# 2005 Frontier Nismo Excessive Oil Consumption - Needs New Engine and Cat Converters



## agrace1410 (Oct 8, 2011)

I am having to replace my engine and 2 front catalytic converters on my 2005 frontier crew cab nismo. It has 118,000 miles on it and the front two converters have already been replaced once (by a previous owner). The truck has also been burning enough oil that I have had to put 2 quarts in it every week. When taking things apart to get the engine out and the truck ready for the new engine, I noticed a lot of black (what seemed to be oil) in the Mass Airflow. There was also a lot of grit (maybe carbon buildup) around the 1st and 5th spark plugs (both on passenger side) and there was a lot of oil around the 3rd spark plug (middle one on the passenger side). Right before I took my truck to Nissan the Check Engine Light was flashing randomly (before this, it was just on due to one bad catalytic converter). The PCV Valve has already been replaced. What could be the cause of my problem/problems?


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## rogoman (Dec 16, 2004)

What engine is in the truck? If it's a QR25DE, then those early engines had many problems with failed CATs causing ingest material from a damaged pre-catalyst resulting in increased oil consumption. Power valve screws in the intake manifold may become loose and result in unstable idling, power loss or damaged engine.


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## agrace1410 (Oct 8, 2011)

My engine is the VQ40DE. It has 4 cats but we can't seem to figure out why they are going bad. We feel like my oil problem may be from the bad cats causing damage to the engine but what is the actual source of the problem? Why do the cats keep going bad? My engine isn't old enough to be going on it's 3 set of cats.


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## rogoman (Dec 16, 2004)

Some reasons for CAT failures:
- excessive rich fuel mixture
- bad O2 sensor
- excessive oil burning
- antifreeze in exhaust


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## azrocketman (Oct 5, 2005)

A flashing check engine light indicates a misfire. The flashing is to get your attention because a misfiring engine is putting raw fuel in the converters causing them to run hot and potentially causing converter damage.

Steve


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## agrace1410 (Oct 8, 2011)

But I had the oil consumption problem wayyy before the check engine light even came on.


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## smj999smj (Jan 1, 2006)

The problem with the QR25DE engine's was...supposedly...the ECM program was causing the engine to run too rich and causing the precat to breakdown. The QR's, rather than use a conventional EGR valve, used the valve timing to open the exhaust valve slightly during the intake stroke to draw in a small amount of the inert exhaust gas from the exhaust system. In theory this sounds great, but what happened was that small pieces of the catalyst were sucked in past the exhaust valve and into the combustion chamber, where it scored the cylinder walls and eventually caused the oil burning issue that many are now familiar with. I've never heard of this issue occuring on any VQ-series engine, but I do believe the VQ40DE does use the same type of EGR system.


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## agrace1410 (Oct 8, 2011)

Do you know if there is a way to avoid it from happening again once I put a new engine and cats on it? I just hate to spend so much money fixing it to have the same thing happen again.


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## smj999smj (Jan 1, 2006)

Not without knowing what was the cause of the oil burning in the first place. It would've been a good idea to perform a cylinder leakdown test prior to removing the engine to isolate the cause for compression loss. I suppose a borescope could be used to look into the cylinders and check the condition of the cylinder walls and tops of the pistons. I would suggest installing new upstream (or "front") oxygen sensors. Driving the vehicle with someone monitoring the datastream on a scantool might be a good idea, making sure the coolant temp is reaching correct temperature and the oxygen sensors are cycling properly and the long term fuel trim stays within the range of approx. 97% to 103%.


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## agrace1410 (Oct 8, 2011)

why the front O2 sensors and not the rear ones or all of them?


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## smj999smj (Jan 1, 2006)

The catalytic converters likely went bad due to contamination. If they were contaminated, the upstream sensors, which sit in the exhaust stream between the exhaust ports and the front catalytic converters, were likely contaminated, as well. The upstream oxygen sensors have a direct affect on the air/fuel mixture determination by the ECM. The downstream sensors, which are used for monitoring the efficiency of the catalyst only and have no influence on the air/fuel mixture, were likely not to be contaminated as they are located after the catalytic converters. The contaminant, likely oil, would probably burn up or clog in the catalyst prior to ever reaching the downstream sensors.


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## agrace1410 (Oct 8, 2011)

Would my ECM not have given me a code for bad O2 sensors? I don't know what code the ECM actually read. I just know that Nissan told me I needed two new front catalytic converters and a new engine.


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## jdg (Aug 27, 2009)

Not necessarily. The ECU only throws codes at what it "thinks" is wrong, not what is actually wrong, although codes thrown are usually a really good guess. Quick example is if you break a wire for the TPS, it might throw a TPS code (although today's ECUs can generally distinguish between a bad TPS and a bad wire, but that's another story). If you had other things going on, oil burning that contaminated the O2 sensors initially caused by the meltdown, oil fouled plugs causing misfires in the first place, etc, the ECU might've decided to ignore everything else and just keep it running good enough to get you home. I would venture to guess that you probably did have O2 sensor codes, but that was the least of the worries of the tech working your engine.


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