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Let's not forget it does fire up and then die. Sometimes, it stumbles and dies. My recommended plan of attack is to jumper the fuel relay and listen for the fuel pump. If no fuel pump case solved. If it does, try to start the car. If it starts that would indicate a fried LH fuel circuit (BTDT). Then swap the GTS LH into your 87 S4 and see if that car starts. If it does then we could look else where.
Mine did the stumble-and-die, caused by a failing fuel pump relay. Fuel pressure test uses a jumper at the relay socket, so the relay is out of the circuit for that specific test. You'll hear the pump (or not...) running at that time. The internal pump should also run, no good way to test that one without disconnecting wiring at the main pump, then listening for the one in the tank.
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There's No Such Thing As A Coincidence. Really. There is always reason.
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The internal pump has a nylon 'sock' strainer at it's inlet, protecting the pump from getting jammed with crud in the tank. You can see the sock looking down from the level sender port on the top of the tank, but I wouldn't consider opening that until a few gallons have been drained from the tank. If the strainer has failed, or is plugged up with new junk from your gas station, you'll see it with the fuel pressure test of course.
Generally, fuel filter/strainer problems won't keep a car from starting. If you managed to get a load of bad gas with a bunch of crud in it, it can suddenly plug and keep the engine from starting. Worse, if the strainer fails and some of that crud gets drawn into the pump(s), they will lock up completely even before the filter sees enough crud to block fuel flow.
Remember there are 2 fuel pumps on a GTS. My in-tank pump just gave out at the 21 year mark.
I hope that isn't it because Porsche just doubled the price to $800.
Driving down I85 in Atlanta this morning, doing about 30mph with heavy traffic and car just dies. Was able to coast into center median. Car will crank and fire but then immediately dies. Have done nothing to her other than fill up at the BP with 93 octane this morning. ...
This is key, I think. If it cranks and fires then that means (a) there is ignition (which means the CPS is OK), and (b) there is at least some fuel pressure, which probably means the fuel pump is OK.
When starting, if the EZK picks up the CPS signal then it does two things: Fires the spark, and tells the LH that the crankshaft is rotating. The LH, in turn, powers up the fuel pump(s) and starts firing injectors-- for a fixed pulse-width (about 3 ms) at first.
At this point the engine fires up (albeit briefly), correct?
Once the RPM's come up above cranking speed then the LH shifts to the "running" mode, and uses the fuel maps to determine the injector pulse-width. At this point the engine stalls, if I understand correctly.
What this tells us that things bad when the LH shifts to the "run" mode and starts using the fuel maps. That could be a LH failure, but perhaps more likely would be a sensor problem. The LH needs two inputs for the fuel map: RPM from the EZK (which comes from the CPS), and engine load which comes from the MAF. (Engine temperature-- Temp-II sensor-- also comes into play as does the O2-sensor-- but they won't cause the start-then-die symptom).
There is good evidence that the CPS and EZK are OK, because the engine fires. while cranking. Ditto fuel pump, it wouldn't fire at all if the pump were toast, or not getting power from the LH.
The MAF is a prime suspect: It is not used for cranking, but is critical for engine running.
The LH has a "limp home" mode for the MAF where the LH, if it detects a dead or disconnected MAF will inject a fixed pulse width (two actually, depending on RPM) to allow the engine to idle and run badly. That is not the case here, this engine won't even run badly, but not all MAF failures are detected by the LH.
What to check first depends on what tools and spares are available. If there is a fuel gauge with a 928 fuel-rail adapter in the toolbox then slap that on first, crank the engine and confirm 55 psi or thereabouts.
The shade-tree fuel-pressure test is to crank the engine (to run the pump, if it runs), then surround the front of the pass-side fuel rail with rags and crack open the cap. If it sprays fuel then there is pressure, if it only dribbles then there isn't. (When removing the cap, e.g. for a fuel-pressure gauge, keep your eye on the bearing-ball as it bounces towards the street. You will need it.
If there is fuel pressure, and you happen to have an '87 S4 sitting in the next bay, then respectfully borrow the MAF and try that, then the LH. Then spend a little time cleaning and polishing the '87, there are things at play here that we do not understand.
If still no go, then confirm spark, confirm fuel pressure, confirm the injectors are firing (e.g. fuel smell in exhaust), then check compression. Also check for flooding, pull the spark plugs and have a look.
So didn't get home till 8pm so no daylight but did a couple quick checks. First pulled fuel pump cover nd had my daughter turn on key, then try to start car and got nothing out of the pump, couldn't feel anything while holding it.
Next I opened up CE panel to install a jumper at fuel pump relay and found this, there is a solid yellow and solid Brown wire going into the fuse holder and it disappears behind the panel Any ideas as to what this is? As Dr. Bob said, I don't believe in coincidences!
Looks like the fire already happened, inside that fuse. Whatever that is needs to go away and whatever it was 'fixing' needs to get fixed. Fuel pump is fuse #38 (the blue 15 amp underneath this auxillary fuse holder in the pic), if that is somehow tied into 38 that might explain your sudden no-fuel pump action.
If you clean the solid brown wire does it turn yellow again? Maybe it just got toasty when the fuse melted.
Last edited by Rob Edwards; 09-08-2016 at 01:17 AM.
Unfortunately when a fuse melts in place, the only fix is a bypass fuse unless you enjoy putting in a whole new CE panel. Which is always the better option, true.
09-07-2016, 01:40 PM
