inokiyo

Hi All,
Thanks in advance for taking time to read a newbie question.
I bought an 1988 928S4 Euro model, my first 928. Now I understand your obsession!
I do/did have a few problems, and thought you may have some advice.
The vehicle arrived with a dead battery and running very rich, and at one point, would not fire up. I cleaned the spark plugs, which was dry but covered in black carbon deposits. The car fired up again, and though still running very rich and rough idle, took it to a shop refered by a porsche enthusiast. During the 1 hour drive, the car started to run a lot better, but I thought I would have the mechanic look at it since I was there anyways.
They found a mess under the intake manifold (valley?) and replaced the following;
New battery
Knock sensor
Thermostat housing
Cooling hose
Breather hose
Idle control valve
vaccum lines
fuel pressure regulator
The car idles a lot better when cold, but after about 15 minutes on the road, it starts to idle rough (rather goes up and down), and is till running a bit rich (better than before). I have no problems driving it, but when at a traffic light, it almost feels like the engine will die at times.
Any suggestions, as to how I can tackle this problem? Is there a moethod where I can diagnose if this is an O2 sensor, temp sensor, or an LH brain problem, etc? Or is it better to start replacing parts anyways; from the least expensive down to the LH brain?
Thank you all for your past tips and future as well!
Ken (inokiyo)

John Speake

Hello Ken
You need to work methodically through the various possibilities, so not to waste your money :-)

Did the repair shop run a diagnostic tester on the car ? That would have shown up the more serious faults.

Temp senor is unlikely to be the problem, as when faulty the LH defaults to "hot" settings.

O2 sensor could be bad, especially if the car has run very rich for some time - it might be contaminated.
Disconnect the battery, unplug the O2 sensor at the round 3 pin connector on the floor infront od the fuse/relay panel, reconnect battery and run the car. This forces it into a default mode, and it should run OK, other things being to spec.

There can be a thermal fault with mAFs when they make the car run rich, after a few minutes running.

To check this, insert a wire into pin 5 of the MAF so you can monitor MAF volts with the MAF connected. Swicth on ignition, no need to start the car. The volts should be about 1.8v to ground. Wait for 10 mins or so, and check again. The volts shouldn't has risen much in that time. Certainly not above 2.0 v If they have, the MAF has the thermal problem.

Let us know how you get on.

inokiyo

Hi John,
and thanks for the info.
I will have to ask if the shop ran a diagnostic sensor, but they were not sure what the problem was either. The mechanic did say it maybe the O2 sensor, or the LH brain. I am thinking of replacing the O2 sensor anyways, since from what I have read, they need to be changed periodically anyways. Please forgive me if I am wrong, but I think I read somewhere that the O2 sensor does not have to be the stock porsche one, but one that is less expensive can work fine. Would you know if this is true? And if so, what should I get?
Nay info would help.
Thanks in advance!!
Ken (inokiyo)

khalloudy

if your LH is dead, it will default and start running rich. I would run diagnostics first... take the car to a mechanic that has either the Bosch KTS-300/350 or JDS Spanner or Bosch 9268. If you mechanic does not have it, take it to Porsche, they have the tester. There are many potential problems, but as John said, I would suspect either LH, MAF or some sensor somewhere. Best of luck.

John Speake

Hi Ken,
There was a very recent thread on this list about alternative O2 sensors. Do a search for it (top of this screen) USe the 928 list filter.

When car is cold it is off loop, and as your car runs worse when hot I might suspect the O2 sensor. If it is the problem, there will be a stored fault code, as Khaled has said

inokiyo

Hi John,
Thanks for the info.
I found the thread and will go about replacing the O2 sensor first. I will talk to the mechanic to see what readings he got, if any.
I'v read alot about the motor running rich when the LH brain has gone bad. Sounds like a typical symptom. What are the symptoms of a MAF gone bad? Is it the same as the LH?
On another note, I noticed the odometer and trip meter is not working. I read If the speedometer is not working along with the odometer, it would most likely be bad wiring in the battery compartment. If it is only the odometer, most likely a gear broken. Is ti posiible that bad wiring can also cause just the odometer to stop working?
Thanks again guys!!
Ken (inokiyo)

John Speake

Hi Ken
Sometimes the LH can fail where it "thinks" the WOT switch is always closed. But I would look for other possible causes of the problem first.

On a diagnostic tester you can check out things like WOT switch.....but also the fault codes.

MAFs can fail rich, you can check this by measureing MAF volts on pin 5 MAF to ground (MAF still connected), slip a thin wire into the connector) It should be abt 2.7v at idle. If it creeps after a few minutes at idle to much higher than 2.85v then the MAF has this problem.

inokiyo

Thanks again John!
I will do all that you have recommended. I will go ahead and order the o2 sensor anyways, although I will do the test first. (Found a universal o2 sensor on ebay for about $20)
http://cgi.ebay.ca/ws/eBayISAPI.dll?...MEWA%3AIT&rd=1
John, Khaled, I really appreciate all the info you have provided me. I was at loss when the mechanic charged me $3800 for the repairs he had done, without having the problem solved. I know he did what he could so no hard feelings against him, as I knew from the start that these are considered high maintenance vehicles. My wallet is empty, but my heart is full from all the help and information you provided! And while my wallet will still be empty for a while, I think it was worth buying this 928 as the more I fiddle around with it, the more I get obsessed with it. Also knowing that I have people like you for support makes it even more better!!
Ken (Inokiyo)

John Speake

Hi Ken,
If you can manage it - in future try and do some of your own maintainance and repairs - it's much cheaper :-)

There's lots of help from people on this list, and sites like Nichol's, Tony's and Pirtle's with step by step procedures with pictures...

Mongo

Just for the record, you can save yourself some bucks by purchasing the Bosch Universal 3-wire O2 sensor and splicing it into your wiring system already. The part number for the Bosch Universal oxygen sensor is 15726. Good Luck.

inokiyo

Thanks Andy, and another thanks to you John!
Is there supposed to be two O2 sensors in the 88 928 S4 euro? I know the one in the exhaust, is there another one? Sorry for the dumb question, but I thought one, but my mechanic is saying two.
Ken (inokiyo)

whitefox

The MAF sensor is located behind the intake manifold under the air filter box.

John Speake

Hi Ken
A Euro S4 should only have the one sensor, but I do remember that Japanese market cars had a second one after the cats but I have never seen any good tech info on that.

Regards

Garth S

John,
My '88 S4 is a 'ZZZ' euro car specified for the Japanese market: All the cars remain OBD I with a single forward O2 sensor. However, there are two sensors at rear of the cat, one per side. They are simple thermistors to warn of a cat fire via excessive temp - and they light up an icon in the pod.
These thermal sensors 'look' absolutely identical to the two screwed into the exhaust manifolds of the '89+ cars that are used to trigger the shutdown of the LH in the event of an ignition failure.The red/green diodes of the EZK mounted relay is a further complexity of that engineering advance .

John Speake

Hello Garth,
Ah yes, now I remember ! Thanks for the info. I knew there was only one O2 sensor to the LH, but couldn't recall the post cat bit.

So Ken, only one O2 sensor !

Regards