Prof Requiem

Greetings everyone,

I have been working on a 1981 Porsche 928 auto since last May and have made little progress on it. There are two issues (among others) that I would like specific help on.

First, the fuel pump relay is missing and was apparently jumpered by a previous owner. Will attach photo below.




As you can see in the photo, it has been jumpered from the top left small slot to the middle right slot. I have tried to replace the jumper with an actual relay on three occasions and the relay sent to me has always been a Kaehler brand. The Kaehler seemed very poor quality and the first one in particular fell apart when I tried to take it back out (the housing is very sketchy and pulls off very easily). None of the three have activated the fuel pump at all (the second and third I bought I was trying to get a Stribel brand but both were mislabeled - the third company's customer service guy said that the Stribel brand has been discontinued) except the first one did somewhat randomly when I had pulled the housing off and wiggled the guts of the relay, but it was intermittent and never could get it to stay on without putting pressure on the internal wiring of the relay (weird).

The car runs with the jumper and activates the pump (audible) as soon as the key is turned to the on position, but I really want to get this fixed with a proper relay. My questions are whether the brand would make a difference and how bad is it that I have been running it with it jumpered in this fashion?

And second, the car has a very strange stalling issue. When first turned on it idles fine but after about 3 miles or so of driving once it warms up the idle gets rough and low (400-600 RPM) and sometimes will stall when stopped. The warning that this is going to happen is that the oil gauge starts to dip, when running at speed it will be up at the 5 mark but as the engine slows it dips progressively until the 1-2 mark range, which warns me that it will want to stall if at idle. I keep it from stalling by braking with my left foot and resting my right foot on the gas just a tiny bit so it keeps the RPMs around 800-1000, but this is an annoyance to say the least and certainly not a long term fix. Also, there is an audible whine from the engine compartment at around 1000-2000 RPMs that is somewhat random and may be related but I have been unable to pinpoint its origin.

I have been chasing this issue since day one of owning the car (not revealed to me by the seller, arg!) and while it has gotten better, it still idles rough and will stall occasionally (although not as often or fast as before). Here is what I have done so far trying to fix this issue:

1. Replaced the Temp II sensor
2. Replaced rubber boot on one of the spider legs that was cracked and split
3. Played around with idle screw to no avail (and it appears a previous owner had done so as well as the idle screw was way open when I first started messing with it)
4. Cleaned the air intake and MAF sensor
5. Replaced the O2 sensor

The idle issue improved some from these operations but it still has not been fixed. Any help would be greatly appreciated and thanks in advance.

Speedtoys

And the jumper isnt hurting anything, but on my S4, it will kill the battery if left there when the car is off.

Brand of relay isnt a big issues, its a pretty standard relay. Use the horn relay for now if you like.

bureau13

I took a quick look at Alan's CE panel sheet for '81...it doesn't look like the fuel pump and horn relays are the same for that year.

My first thought was "Hey that's not the fuel pump relay" because I'm used to the '86, so I had to check it out. Of course, it is the fuel pump relay, but it's not a 53B (doesn't appear the horn was either, back then, but I digress)

Mrmerlin

first find your correct relay for your car, go here ,
it should be
928.615.113.01

www.928gt.com

find the fuse relay chart.
print off a copy .

NOTE as Jeff points out the 53 relay is not the correct part for your 81

soontobered84

Jeff,
On an '81, the relay is specific to the fuel pump, and a horn relay will not work. Horn relay swap only works from '85 and up when Porsche used 253B relays in most applications of the Central Fuse panel.

Prof,
Are you getting your parts from one of the Big 3 or 4?

A jumper should not hurt anything at all if jumpered between 30 and 87. I would add wire length to your jumper and also add a switch, so I could turn it off and on at will without climbing up into the passenger footwell while testing. You just have to remember to shut it off when done testing.

I think your PO is a culprit as far as your stalling goes. The car is running on the WUR, gets warm, and then the previous mal-adjustment of your idle screw is giving you fits when the engine is warm. Get the idle screw adjusted correctly and that should solve some issues.

Prof Requiem

Thanks for the quick replies guys. The relay is a Kaehler brand that was sent to me and it is the right part number, still doesn't work though. I have the third one still haven't sent it back yet. I can post a pic to be sure, but it's still a mystery to me why it (or the other two previously sent to me) won't work. Also the jumper splice runs the pump but only when the key is turned on, so it hasn't been draining the battery. It's audible when it turns on as soon as the key is in the on position, whilst the Kaehler relays don't make it hum and the engine won't start with them. The first one I got did make the pump turn on, but only after the housing had come off and I literally squeezed the guts but would turn back off as soon as I let go, very strange.

Thoughts?

Also any insights on my stalling issue are greatly appreciated as I have been tracking that down for many moons.

Prof Requiem

Prof,
Are you getting your parts from one of the Big 3 or 4?

- Most recently I got my part from import replacement parts, but it is the same item everywhere as far as I can tell, and all three relays have been the same Kaehler brand.



I think your PO is a culprit as far as your stalling goes. The car is running on the WUR, gets warm, and then the previous mal-adjustment of your idle screw is giving you fits when the engine is warm. Get the idle screw adjusted correctly and that should solve some issues.[/QUOTE]

-Sorry but what do you mean by PO? Also, is there any sure-fire method for adjusting the idle screw? I had tinkered with it some which was when I realized it had been opened way too much and so I tightened it down as tight as it would go reasonably so without forcing it and haven't messed around with it again, figured it was time to get on here and start a thread...
Any idea why the oil gauge would dip?

bureau13

Big 3 = 928srus, 928 International, 928 Specialists...I would be sitting in a pile of car parts, weeping uncontrollably were it not for them. They'll make sure you're getting the right parts, as they're owned and run by enthusiasts who drive and love these vehicles like we all do.

PO = Previous Owner.

If the relay is actually good, then whatever signal is supposed to come in and turn on the fuel pump is not getting sent. This is from one of the computers on my car, after it sees inputs its looking for, but your car is likely different. Still, the relay is not just a jumper in a box, it needs someone to tell it that it's OK to turn on the pump. That being said, your description of the relays you've been getting doesn't make me confident that they're good, working relays, unfortunately...which is where the afore-mentioned Big 3 come in...

soontobered84

Open the idle screw 1 full turn to start. See if that will keep your warm engine running. From there, use increments of 1/4 turn (open or closed)

My inclination is that oil pressure dips when engine bogs down. What oil viscosity are you running? It should be 15-40 or 20-50.

James Bailey

having the fuel pump jumpered to run is a safety problem as well as a potential to damage the engine. First if you were to crash the car and catch on fire...the fuel pump will pump gasoline until the battery wiring or the fuel pump burns up...NOT GOOD !! The relay shuts off the pump IF the engine is not running.
Second if you or someone else levels the ignition switch key on to listen to the radio...work on electrical issues etc. If you have a leaky injector it could fill a cylinder with gasoline....then when you crank the car the piston tries to compress liquid gasoline if it starts on the other cylinders then all that force will hyrolock the cylinder full of fuel and ether crack the cylinder, bend the connecting rod or lift the head.....none of which are good things.

dr bob

[apparently typing while Jim and a couple others were typing...]

So... I'm away from the critical docs, so bear with me.

The early fuel pump relays use a tach/strobe signal from the ignition box to close the relay (pin 31b, the narrow one), sending voltage/current to the pump only when the engine is running. The relays also send a couple seconds of current to the pump immediately on key to run position, with circuit 15 active. The proper relays aren't cheap as the 53b's can be, and a failure on the road leaves you stranded somewhere. In a closer to perfect world you'd pop in your known-good spare, in the less perfect than that world you'd put your handy jumper in from the 30 (power supply from battery) pin to the 87 (wire to the fuel pump motor) pin, and it would stay there long enough to drive directly home. At home you'd pull the jumper, install a new relay, and everything would move back to perfect-world status.

But... It's also possible to put a jumper directly from the 15 circuit (shown as relay pin 15 in the wiring diagram) to the 87 pin, and have the pump run whenever the key is on regardless of whethjer the engine is running. If that's the condition you find when you turn the key to 'run' position, this is the way your jumper is installed.


The downside of this configuration is two-fold:

1) The original relay configuration is for safety reasons, so that when you hit something hard enough to stop the engine and daze the driver to the point where he/she doesn't turn the key off, the pump won't continue to feed the underhood and soon whole-car fire.

2) The relay also brings pump current through the relay contacts between 30 and 87, via the fuel pump fuse that's in the circuit between the battery and the 30 pin in the relay socket. With the jumper configured as you have it, the fuel pump motor is drawing current directly through the ignition switch contacts, the too-small wiring between the switch and the 15 pin socket, all with no fuse protection.

Recommendation: Don't turn the key on with the jumper between 15 and 87.

-----

The workshop manual wiring diagrams show you the relay connections, including the 31b connection to the ignition unit. The pulse connection is shared by a couple circuits including the tachometer, so if your tach works it's likely that the ignition module is OK. That said, amatuer technicians, in the heat of trying to get things to work, sometimes poke probes and wires into places they shouldn't. The fuel pump relay depends on seeing a pulse drawn low (close to ground potential) by the ignition box. If said technician decided to put current from the 30 pin into the 31b pin, it's easily possible to damage things. Verify that the connection between 31b in the relay socket is intact to O7 and Z4 plugs. If you have an old analog voltmeter (the kind with a moving needle instead of a digital readout) you should be able to see the needle swing as you crank the engine on the starter, with meter on DC volts scale, black grounded, read on any of those terminals (31b in the socket, O7 and Z4 in the CE panel) with all plugs still inserted. If that doesn't show up, verify wiring integrity with an Ohm meter, battery disconnected and ignition box unplugged. You may need to replace the ignition box if the tach strobe circuit is weak or non-existent.


Hope this helps!

James Bailey

Bob I like your post the best

soontobered84

Jim and Bob,
I liked both of your posts, so I edited my previous post to reflect more what I meant to say

bureau13

I was just keeping the OP interested until one of you smart people showed up

James Bailey

I was going to post... "It takes a Village" but then realized who made that phrase famous ......