Well now my gas mileage is starting to get out of wack-I replaced the rpm sensor
(original one for 225k)I got all my vacuum hoses up to snuff,good plugs and air filter and mileage has recently dropped from 27-28 to 24 open road. Car runs great,really got to watch it out on the road it wants to cruse 80 and better,idels well and has good power-but mileage is down. Last time O2 sensor was changed was about 100k ago. Do I change both out or just the front one or shoul I look elsewhere for the problem. The universal 4 wire units any good? Has anyone used these, about a hundred dollars cheaper than the OEM units.
Thanks all
Poolman

Hi Poolman,

I would check the reading on the font sensor if you could with a analog VOM. As CJ said, the front O2 sensor is the important one (the one that affects A/F ratios). I have never been a fan of 'generic' O2 sensors unless the connector was a perfect fit. If you have to clip and solder I would pass. These sensors put out very low voltages and any resistance (as in crimp on connectors and such) will detrimentally affect their contribution.

DEWFPO
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1998 S90 071,245 and 1995 964 154,100

Whether this will help you or not, I can't say - but my experience with two-sensor systems (on VR6 VWs) is that the rear sensor is solely responsible for monitoring the effectiveness of the catalytic converter. The value returned from that should consistently be about 0.2v, and any substantial variation would indicate the converter is not operating efficiently enough to control emissions, or some other condition is causing more unburnt fuel to be dumped into the exhaust than what the cat can handle - such as bad wires, or a faulty coil pack - or a faulty sensor. A fault in that should set a CEL (and possibly trigger limp-home mode - it does in the VW Motronic system). So it seems to me that the front sensor is the one you'd need to check since it's the one that would control mixture. 100k isn't all that old in my opinion, but it depends on whether any other contaminants - coolant, for example - got into the exhaust and shortened the life of the sensor. Sorry I can't be of more help.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina

Thanks Guys-I will tackle the O2 sensor if this new thing I found isn't the culprit-I posted about it just a little while ago.
Poolman