Porsche 930 Hot Restart Problem

[Mark93077]

I have a 1977 930 that I have been working to get back on the road with great success other than a problem restarting after the warmed up car sits for more than 10 minutes. The car runs absolutely perfectly after pulling and cleaning the gas tank, replacing tank filter/drain plug, both pumps, the accumulator, the filter, and making the wur adjustable so that pressures (cold, warm, and 10/20 minutes post shutdown) are all in spec. I have left the pressure gauge installed so I can monitor while running and after shutdown. When shutdown it will restart immediately in the zero to 5min range with no issue but not when left for 10-30min. In the 10-30 min period it will start with starter fluid with a little bit of effort and then runs great and restarts as long as it does not sit for more than 10min. Looking for what I could test or try next. Is there some component that would cause this behavior if cis pressures all in spec and clearly it’s getting spark.

[boyt911sc]

Test your residual fuel pressure. What are your control fuel pressures? System pressure?

Tony

[Noah930]

I had a hot restart problem on my '87. In the end, it turned out to be a broken solder joint in the speed relay below the driver's seat. Mechanic figured it out by swapping in a known good relay from another car in the shop. The relay is NLA, so it was repaired by resoldering the joints.

[Mark93077]

The system pressure is about 6.5 bar. And when the WUR is plugged back in it gets to the 2.8 -/+ 0.2. Then the leak down after 10 and 20 min well above minimum.

I am not sure I have a speed relay on my car but that’s the type of component that or clue I want to look at next

I am thinking that perhaps even though the pressure stays up and within spec that maybe injectors are still leaking fuel to effectively flood it when I try to restart so that’s why I need starter fluid to get it going again

[Mark93077]

Originally Posted by boyt911sc

Test your residual fuel pressure. What are your control fuel pressures? System pressure?

Tony

System is 6.5 bar, when WUR plugged in it gets to 2.8 bar +\- 0.2. And over 2bar after 20min post shutdown. Have kept the gauge in. It’s between the fuel distributor and the WUR.

[kaskander]

Mark.

Had same issue (see my thread). My issue was a dying battery. How is yours?

[PeteKz]

If your injectors were leaking, you should have too much fuel in the engine, i.e. flooded. Adding starting fluid would not help it start. If it takes starting fluid to get it to start warm, that suggests the mixture is too lean. I know you measured the residual pressure, which indicates that the fuel accumulator is holding sufficient pressure, but these symptoms suggest a bad fuel accumulator. Here's a quick check of the FA: disconnect the return line from the FA, then run the pump. If you get fuel leaking out from the fitting on the bottom of the FA, it's bad. At this age, the rubber diaphragms inside them are leaking, or about to. Yes, I replaced mine recently because it leaked a lot of fuel when I disconnected the return line, and that fixed the warm start problem.

[bkreigsr]

Symptoms similiar to mine.
My front pump was working sporadically when warm.
'Sporarically' turned into 'not working'.

(FWIW, the car will start/run with only the rear pump.)

When the rear pump eventually died, I was on my way to a Werks Reunion in Florida. (2 nights, 3 days in Columbia SC)
Now I carry a spare for both pumps.

Bill K

[stevie 77 930]

As pointed out, a failing pump can cause this symptom especially when warm.

But before you start dolling out $ for fuel pumps, be sure to check your fuel pump relays. check and see if the get hot to the touch when the car is running (or at least when both fuel pumps running)
Try replacing these 2 relays (or even swapping them) to see if the condition persists.

One additional thing to check, is you over boost switch. These can fail also when warm, cutting the power feed to the fuel pump relays. Simply just disconnect the wire on the over-boost sensor. And one last relay in the engine compartment... the over boost sensor relay. This too may cause a power cutoff to the fuel pumps. These few items are not difficult to check nor expensive and can help lead you to solving the warm start issue.

But as Bill pointed out a failing fuel pump will need to be replaced

[RarlyL8]

I'd back up and start at the beginning.
When it fails to start pull a plug and check for strong spark and fuel.
If spark is not strong work your way back until the failing component is isolated.
If the residual fuel pressure is good and you are getting fuel then the mixture must be off.
Pull the fuel safety switch on the back of the air meter housing so the pumps will run when the key is on. Turn the key on and verify the fuel pressure is still within the warm range (engine heat should be warming the WUR even after 30 minutes). If slightly lean give it a tweak, if significantly lean check the pumps.
Also if you have an AFR meter pay close attention to what it is doing at idle when the engine is fully warm/hot. If the fuel pressure is in spec but the AFR shows lean you may have a small vacuum leak that opens up when the engine is hot and closes when it cools off.