Setting mixture on 86 GTI

I was recently successful in resolving a nagging problem with my VW at cold-start and idle. I usually do a "lessons learned" write-up for my personal files, and I thought I'd share the story with the group. I hope it's not too lengthy...

1986 VW GTI, RD gas engine, CIS-E injection.

For quite awhile, my GTI has had a couple of problems with idling, which seemed to happen after the last repair session at . I recall that they had had a problem starting the car after they had done major transmission/engine seal work. They had tried to start engine, and it wouldn't idle, and I think the mechanic started adjusting things. Eventually, I think they found that something was disconnected, they connected it, and it started and ran, but I don't think they set everything back to original settings. They also may have played with mixture, since the sealing plug was not in place on the fuel distributor. I wasn't able to look over their shoulder while they were doing this.

Anyway, my problems were: - Started very easily, but after about a minute, idle got very rough. You needed to keep foot on pedal, or pump every few seconds. Eventually, when engine warmed, it ran pretty well, and overall, engine power and acceleration were pretty good. - Looked at readings for Oxygen Sensor (OXY) and current for Differential Pressure Regulator (DPR). OXY voltage, when engine warmed up, should have been varying between about 0.3V and 0.7V. However, it started off at about

0.5V, and then slowly went down to about 0.2V. According to Bentley manual, this suggests the sensor is seeing a mixture that is too lean. Is the sensor okay, or is it really reading a lean mixture? (My OXY sensor dashboard reminder had recently come on, at around 100,000 km.) - Current for DPR starts off around 80mA or so, and is supposed to slowly drop to around 10mA after about 20-30 seconds. However, my current went down to about 20mA, and never went lower. - During troubleshooting, I discovered that the idle switch on the throttle body was defective. The switch was broken, and would not close when the throttle pressed it at idle position. Thus, the fuel computer did not know that throttle was at idle position. Did this switch break at the repair shop??

I didn't want to start adjusting the mixture too much, and wanted to understand the sensor readings. Read over Bentley many times, and also consulted rec.autos.makers.vw.watercooled archives - good stuff there!

To look at the idle problem, I made up a 15Kohm pot with wires to simulate the coolant sensor. This sensor is about 3-6K when cold, and drops into the hundreds of ohms when hot. If nothing else, I might be able to fool the system into thinking it was colder than it was, and resolve the rough idle at initial startup.

I set up my digital multimeter on OXY sensor, and an analog current meter on DPR, and the pot on the temp sensor. Set temp sensor to around 3-4 K (about

10 deg C).

Started engine; it started easily. OXY was about 0.5V, and DPR was around

100mA, and slowly went down to about 30mA after about 30 seconds. Wasn't stalling, and as I adjusted the fake temp sensor, I could see the DPR current change. Hotter engine made current drop, which caused a lower commanded richness. Mixture should be richer when engine is cold. I lowered the temp sensor resistance a bit more, and now current around 20mA, with OXY still around 0.2V.

I decided to try a mixture adjustment. Using long 3mm allen key, turned the mixture 1/4 turn clockwise, which makes mixture richer for a given DPR current. OXY rose slightly, and DPR current dropped to about 15mA(?). Hmmm... this seemed good. I turned the mixture another 1/4 turn clockwise, and now the DPR current dropped to around 8mA. More importantly, the OXY started rising to about 0.6 and 0.7V, and then it started to waver between about 0.7 and 0.4V. THIS IS THE CORRECT OPERATION!!!

Revved engine, and it worked fine. Temp sensor setting now around 1K or less. I decided to shut off engine and connect real temp sensor. Did that, and engine started and idled fine. This time, OXY starts at 0.5V, and after

30-40 seconds, starts the 0.3 to 0.7V varying, as it should. Current still around 8mA, with some varying when running. (Couldn't actually tell DPR varying, since this meter has quite heavy damping.) I decided to tweak mixture back about 1/8 turn CCW (less rich), and now DPR current around 10mA average, as Bentley manual says. CORRECT!

Took car for a drive, and it went just fine. I also reset the OXY sensor service reminder (on firewall behind coolant overflow tank).

THEORY: ======= I think that somehow, the auto service tech had turned the mixture screw about 3/8 turn CCW, making mixture too lean for a given DPR current. The OXY reading of 0.2V was telling system mixture was too lean, and the module was driving DPR current to 20mA, trying to make the mixture richer. 20mA was as hard as it could drive the DPR, given all other constraints, like engine temp. When I then turned the mixture screw CW, it made mixture richer for a given DPR current, and the OXY voltage showed that. Eventually, when I turned the mixture screw far enough CW, the mixture actually got richer, and OXY went above 0.5V. From that point on, the OXY sensor/DPR current control loop was working correctly.

The real proof was the next morning, when it started up instantly, showed no stumble or hestitation when idling, and ran smooth and easy on the road - like a new car!

Thanks for reading, and for sharing your own repair stories!

WT, Winnipeg, Canada

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