Chuck Jones

Sandman and Stephen: thanx for the info on the yellow relay....I took it out and noted that some of the pin contacts were a bit schmootzy...so I used some CDC to clean them up...car fired up but still has that sometimes hum that definitely comes fromt the front trunk area that is driving me nuts. I see three other porcelain fuses in there with the yellow relay that look like they came out of a Model A roadster. That's the first time I've been into that fuse box, so as a learning experience it was good. All I can figure at this point is that it may be the fuel pump trying to die...and doing so by degrees. If the pump were to get hot for whatever reason....would that cause it to stop....then reset itself when it cooled off? In thinking back over time....every time the car has just stopped....it always restarts after it has set for a while and cooled off....usually between 10 and 15 minutes.

Chuck Jones

OK....last entry....I went thru all the fuses and relays including the engine compartment and think I've narrowed this down to the front fuel pump. I've noticed that the first red relay closest to the firewall is getting warm to the touch....not hot, but warm.....much warmer than any of the rest of the relays in that entire row. According to the chart on the cover...the first and second red relays are marked Fuel Pump I and Fuel Pump II. All I can think of is that it's drawing more power due to some sort of short.

Is there a way to test the pump to tell if it's somehow causing the relay to heat up? I ordered another front fuel pump should be here in a day or two...Now I'll have to dig up the How-to on changing out the pump. Thanx for the input guys.

PorschePhD

The relays will get hot. No quesiton about it.

I wish I could hear how loud this hum is. There is always some noise form the pumps. It is when they make none you need to worry

Chuck Jones

I don't know how to do a sound clip....but I could tape record it and give you a phone call... How's that for a real hi-tech approach???

Someone said that the CDI unit will make a sound...but I replaced it with the MSD 6AL when it burned out. Does the MSD make any humming noise just as you switch the ignition on?

PhD: Are you saying that the fuel pump relays get hot through normal use? If that's the case, then maybe I'm making much ado about nothing. Just to be sure, I put in new relays....but it still doesn't solve the intermittent dead stop issue... Sandman had me go thru the yellow relay in the engine compartment so I cleaned up all those contacts and it seems to be working fine.

Is there any possibility that if the fuel pump gets too hot a thermal disconnect kicks in? I dunno...hope that wasn't a stupid question.

Maxx1

I checked my red front fuel pump relay when my car died and it was warm as well. I never hear anything making noise until my car starts. Can't you get the pumps to turn on by pressing down that air intake flap at the base of the air filter housing long enough to hear if the pumps are working or not?

JFairman

Hi Chuck,
I think you've narrowed it down to a bad relay or fuel pump. If you ever take a bosch fuel pump apart it's impressive. Fuel flows right around the electric motor armature as it's spinning so it is continuously cooled by fuel flowing through it. They don't last forever though and the normal buzz they make usually gets louder as they are on their way out.

My fuel pumps come on with the ignition before turning the key to start and sometimes they don't. They shouldn't come on till the starter motor is engaged.
I traced that to the air flow sensor plate not always coming all the way up to the rest position and triggering the green switch on the CIS unit that shuts off the pumps when the motor stops turning over and sucking air.
To check that take off the intercooler and then the aircleaner housing.
Then turn the ignition on. If the pumps are not running gently press down on the center of the air flow sensor plate.
You should hear the pumps come on as soon as the plate moves down a little and you will feel fuel pressure try to push it back up when the pumps come on.
Don't keep doing this though because fuel gets sprayed into the intake ports while the motor is not running, and thats not good.
If the pumps come on with the ignition then try lightly tapping on the plate with your finger to see if it comes up a little more and triggers that green switch which turns off the pumps.
That did it on mine.
I don't know if it's old greasy crud buildup somewhere in the mechanical linkage of the sensor plate or a small adjustment that mine needs. I've tried spraying carb cleaner around the sensor plate.
That little green switch next to the sensor plate could be going bad too.
If you remove the electrical plug from that switch the pumps will always run when the ignition is turned on.
Thats ok, but could lead to a fire if the cars in a bad accident, the motor is off, the ignitions on, and the pumps keep running...

It's a small issue, it's not easy to get at, and it runs fine so I havn't messed with it yet.

Hope this helps....

sand_man

Yes you can. Without having to remove the airbox (if still equiped) to reach the flapper, I guess you could give the spring loaded adjustment screw a good push down. You can also reach around the back (from the driver's side) and unplug the bluish/green electrical connection on the air/fuel metering housing. With this unplugged, and the key "on", you should hear pumps running...they make a lot of noise when the engine isn't running.

sand_man

picture of plug (backside view):

sand_man

I think maybe the only problem with pushing the flapper down to see if the pumps will come on is that you're also causing the fuel injectors to fire which could flood your combustion chamber. By just undoing the plug I pictured above, the pumps will run, but the injectors don't fire.

Maxx1

Yeah, that's a good point.... wouldn't want to hydro lock the motor.

I'll be watching this thread carefully. If the MSD doesn't fix my problem, I'll probably be taking this same path.

JFairman

Pressing the sensor plate down momentarily just enough to see if the pumps come on won't flood it or come anywhere near hydrolocking it...
Also the injectors don't "fire" like an electronic injector. They open mechanically and stay open when the fuel pressure gets high enough to open them.
CIS stands for constant injection system, also known as K jetronic.
Then they close when the pressure drops to certain point, like when you shut off the motor. Then the spring loaded diphram in the accumulater next to and behind the fuel filter holds that pressure against the check valve in the fuel pump for a day or so so the car starts faster next time.
I can't remember the opening pressure for the injectors but I think it's around 35-40psi.

A BMW 320i with CIS injection I used to have would have about 60psi fuel pressure at idle. I havn't checked it on my 930 yet.

When CIS injectors get dirty, and they get dirty because they stay open.. the spray pattern is no longer a finely atomized conical spray pattern, and they will not hold fuel pressure after shut down. They will then leak down or dribble fuel into the intake ports after shut down until the diaphram in the accumulater is at it's rest stop and pressure is near or at zero... flooding the motor and making hot starts take a while tll the excess fuel clears out.

Electronic injectors as used in L jetronic and motronic are self cleaning to an extent because the solenoid actuated valve is constantly opening and closing as the engine runs which keeps carbon from building up on them.

sand_man

Correct...I was using the term "fire", figuratively. I'm well aware they're not elctronically controlled. However, I do belive it's the downward action of the flapper that can cause the injectors to operate, provided there's adequate pressures. I only pointed it out so that Chuck wasn't constantly activating the flapper.

Chuck Jones

Great info here guys....good and easy-to-read roadmap for how to go about checking out the pumps. I had the intercooler off yesterday so doing it again will be a snap.....and seeing the internal parts of the CIS system is a great way to pick up a visual picture of what you folks are talking about. I'll go thru that flapper sequence this afternoon.

Could I get some feedback from the rest of you wheter or not the fuel pump relays (usually the first two red ones closest to the driver)...whether the fuel pump relays get hot or warm? None of the others in that whole line of relays gets warm.

Chuck Jones

I was in the process of taking off the air filter housing then re-read the thread Sandman posted about reaching around back and taking off the green plug....Since the air filter housing is a bit difficult to get to that 10mm nut underneat it....I tried reaching around for the green plug.....and I found the end of the conector hanging loose....someone had unplugged it.

I have no idea why it's unplugged or what that's going to do to the performance if I plug it back in....but Yutta' Hey...Porsche put it there for a reason....so I guess I'll just try to clean up the contacts with some CDC cleaner and plug it in. Maybe that's why I'm hearing the hum from what I assumed was the fuel pumps....because they were in fact runing due to the green connector being unplugged. This is getting curious. Why would someone opt to unplug that connector? Why would you want to bypass the safety feature that would shut down the fuel pumps in the event of an accident?

sand_man

It's a very comon thing to do: disconnecting the plug to insure the fuel pumps engage when the key is switched on (car not actually running). For a short while, I did the same. Some just like to know that they're (the pumps) actually running. And honestly when testing/checking your WUR and other CIS fuel pressures, the manual requires the plug be disconnected. I dunno...tough to say, Rocket, probably has a good thing going with the intertia switch. At least we know now , that the noise you were hearing was the fuel pumps!