Check engine light is on

  • 2.7L
  • 6 CYL
  • AWD
  • 138,000 MILES
When I put my foot on the gas the rpm goes up but the car will not go above 25 mph. No power and car sputters. Local mechanic changed the o2 sensor and the evap canister. When each was changed and put on computer again they went from rich to lean and lean to rich. Mechanic said he's never seen anything like this before and said to go to dealership as he could not diagnose problem. The sensors he did change did not solve the problem, maybe even made it worse. Dealership wants $99 per hour to diagnose. Please help! Thank you.
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Monday, March 2nd, 2015 AT 5:01 PM

1 Reply

Anything under $100.00 per hour is real good deal, especially if it saves you from wasting money on unneeded parts. If I were to post my list of taxes, insurances, government regulations, utilities, wages and benefits, freebies, and all the other things repair shops have to pay for, you would wonder how they could afford to stay in business by charging so little.

The place to start is by reading and recording the diagnostic fault codes. You need to find out which ones were set. It's best to post the exact code numbers because there can be a lot of different potential codes related to any one sensor, and those codes mean very different things. It's important to understand that fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. About half of the time when a sensor is referenced in a code, it is not the actual cause of that code. There can be wiring and connector problems too that have to be eliminated first.

The fact that your mechanic found the oxygen sensor switching between "rich" and "lean" proves it was working. It is supposed to switch about twice per second.

Since engine speed is increasing but the vehicle isn't, the things to look at are the transmission staying in a lower gear, the transmission is slipping, or the brakes are dragging. You can identify a dragging brake by feeling the heat by that wheel, and by stopping in neutral on a slight incline and the vehicle doesn't creep downhill on its own. You can feel the shifts and when they don't take place.

There isn't enough information to make a better diagnosis. You need to see a competent engine performance specialist, but that doesn't necessarily mean going to the dealership. Most independent shops have one or two people who specialize in driveability problems.

Due to a house fire, I have to drive into town and sit in the library to use the internet. They'll be closing soon so I won't be able to reply right away. Try to find out exactly which fault codes were set, and any other information, observations, or clues that might lead to a better diagnosis. You might consider having the charging system tested too. Of particular interest is the maximum full-load current and "ripple" voltage. GM generators starting with 1987 models develop a real lot of voltage spikes. If ripple voltage is high, those spikes will be worse, and they can damage the internal diodes and voltage regulator, and they can interfere with computer sensor signals. Defective GM generators have been the cause of a lot of elusive running problems.

Your mechanic should also use the scanner to view live data while the engine is running. That will display sensor voltages. Each one has a set range of signal voltage that is acceptable to the computer. Fault codes are set when the voltage goes out of that range. The problem is a sensor can send an incorrect voltage, and the computer will try to run on that, but as long as it's within that acceptable range, no fault code will be set. This is where the mechanic has to interpret the data. He can also manipulate things to introduce a rich or lean condition, then watch to see how the computer responds.
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Monday, March 2nd, 2015 AT 5:51 PM

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