Don’t blame shifting problem on transmission

Hey there, time traveller!
This article was published 11/06/2004 (7894 days ago), so information in it may no longer be current.

QUESTION — I have a 1998 Chevrolet half-ton with a 5.7-litre V8 engine and a four-speed automatic transmission. I think I need a transmission. Sometimes, after starting the truck the transmission doesn’t shift. The engine runs fine. If I turn it off for a couple minutes and restart it, it may then work. I had the transmission oil changed but the problem is still there. Should I put in a new transmission or is there something else I should check?

ANSWER — Don’t put a transmission in your truck. Your problem is intermittent which usually suggests an external problem. This transmission is electronically controlled by the powertrain computer, as are most modern automatic transmissions. Two shift solenoids on the transmission valve body are switched in various on/off combinations by the computer to provide four forward gear ranges. If the computer senses a problem with the electrical circuits for these shift solenoids, then it prevents them from operating.

With both solenoids turned off, the transmission is stuck in third gear. A poor electrical connection at the transmission or at the computer could cause this problem, but yours only seems to occur at startup. I have seen this same symptom occur after a remote starter system has been installed, but your truck’s problem is most likely in the ignition switch.

The ignition switch comprises several switches combined into one unit. One of the switches provides power to the powertrain computer. Another switch provides power to the transmission solenoids. If the computer is powered up before the transmission gets power, then the computer “thinks” the transmission solenoids or wiring are faulty. The computer keeps the transmission in third gear until it can test it again at the next vehicle start sequence. If everything is then powered up at the same time, then the transmission shifts properly. Check out the ignition switch circuits first.

QUESTION — My 1989 Plymouth Voyager’s four-cylinder engine has started using oil. There are no leaks or puffs of blue smoke from the exhaust. The engine has 140,000 kilometres (86,000 miles) on it and the compression is good. Is there an additive that can stop the oil consumption?

ANSWER — When an engine uses oil, there are only two ways it can disappear: leak out the gaskets and seals, or go out the tailpipe. There are no leaks, so then it must be going out the tailpipe. Blue smoke from the tailpipe is difficult to see while driving unless the engine is using a lot of oil.

An engine that burns oil has oil getting into the combustion chambers from several possible places. Worn valve guides or broken valve guide seals allow oil to run into the engine when it is turned off. This usually causes a puff of blue smoke when the engine is first started after sitting for several hours. Your car doesn’t seem to have these symptoms.

Worn piston rings or engine block cylinders allow excessive oil from the crankcase up into the combustion chamber. Your engine has high mileage on it and the rings or cylinders may be worn or you could have a broken piston ring. The engine may run fine and have good compression because the extra oil on the cylinder halls helps seal the piston rings. It will use the most oil at higher engine speeds such as on the highway, but almost all the oil is burned inside the engine, so very little smoke is produced out the tailpipe. New parts are the only repair for worn or broken parts, but there is one other item to check first.

Check the PCV valve and hose to see if there is a lot of oil in it. Oil may be sucked through the crankcase breather system and into the intake manifold and burned in the engine. A wrong PCV valve could cause this, or the baffle inside the valve cover may be broken or missing that prevents oil from being directly sprayed at the PCV valve hose inlet. If there is no baffle, then the engine can use a lot of oil. The PCV system is simple and cheap to check so do this first.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca