jwillman

I have been trying to address some starting and idle issues for a while.

I have good pressures at the WUR, verified operation of the aux air valve, replaced injectors, did on car test of FD, new plug wires and a top end refresh last spring.

Been using the car as a daily driver for the last month here in central Texas and as usual we have had the full spectrum of weather, freezing temps at night, to near 80 degree days. Car sits outside.

What it does:

Car starts immediately no matter outside temp but takes ~3 seconds of rough running in the 700RPM range before it accelerates to about 1500 RPM and run smooth.

Once it warms up idle drops to 1000 and then on occasion will drop to 750 and then begin to hunt/ surge and on occasions can seem like it will almost stall during a dip down. If i give it a little gas and idle faster, anything above 1000 RPM, the surging stops.

As I drive and go from stop light to stop light the idle is a bit different at each stop, sometimes 1200, next light it settles at 1000 etc.

Yesterday I spent an extended time at idle, it was stable at 1000 but when I went to pull away it stumbled badly and I really had gas it up to 1500 or so before it would accelerate through clutch release without acting like it wanted to die. It did that a couple more times on the way home.

The other symptom I have had twice over the last two weeks is I can start the car and go for a few mile drive. During the drive the car will start making what sounds like a loud lifter noise from the bottom of the engine/ exhaust at idle. if I rev the engine up to 1400-ish RPM it goes away but immediately returns when the car begins to idle. I pulled into a parking lot, shut off the car and come back out 15 minutes later and the car starts and idles fine, no clatter until I get down the road a bit and it starts again at idle at the next light.

I pull into the driveway with the car sounding like its going to explode. I check under the hood, levels etc and start the car and the noise is from underneath.
Neighbor heard it and suggested it sounded like detonations so I adjusted the AF mix screw and sure enough it stop. Drove it several times over the next week with not issues and then one trip it did it again. Shut car off for a bit and started it and it has not done it again for a week.

So seems I have some gremlin/s. Want to think this sounds like a vacuum issue but not sure of the best approach to track it down using my mighty vac hand tool. Collin suggested fuel accumulator at 3rd coast but my leak down pressures are in tolerance.

Thoughts.

LT Texan

wild *** guess. Is the FD plunger or air valve sticking?

I really have no idea what mine was doing until I put an A/F ratio gauge on it

jwillman

Possible I guess. The test I did checked for no flow with A/F mixture full closed and then all flow simultaneously when adjusting A/F screw to open point.

Are you talking the aux air valve?

GlenL

My guesses:

A) Sticking sensor plate arm.

B) Vacuum leak.

C) Both.

LT Texan

I meant the fuel metering plate that drives the FD plunger.

vze2jshn

Try a bottle of Techron.

jpitman2

Sticking throttle shaft ? These can get very unreliable at closing to the same place all the time.
jp 83 Euro S AT 56k

jwillman

JP,

Are you talking on the side of the throttle body where the accelerator cable attaches?

jpitman2

yes, the throttle plate shaft. There will be a coil spring on one side of the throttle body, and drag on it can result in the throttle niot comeing back to fully closed.
jp 83 Euro S AT 56k

Rich9928p

If your 928 is a 5-speed another item to look at is the vacuum limiter (p/n 930.110.190.00). My 1979 928 would occasionally drop RPM very quickly between shifts, even stall out. A test showed unit wasn't holding vacuum. It is expensive (only in stock in Germany), but when replaced the engine ran much better and doesn't drop ROMs like it previouslly did.

jwillman

Any way to test the Vacuum limiter?

Jim

GlenL

Remove and plug the vacuum line. The engine will act differently when you suddenly close the throttle but if it operates consistently then you can replace it...or just leave it alone.

I was going to suggest this as well. It can stick partially open.

Rich9928p

Use a vacuum tester like a Mighty Vac ... it has a vacuum gauge. Connect the hose to the vacuum fitting. A few pump handle squeezes creates the vacuum and you watch the gauge to see if it holds. If the vacuum leaks out the limiter requires replacement.

You also should inspect the hoses that connect the limiter to the throttle body, if they're original they too could be leaking.

Rich
'93 928 GTS cover girl
'87 S4 (now with working cruise control)
'79 928 5-speed (now with working cruise control)
'79 928 5-liter track beast

jwillman

I found a problem. There is a pressure converter (#14) that has 4 tubes coming in/out of it on the passengers side between the intake tubes. It has a red tube that runs down the passenger side to a metal tube that is threaded into the passenger side exhaust manifold. Diagram shows it with the EGR valve plumbing (#1). The metal tube (#20) was burned / corroded through.

I have the threaded pipe fitting sprayed with some penetrating fluid but it is a rusted mess threaded into a hot exhaust manifold so concerned about getting it out.

What is this? Do I need it? Can I cap it off etc? If I need it is this part that still available or can I have something made that will take the heat and work?

What about just using some heat tolerant rubber tubing and splicing the break in the tube together? having given it more thought I will try and find a metal tube to act as a sleeve, inside or out, and JB weld it together.

rjm65

#20 (928-111-147-02) lets the converter/amplifier know how much exhaust gas back pressure exists in order to control EGR opening. Is there enough metal left to slip some silicone tubing over it?

Raymond

Came across this a couple years ago, it gives a great overview on the EGR system on the 78/79's: