Dear John,

You have gone off half cocked like Snowball. A lot of technical mumbo jumbo, but no technical facts to back up you premise that the operating fuel pump pressure is controlled by an electric pressure switch. <img src="graemlins/nono.gif" border="0" alt="[nono]" />

True the fuel system, with the addition of a pressure accumulator and pressure switch, could be made to turn the fuel pump off and on. The problem in using that sort of set up would be that the pump would need to be turned an few seconds before starting the car to build up pressure and continuously cycle in operation. Some fuel systems are set up this way. I do not see anywhere in the system electric or hydraulic circuitry that indicated that this is the way Porsche has set it up on the 928...if you do please explain where.

It looks like Posche opted for a performance trade off and went for the advantage,without having a pressure accumulator in the system, the 928 fuel pump would not be rapidly going on/off as the smallest amount of fuel is used by the engine.

The advantages in not shutting a fuel pump 0n/off frequently are: 1) less fuel pressure flux to deal with 2) less heat build up in electic fuel pump from starting current. 3) less rapid relay solenoid and contact point cycling load.

If you have a factual basis for your ranting about the Kids from Zuffenhausen not being that stupid....please enlighten us all with facts.

hawke <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

 

Boy, this is nearly as much fun as the V word controversy!
Even before EFI came along there were (and are) engines with recirculating fuel systems - arduous duty or high engine bay temperature units. My Carbed 1974 Rover V8 has an AC mechanical pump with a return line to tank, also common on Range Rover Carb models. My pump has been overhauled, but not in 7 years of my use. All in the interest of preventing vapour lock by cooling the fuel back in the tank, instead of letting it cook in the engine bay.
The easy way to find flow rate is ... RTFM. The pump test usually gives cc/min delivery rate - does in LR manuals anyway?
jp

 

Tremendous help and many thanx---

-I am pretty sure that the problems with my beast have to do with the injection. But when I get home, I am going to do something that may sound a bit silly just to be sure.....I'm going to take it to precision tune and let their "technicians" put the ignition on their occilloscope ($30 = cheap and accurate check)just to be sure.

Some thoughts:

-I'm pretty sure it runs on 6-7 cylinders for a while after I return to Orlando. No, I haven't pulled plug wires while this is going on in order to find out just which ones are misbehaving, as I believe that the real problem lies with contamination of the whole system, and that these two injectors are probably just the worst of a rather unhappy crew. I was shopping the injectors (a parts store in Orlando had them for $99 each; they are unique to the S2) but I suppose it is best to remedy the contamination in the lines before I drop eight bills for new squirters!

Has anyone actually gone through the trouble of cleaning out their undercar fuel lines? Have any of you taken apart the fuel pumps and cleaned them out by hand?

Now, the idea of running the tank low, adding a bunch of injector cleaner (like....several cans), and then jumpering the pump relay socket to run the pump with the engine off appeals to me. This sounds like a great way to clean the system without plugging the injectors, provided one flushes the tank out after a while, adds fresh gas, and then jump the fuel pump again for a while to make sure nothing hits the injectors. A filter change would be necessary before starting the engine, of course.

NOW....we are certain that the pump runs continuously, that this type of use will not hurt it...right? Any idea how much injector cleaner should be put into the tank in order to do this?

And wouldn't it be wise to pull the connector off the injection computer while a battery charger is hooked up?

Your thoughts are appreciated!

N~

 

Norm,

Thirty bucks is the cheapest mechanics time cost you are going to find on a 928, so why not. The problem is that I am not sure if dirty injectors will show up on their normal engine test which would allow you to pick out which injector is the problem, other than it running rough. Maybe one of the experts in this area can add something about test equipemet capability.

Pulling the plug wire would work only if the injector was fully plugged. Partially plugged and everything in the idle management system is thrown off. The system is getting bad readings and sending bad responses to correct for the bad readings. Any plug wire pulled would make it run even rougher.

When I installed my new injectors, I accidentally didn't make a good electrical plug connection on one. <img src="graemlins/nono.gif" border="0" alt="[nono]" /> When I first test drove the car, with one injector not working totally, it was nearly undriveable and ran much worse than simply with a dirty injector partially plugged.

Running the fuel pump without running the engine, to keep he injectors from plugging while cleaning sounds like a good idea. But, if you think about what is happening at idle it may be over kill. Getting back to the erruptive recirculation flow numbers discussion. <img src="graemlins/c.gif" border="0" alt="[ouch]" /> At 60 mph (4 gal/hr of fuel)...90% of the flow is going back to the tank. At idle, you are using even less fuel so, 95% plus is going back to the tank. Less than 5% is going to the injectors reducing the chances further of the build up particles breaking off from the fuel lines plugging an injector.

With that said...I still like someones idea of running the fuel pumps without running the engine to clean the system up.

Less than a half full tank and three cans of Berryman worked for me. That way after it is done you can top off the tank and have not much more than what is recommended on the can of injector cleaner in the tank.

hawke
<img src="graemlins/burnout.gif" border="0" alt="[burnout]" />