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98 Maxima runs rough once it warms up/doesn't idle well

My 98 Maxima won’t idle without dying, it sputters when you accelerate, and it tries to die while coasting. I’ve changed two O2 sensors (AutoZone gave me code for bank 1 sensor 1 & 2) changed spark plugs, tried fuel injector cleaner, oil change, and cleaned MAF sensor & checked wiring. It;s still spitting out black exhaust. Although after changing the O2 sensors it quit for about a day or two, but started again (I’m guessing the other sensors need to be changed to) But the main problem is once it warms up, it really runs like crap, and is a major gas hog. I’m getting about 12 Mpg. HELP!!! lol

“code for bank 1 sensor 1 & 2” is way too vague. Do you recall the specific code numbers? (e.g. P0171 maybe? P0172?) Lots and lots of codes. (If you’d like, go here. First do a search for “sensor 1” and then do a search for “sensor 2.” You’ll quit before long).

I’ll guess this - I’ll guess it was a code that indicated that you are running rich (e.g. P0172) - because it sounds like you are. And I’ll further guess that you let the Autozone person help you fall for the trap that codes tell you that sensors are bad? (Sometimes, sure).

Anyway, I’m guessing that the engine light is back on and even if it isn’t have it scanned for codes again.

It sounds like the trouble begins after the car enters closed loop or should be entering closed loop, and I would check the coolant temperature sensor. You changed the plugs. How are the wires?

I would also check the fuel pressure and fuel pressure regulator. But having codes would help. If you can get it on a scantool that does live data a peek at the fuel trims would probably say a lot.

I don’t recall the specific code #, if I was at home I could look at the slip. I just remember it said something about the sensors being bad, so we changed the 2 front ones. The CE light is back on again but I haven’t been able to have it checked yet.
I’v e heard all kinds of things. Like I need a new MAF sensor, or it’s the catalytic converter (majorly expensive…) or even fuel filter.
What’s the difference between a live data scanner & what Auto zone/Oreilly’s uses?

“I just remember it said something about the sensors being bad”

That’s what I was trying to say - there are no codes that tell you that any sensors are bad. All a code ever tells you is that the computer sees something [consistently enough] out of spec. The actual cause for what the computer sees is always TBD by a human.

You will hear all sorts of things. That’s because a) lots of people don’t know what they’re talking about (leave open the chance that that is self-referential) and b) people don’t have the info they need. There are 101 things that will make a car run poorly. Intelligent testing is how to get the 101 down to 1. That often starts with diagnostic codes. So get the codes read again, but this time don’t buy anything. Just write down the “Pxxxx” number and post it.

When you do that report more about the car. How many miles are on it? What is the overall state of maintenance & its history? Not “oil changes” - but all of that stuff the owner’s manual lists. So far we know the plugs aren’t that old.

Great answer. cigroller!

The most common DTC reading can usually done with the engine turned off. It just reads the codes that have been stored in the computer’s memory. Might be codes from yesterday, or two weeks ago. Live scan means they hook a scan tool up while the engine is running and look at various measured engine parameters and how they change over time, like as the engine warms up, they’ll look at parameters like fuel-trim which can be diagnostic in a situation like this. Fuel trim is how much extra gas (or reduction in gas) is required to be injected to achieve the required O2 reading, beyond what the computer thinks would be required for that situation.

Since the problem occurs once the engine is warmed up, this would be consistent with an air/fuel mixture problem. The O2 sensors are the primary method of controlling air/fuel mixture, and they don’t start working until the engine is warmed up. Before that, they computer uses it’s open-loop parameters to calculate fuel injection rate. In your case the O2 sensors, once they come online, they may be saying there’s too much O2 in the exhaust, so it thinks more gas is required. But it is possible the O2 may be coming from an exhaust leak, not that there’s too much O2 in the intake manifold. When the computer attempts to correct for too much O2 in the exhaust, it injects extra fuel into the intake manifold, which makes the combustion mixture too rich, causing poor performance and poor mpg.

You might consider asking your mechanic to do a fuel trim measurement.

I’ll have to double check then LOL. Although when we changed them, they were clogged & black.
Also, when my husband was taking stuff apart, I don’t remember exactly what it was, but it was up by the accelerator somewhere, some black piece he took off, it had tiny little pieces of what looked like charcoal in it?
The car has 262K miles on it…When I got it, there was no maintenance records, nothing. just that the person I bought it from only purchased it 5 months before…
But as soon as I can get the codes read again, I’ll do that!

That gadget with the black stuff on it might have been the idle air control valve, or it might have been the throttle body. No harm done to clean them. Probably did some good. But it sounds like there is at least one more problem remaining.

“when we changed them, they were clogged & black”

I assume that is about the O2 sensors. They were clogged and black for the same reason you have the smoke out of the tailpipe - you’re running rich, or burning tons of oil. Do you have to add oil a lot? Anyway, as George mentioned the O2 sensors report to the computer on the air/fuel mix. A lot of people assume that a code that says a sensor is reporting rich/lean means the sensor is bad. Meanwhile, the sensor is doing exactly what it’s supposed to do. So changing it is shooting the messenger. You said the car is running rough and getting lousy mileage and blowing smoke. All of that adds up to running rich - so the codes probably say about the same thing. So then you look for reasons there is unburnt fuel blowing through the exhaust - fuel pressure problems, fuel injector problems, spark problems, “information” problems (like a bad coolant temp sensor), etc.

P.S. running rich can gum up the O2 sensors, so you may have needed them anyway. But if the new ones get gummed up…

Yes, the O2 sensors were clogged. And I checked the slip, the codes were P0134 & P0154.
The oil level is usually fine,

The part w/ black stuff, it wasn’t ON it, it was IN it. Like tiny little charcoal-like pellets rolling around inside.

Have the code read again and report back. If they are the same, then note that those codes are about the circuit not the sensors. The sensors are part of the circuit - the rest of it has to do with the wiring, including the connections at the computer. Both codes say that the computer see nothing from those sensors. So, the first thing to do is check the wiring. If you go to Autozone’s website and register an email address you can plug in the car’s info and get access to wiring diagrams to help with that.

But the car is running very rich. This could have something to do with the O2 sensors since they are needed for the computer to do the fuel / air mix. But it could also be other things causing the rich running - and then gunking up the sensors again. It’s a non-virtuous cycle. For running rich go back to my earlier notes - spark plug wires, fuel pressure and fuel pressure regulator, coolant temperature sensor… The car isn’t running cool is it? As in, not getting all the way to temp and maybe producing very little heat?

Alright, I’ll try that. Thanks :slight_smile:
No, temp runs normal, right in the middle. There’s also issues with it idling low. We’ve raised the idle some, and that helped w/ it dying, but not for long. It always seems to get loosened up.

Might be worth it to have a compression test done too. A burned exhaust valve can allow unburned gasoline directly into the exhaust steam. That would usually show up on a compression test.

The problem might be with the fuel pressure regulator.

If the fuel pressure regulator developes a leak at the diaphram, the engine will draw in excess fuel causing a rich condition.

The fuel pressure regulator is located on the fuel rail. Start the engine and let it idle for thirty seconds. Shut the engine off and remove the vacuum hose from the regulator. If gas leaks out of this connection replace the fuel pressure regulator.


It’s been a while, but I finally managed to have the CE light checked again. The codes are:
P0154, P0134( this is one of the sensors we previously changed) , P0100, P0300, P1320…

Have you checked the fuel pressure regulator per @Tester?

Yes, and nothing was leaking out. But that was a couple months ago, the car has gotten worse since then so I may check it again. Before, despite the way it ran w/ all the sputtering, the engine would get up & go it ran fine. But now it’s gotten to where it’ll barely register you’re even pressing on the gas…

You’ll be money ahead to take it to a good mechanic and authorize at least an hour of diagnostic time. It’ll be the best $100 or so you ever spent on this car.

Probably so!

You didn’t list out all of the codes. This doesn’t help.

I’m going to assume that you have a bad wiring harness someplace. One of the things this is doing is killing the signal from your upstream O2 sensors (P0154, P0134). Once the car warms up the computer needs these signals to do the air/fuel mix. It is also messing with the MAF or MAF signal (P0100). The result is that the car will run like crap once it’s warmed up and perhaps misfire a lot (P0300). If you had actually listed all of the codes, it might be easier to see a picture forming.

For now, when you do take it in, no codes refer to sensors. It’s completely impossible. The only way for the computer to “see” a sensor is via wires. Codes refer to circuits. You probably have a problem in one or more. Either that or you’ll be looking at a new PCM. Whatever you do, don’t let someone just start throwing parts at the thing.