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Diagnosis – Water Pump causes lack of power on 1995 Corolla

This is a story about a Corolla that would not rev over 3500 RPM. Once it hit 3500, it would cut out rhythmically — guh, guh, guh, guh it would say as the throttle was held to the floor. Sort of like a car with a rev limiter that kicked in at an abnormally low RPM.

Someone had already done a full ignition tune up in an attempt to solve the problem, but it had not improved.

The symptom reminded me of what 1990s era Honda and Acura products do when they have distributor problems, so my first thought was to check the cam and crank sensors to see what their output looked.

cam and crank sensor waveforms. crank sensor waveform amplitude is too low

At first glance, the cam sensor (upper red trace) and the crank sensor (lower blue trace) looked normal. However, then I noticed how low the crank sensor amplitude is.

The cam sensor and crank sensor should produce roughly the same voltage — around 5V to 7V. Notice that the scale on the cam sensor is 2V per division? And the scale on the crank sensor is 0.5V per division?

Even though the crank sensor pattern looks normal, the output is far too low. About 760mV peak to peak, when it ought to be about 10x higher.

This is a 15 year old design, and I’ve never seen a crank sensor failure on a 4AFE or 7AFE engine, so I was a bit surprised to find an old design with a heretofore not encountered problem.

I decided to remove the sensor to take a look at the reluctor, and here’s what I found when I peeked down the bore where the crank sensor mounts.

The aftermarket water pump someone had installed was leaking coolant from the seal, and the trail lead straight down to to crank sensor. The coolant was leaking through and around the crank sensor and had formed a thick pink crust between the crank sensor and the reluctor. Apparently the crust was doing a pretty good job of blocking the magnetic pulses emanating from the spinning reluctor. A little poking with a pick followed by a blast of compressed air was enough to remove all of the crust.

Honestly, I don’t know for sure that the crust was the only problem, because I replaced the sensor too. I wasn’t expensive, it was soaked, and I didn’t want to put it in only to remove it again.

I strongly suspect the crust was the main problem, but don’t have any scientific proof since I replaced the sensor at the same time. However, I do know the car was fixed by cleaning and replacing the sensor.

If your Corolla won’t rev up, a quick visual inspection of the water pump may save you some time and point you in the right direction.