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  Tech Tips

Cutting Corners Can Be Costly

Posted 9/10/2009
By Paul Kujawa

Nissan Quest comes in running rich.

Recently we received a call on a 2000 Nissan Quest. The vehicle came into the shop running very rich, both at idle and at higher rpm, and the O2 sensor was reading rich. The engine ran better at higher rpm but was still too rich.

"… a 2000 Nissan Quest came into the shop running very rich, both at idle and at higher rpm, and the O2 sensor was reading rich. The engine ran better at higher rpm but was still too rich. Unplugging the mass air flow (MAF) sensor caused the engine to run much better."

Unplugging the mass air flow (MAF) sensor caused the engine to run much better. The only trouble code in the engine control module (ECM) was a knock sensor code, P0325. On this vehicle, the knock sensor code will not turn on the malfunction indicator light (MIL) or cause the engine to run poorly. Checking scan data at a warm idle, the MAF sensor grams/second reading was high at 9.9 grams. A normal reading is about 4.4 grams for this vehicle. The scan data also indicated the calculated load was reading very high at 99 percent.

The MAF sensor output voltage on the white/blue wire was checked and read high at more than 2.0 volts. The MAF sensor ground voltage was checked on the blue/orange wire and read OK but the technician added a ground to the MAF sensor anyway ... no change. With the MAF sensor unplugged, the engine ran much better and the front O2 sensor started cross-counting above and below 0.5 volt. When the MAF sensor was plugged back in, the engine started running rich again. The technician had already installed a remanufactured MAF sensor and the problem did not change. The MAF grams/second and calculated load readings still read very high.

Scan tool data showed short-term fuel trim (STFT) reading at minus 25 percent, then resetting back to 0 percent, and counting right back down to minus 25 percent. This indicated the ECM was trying to take fuel away. Long-term fuel trim (LTFT) was reading minus 8 percent. When the STFT dropped to minus 25 percent, the engine ran better and when the STFT reset back to 0 percent, the engine ran worse.

The front and rear O2 sensors both indicated rich with voltage over 0.8 volt. Then the O2 sensor voltage was checked with the O2 sensors unplugged and the key "on." There was a 0.29 bias voltage to the O2 sensors from the ECM, indicating that the O2 sensor circuits to the ECM were good. The injector milliseconds were checked and were high with more than six milliseconds at warm idle. The engine coolant temperature reading on the scan tool was normal. The ECM powers and grounds were tested for voltage and were OK. The charging system voltage was normal and there was no excessive AC voltage in the charging system.

We have seen high MAF grams/second readings and negative fuel trims due to a fuel injector resistance problem, so the fuel injectors were tested and all had resistance within the specification of 10 to 14 ohms. All six spark plugs were black, which indicated that all cylinders were running rich. We suspected a defective remanufactured MAF sensor, so another was installed, and the problem was still not corrected. The engine continued to run rich and the MAF sensor and calculated load readings were still high as well.

Now that the MAF had been replaced twice with no change, it was ruled out. There must have been something causing excessive air flow through the MAF sensor. Next, the idle air control (IAC) valve command was checked on the scan tool to see if command was high. It was 20 percent to 32 percent, which was a little high, but likely caused by the ECM opening the IAC valve more in an attempt to keep the engine idling.

The timing belt alignment was checked to determine if the camshaft timing was off a tooth, resulting in low engine rpm and the increase in the IAC valve command as the ECM tried to keep the engine running. The timing belt checked OK. The ignition timing was inspected and was correct. The start signal was "off" with the engine running. The throttle position sensor (TPS) was checked and showed zero percent closed throttle at idle. Scan data showed no other inputs with an incorrect reading. Next, the technician tried a used MAF and ECM but still got no change. This made absolutely no sense.

I mentioned seeing high MAF readings from a similar case where the MAF sensor screen was restricted, causing increased air velocity through the MAF sensor. Since our MAF sensor had been replaced three times, that didn't make sense. Then the technician told me that he was only taking the old sensor element out of the MAF sensor housing and replacing it with the new element. He did this to make it easier to repackage the part and return it if it didn't fix the problem. Upon inspection of the MAF sensor screen, he found the same problem with this one. There was debris on the screen, increasing air flow velocity through the MAF sensor. The screen was cleaned and the problem was solved.

Lesson learned: Trying to cut corners to allow an easier return on unwanted parts can turn out to be costly in time and money.

Paul Kujawa Paul Kujawa is an Identifix Asian specialist. He is a Nissan master and ASE master with L1 certification.
Experience Identifix This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.

For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time.

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