heyadamhey

I just got an 87 S4 which had been standing for a few years. My first ever Porsche, so I'm a total noob to the 928 world. I'm handy with a wrench, but not a mechanic.


Not sure exactly how long it was standing for, but the fuel smelled very old - like paint. The PO passed away and his daughter was the seller and she wasn't sure about what maintenance had been done, but insisted that it was well looked after
When I first saw it, the seller started it and it idled ok, but wouldn't rev. I assumed it was the old fuel that was making it run rough.

I did the timing belt, water pump and porken tensioner. I drained the old fuel, cleaned the inside of the tank and put fresh fuel in with some fuel injector cleaner. The "old" timing belt and water pump looked new and the oil filter as "08 2022" written on it, so it seems like it had some fairly recent maintenance carried out
Massive thanks to the amazing how-to on Dwayne's Garage. It was quite an intimidating job, but I followed the instructions very carefully and got it done. I removed the air pump and currently have the air pipe crimped before it goes into the valve next to the cam cover (where it is directed to the exhaust or to the air box)
It starts fine and idles relatively ok, but it's still reluctant to rev. To me, it sounds/feels like the timing is out, but I believe a vacuum leak can cause the same result.
I've checked the following:
- All plugs have good spark, but I have not checked each plug yet
- I did a smoke test on the vacuum lines and I couldn't see any major leaks. A little bit of smoke seemed to be emanating from beneath the intake, but it was hard to see the source. I ordered a vacuum gauge from Amazon, so I will check the vacuum at different points when it arrives tomorrow.
- The connector on the cam speed hall effect sensor (on the left-hand side bank) came apart during the TB/WP job but I managed to get the wires reconnected - could this cause the rough running? I can't hear any change when connecting/disconnecting these wires while the car is idling. I was told on Facebook that it can affect the timing by 6 degrees or something to that effect.
- I've checked every single fuse and they're all fine (never seen a fuse tester among the relays before! Pretty cool)

I was hoping you clever folks could help with some suggestions of what I can check or how I can further diagnose the issue.
Thanks in advance!
Adam

Mrmerlin

Nice car you got have fun with being the new caretaker.
Based on your info I would send out the LH and the MAF and have them tested.
My 88 S4 started ok after the intake refresh but wouldnt rev it would stumble and then die ,
A fresh rebuilt LH cured that.

see post 55

1988 Grand Prix white just rolled in, Project

heyadamhey

Thanks, MrMerlin.
Forgive my naivety, but what is the LH?

never mind. Did a google and found this: http://www.928-ecu-repair.com/928_lh_repair.html

FredR

The male connectors on the knock sensors, the CPS [crank position sender] and the Hall Sensor have a tendency to crumble for some reason. Failure of the Hall sensor and either knock sensor will retard the ignition by 6 degrees but will not cuase what you describe just a loss of some top end power. Failure of the CPS tends to be a no start condition.

The LH computer is a very common point of failure and can easily cause the kind of issue you described. If it has been repaired it will likely have a sticker on it saying the date the repair was coarried out. If the unit has not been repaired already it should be done pre-emptively before it fails and leaves you stranded. The MAF tends to under read air flow as it ages but can fail to read at all. When this happens it defaults to what is called limp home mode wherein you get a couple of operating points to support low speed operation. The usual test for this is to remove the MAF plug. if that makes no difference then the MAF is the likely suspect.

Beyond that there are other conditions that can cause problems- the wiring on the inlet to the MAF has a tight radius and the cables and plug terminations can degrade somewhat chronically as can wiring to the sensors and fuel injectors. Another common failure point is the diaphragm in the fuel pressure regulator and/or dampers.

John Speake

LH is the computer that deals with engine fuelling. They often fail with time. EZK is the spark computer, they are pretty reliable. They both sit togetther in the passenger side footwell, under the curved carpet.

As Fred says, sounds like the car is in limp home mode, usually when the MAF has no output.

heyadamhey

Thanks everyone. I'll try unplugging the MAF and see whether it makes a difference. If it is then, then I'll get it rebuilt

Mrmerlin

odds are the LH needs to be rebuilt as well
post some engine pictures for a better diagnosis

heyadamhey

Thanks. Will do. Is there any specific angle that would be helpful?
I've got the following while I was putting things back on after doing the TB/WP. I'm not sure what the little square device on the right-hand cam belt cover with two vacuum lines is. In the photo only one line is connected, but I have since connected the second one


Cam sensor plug. I've "McGuyvered" three wires to connect the two ends of the plug, but FredR said this should present a different symptom

soontobered84

Adam,
As an addition to what has been posted here previously, I would send the injectors out for cleaning. Chances are that ethanol has glued some of them shut.

FredR

We refer to the 1/4 bank às the right hand side of the engine. I suspect the device you refer is the flappy valve solenoid located on the 5/8 bank cover. This sends vacuum from the vac reservoir to the flappy valve actuator as and when programmed to do so.
I spliced a new section of cable onto my Hall sensor. Needless to say the correct assignments need to be retained .

heyadamhey

yeah, I googled "flappy valve solenoid" and yes, that is what I was referring to. So it is then on the "left-hand side" of the engine from the perspective of the driver's seat?

FredR

Indeed- I prefer to use the bank reference.

The flappy being non functional also takes a small increment out of the top end performance but would not explain your symptoms.

545svk

On my '87, I spend years doing everything but have the MAF rebuilt. Car always felt a bit heavy and sluggish. Had the MAF done nearly as an afterthought and the improvement is vast. Mine was about 7% out of spec.

I'd suggest do the MAF first. It will probably need it, and then it is a known entity to build on.

heyadamhey

I recorded a video of the car running with and without the MAF connected. It is initially disconnected and then makes a strong sucking sound when I connect it. Is that normal?

There's a big difference when I unplug it. Could it still be the cause of the problem?

perhaps this is my issue?

Thanks again for all your input - it's much appreciated

Mrmerlin

Adam at this point you should send your MAF and the LH computer out and have them tested,
odds are good both of these parts will need to be refurbished,
having them tested and found good will rule out parts of your injection system that could be questionable.

NOTE even though the MAF was rebuilt 5 years ago and the LH is unknown ,
get them tested and inspected for function.

Who knows these parts could be part of the reason the car was put into the garage so long ago.
And they are known failure points.


Based on that video you need to do an intake refresh,
that will include the work as I have outlined.

Budget about 4 K in parts and PC if your doing the work