Hello everyone,

My 86.5 Auto has been suffering from a hot start issue. Starts fine when cold, but never when warm. Only way to start it when warm is to pour a few drops of fuel down the air intake.

I've gone through all the general troubleshooting steps until I realized the front fuel dampner is not holding vacuum.

How do I safely bypass this dampner so I can continue troubleshooting or run while waiting for parts?

Thanks a bunch!
Tarek

 

if it doesnt hold vacuum, it could mean that the fuel is leaking into the intake . disconnect the vacuum line and see if fuel drops out. you dont need the vacuum in the system to run well anway. I usually run no vacuum on the system during my racing.

if it is leaking, then to by pass it , you need to put something in place of it.

what year do you have? i forgot

mk

 

Its an 86.5 . Read the post again Mark.

Tarek, I found this while searching. It was originaly posted by 2v4v.

Do NOT operate the car with a leaky vacuum diaphragm on either regs or damper. There are two possible (and likely) outcomes -

1: FIRE! (this is bad. like terminal bad. I like shark, but not cooked with gasoline)
2: HYDRO! (hydro-locking the motor by filling it with gasoline, this is also distinctly non-bueno. can bend pistons, valves, and your leg from repeatedly kicking yourself in the *** for trying to get away with it)

Both L and LH feed fuel from the front of the rail to the rear. The "IN" from the pump feeds to a damp(er/ener) in either L or LH, then the fuel flow is 'split' to feed the individual fuel rails on either side. This initial damper is to help mitigate the pressure waves that come out of the Bosch roller cell pump.

This is where the divergence occurs between the L/LH 2V and the LH 4V.

On the L/LH 2V cars, the fuel exits the back end of the rails, pressure is regulated in the rails by allowing a certain amount of bypass flow. The bypassed flow leaves the rails and returns to the tank.

The LH 4V cars have a setup that is, well, not exactly the same. The entrance to the rails through a damper is the same but then things change. Upon leaving the driver's side rail, the fuel passes through another damper (this is not a bad thing, since a damper really belongs on the end, or even in the middle of the rail, to damp the fuel pressure spikes that occur when the injectors cycle) then gets to the lone fuel reg. The fuel rail on the passenger side does not get equal treatment - it goes straight the same reg without the benefit of a damper to help mitigate the same pressure waves that happen on the driver's side rail.

Fair? Nope. Functional? Well, at the time it was ok.

Current knowledge dictates neither setup as ideal, the later model even less optimum than the earlier setup. Knowledge was not nearly what it is today, so by the time Porsche really knew they could do a much better job on the fuel deliver system, the 928 was selling nowhere near enough units to justify an update. Hell, it made plenty of HP, why mess with it?

Dampers are an integral part of effective fuel pressure management. They absorb the spikes that happen in the rail due to injector cycling and line pressure variences due to the output characteristics of the Bosch roller-cell pump.

For those who are going to ask, the 2V L/LH update/grade fuel rails are ready to test as soon as I get a coupla adapters that are on backorder. Ugh. That project is sooo far behind schedule.


Here's the commonly accepted current state of the art (applicable to the 928 anyway)...

Damper on the end of each rail, follow through to the reg.

There are 2 different philosophies when it comes to regs.

Phliosophy one: One reg per rail. Logic is that each rail is it's own environment, and the reg can react fastest and most accurately to the individual needs of the rail it controls by being close and not having to deal with pressure waves from another rail. Those waves (from another rail) may be out of sync with ones in the other rail, "bumping" the reg to bypass to release pressure which it doesn't actually need to do for the other rail.

Philosophy two: One reg is adequate and can handle the fuel needs of most street cars. The pressure waves are not a serious issue on a street car. Also, by running both rails to the same reg, you "average" the pressure to both rails and it's close enough.

Strictly speaking, you could do it either way. Some of the aftermarket regs (S/X, Aeromotive) even have a diaphragm built in to the reg to help mitigate pressure waves (their claim, not mine) without actually running a seperate damper at all.

As far as the solution suggested by the best available science (balancing against a realistic budget) it would be the following:

Damper on the exit of the rail. Pass through style (like the 928's OEM Bosch). Go straight to a fuel reg for each rail. Vacuum reference on the regs, not really needed on the dampers. "Dead-end" style dampers can also be mounted at a right angle to the rail, and are still pretty effective, though not quite as good as a pass-through style at lower pressures.

Dampers are really most needed on a lower pressure (~40PSI) fuel system. If you run a higher line pressure (~70 PSI) the effects of the injector cycling is less pronounced. Really high pressure (100+ PSI) race setups don't bother with dampers at all.

Rising Rate Regs: A solution, but more of a "workaround". They do provide extra fuel under high draw conditions thus providing you with (some) of the fuel of larger injectors when needed, without actually installing larger injectors and resetting the ECM pulse width to match the new injectors.

They work, and for some minor performance improvements like headers/cams/porting/etc., a RRR may be enough.

If you want optimum performance from a force-inducted scenario, there is no substitute for larger correctly sized injectors, and some sort of programmable engine management. Or at least, correctly sized larger injectors.

 

Thanks Mark. It's an 86.5 Auto. I did disconnect...no change(s). This hot start problem is kicking my ***. Today I'm tackling the PITA fuel pump check valve replacement to see if that makes a difference. Thanks again.
Tarek

 

Nice find Shawn!! I felt like I should study this in case there is a quiz about it. I am at this point trying to identify that thingy that is hanging off the cross brace, so I make sure I'm troubleshooting the right thing. My computer with data/info is still in the garage since the flood so the forum is the only resource I have right now.
Thanks again!
Tarek

 

if you remove the vacuum line then there is also a possibility that raw fuel will drip/run out of the now vacant damper, so if its leaking its best to plug the damper and the vacuum line till a replacement part is secured and it would be a better choice to not run the engine till the correct parts are fitted. If one damper has failed it might be a good idea to replace the other if you have one and the fuel pressure regulator then your good for another 20 years or less

 

No leaks from the fuel dampner or the two regulators.

So far the following was completed:

New FP and check valve
No leaks in dampner or regulators
Checked relays and fuses
Swapped ECU's
Verified pressure at the fuel rail

What I had original thought was fuel dampner turned out to be a vacuum limiting valave, but I did check the actual dampner and regulators and none were leaky.

This is starting to **** me off!!! I do not know how the alarm system works...yet, so I would like to disable it to see if anything funky is going on there

 

https://rennlist.com/forums/showthre...ight=hot+start

 

Thanks Shawn. Note that I posted on that thread...but mine was a cold start issue then. I did check the battery posts and chassis ground connection. That was my first suspicion, actually, knowing that Stan had gone through this before...no bananas. I will go back and re-clean, though, just to be sure
Thanks
Tarek

 

I guess one way to check is to reproduce the failure and go out and spray something on eth battery post to cool it down and see if that makes a difference!

 

Here is the part in question

 

If the starter can crank the engine, I don't think a faulty ground strap on the battery would prevent the car from starting. The starter draws more power then just about anything else so if the ground prevents the vehicle from starting, it should certainly keep the starter from operating. Then again, nothing surprises me.

My '85 also has a hot start issue from time to time. It's most evident in city driving conditions at low speeds that allow the temp to climb. If I shut the car down when it's hot and leave it for a while it can be difficult to restart. I think there's an issue with fuel evaporating up near the engine.

 

I actually had some vapor-lock problems with my car, and solved it by wrapping the fuel-feed line on the pass. side with high-temp insulation blanket. It's wrapped all along the section closest to the exhaust. Seems that heat-soak would boil the fuel out of the line, and the car wouldn't start or run until the line was purged by cranking forever.
It seemed odd to me that it would suddenly start this, but everything else checked fine- pump, pressure, accumulator, damper, reg, etc...and the insulation was all I did to fix it. Hasn't happened since...
Maybe or lovely Kalifornia fuel mix has higher aromatic content lower vapor pressure, or something.
I noticed when attempting to figure this out, that the fuel in the tank was hot enough to burn my hand when I changed the fuel filter, so that got me thinking about the insulation. Return fuel is still hot, but at least it doesn't get heated up even more by absorbing radiant heat into the line. Since then, I haven't had a problem. I have been thinking about some way to cool the fuel (besides running the A/C) that doesn't create a hazard or cost a fortune....

Hope that helps, Tarek.

 

I want to thank everyone who responded and provided insight/help. My problem turned out to be caused by bad injectors. One specifically had a broken tip sleeve. I'm off and running again. Thanks again

Tarek

 

thats great news so what did you replace your fouled up injectors with ? and where did you get them/price???