and fuel flows as expected. How long is it supposed to take to pressurize the system, and why does it react this way only after 1/2 to 3 hours? (Is it the same root cause at 1/2 hour vs 3 hours or are they an overlap of issues) You'd think that the first start of the morning would suffer the same delayed fuel pressure build up, but it starts fine. What's different about first start is the aar valve is open wider, driving the sensor plate up higher and harder, the control pressure is sitting at a different point on the curve and the residual fuel pressure will be at it's lowest value. Don't know if this helps, thinking out loud. (I trust your fuel measurement are okay for the model year)

Phil
____

[ZV8888]

Thanks Phil. Good points, but in the warm start scenario, when the residual is say 1.5 bar, the control pressure re-pressurizes to 2.8 bar instantly when I turn on the FP (or try to start) and the system pressure goes to 4.8. I didn't look into the AAR, but I'm not sure if that will make a huge difference to the fuel supply before the engine is up and running.

Maybe the warm start control pressure of 2.8 is too high? As during a cold start it would be down about 2. This would mean that the WUR isn't cooling down quick enough. But I wouldn't expect it to cool down quickly, given its position in the engine etc.

[ahh911]

When the control and system pressurize instantly, does it still take several seconds more of pushing the sensor plate up before the injectors spray? I'm not sure what that could mean.

[ZV8888]

Yes, that's exactly the situation - When the control and system pressurize instantly, it still takes several seconds more of pushing the sensor plate up before the injectors spray. I am thinking along the lines of a vapour lock, but I can't figure where or how it could appear.

[PeteKz]

Your pressures look correct, but since I can't see your gauge setup, let me ask the dumb question: Are you sure you have it hooked up correctly, so that you are reading the right pressures?

[ZV8888]

Hi Pete

I think I have things hooked up correctly: Between the WUR and the fuel distributor head, with the valve on the WUR side. Well bled before use. Also, I'd imagine the chances of getting the 'in spec' readings with it incorrectly hooked up would be slim.

I really have run out ideas on where to look next to solve the problem since both hot and cold starts are fine.

If the injectors require 3 bar to release fuel, maybe the control pressure is too high on warm start? It measures 2.8 bar, when it is only 2 bar on a cold start. Maybe I should remove an injector and replace it with a pressure gauge in the warm start situation and take a reading?

[PeteKz]

This is one of the stumpers.

Phil and a couple others had a similar problem that he traced back to the plunger in the FD lifting under a vacuum or siphon, and not being in the fully down position to start during a warm start. I think he eventually solved it by adding a spring to the top of the plunger that he got from Tony. I'll let Phil explain what he went through.

Another hypothesis I would be entertaining is that the residual pressure bleeds off sometimes, but holds other times. Tony saw an FA that would do that. I know from my own experience with a bad FA years ago that if I was having a hard warm start, and lifted the plate to hear the injectors squirt, then it would start up quickly. I started having the problem intermittently, then consistently. I eventually replaced the FA which solved the problem. Consider leaving the CIS gauge attached, and see what your pressures are when you have the hard warm start problem.

[ZV8888]

Thanks Pete. I actually did keep the gauge on and have found the pressures are consistent and in spec, yet the car doesn't start. I had the fuel distributor rebuilt a couple of years back and it's been on the button ever since and even balanced with the individual injectors so I'd doubt the plunger is problematic.

I'm thinking about putting the pressure gauge in line from the output of the fuel accumulator to see what readings I get. But I'm clutching at straws. I'm close ignoring the advice and just buying a new fuel accumulator and hope for the best.

Maybe there will be a simple solution someone will come up with. Fingers crossed.

Maybe

[PeteKz]

I get your frustration, but don't resort to throwing parts at the problem. If the RP is holding above spec for an hour, and it won't start, I doubt the FA is the cause. As you suggested, try measuring the SP and RP between the filter and FD. Let's see if that shows any strange behavior.

Not related, but let me ask: When you rebuilt the FD and "balanced" the injectors, did that include measuring the delivery from all injectors at the same time?

[ZV8888]

I'll post the system pressure and residual pressure with the gauge between the Fuel Accumulator and the Fuel Filter.

Since I'm at it, I'll change the fuel filter (It's not old - 1 year / 3k miles) but it's worth a shot. I've been putting additives in my fuel for the past year or so to try and combat the ethanol so maybe they have had a negative effect on the filter. Although it's worth noting that the car is pulling well right up to 6k rpm, when I chicken out so I think the fuel flow is fine.

Regarding the balanced WUR, yes, it was tested with all injectors at the same time. In fact, all the injectors were pressure tested individually to make sure they were within spec, then attached to WUR, then each port output on the WUR was matched to each injector to get a precise volume throughput.

[boyt911sc]

Originally Posted by ZV8888

I'll post the system pressure and residual pressure with the gauge between the Fuel Accumulator and the Fuel Filter.

Since I'm at it, I'll change the fuel filter (It's not old - 1 year / 3k miles) but it's worth a shot. I've been putting additives in my fuel for the past year or so to try and combat the ethanol so maybe they have had a negative effect on the filter. Although it's worth noting that the car is pulling well right up to 6k rpm, when I chicken out so I think the fuel flow is fine.

Regarding the balanced WUR yes, it was tested with all injectors at the same time. In fact, all the injectors were pressure tested individually to make sure they were within spec, then attached to then each port output on the WUR was matched to each injector to get a precise volume throughput.

ZV8888,

Maybe you meant FD instead of WUR to the highlighted WUR above.

Tony

[ZV8888]

Tony, yes, you are correct. I meant to say:

Regarding the balanced Fuel Distributor (FD) yes, it was tested with all injectors at the same time. In fact, all the injectors were pressure tested individually to make sure they were within spec, then attached to then each port output on the FD was matched to each injector to get a precise volume throughput.

No excuse for

[ZV8888]

Two things to report:

(1) Somewhere in my search for solutions online there was a suggestion that for warm start, try keeping the accelerator pedal fully depressed while starting and release as soon as the engine starts.

(2) I changed the fuel filter since I was pulling fuel lines there trying to connect to my pressure gauge but I couldn't find the right connections for my gauge so I had to give up on that for now.

I was driving the car today and I was able to warm start twice without issue, using the advice (1) above. I am not confident it is resolved, however the technique seemed to work. Also, of course it could be the new fuel filter has resolved the problem, but I'm not confident about that either. Could it all have been caused by a bad filter?

[ahh911]

Option (1) must drive up the air plate higher (probably with more force too), this should cause the multi second delay in spritzing to speed up considerably as more fuel is driving through the fuel distributor, you'd think?

(Funracer added the springs Pete, not me)

[boyt911sc]

Originally Posted by ZV8888

Hi


I have pulled an injector when the problem happens and I see there is no fuel coming out. So I have to I remove the air filter and raise the plate for a few seconds with the ignition switched on to get the fuel into the system. Then I can start the car.

I can't think of anything else I can check to solve this. (Fuel pump / FP Check valve / FP relay all good too) Any ideas?

ZV,

Was the fuel pump running when you pulled out one of the fuel injector? Probably not. Please advise. Thanks.

Tony

[30westrob]

I have the same issue with my 81SC when I first bring it out of winter storage, in the spring. The fix is to run the fuel down to less than 1/4 and refill with fresh gas
I suspect the old fuel vapor locks at a much lower temperature, and fresh gas returns normal hot starts. Rob

[ZV8888]

Originally Posted by boyt911sc

ZV,

Was the fuel pump running when you pulled out one of the fuel injector? Probably not. Please advise. Thanks.

Tony

Hi Tony

The fuel pump was definitely running at the time as I tested during starting and with relay bypassed so I could hear the pump running before engaging the starter motor.

Duncan

[ZV8888]

Originally Posted by ahh911

Option (1) must drive up the air plate higher (probably with more force too), this should cause the multi second delay in spritzing to speed up considerably as more fuel is driving through the fuel distributor, you'd think?

(Funracer added the springs Pete, not me)

That makes sense and explains how the trick works, but I'd like to get to the bottom of the problem and resolve it.

[ZV8888]

Originally Posted by 30westrob

I have the same issue with my 81SC when I first bring it out of winter storage, in the spring. The fix is to run the fuel down to less than 1/4 and refill with fresh gas
I suspect the old fuel vapor locks at a much lower temperature, and fresh gas returns normal hot starts. Rob

You're making me wonder if the additive I am using to reduce the effects of Ethanol in the fuel is causing the problem ..... I have been using Millers VSPe (https://www.millersoils-shop.co.uk/vspe-power-plus-multishot) for about 6 months, maybe I should stop and try a different brand or forget about ethanol treatment completely. Over the last year they have increased the ethanol in our petrol to 10% here in Ireland.