MPG dropped, running rough. Bad O2?

[toovira]

No CEL came on. The car runs rough from time to time. I examined the plugs and it's a little lean on all cylinders. I replaced the plugs last year. So I think they are still good. My MPG dropped from 27 to about 16MPG. Fuel pressure tested at about 40PSI, within specs, even under rev.

I figured if it's the dirty injector, I will find some variation in how the plugs look, but in this case, there was little variation.

I followed the Service Manual on how to test O2 and tested the upstream O2...voltage seems ok, although high variation on voltage...between 0 and 1V even with no variation in throttle (constant speed with minimum load change).

I've read that O2 should be replaced every 30k miles as a maintenance - that's the first time I've heard of that....is this true?

I'm pointing towards O2 because I think it's the only thing in this point that can lean on all cylinders because inaccurate reading. FP is good....I'd point to Fuel pump if FP was low.

[TedL]

It helps to know year, engine, mileage.

Replacing O2S's at 30k miles as PM is only recommended by those selling O2S's, not those buying them. 90- 100 k miles is reasonable for PM. I'd look at the fuel trims shown by my scan tool, but I'm guessing you don't have that available to you.

Fuel pump/pressure/filter failure does not cause fuel mileage drop, at least not 'til you're to the point of misfiring. Sounds much more like a bad AFR sensor that's not bad enough to be totally out of valid range. But I'd like to see the ECT that the computer is seeing. The problem could be there.

If you're into parts swapping rather than diagnostics, most Toyota AFR sensors can be had for around $100 online.

[ShawnTRD]

Get the CEL code checked then come back. Autozone, Advanced, they all will check it.

[TedL]

Originally Posted by ShawnTRD

Get the CEL code checked then come back. Autozone, Advanced, they all will check it.

???? The first two words of the post are "No CEL.....".

[ShawnTRD]

Sorry I read it as "CEL came on"

[thaharlequin]

I've never had a significant drop in mileage or a rough idle from an O2 sensor going out. In fact, the only way I've gotten it to idle horribly even with all the O2 sensors unplugged is to really really lean out the AFM (cog), and then it just bounces up and down. Otoh, I have had really rough idle when one of the spark plug wires broke off on the plug, and the would decrease mileage by a good 15+% or more if the trans ends up unlocking more often.

[fenixus]

... and the year, model and mileage is ... ?

you could have a bad ECT for EFI sensor, just a wild guess.

[toovira]

1995 5S-FE 2.2L I4. 340k miles. The O2s have about probably at least 150k on them. The problem with the diagnostic in the service manual is that it's very vague. It did meet all the specs (2500rpm rev and E1/VF1 switch to 5V more than 8 times in 2 minutes and the other test...O2#1 voltage measure in ref to E1).

I tore apart the disty, changed the disty rotor, ohmed all plug wires, found that one plug gave inconsistent reading. After probe a little more, all of the sudden, it ohmed correctly. Reinstalled everything and re-did the timing. I got shocked while grabbing the distributor (right at the plug wire connectors) while the engine is running. So I don't think properly insulated wire would've shocked me. But now, since it seems a lot more consistent and smother. Once in a while, she'll sort of choke at constant speed.

So new plug wires are on order...think I'm going NGK instead of shitty Autozone, etc.

Sorry guys, this is Pre-OBDII. Otherwise, I'd data log the damn car while driving. No Diag Trouble Code anyway....so O2 might actually be good.

[toovira]

On a side note, the plug wires appear OK through visual inspection....but I still insist if they're good, I should've not been shocked. It was dry and no oil or liquid near it....and I grabbed the plastic wire secure tab....which is fairly far away from the actual conductor.

[ShawnTRD]

Air filter, and Fuel filter good? Maybe run a can of BG 44K in the gas tank.

[fenixus]

if you've got shocked by touching live spark plug wires' insulation then there must be a fissure in them, so the spark plug wires are bad. change them. I would suggest doing a minor tune up for staretrs (plugs, wires, belts and A/F filters) then go see about the sensors.

at 150k miles though, I would just go ahead and replace all o2 sensors and also an ECT for EFI sensor.

[toovira]

As I mentioned before, the fuel pressure, which is measured after fuel filter, is well within spec. The air filter is slightly dirty....I don't think air flow can cause a sudden stall unless a squirrel is living in the airbox. Plugs inspected...less than 20k miles on them...slightly lean on all cylinders. First order of business is plug wires.