Fly911

My 1985 is running at 1400rpm on idle, warm engine. I have had the entire intake off numerous times, changed every hose and vacuum line, checked every vacuum component, changed intake runner gaskets and rubber sleeves, changed the ISV, changed the FPR back and forth, measured the fuel pressure (is OK), changed MAF, changed all temp sensors, changed all fuel injectors, checked ignition timing, changed and checked TPS switches, changed LH and EZF chips back and forth, the engine is idling at 1400rpm no matter what I do... I have sprayed WD40 and starter fluid on and around the air intake runners and TB, but no change in idle speed. I would assume if there was a leak, that the idle speed would have gone up...

How does the ISV work? Is that an on off valve, or is it modulating to achieve the correct idle speed? If so, what is determining the modulating output signal from the LH? Or what input signals would determine the LH to cause a high ISV output? Or what other things could cause a high idle?

Other than the high idle, the engine is running flawlessly. However, it is running a bit rich. Not a big problem, better running rich than lean.

Any LH gurus that would know where to start?

monkez

So fair warning, i'm an accountant, not a mechanic. Listen to the experts here.

However, I did have the same issue. I sprayed starting fluid all around and had no noticeable increase to the idle. However, it did end up being a vacuum leak. The intake runners can have space where they attach to the heads. hard to notice and not easy to spray starting fluid right where it will affect the idle.

Dwayne's garage shows a great procedure on how to seal up the engine in his refresh write-up. you really want to seal it up this way and do a smoke test. I would argue you cannot rule out a vacuum leak until you do this.

Other's may disagree and they're probably right.

928NOOBIE

What happens when you close the idle bypass screw all the way? Does the idle speed change at all.

Fly911

Turning the idle bypass screw has no effect... Open or close. I will take the intake runners off (again...), and try to use dual gaskets and sealant. The supposedly better metal gaskets are out of stock from Porsche, they are thicker and would probably seal better than the paper gaskets. Maybe double gaskets with sealant on top, bottom and in between would do the trick...? If that doesn't work, I will try to put an orifice restriction in the ISV hose.

Does anyone know how the ISV works? Is it on/off or modulating?

Speedtoys

Nothing like making a simple problem, more complicated.

And people wonder why previously owned 928s are such basket cases for the new owners.

You could just fix it..it will be apart anyway...

How did you clean off old gaskets? With a blade..which is OK, but the blade at 90 degrees to the surface, or lower, so it would have caught on the aluminum and generated a leak(s) opportunity?

gazfish

Listen to the advise of smoke testing the intake instead of taking it apart again, to remove the guesswork.

davek9

Swap the LH ECU with a known good one, as you have failed ISV control / driver Transistors, swaping the "chips" won't solve this, only a repair of the ISV drivers will.
It' been spoken of on here before, I've see this on two 85-86's.

Note: I have an LH that works perfectly when cold, after 15 to 20 mins of driving, the idle goes to 1400 RPM and stays that way until the LH completely cools off, one of these days I'll repair it or get it repaired.

Dave K

EDIT: and you don't need to double up on the gaskets, things will start to not fit correctly, some sealant YES, and follow the directions on the tube(give it a chance to setup before torquing) .

928S MN

Have you checked to make sure the throttle cable isn't hanging up somehow?
I like to start with the simple things.

928NOOBIE

I wonder if that's what's going on with my car...symptoms sound familiar and I sure as heck don't want to go after the ICV and would like to not do yet another smoke test.....has anyone thought of using heat sinks on the outside of the case behind the board. maybe its getting hot and deteriorating like you're describing Dave....thinking now about maybe some of that stuff you use to put heat sinks on processors in computers...with several low profile sinks (can't remember if the side of the LH with the board on it is on the side of the EZK or no)....I think I'm going to give this a go....I'll do some pictures. It's supposed to rain here for next few days so I may get thee to Micro Center and hook myself up. I'll scuff up the case somewhat..maybe some 100 grit...then emory or 100 steel wool to give it a real nice smooth surface for best transfer.

GregBBRD

The intake runners that originally had the paper gaskets pull down where the bolts go through (on the corners). Over the years, the gaskets get thinner in the bolt area (probably from tightening by mechanics.) They are famous for leaking when the gaskets are replaced....the new gaskets will not compress enough to seal in the middle.
We end up having to resurface every single set of these manifolds.
And then we will only use the later metal gaskets with the 0-ring seal around them, so it never happens again.
Yes, the metal rubber gaskets are expensive....but are a "rounding error" when you consider the time you've spent, so far.

FredR

The obvious thing to check [if you have not done so already] is that the temp2 signal is getting through to the computers correctly.

A new sensor is a good pre-emptive part to replace but if the parameter it is measuring simply does not get through to where it is needed....... 1400 rpm sounds suspiciously like a cold idle type of speed.

davek9

No need for adding any external heat sinks, just replace the ECU or have it repaired.
It's been speculated that a worn out ISV (ICV) makes the drivers work harder and leads to their failure or it could be that there was just a few bad components in the batch.
The S3 ISV system was only in production a few years, it was redesigned for the S4, and even early 87 ECUs had their know failure rate until they worked out the bugs

Rich9928p

The 1985 - 1986 928 Idle Speed regulator Valve (ISV) has three wires connected to it, the center is +12V and the two others connect to LH fuel injection module connector pins 10 and 23 to open and close the metering valve. A variable duty cycle square wave is the driving signal. If one or both of the two idle speed motor transistors in the LH injection module fail, the ISV will not properly function. The later 1987 - 1995 928 S4 - GT - GTS ISV has only has the +12V and one connection to the LH fuel injection module, connector pin 33, with one transistor controlling the ground switching.

The idle speed motors do get caked with carbon and cleaning them may fix the problem if jammed or slow moving. I recently replaced the ISV on my 1987 928 at 187,000 miles along with a well needed intake manifold refresh. I cleaned up the old one and was surprised that it was working well in my test bench.

I occasionally see failed idle speed transistors when I repair these LH fuel injection modules. Testing them is part of my repair process.

Rich
928 Owners Club Membership Chairman
1993 928 GTS cover girl
1987 928 S4
1979 928 5-speed rescue cover girl
1979 928 Euro track beast