Hi everyone
Have checked search without success, so maybe one of you has encountered this problem:
Firstly my car is an '87 S4 auto. It has a slight miss at idle but otherwise steady 670-690 rpm (have changed plug wires and plugs, but no improvement) Under way, there's some vibe at part throttle and power seems slightly down. Back at idle again, when the a/c cycles off then the idle surges a few times and finally stalls out. Could this be impending LH failure?
Dist caps and rotors still look good and a timing light on each wire seems steady. Coils probably original. The car is very low mileage.
Thanks for any suggestions.

 

Is it a cat equipped car or non-cat ?

If it surges then that's a good sign as it indicates that the ISV is being varied by the LH ECU.

First, make sure the closed throttle switch is working correctly. Then report back here.

 

Hello John
I can hear the microswitch clicking and the ISV/LH generally do a good job of holding the idle to 675 +/- . The car is non-cat so no O2 sensor, the manifold seems tight and no other vacuum leaks - just put in a new evaporator so refreshed the hyvac diaphragms and all is now leak free. As soon as I turn off the a/c or just sitting at the lights the idle will start to surge and usually die and the engine is quite rough under part throttle. The plugs are a good colour, but I just changed them for genuine Porsch items anyway. Maybe it's something obvious I'm missing here, but the potential for wasting money changing stuff out is huge!

 

OK, if non-cat, then you need to set the A/F ratio at the tail pipe to 1-1.5% at idle. It needs to be set there for the car to idle correctly.

However the roughness you describe may well be ignition leads/rotos.caps or a rogue injector.

 

Thanks for the advice John, I'll get some distributor parts and swap them around. I've already run a couple of shots of injector cleaner through the fuel system, is there any method of detecting a bad injector other than listening for the clicking?

 

I was just about to post more or less the same problem. Idle is good and steady then a sudden drop in rpm, usually picks up to idle again but it has stalled once..... my car is 89 euro auto with cat and RMB also low kms.

chris

 

Not that I know of, you need to put them on a flow test bench and measure cc/min and spray patterns. But do check and adjust your idle CO first. You should be OK with your low mileage MAF, but it's worth a check, especially if the car has done short journeys most of its life.

 

Cat equiped car is something different to Asiandude's non cat. Even if your MAF was aged, the O2 loop will adapt somewhat for MAF ageing. Certainly at idle rpm.

Try a battery ground strap disconnect for a few seconds, then drive the car for 10 miles or so, see if idle settles down.

 

Ok, a CO test tomorrow (but where I'm not exactly sure!) and I'll get back with the result. Thanks John.

 

Idle mixture pot is above the fuse/relay panel, about 1/2 way along. Flat blade screwdriver slot in adjuster, 20 turn pot with no end stops. Clockwise to richen, anti-clock to weaken. Once mixture stops changing don't keep turning pot or it can be damaged.

 

will do.
i just changed the original plug leads and plugs, I think the leads have never been disturbed, (not the easyist of jobs I must say, no sence of humour? I think these German techs knew what they were doing!)
Hope to see a change after this.
Chris.

 

Long shot, but easy fix. Did you check both ends of coil wire connections? Frequently find corrosion on them. Also grounds from coils? SE Asia must be a damp climate.

 

Yup, spotted that when I first got the car (which was in a sad state and barely ran) now if I was rich I'd just go out and buy dist rotors, caps and coils and be done with it.....but I'm not. The surging then dying seems odd though, maybe the closed throttle switch is on the way out.

 

Changed my leads,plug's all well for a while then at the lights I had the revs drop. I noticed the volts drop also but when the tick over recovered the volts remained lower 11v again at the next lights the revs drop along with the volts again pick up but volts lower 10v. Is my problem with the charge circuit?

chris

 

Chris: Some people search the titles of threads. May I suggust you start your own thread, specific to the possible "VOLTAGE REGULATOR?" alternator issues you are having. You should get a better response.