'81 928S Euro - No idle - loosing hope

[Attylla]

Hi Pelicans

I've reached the final stage of desperation.
I guess next stage is to push this car to the nearest river

1981 Euro had no O2 sensors yet.
The problem is the car stalling on idle. No matter if hot or cold.
If I start it with gas pedal pressed it starts and runs fine but as soon as I take my foot of the gas it will die.

The car has new fuel pump, new filters, new injectors, new WUR, new Cold Start Valve, Decelerator valve, Fuel Accumulator
Control pressures bang on hot and cold.
System pressure is ok
Leak down test is ok.
AAR checked and closing fine.

Throttle body Air bypass set to Full Cw + 2 rotations CCW.
CO2 at 2,4% at ~1000rpm (not able to measure on idle as there is no idle)

All rubber hoses are new including vacuum lines. No obvious vacuum leaks like broken hose etc,

Any thoughts what could be causing this?
I was so happy when I bought this car but by now I've already lost all the enthusiasm for both K-jetronic and 928

Any help much appreciated
Marcin

[JK McDonald]

Hey Marcin - Looking over the list of all the parts you have replaced and the system/control pressure tests you have made - has pretty much eliminated the typical problem areas on the K-Jet injection.

To your knowledge - Has the car ever run correctly or did you inherit the problem when you bought the car ?

Are you saying that as long as you manually hold the throttle plate slightly open - Off Idle - the car runs fine ? And only when you release the throttle plate that the motor dies ?

This may indicate a blockage in the Idle Air Passage that bypasses the closed throttle plate. You might try removing the Idle Air Passage Adjustment Screw and blowing compressed air down through it's opening. When you now start the car if this has helped to restore the ability to keep the motor running at idle - it would probably be best to remove the air control assembly for a good cleaning.

Good Luck and Keep Us Posted - Michael

[Mal81]

JK, are you referring to the air/fuel screw on the CIS fuel distributor?

If not, Attyla, you might want to also look at MP Dano's post about how to make adjustments (VERY ::small:: adjustments) to it. I think they're in the Common Fixes sticky thread.

I say this, because you have replaced a whole lot of stuff with new, and the last time someone messed with your fuel distributor adjustment, your car likely had some vacuum leaks, clogged fuel injectors, poor ignition, well, you get the idea.

Also, you mentioned you don't have an o2 sensor. As an owner of an 81 Euro too, I can attest that the single best diagnostic and tune up tool that I've bought for the car has been the same wide band o2 sensor and digital guage setup that MP Dano also has written about in the same thread above.

[Danglerb]

Trouble with dumping in a lot of new parts is that what you usually want to check with problems is the last thing you changed, which is now unfortunately everything. Everything except fuel dist.

How badly does it run when you hold the accelerator down to keep it at 1000 rpm? You say fine, so I am going to take a wild shot that that vacuum lines especially the one to the distributor is not routed correctly. The vacuum ports on the throttle body switch once the throttle is slightly above idle. What does the ignition timing look like running/dying at idle?

[Attylla]

Hi Guys

Thanks for replying. I will try to investigate further.

@JK I will remove the throttle body to have a closer look at the bypass.
@Mal81 AFR should be set on the idle running engine at operating temp. This is what I'm trying to achieve here.
And yes the engine was running fine before replacing injectors and fuel accumulator so all the blame is on me purely.

@Danglerb not sure which vacuum lines do You mean. I don't have any routed to the distributor.
I do however have new vacuum line connecting WUR and throttle body.
They are connected diagonally so:
1. Passenger side WUR vacuum port is connected to driver's side throttle body port.
2. Top WUR port is connected to passenger's side throttle body port.

Couldn't find a detailed diagram of vacuum line connections but I've re-routed all of the replaced hoses back to their original positions.

[JK McDonald]

Hey Marcin - There has been a lot of good advice but I think we had all made some assumptions about what had generated the problem. My suggestion for the potential need to clean-out the throttle plate bypass idle passage was referring to the large (flat tip screw driver) screw on the front of the throttle body. This adjustment is normally used to set the idle speed. If this passage is blocked for some reason the motor will strangle from a lack of air when the throttle plate is closed. At this point it's more than likely NOT going to be your problem since you have now indicated that the car was running fine at idle before you started throwing parts at it.......

Just a word of Caution : During your troubleshooting process don't attempt to adjust the CIS Air/Fuel calibration that is in-between the fuel distributor and the air sensor plate housing. This recessed adjustment has a major affect on how the car runs by conrolling the air/fuel mixture. You'd know if you have tweaked this setting because it requires a very long thin 3mm Allen wrench that must be fitted down through the fuel distributor housing. Here is some information on a CIS 911 but the theory is the same. Unless you are very familiar with the air/fuel adjustment procedure "DO NOT MESS WITH IT" . Turning this 3mm Allen screw either Clock Wise (CW) - Rich or Counter Clock Wise (CCW) - Lean, can easily throw the Air/Fuel mixture way out of kilter and cause all sorts of problems.

Porsche 911 CIS Fuel Injection Tuning and Adjustment | 911 (1965-89) - 930 Turbo (1975-89) | Pelican Parts DIY Maintenance Article

Mercy - At this point I think we may all be trying to trouble shoot this problem "By Guess and By Golly". It sounds like you have simply connected something up incorrectly. I had assumed that you had changed out all the new parts while trying to troubleshoot a new problem. I hadn't realized that the "No Idle" was self inflicted.

Good Luck, Michael

[Mal81]

Hey Attylla,

When you get this all worked out, please be sure to report back with the whole story. So many threads like this get to a certain point, and then the starter doesn't post the conclusion.

Oh, and BTW, the final stage is 'acceptance,' so you still have a ways to go yet. LOL

[Danglerb]

Sorry I don't know Kjet CIS that well, but doesn't your ignition distributor have a vacuum connection to advance and retard ignition timing?

On non electronic injection cars the way the engine control system knows that it is at idle is from the vacuum signals from the throttle. The various ports on the throttle body have openings either above or below the throttle plate with at least one that changes from no vacuum to high vacuum (and/or vice versa) when the throttle is slightly opened and controls the idle/not idle ignition timing and various other bits.

Since you have problems only at idle, that points to a wrong/bad vacuum connection.