thepurpleblob

I acquired my '86 (16v) 928 at the end of last year with very little history so almost immediately set about replacing everything - timing belt, oil seals, all the vacuum pipes, all the gaskets and o-rings around the throttle body and so on. All the injectors have been cleaned a refurbished professionally and declared fine. The MAF has a sticker indicating that it was rebuilt at some undefined time.

I'm just not happy with the idle. It hunts considerably and sounds like it might even be missing sometimes. On the other hand, it pulls like a train - when you get going it runs great. I've glared menacingly at all the vacuum pipes and re-checked all the sensor and other electrical connections but am struggling to find an explanation.

Any thoughts and/or things I might not have checked?

gazfish

Only a couple of things I can think of, when you checked all the sensors did that include the throttle position switch? Have you had the AFR checked at idle, it might just need the adjustment in the MAF twiddled a bit if it's too rich or lean.

thepurpleblob

Good idea (to both). I have not checked either

Hafiiz

Verify fuel pump and check valve are working. I had the same issues awhile back and replacing it with a genuine (don't even look at those generic 044 crap, I tried and never again) bosch pump fixed the hunting problem. Had no issue with driving hard but when left on idle will hunt between 300-700 rpm.

thepurpleblob

Should a fuel pressure test show that up?

FredR

The prime suspect to a bouncy idle is the ISV. For whatever reason the earlier ISV's seem to be more problemmatical and they are a service items that I reckon are good for about 20 years or so. The shuttle can get a bit sticky with fouling or whatever nad thus when the controller demands a correction it does not move so more correction is applied and then it hunts some. Not sure regarding your MY but AFAIK tey need the inlet manifold removing to get at them- a royal PITA.

Some folks squirt WD40 into them- not sure I like that idea but if it works all well and good.

thepurpleblob

Yeh - in a previous life we used to constantly have to un-stick Bosch idle valves on a variety of cars using L-jet. The later Motronic "buzzy" ones are much better. A good blast of carb cleaner often did the job.

Yes - I think that the branches for cylinder 3 and 4 will have to come off. Could be worse.

gazfish

There isn't one on the S2, its still the warm up slide valve.

FredR

OK I must be confused with the 32 valve models in the US- I tend to look at the year rather than the model.

What engine management system does this model use?

gazfish

Hi Fred,

It's the lh-jetronics and the very last of the 16v with twin dizzy so it's probably very similar to the 32v controller but doesn't have the idle control, closed loop or any adapting.
This model also has the S4 interior, brakes and 310hp so a bit of a transition model.

Cheers,
Gary

FredR

Hi Gary,

No wonder I was a bit confused. My assumption was that the ISV went with the LH/EZF combo. To be fair, the list mostly covers US models by weight of numbers so obviously we read more about them.

I understand that some UK pre 32V models turn out the 300 plus BHP and as a consequence of the cams have interference engines and that some pre S4 models have some S4 features like the brakes etc.

Makes one wonder what Porsche were thinking of- presumably they were bogged down with environmental issues primarily in the States so responded to that market first.

thepurpleblob

Yep - that's the one.

Wisconsin Joe

It's a bit complicated, and I likely have some of it wrong.

The Euro S2 (84-86) was the twin dizzie 16v. EZF & LH 2.2. Rated at 310 HP, but reportedly a fair amount higher. Some claim close to 330.
The US saw the first 32v in 85. Emissions was at least part of it. Beyond my understanding, but the 4 valve chamber gave some advantages in that.However, it was rated 'only' at 288 HP.

The 86 Euros (all of them) and the 86.5 US got the S4 brakes, exhaust, suspension and a few other bits.

thepurpleblob

Can somebody clear a couple of things up then...

- the air valve thing under the manifold branches on the right bank is not an idle control valve (it doesn't look like the common Bosch one of that era, granted)? It's what? Cold running valve of some description?

- the two temp sensors on top of the thermostat bridge. What are they / which is which. They are in line. The one at the front has a blue "Bosch style / fuel injection" stylee connector. The rear ones has two (rather corroded in my case) spade terminals.

- While we're here, there are also sensors in one of the manifold branches (two more spade connectors) and one under the air cleaner box. Do they have anything to do with anything?

FredR

The temperature sensors are the temp2 sensor and the dash panel sensor. The temp2 sender has two channels- one for LH and the other for the EZ unit. Not sure if your model is similar to mine but if it is the temp2 sender is on the passenger side of the bridge and the dash panel sender more to the driver's side.