Kevin in Atlanta

I am going to have recheck my findings. As of now the fuel pump only runs when its relay is jumped. Earlier tests indicated there was continuity from the connector to 85 pin. Injectors only click when the engine is cranking. I am using a known good set of computers. Unrelated to all this is the fact that no fuel is getting to the rails. I know there is gas because it was present when we swapped out the fuel pump over the weekend. Very interesting.

soontobered84

Are you sure that the fuel pump is pumping and not just humming?

Kevin in Atlanta

That is what we are going to find out tomorrow.

soontobered84

Are you going to remove the cap and ball on the passenger side fuel rail, attach a rubber hose leading to a fuel container, and jumper the fuel pump relay expecting to see fuel seething from the aforementioned rubber hose into the fuel container at an acceptable rate of flow all the while admiring your close, accessible, fully filled and newly certified fire extinguisher?


That's how I think I'd do it.

Kevin in Atlanta

Car came equipped with electrical shutoff at the ground strap. So the plan is to jumper the fuel pump relay, leave the shutoff in the off position, run a hose from the metal fuel supply line to a plastic gallon jug and then turn the power on briefly.

Kevin in Atlanta

Nothing at the injector rail cap. Have fuel into the front regulator. Brand new regulator per the PO. PO said the car ran prior to his son replacing the regulator.

Going to pull a used regulator from my 87 parts bin.

John Speake

That unit is a fuel damper, not a regulator...

soontobered84

This is gonna be an easy fix.......

Kevin in Atlanta

We have fuel pressure! Waiting on my son to return with the keys he ran off with by accident last night.

I always confuse dampners and regulators!

Kevin in Atlanta

When jumpered the car starts and runs. Idles but dies when gas is applied. I am going to swap in another MAF sensor

Also, I tested the computer that I used to get the red S4 running in my 87 S4 and it will not start. This was a known a good computer from a recently acquired 88 S4.

Can a corroded fuse (seen above) fry a computer?

Hilton

Is the known-good computer a rebuild? Known good on an original LH is just tempting fate - I've had an original LH which tested good (and worked with ST2 during testing) fail doing nothing but sit on the shelf inside the house for 12 months. I've also had an LH which I spent 100 hours of my time chasing emissions issues on and 200 km's of driving, before it finally died completely (I actually pushed the car out of the emissions test facility with clicking injectors when not running, about 2 minutes after it passed the test on the 6th test centre visit!).

The one you took pics of in the OP is not a rebuild - it has the original Bosch "tile".

It appears you're looking at more than one issue here. The good news is it appears you have two S4's handy, which makes diagnosis easier because you can verify parts between them. I think you're right questioning both the LH and the MAF as a starting point.

I'd also be testing the other variables like temp II sensor readings at the LH/EZK connector pins, throttle idle switch at the LH, and fuel pressure with the pump running as well as fuel delivery into a container. (amusing story - one of my 928's I bought sight unseen had a cheap ebay imitation fuel pump fitted which was sold as a pump for a 930)

If you have an o'scope then also testing the CPS pulse, as well as the injector circuit pulses would also be a good idea.

Other stuff like knock sensors and hall sensor will affect power when running, but not starting, so don't worry about them until later.

Kevin in Atlanta

After swapping in the MAF from my 91S4 the car ran great, albeit with the fuel pump relay jumped. I know the LH out my 88S4 was good because fellow rennlister jettison used it on his 87.

I just think I faced with rebuilding a couple of LH computers and MAF. I also think that the CE board needs a serious deoxit treatment.

soontobered84

On both my 84s, the fuel pump had to be jumpered because IIRC, the fuel pump relay gets it's ground from the LH. Both of my LHs needed rebuilt. However, I did drive the Stepson to and from SITM in 2005 with a fuel pump jumper on which I installed an inline switch so I could turn it off without disconnecting the jumper. Once I installed the rebuilt LH, I didn't need the jumper.

John Speake

If the car runs with the fuel pump jumpered but not without the jumper wire, it means that the relay has been jumpered with the incorrect pins at some point. I get quite a few where the FP driver stage has been blown up due to that mistake.

soontobered84

See?? You learn something every day on this forum. Thanks John for the information. I can thank my POs for blowing up the FP driver stages.