I need some help.

[Dirtbag]

Having a problem with my 1998 Nissan 240SX with a stock KA24DE engine. The "service engine soon" is on and when read by an OBDII scaner it says that the O2 sensor reads the system is too lean and that there is a misfire. My fuel consumption is through the roof at about 15 MPG and when I turn the A/C off the car's RPM's drop to about 500 and the car shudders.

[moso]

sounds like a bad o2 sensor.... 15mpg is pig rich!!

[compactfean]

Id say cts with that bad of fuel economy.

[Dirtbag]

Would a bad O2 sensor cause the engine to drop in revs hen I turn the A/C off? Also, I'm kinda a noob, CTS?

[compactfean]

Coolant temp sensor. And no, o2 sensor wouldn't cause that.

[Dirtbag]

What about just general carbon build up? Causing the O2 sensor to fail and read lean and clogging the IACV? Perhaps seperate issues? Looked into vacuum lines via the propane torch method and no leaks. Now where can I find the coolant temp sensor and how do I test it exactly?

[moso]

the ecu is programed to idle up then the ac is on to compensate for the a/c compressor running. usually when the cts goes bad the car will run like crap when it warms up, it will also have bad gas mileage and want to buck and stall out. it usually wont want to start when its warm either. your car is throwing a code that says "hay buddy, my o2 sensor says the engine is lean but I'm dumping crap tons of fuel in the motor....something is wrong". that being said having the actual code numbers would be quite helpful, but im still going with bad o2 sensor.

[compactfean]

Word! ^ codes please.

[Dirtbag]

P0171 is the reliable code.

The Misfire is not present right now i don't know if it was a false code but I cleared the ecu and the misfire hasn't shown up again.

[Dirtbag]

Alright. I reset my ECU to see if it would re map the fuel mix. The engine light hasn't come back on yet but the idle is still very bad after I turn the A/C off. Too soon to tell if it had any effect on fuel economy.

[DanielRuff]

did you get you car running good, again? I am trying to fix a 1989 240sx automatic w air. So far I have had to clean the gas tank, replace the fuel pump, fuel filter, something ate 12 wires in two, and the computer said my engine temp sensor was bad...(code13). Now my computer says code 55 all ok but it idles like crap and has no power at all. It had some noise I couldn't place, so I take the valve cover off and I see half of the timing chain plastic guide (facing the car on the right side) I see half of this laying about half way down the engine, I guess the rest of it is stuck to the pick up tube in the oil pan in pieces. Anyway, what causes that to break and what would cause it to run that bad?? Thanks for any help.

[Dirtbag]

Ahhh. Someone please help. This is what I've tried so far:

Cleaned the MAF.
Seafoam
Change oil and filter.
Change fuel filter
Change spark plugs and wires.
Checked the distributor and rotor. They are fine.
Checked for vacuum leaks. None found.
Changed front O2 sensor.

The car has a surging idle and horrible gas mileage. Maybe 15mpg combined on 93 octane. I need this problem fixed soon. I can't afford the gas waste.

[Dirtbag]

Forgot to add. The CEL is on and displaying code P0171.

[moso]

that code p0171 means your getting non-metered air in your intake, i.e. you have a vacuum leak. that or your mafs or mafs wiring is faulty.

[compactfean]

I've thrown that code and ended up being a cracked stock rubber intake. A little duct tape worked wonders till I replaced it.

[Dirtbag]

But I checked for vacuum leaks and couldn't find any. I used a propane torch method. Ugh. This is getting old. Where are thw most common places for these leaks if there are any? Looking for an answer other than "the vacuum hoses" or "any of the rubber hoses on the vacuum line"

[moso]

the propane method dosnt really work, your ecu is dumping fuel, the propane is going to further richen an already rich condition.

[Dirtbag]

Then...how the hell do I check for a vacuum leak? I thought that was a solid way of testing for it. I am tempted to buy one of the vacuum line kits I see online.

[compactfean]

You can use a spray bottle with just water, or my personal favorite.....a boost leak detector. (yes it can be used on n/a motors as well, do it all the time.)

[moso]

propane will work for a vacuum leak if your running lean, if your running rich it does nothing

[Dirtbag]

Ok. Confirmed. No MAF issue, no vacuum leak, no iacv or throttle body issue, no TPS or CTS issue, no timing issue, no rotor/distributor/spark issue, and no O2 sensor issues. The only thing I can think of now is fuel injectors. Anyone else?

[biggie]

Tried a different ECU yet?

[compactfean]

Egr valve could have a, vacuum leak....that'd one that's easily overlooked.

[Dirtbag]

Where is the EGR? I havent seen it yet

[biggie]

EGR is the system that has a pipe running behind the engine from intake mani to the exhaust mani.

[Dirtbag]

Ahhh. I just realised I never updated this. Haven't been on in months and figured I should inform you what I have found so that it may further assist any future issues.

After checking, double checking, and triple checking everything including going through 2 O2 Sensors and quite a bit of brake cleaner, as well as damn near rebuilding the engine I found the issue. It was a single wire in the wiring harness. I finally realized while on the diagnostic that when I played with the O2 sensor lead from the wiring harness it would intermittently show that it is reading the A/F Ratio. so I took that nasty heat shielding off and found that one of the O2 lead wires were broken and shorting. After replacing that section of wire there was never an issue again. Hopefully this information will help someone.