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Matt, Engineer
Category: Nissan
Satisfied Customers: 20366
Experience:  BEng hons Mech engineering, worked in auto industry for 22 years on engine, transmission and suspension design
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2004 1.8l. It started on Saturday morning running very,

Customer Question

2004 1.8l. It started on Saturday morning running very rich, black smoke out of the exhaust. Looked at it yesterday. Checked the air filter and put a new one in it. Still running rich but not as bad. Determined that it was in 2500 rpm limp home mode. Checked and cleaned the maf sensor, sensor wire didn't look too bad. can't determine why it's running rich. Fuel pressure is at 52 psi. All 4 spark plugs were sooted, cleaned them. They are only 2 months old. Unplugging each injector made it miss so it seems they are ok. I replaced the front o2 sensor, didn't think that was the problem and it wasn't. With it not in limp home mode the front o2 sensor is reading around .250 volts. The stft was at 0. The maf sensor is reading around 3gal/min on all with my scan tool. I have a maf sensor ordered, not confident that is it but I have to try something. The car has 252000mi on it and I have had it since 138000 and have only put in spark plugs. This car doesn't have an egr valve either. Ideas?
JA: Have you located all of the vehicle's oxygen sensors? Are the wires leading to and from each intact and in good shape?
Customer: Yes they look good.
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: yes.
JA: Anything else you think the mechanic should know?
Customer: I am an ase auto certified, ase master truck certified and master evt fire apparatus certified tech.
Submitted: 2 months ago.
Category: Nissan
Customer: replied 2 months ago.
BTW, there are no codes stored or active.
Expert:  Matt replied 2 months ago.


if its runnign very rich then I'd 1st check for incorrect fuelling due to a faulty water temp sensor. If this is the problem then the ECU will be leaving the car constantly ' on choke' and not reducing the fuelling as the engine warms - so unplug the sensor and clean the contacts on both sides with some switch / electrical cleaner (not WD40) and inspect the wiring for any signs of damage to the insulation especially where it runs over other components and through the bulkhead- if this doesn't help consider replacing the sensor as they're not expensive

Check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

Customer: replied 2 months ago.
I replaced the coolant temp sender. No change. The maf sensor that I ordered last weekend was delivered yesterday and that was the problem. The old sensor was reading 3 gal/min, the new sensor is reading .4gal/min. It thought it was getting more air than it was so it was putting in the fuel it thought that it needed.
Expert:  Matt replied 2 months ago.

thanks for letting me know and well done for solving it