A/C Component Causes Loss of PowerPosted 11/16/2004
By Mark Owens
One summer when I worked at a dealership, a customer brought in a 1995 Chevrolet C1500 pickup. It was a standard cab with 5.0L throttle body and automatic transmission. The customer complained about a loss of power on acceleration, but I wasn't able to duplicate the complaint. And after checking the obvious fuel pressure, secondary ignition, prom updates and bulletins, I wasn't coming to any good conclusions.
The A/C system was an aftermarket add-on. There was a cell phone mount, electric door locks were added, and an aftermarket stereo was also installed. These accessories were always in the back of my mind as I worked on the car's diagnosis. I returned the vehicle to the customer and advised her to make a mental note whenever she lost power on acceleration.
The symptoms came and went at random and still could not be induced. Finally, the customer determined that this problem only happened on a freeway acceleration ramp, a short distance from her home, and only after the vehicle had been sitting for an extended period of time (overnight). The hesitation was consistent with the time frame of operation, but it did not happen every day - only when she took the freeway to work. I checked all of the obvious items a second time. Still found no problems.
I started to suspect that the engine's air/fuel ratio was incorrect; maybe caused by an early fuel evaporation (EFE) problem. This was far more common on older carbureted engines with EFE systems. The only dedicated system on this application was the hot air intake tube from the exhaust manifold to the air cleaner housing and a thermal valve used to control the hot air inlet.
The engine often runs good without this hot air intake tube in this application, especially in the summer since this engine uses engine coolant to warm the intake manifold. No problems were found during a visual inspection of the air intake system. Internally, the engine was in "new" condition.
I realized that every time I worked on this truck it was a hot and muggy day. That is unusual in Minnesota. Most hesitation complaints come during cold weather. Except for a few scattered warm fronts during the summer, we'd drive with the windows open on the way to work and the A/C on the way home. I started wondering if the seldom occurrence of hot weather could also be related to this intermittent condition.
Could the customer be running the A/C in the morning on the way to work and somehow be supporting my theory of an EFE problem? I anxiously went to the parking lot, opened the hood and performed another visual inspection. When I started reviewing the A/C system more carefully, I figured it out instantly!
I couldn't believe I hadn't thought of it sooner. But then again, things usually make sense after you figure them out! The aftermarket A/C installation contained a hot water shut-off valve located in the heater's core inlet hose. When the A/C was on, the water shut-off valve stopped coolant from entering the heater core. This coolant hose is also used as the engine coolant bypass if the thermostat is closed.
When the customer ran the A/C in the morning, coolant flow through the engine was being stopped by the aftermarket installation of the shut-off valve. This coolant must flow to warm the intake manifold and keep internal engine temperature consistent. With the engine warming up and the thermostat still closed, the coolant temp sensor, located near the closed thermostat, was not reading engine temp accurately. During this condition of a closed thermostat and a partially warm engine, the air/fuel ratio was incorrect for the engine on the acceleration ramp.
When the customer took an alternate route to work, she "drove through" the problem because no rapid change in fuel delivery was needed, and the condition did not exist. When the thermostat opened, engine coolant flow was restored, and the problem was gone.
The customer left the truck for me to drive in the morning. I started with a cold engine and ran the A/C. After a short drive, I accelerated hard and it fell on its face. I was so happy to finally verify the complaint! The next day, when I disconnected the hot water valve and performed the driving sequence, A/C performance was barely affected. It was fixed - without any parts replacement! The aftermarket installation of the added components had been the problem all along.
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