AutoFocus: Initial fix doesn’t fix acceleration problem

July 14, 2010

The technician was working on a 1995 S/T truck with a 4.3L V-6 CPI engine. The complaint was a surge on acceleration and at steady cruise speeds.

The tech had determined that the surge was gone when the linear EGR valve was disconnected. Another sign of a possible EGR problem was a reoccurring code 1406, EGR pintle position error. He installed a new EGR valve, but to his dismay, the surge and EGR position problem were still present on the next road test.

That's when the technician called the hotline for assistance. He brought me up to speed with the driveability symptoms and what he had done so far. The truck idled fine and the surge could not be reproduced in the shop. I asked the technician to:

1) Road test the truck and monitor EGR desired and EGR actual positions on his scan tool while the surge was present. He found that as the surge became more severe, EGR actual and EGR desired positions moved farther apart on the scan tool. At idle and at lower speeds, the EGR actual and EGR desired positions matched.

2) Check EGR actual and desired positions with key on/engine off, using the scan tool's bi-directional capability to control the EGR valve solenoid. At all positions, the EGR actual and desired percentages matched. Since the EGR position error only showed up with road load, I thought that we might be dealing with excessive exhaust backpressure although the owner had not complained of a lack of power.

3) Loosen the exhaust pipe ahead of the catalytic converter to relieve backpressure and road test since exhaust restriction still seemed the most probable. The technician called back happy, reporting that the surge disappeared and that EGR actual and desired positions now matched at all times.

4) Check the pipe and muffler to be sure that the problem was indeed the catalytic converter. After the technician replaced the catalytic converter the surge was gone, EGR positions matched at all times, and the truck had a noticeable increase in power. Apparently the lack of power had occurred gradual enough that the customer hadn't noticed. The truck now ran good at all speeds with no signs of the surge or the EGR code.

I told the technician that we should do more checking to look for a possible cause. The likely causes for catalytic converter failure are coolant ingestion or excessive hydrocarbons in the exhaust stream, either from an engine that uses oil, or an engine that is allowing raw fuel past the exhaust valve(s).

* The technician quizzed the customer about coolant or oil usage... no history of either problem.

* I had him check short-term and long-term fuel trim values at idle and while driving with a steady throttle. Short-term values were in range, staying near 130. Long-term values were slightly high, averaging 138. All other sensor data appeared normal, so no real indications of a rich run condition other than the long term fuel trim values.

* Initially, with key on/engine off, fuel pressure came up to 60 psi, but slowly dropped to 30 psi after a few minutes. I had the technician repeat the test, this time with the fuel lines blocked just after the pump had stopped, pressure was still dropping. At this point we knew that the leak down must be under the upper intake plenum, since external fuel leaks were not visible.

* The technician removed the upper plenum and could see an area near the fuel pressure regulator that was washed clean. He replaced the fuel pressure regulator and put the vehicle back together.

* I asked him to clear the long-term fuel trim memory with his scan tool and road test again. Now the fuel trim numbers, both short and long, stayed within a 110-135 range at all times. In fact, the long-term now averaged close to midpoint, 128.

What started as an undetected fuel leak under the intake plenum caused the catalytic converter to eventually overheat and begin to melt down (it was now a partial exhaust restriction). This caused an increase in exhaust backpressure. The increased backpressure caused the EGR valve pintle to open further than commanded when accelerating or at a steady cruise speed. The excessive EGR position caused the code 1406 to set, and the excessive EGR flow caused the engine surge.

From the early test results the problem appeared initially to be a failed EGR valve.

The customer complaint was only of a surge and a “Check Engine” light coming on. When peeling through the layers of the driveability condition, the true culprit was evasive at first, but with some persistence the root cause was found, and a repeat of the catalytic converter failure was avoided.

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