Dr_Goose

My friend just bought a 928 S4 and has some starting/idling issues. The diagnostics listed here seem to be a good place to start.

Is there instructions or an FAQ on how to find and jump the right relays?

Thanks.

I have a friend with a 88 928 with a devek 6 liter stroker on it. He called me trying to find out why he couldnt get it started, plus when it was running, it was not idling right. It was stalling at idle, with the idle hunting badly, like the idle stablizer valve was bad or the idle switch was disconnected.anyway, we got it started by going through the normal tests. spark, yes........... jump fuel pump relay, yes, it humms................ turn the key, no hum from relay. jump the EZK relay, no change............... jump the LH relay and............YEP, it starts.
928 international recommended a relay that can be bought for cheap. $12 and he put the relay in, not only does it start rigth up as expected, but it runs like a champ and the idle issue is completely gone. Has anyone heard of this? Idle is higher, near 1000rpm vs the 600 and hunting it was doing earlier. i wonder if it was a voltage or relay issue feeding the LH box??

fraggle

The LH/EZK training guide is a good one. I think it's on the moorehouse Cd's, but I have a real paper copy.

dr bob

For a new owner with a new car and unknown history, it's probably a good PM task to replace all the critical engine relays. Premium relays can be a bit more than $5, worth getting with one of our sponsor/vendors to make sure you don't end up with something less that totally reliable.

The short list:

Fuel Pump relay in position XX
LH (fuel injection brain and injectors) in position XXV
EZK (ignition brain) in position XVI

All of these can use the common 53B relay. The EZK relay installed from the factory has a fuse tester built in, but otherwise is a 53B.

The ignition X-bus relay in position IV is also a candidate.


The car has/had a nice fuse and relay chart attached to the door on the central electrics panel, under the forward carpet in the passenger's footwell. 928 Specialists has a copy available on their website www.928GT.com, with handy links to make it easy to buy replacements.



All the relay stuff aside, there are more than a few things that will cause the poor idle/stumble/hard to start symptoms, too many to just say that there are one or two fixes that cover everything. First thing's first, and that first list should include catching up on all the normal maintenance stuff. New plugs, caps, rotors, and wires make that list if there isn't documentation showing that they have been replaced in the last 50k miles or so. New oxygen sensor too. New plugs regardless, since they are cheap and it's an easy way to see how the cylinders are firing.

Poor idle is often the result of partially-fouled injector nozzles, and/or a tired mass airflow sensor. These aren't cheap enough to just replace them without a little confirmation. But if the originals are still in there and the car is over 100k, it's probably due.

Vacuum leaks from tired or split hoses will cause the car to idle poorly. There are a bunch of hoses and connections that gather under the intake manifold. A bit of false air leaking into the engine confuses the controllers.

The idle air control valve under the intake is sometimes a problem, and function is tied to the throttle position switch. Both would be checked and usually replaced as part of a full intake refresh effort.


Lots of things to think about. Relays are relatively cheap and easy place to start.

Eric McCall

Hi Team
I have been plagued with rough idle now for years on my 89' S4. I give up on it for a while and continue to drive my 85' S2 which I adore after owning it for 27 years.
My 89' starts well but after a few minutes warming up it's starts to idle rough and stall. If driving it and exceeding 1000 rpm plus it runs beautifully at smooth power unless I strike traffic or traffic lights. Rev's drop below 1000 rpm and it stops. I have to find neutral, restart and try to keep rev's up and ignore the toots from the SOB's behind me. I have to ride the foot brake with my left foot like a clutch and keep the gas up for idle to exceed 1000 rpm to stop stalling.
I have replaced the spark plugs, The O2 sensor, the Idle stabilizer valve twice ( once with a 2nd hand unit then a brand new Porsche one ). The car improves for a moment giving false hope Do I do the replace all the relays as mentioned in this thread ?

I have had the car recently in the Porsche main workshop for a week and they said they couldn't work on it because it won't idle. The said they worried it might bend a valve so I took it back ( with a flat battery ) and drove it home in pouring rain, bumper to bumper traffic riding the brake and gas pedal hoping I didn't stall it as I had no jumper pack.

When I dropped it off at Porsche in the first place my Son drove it in and I followed him. I did notice the car was running RICH and after the freeway trip in and reaching traffic it was running fuel smoke and smelling rich. Is this a clue .

Help please,
I will order the following - Fuel Pump relay in position XX
LH (fuel injection brain and injectors) in position XXV
EZK (ignition brain) in position XVI

All of these can use the common 53B relay. The EZK relay installed from the factory has a fuse tester built in, but otherwise is a 53B.

The ignition X-bus relay in position IV

Regards

Eric
Dragon Red 79, 85 S2 and 89' S4

John Speake

Where are you ? As you talk about an S2 I assume you are in the UK ?

Eric McCall

Hi John
I am in Auckland, New Zealand.
I have been to the UK and have been on Rennlist for years & years as Dragon. Many mutual friends on this list.
I have three 928's but its the 1989 S4 that I have all the problems with.
Regards
Eric

John Speake

OK, thanks. If you think the car is running very rich, then the idle control loop can't cope with that situation. Is the car cat or non-cat ?
Try unplugging the MAF to force limp home mode. It should then idle reliably, although the mixture will be a little rich.

I assume you have checked the MPG display shows a very high MPG when coasting down from 3,00rpm with the throttle fully closed ? That will confirm the
close throttle switch is OK.

Eric McCall

Thanks John, the car has no cats as they are not required in NZ at the moment on older cars. I bought a O2 sensor replacement only to discover I don't have an O2 sensor fitted lol. I will unplug the MAF in the morning and try. It's nearly midnight here in NZ. I have never checked the MPG display as I never thought about it. After having the S2 as my daily driver for years I never worried about fuel consumption, I drove it like it was stolen.

FredR

Eric,

Based on what you have described the two things you need to prove are that the idle switch closed contact is making [John described this]- if this fails the ISV cannot function and your idle is toast to some extent- however my experience of this failure is that the neutral position in the ISV allows sufficent air to form a reasonable idle without the a/c operating. If the a/c is operating there is no correction for the aditional load and the engine will die unless you fether the throttle.

The other system that seems to cause issues with hot idle is if the temp 2 sensor fails to read correctly and the the system throws in fuel for a cold engine when in fact it is warm - these things are well known failure points.

If the car runs fine when above 1k rpm then I doubt you have a serious MAF failure but if the car has the original MAF fitted to it then coincidentally but not relevant to your original query, there is a good chance that your MAF will need John's magic touch sooner or later. I assume you know but just in case- John is the "go to man" for issues with the MAF or the LH computer which every S4 will suffer from sooner or later and these days mostly sooner [or already]. My late S4 suffered from a tired MAF about 12 years ago and same for its LH computer that fortunately I had an available spare at the time.

I also note your system is similar to mine in that I do not have cats fitted. Such configuration has a resistance pot that has to be adjusted correctly to give the correct idle mix- I doubt you will have an issue with this relevant to your current problem but it is something you should be aware of. I tune my motor with John's ST2 kit and idle can be a bit quirky - I find that idle is more stable when the AFR is at 14:1 - a tad rich.

Rgds

Fred

Shark_gts

Combustion aka Fuel/air mixture is controlled by the O2 sensor loop post warm-up. So what about the signal from the sensor?

If it's running "rich" then LH could be thinking there is too much O2 coming down the pipe.

The control loop will trim based on that reading.

Sensors should be replaced every xxx mi for precision control

FredR

What O2 sensor?