Fuel trim Issue 2005 Chevrolet Colorado

A few years ago a fellow showed up with a 2005 Chevrolet Colorado with a 4.2l 6cyl. It had a misfire at idle that was proven to be valve seat failure which was common for those engines. We didn’t get to do the repair because of a special policy set up by the manufacturer.

Just this past week the customer came back to us after numerous attempts at other shops to have a check engine light issue repaired. The vehicle couldn’t even have its emissions test run because of two incomplete monitors, the catalyst and the evaporative emissions systems.

When dealing with one of these the first step is to retrieve and write down any codes, check and capture freeze frame and failure record data and then get a good road test in so that you have a good baseline on how the emissions system on the car is functioning.

There was a P0171 set in the computer’s memory, that’s a lean air/fuel ratio code. Checking the freeze frame the code set at 25% engine load and 33 mph, that’s a light throttle condition. Now the question that has to be asked yet is whether this is the only time that the engine is too lean or not. The answer to that is easy, start the engine and get the truck into closed loop. Check the fuel trims at idle, no load and what I saw was the long term trim at +25%, and the short term trim +20%, that’s a total correction of 45% which is a very lean condition. The next step is to try and determine if this is a vacuum leak or some other fuel issue. To do that the engine speed is increased to 2500 rpm while again monitoring the fuel trims. They were still +25% long term and +20% short term just like the no load idle. Generally if the problem is a vacuum leak there will be a smaller correction at the higher speed because there is less time for the leak to impact the air/fuel ratio. The next step is to put the car into gear against the brakes and speed the engine up to around 1500rpm. Now there is almost no manifold vacuum, and a vacuum leak would be nullified if that was the cause of the problem.  The trims were still a total correction in the 40% range.

This is information that is gathered before the car ever moved from the spot the customer parked it at. Think about what the next step should be, and what the possible causes are. I’ll post the next part of this vehicle in a few days.