Got a problem with a '94 Ford F-150 1/2 ton pickup (4.9l I-6, 4-speed auto trans, 2wd). Every now and then, hot or cold, the engine will idle just fine, but responds to any throttle input by stumbling, sometimes stalling, then after nursing it for a little while, runs fine until the next episode. This may be five minutes later or the next week. While it's having its fit, the truck spews black smoke from the tailpipe, which tells me something is telling the PCM to dump more fuel into the cylinders. Vacuum at idle is around 20 in., spark plugs, cap and rotor, MAP sensor and throttle position sensor were recently replaced. I'm thinking I need to replace the O2 sensor. I've heard it should be considered part of a tune-up on any modern vehicle, but at $50- $60 for one sensor, I'd rather test it and replace only if needed. Does anybody know the test procedure and/or resistance readings? I believe this is a heated O2 sensor. Thanks for any/all input - Chris

 

Chris,

The only way to tell "for sure" if the O2 is working is to check it with a graphing multimeter so you can see the amplitude and make sure the sensor is switching properly.

You didn't say how many miles are on the truck or when the O2 was last changed, but it might be a good idea to replace it. I had a similar hesitation, stumbling problem on my 86 Accord recently and a fresh Bosch O2 from AutoZone did the trick.

Make sure you get one that has the connector already on the sensor. Although you can solder them, they work much better when they're the "plug 'n play" variety.

As for replacing the O2 during regular tuneups, it's entirely up to you, but it IS cheap preventive maintenance.

HTH

Matt

 

Thanks for the advice. The truck has a little over 100,000 miles on the odometer, and it sat for most of the last two years. I'm inclined to agree that even at $50 a whack for an O2 sensor, it should be at least a once-a-year item. I'm starting to feel the same way about in-tank fuel pumps, but that's a whole 'nother story. Thanks again; I'll give it a try. - Chris

 

Although it may be time for a new sensor, it's not what's causing your current problem and in fact may be getting damaged from the over rich condition. I suggest you resolve that issue before changing the sensor so you don't ruin the new sensor right away.