Check throttle position sensor for worn spot

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This article was published 25/08/2006 (6952 days ago), so information in it may no longer be current.

QUESTION — I have a 2005 Toyota RAV4 (automatic, 4-wheel-drive) with about 28,000 km. The problem is that when I’ve slowed down for a tight right turn, or when I’ve almost stopped for a red light which then turns green, there’s a definite hesitation when I step on the gas. It seems like the engine (or transmission) just isn’t getting the message for a second or two. I’ve also noticed the problem when I was stuck behind a slow tractor — when I tried to pass there was just no power for a second or so.

The dealer checked to see if there were any new codes for the computer, but apparently there weren’t. They then reset the computer, but that hasn’t made any difference either. Any ideas?

ANSWER — This could be a tough one to find, but I would start with testing the throttle position sensor. It sounds like the sensor may have a worn or bad spot on the variable resistor inside it and the voltage signal to the computer momentarily drops out at certain throttle openings. It is not common to have a bad throttle position sensor at this low mileage, but I have seen it happen at even much lower mileage.

The technician should graph the voltage output of the sensor and watch for sudden changes in voltage. It should show a steady increase in voltage as the throttle is opened slowly. Graphing the voltage with a scope or computer is the best way to locate this problem. Watching the numbers on a scan tool may not show the voltage dropping out because it happens for such a short time period. Fortunately, the fault should be covered under the vehicle emissions warranty.

QUESTION — I have a 97 GMC half-ton 5.7L that is giving me a lot of problems. First, the fuel pump went. After I changed it, the truck fired up but every time it was under a load it would stall and then it quit running altogether. I have full fuel pressure and spark. I put in a used injector spider system in the intake plenum. And I’ve changed the air mass flow sensor, computer, throttle position sensor, coil and coil driver, crank sensor, distributor, cam sensor and more. It still won’t run and I have no idea what else to do. I am desperate to get the truck running and not waste anymore money on it. Can you help me?

ANSWER — You have fuel pressure and spark, so the crank inputs to the computer must be good. Start by doing a basic mechanical check. Do you have compression? If not, then the problem could be camshaft out of time because of a skipped or jumped timing chain.

Do a visual inspection of the engine compartment wiring harness looking for worn or cut spots. Also check the ground connections on the engine and on the body and frame. A loose ground could be causing the no-start problem.

Two sensors, the temperature and throttle sensor need to be checked next. If the temperature sensor or wiring is shorted, the computer will inject too much fuel and the engine will not start. If the throttle position sensor is signalling the computer that it is at wide open throttle, then the computer turns off the fuel injection during crank and the engine doesn’t start.

Finally, the cam sensor (distributor) may be out of time. The distributor doesn’t need to be timed with a timing light like older systems, but it does have to be close for the engine to run. Since you have had the distributor out, perhaps it is not quite in the correct position.

An engine needs air, fuel and spark at the correct times to run. You have spark and fuel pressure, so make sure the injectors are triggering, and that the engine is pumping air (compression). That would leave you with spark timing to check.

Jim Kerr is an experienced mechanic, instructor of automotive technology and member of the Automobile Journalists’ Association of Canada. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net