I've read most of the posts on the forum concerning compromised gas mileage. Seems the most obvious is the o2 sensor.

Here's what I've done recently. I thought I should start work on the mass air flow sensor. Well, Murphy and ignoramus here pulled the wires out of the Mass Air flow sensor..big mistake...I got it back together, but still need to solder some of the connections back. While I had the top off the sensor, I noticed that it rotates and opens the air intake as the engine rpms increase. I also noticed the inside of the air intake had a lot of oil residue in it where the air intake goes to the carberator. So, I decided to keep the engine rpms up by holding the air sensor open and spray berrymans carbuerator cleaner in the intake to clean the oil residue in the carbuerator. It cleaned it up, but when I put it back together the engine rpms stay around 1500 and it fluctuates up and down between 1000 and 1500 at idle and the gas mileage has gone from bad to worse...

I'm thinking the o2 sensor is bad and it needs to be replaced... I think the sensor is located on the exhaust manifold end closest to the firewall. The bolts on it look like they could twist off if someone like me get's too rough with it.

I looked at the pictures on the o2 replcaement article in the tech section. I think that is on a later model truck than mine.

If someone has done the replacement on a 91 with the 4 cylinder engine, I'd like to have some coaching here if you would be kind enough to help.

thanks in advance for the response.

NJ
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I no expert and I haven't experienced this on my truck, but sometimes PCV valve will allow oil to get sucked into the airbox, which will get your air filter oily (if it's paper type).

I don't know why your engine idles so high now, unless you dislodged some stuff and it stopped up a vacumn port/line somewhere....?

I replaced the PCV valve on last week. I don't think that had anything to do with my compromised gas mileage, but it was only a couple $ so I opted to replace.

I'm wanting to replace the o2 ensor, but I can't find any satisfactory guides on how to do it. Since I screwed up in my attempt to clean the Mass Air Flow sensor...(at this point, I don't think this model truck has one). I'd like to know a bit more about what I'm doing when I replace the o2 sensor.

Many of the posts on this forum indicate the o2 sensor needs to be replaced after 75k....my truck has over 201k... but I've only had it since 197k..

I'll look forward to responses...thanks for yours..

NJ
<img src="/forums/images/graemlins/kewl.gif" alt="" />

O2 Sensor is basically a bolt-in. It's located on the top side of the exhaust just in front of (on the engine side) of the catalytic converter. There are 2 studs made onto the bung where the Sensor goes. The studs/nuts are likely rusted in place due to heat and exposure to the elements. Squirt them with a LOT of PB Blaster or equivalent penetrating lubricant. ( Do this with the exhaust pipe COOL ) Let it sit overnite. The nuts should come off without too much trouble. The wires to the Sensor just plug in. Be SURE to use a Denso or other high quality O2 Sensor as a replacement, or you'll be replacing it again in short order. Unfortunately, the Denso and better units are kinda pricey. Mine was $89 when I replaced mine a couple of years ago. With the better units, you don't have to worry about figuring out which wires connect,, because the connector just plugs in. Cheaper units don't always have a connector made on them.

Mike in AR

Thanks for the post... I've looked under the truck and I'm not seeing what you are talking about.... I'm wondering if the previous owner took this off. I was looking up close to the exhaust manifold..... come to think of it, I don't think I saw a catalytic converter... I'll check this afternoon. I hope if he butchered it that he left the wires in place... I had the muffler and tail pipe replaced about two months ago...

Is is possible this truck didn't come out with the o2 sensor? That seems unlikely.

Thanks for the post!!!

NJ
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Being a 91 model, I'm sure it had one.There should be some wireloom running alont the bottom of the floorpan. I believe mine has some heat protectant foil around part of the wiring, because it's so close to the exhaust.

I'll try and take some pics, if you can't find it.

Mike in AR

yes, you had a catalytic converter if your car was sold in USA. You can check your hood for emissions info. If it is missing, you should get one and get an O2 sensor, your computer will thank you.

You are requited to have a catalytic converter, it is a Federal law. Also, without one you are doing nothing good at all, cats create almost zero back pressure, and many times flow better than the exhaust pipes they are attached to. By running without a cat you are contributing to pollution and making our world a dirtier place to live. There is a reason why they are required on vehicles, go get one, they are not expensive and your great-grandchildren will thank you.

Funny to hear this from someone driving a truck and not a Pruis. <img src="/forums/images/graemlins/pfft.gif" alt="" /> You'd be amazed at how many folks here don't have 'em. CHEAP? Roomie's Corolla CC went for over $200 in the past year.

Some states don't require/inspect for emissions stuff. And up here, rigs are IM'd only in the municipal areas, not the outlying areas. We have cold weather inversions in the winter that don't let CO dissipate like it would elsewhere. You can walk down a hillside with a thermometer in your hand and watch the temps drop about 20 degrees. <img src="/forums/images/graemlins/cyclops.gif" alt="" />

Yes, very likely the PO took the cat off. On the 02 sensor, get some PB Blaster from NAPA and give the bolts a good soak before reefing on them. www.sparkplugs.com has Densos for a reasonable price. Also inspect the wire harness for it.

Hey! My rig passed emissions with flying colors!! <img src="/forums/images/graemlins/notooth.gif" alt="" /> And propane is in the works for the up-and-coming '85 <img src="/forums/images/graemlins/kewl.gif" alt="" /> <img src="/forums/images/graemlins/kewl.gif" alt="" />