Low Mpg -- Straight Pipe/O2 Sensor?
12-07-2021
#1
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Low Mpg -- Straight Pipe/O2 Sensor? Hello all,
New Civic Owner here. Picked up a '92 5MT Sedan a couple weeks ago, and was upset to find out that it got HORRIBLE gas mileage (15mpg). The guy who sold it to me said it was his son's car, and while the kid had put in some maintenance (new master cylinder, new muffler, new timing belt...), he'd also kitted the thing out with a straight pipe. Muffler but no cat. I'm no mechanic but I can think few things... I've done brakes, engine mounts, etc on previous vehicles. So I'm trying to figure out what could be causing the poor mileage. The car also smells like gas, idles very high, and feels underpowered. When I changed my oil, the folks at Jiffy lube said the throttle distributor was opened all the way up. They went ahead and adjusted it down, and the car wouldn't even hit 60mph so I drove back and had them open it up again. So I'm trying to think about root causes here:
1) The clutch is probably past its prime. It's a little rough getting into 1st, and if I rev super high in a particular gear (say, 40mph in 2nd) it might start to slip a little. But in regular operation I don't have any issues.
2) The dude said his kid installed a timing belt. If he had done it himself, could it be a timing issue?
3) I don't see how lack of a cat in and of itself would cause mileage problems, but then I remembered the O2 sensor... if that sensor is expecting cat levels of emissions, then it could be asking the engine to run rich and pissing away all my gas.
I would *guess* option 3, but what are your thoughts? And also probable solutions? I am not in a state that tests emissions, and if we're dealing with scenario 3 then putting a cat back on should solve the problem, but in the very short term I'm looking to save as much money as possible. If I have to put a cat on, I'll probably follow tutorials and do it myself. They are $150 on rock auto. There's a cost/benefit analysis to be done, however, in how long shipping will take... I commute 2 hrs/day. Other than that, is there a way to program the computer to the fact that there is no cat and adjust the engine to run properly with the straight pipe on? If there is a way to do that, then I'll probably try to do that in the short term and let a few paychecks stack up before I throw the cat back on. Thoughts?
New Civic Owner here. Picked up a '92 5MT Sedan a couple weeks ago, and was upset to find out that it got HORRIBLE gas mileage (15mpg). The guy who sold it to me said it was his son's car, and while the kid had put in some maintenance (new master cylinder, new muffler, new timing belt...), he'd also kitted the thing out with a straight pipe. Muffler but no cat. I'm no mechanic but I can think few things... I've done brakes, engine mounts, etc on previous vehicles. So I'm trying to figure out what could be causing the poor mileage. The car also smells like gas, idles very high, and feels underpowered. When I changed my oil, the folks at Jiffy lube said the throttle distributor was opened all the way up. They went ahead and adjusted it down, and the car wouldn't even hit 60mph so I drove back and had them open it up again. So I'm trying to think about root causes here:
1) The clutch is probably past its prime. It's a little rough getting into 1st, and if I rev super high in a particular gear (say, 40mph in 2nd) it might start to slip a little. But in regular operation I don't have any issues.
2) The dude said his kid installed a timing belt. If he had done it himself, could it be a timing issue?
3) I don't see how lack of a cat in and of itself would cause mileage problems, but then I remembered the O2 sensor... if that sensor is expecting cat levels of emissions, then it could be asking the engine to run rich and pissing away all my gas.
I would *guess* option 3, but what are your thoughts? And also probable solutions? I am not in a state that tests emissions, and if we're dealing with scenario 3 then putting a cat back on should solve the problem, but in the very short term I'm looking to save as much money as possible. If I have to put a cat on, I'll probably follow tutorials and do it myself. They are $150 on rock auto. There's a cost/benefit analysis to be done, however, in how long shipping will take... I commute 2 hrs/day. Other than that, is there a way to program the computer to the fact that there is no cat and adjust the engine to run properly with the straight pipe on? If there is a way to do that, then I'll probably try to do that in the short term and let a few paychecks stack up before I throw the cat back on. Thoughts?
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LunarStrider (12-12-2021)
12-13-2021
#3
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Thread Starter
Re: Low Mpg -- Straight Pipe/O2 Sensor?
Well, at this point, I'm going to change the cat and probably the o2 sensors, and then take it in to have the timing assessed. My Haynes manual won't get here for two weeks... I'm trying to figure out which is the correct sensor and I'm having trouble. YouTube seems to indicate that there is one upstream sensor and one downstream sensor. But on Rock Auto they only list "upstream" for the 1995 1.5L engine. Is it going to be the same sensor for both?
Also I'm assuming if I'm replacing one, it's probably best to replace the pair?
Also I'm assuming if I'm replacing one, it's probably best to replace the pair?
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