# 98 Frontier runs rough



## lowdownfreedom (Nov 1, 2007)

I've got a 98 Frontier that is running rough. When I start it, it idles nornally for about 10 seconds and then starts running real rough. It runs OK when I'm driving but runs real rich.

I've changed the oil, replaced the fuel filter, plugs, and air filter, as well as the fuel rail (which was leaking), mass airflow sensor.

THe check engine light is on, but when you plug a diagnostic computer into it, it reads that everything is normal.

I'm thinking the computer (brain) is bad. Any suggestions?


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## boogyman (Jan 24, 2005)

Have you check the oxygen sensor to see if it's changing voltages. I think it should go between 0 - 5volts. I don't have the specs on your year, so I can't tell you where the lead is to check. It could also be the ECM(brain), but that should throw a code. Maybe someone else might have a clearer idea. Good luck.


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## lowdownfreedom (Nov 1, 2007)

Thanks, Boogeyman, I'll try checking the O2 sensor. Anyone know how to check the ECM to see if it is functioning properly?


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## lowdownfreedom (Nov 1, 2007)

Oxygen sensor is working fine. I think this should have given a code as well if not working properly.


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## azrocketman (Oct 5, 2005)

lowdownfreedom said:


> Oxygen sensor is working fine. I think this should have given a code as well if not working properly.


How do you know the system is running rich? Is there black smoke or a heavy smell of unburned gas? How's you gas mileage? Is it down significantly?

Are you sure your OBD II diagnostic tool is working properly? If you disconnect a sensor, say the MAF, does the tool show a code that indicates your inserted fault?

Steve


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## lowdownfreedom (Nov 1, 2007)

Gas mileage is definitely down significantly. It is the 2.4L 4 cyl. and I'm only getting about 18 mpg on the highway. Exhaust is on the dark side with smell of unburnt fuel and oil is was real dark in color shortly after I changed the oil.

The diagnostic computer belongs to a friend that is a mechanic. According to him, he has never had a problem with it. I will try diconnecting MAF to see if I get a code.


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## boogyman (Jan 24, 2005)

Is your throttlebody working ok? What I mean is the butterfly opening all the way and not sticking. Other than that, I think it's the ecm. This problem is beyond my knowledgeable guess'. I would call around to some junk yards and see if they have any. Otherwise try and get some prices from auto stores or dealers near you.


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## azrocketman (Oct 5, 2005)

I'll make a guess that you have a pretty high mileage engine (maybe better than 160k miles). I'm thinking that you may have a PCV valve problem (valve stuck open). I'm thinking that the dark oil is indicating a lot of blowby and an open valve is not metering the blowby gases properly into the engine manifold. The good news is that the part is cheap. The bad news is that it is an SOB to change on the 4 cylinder engine (located low on the engine behind the alternator). I'd replace the PCV valve as my next step.

Before I'd conclude that the computer is bad I'd have a more sophisticated OBD II diagnostic machine, e.g. the Nissan CONSULT system, read the system codes. The sophisticated systems can look at system settings or "trims" that may give a better indication of what is happing in the ECM's "mind". 

Steve


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