BACKYARD MECHANIC: Throttle grief resembles learner’s fumbling

Hey there, time traveller!
This article was published 11/05/2012 (5224 days ago), so information in it may no longer be current.

QUESTION: I have an ongoing problem with my 2005 Chevy Optra. There is a problem with the throttle-positioning sensor getting low voltage, which affects the car’s driveability. I describe the car’s problem as if it’s being driven by somebody just learning to drive a standard transmission vehicle (although my car is an automatic). It doesn’t ever seem to know exactly when to change gears and is often in the wrong gear or will hesitate too long when shifting. It has never stalled but sometimes spikes in RPMs, especially when in Park or Neutral.

It often hesitates when going into first gear or from first to second, making pulling away from a stop sign very jerky. Sometimes on the highway, it will rapidly switch back and forth between gears (and, therefore, spiking the RPMs) if I hover around 80 km/h or 120 km/h.

We’ve had the car diagnosed by many different auto-repair shops as well as Chevy dealers with no success. The TPS sensor has been replaced several times, the terminals have been replaced, and the wiring going into the TPS has been checked. I’ve been told the transmission is OK and the battery is new. Any ideas?

— Laura

ANSWER: The problem may not be with the throttle-position sensor itself but instead the wiring connected to it. The sensor shares ground connections with the oxygen sensor, the high idle switch (part of the throttle sensor) and the exhaust-gas recirculation (EGR) valve. It also shares a five-volt power connection with the EGR valve.

The first thing I would do is a visual check of the wiring to these other components. Look for worn or damaged wiring harnesses, loose connections or places where moisture could enter the wiring.

If everything looks good and you can duplicate the low-voltage connection at the throttle sensor, start disconnecting the other sensors to see if the correct voltage returns.

A fault internally in another sensor can cause the low-throttle sensor voltage. If diconnecting other sensors doesn’t help, disconnect the wiring connector at the engine computer and the above sensors, with the key off of course and test the five-volt wire (grey) at the throttle sensor for voltage or a connection to ground. Since everything is disconnected, there shouldn’t be either voltage or ground.

If you get a reading, then the problem is in the wiring harness, but I would try disconnecting other sensors, too, to see if the reading stops. Sometimes there are changes to vehicles that are not indicated on wiring diagrams. If there is no reading on the wiring harness, the most likely culprit is a bad voltage regulator inside the engine-control computer. This is rare but it does happen, so I would change the computer as a last resort.

QUESTION: I am in the process of purchasing a 1960 Chrysler Imperial with a 413-cubic-inch engine. My question is how well do today’s fuels work in these engines considering the octane rating is much lower, they no longer contain lead and some contain ethanol? Does anything have to be done to the engine or fuel system?

— Wayne

ANSWER: Older high-compression engines will operate on premium fuel without too much problem. The ignition timing may to be retarded slightly so the engine doesn’t ping (pre-ignition of fuel in the cylinders) but if the cooling system is in good condition and the engine isn’t using oil so there are few carbon deposits in the cylinders, pinging can typically be controlled.

I recommend using premium fuel not only for the higher octane rating. Look for a premium fuel that doesn’t contain alcohol. Alcohol does raise the octane rating of the fuel because it has a cooling effect in the cylinders as it evaporates, but it also loosens varnish and dirt deposits in fuel tanks and can harm some older types of rubber fuel-system parts such as mechanical fuel pump diaphragms. If you do use gasoline that contains alcohol, you will likely find it necessary to change fuel filters often, as loosened deposits flow through the system.

One other note: Gasoline used to contain lead additives decades ago. This would coat valves and valve seats and protect them from pitting. Gasoline today contains no lead, but if engines operate below 3000 r.p.m. most of the time and are not placed under constant heavy loads, valves and valve seats will last for many thousands of miles, probably much longer than you will drive the car.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.