Check Engine Light

[Tim]

As long as I’ve had my car the “check engine” light will randomly turn on and off when the temp needle just starts to move, and eventually stay off when the engine reaches operating temperature. It won’t turn on again until the engine has cooled off.

This doesn’t affect drivability or gas mileage, but when the car is in Park and the CEL comes on, the car does some odd things. When the light is on, the idle will go up by a few hundred RPM, and when it turns off the idle will dip very low, and the engine will sometimes die. When the engine dies, it’s hard to start again and I have to push the pedal to the floor to get it going. It will sputter to life, and won’t cause any more problems until the engine cools off again.

I’m guessing that the coolant temperature sensor is the cause, but I lack even the most basic diagnostic equipment so I can’t read engine codes or get resistance/voltage readings. Since the “shotgun” diagnostic approach is kind of expensive, can anyone narrow down my problem from the symptoms? I’d be eternally grateful to whoever can point me in the right direction.

The car is an ’89 Auto with the oddball EFI.


Thanks!

Tim

[MONSOON]

Ive found that startegically placed electrical tape cures that headache!

[worfdog]

I have seen oxygen sensors do like that before. Light will come on, then when engine warms up it will go out,but mine on my s-10 does not have the hard starting problem. Been meaning to replace the sensor, just have not gotten around to it. I know it's the o2 sensor because I read the codes.

[resuwrecked]

sounds like the cts is bad. telling the computer the engine is colder than it actually is will result in a rich condition, this is why you have to floor it to restart- cuts fuel while cranking. the o2 sensor doesnt affect the fuel trim that much and the engine will run fine with it unplugged even. also check resistance between sensor and ecu to make sure thats ok.

[batstiva]

Ive found that startegically placed electrical tape cures that headache!

Ha, thought I was the only one who fixed things that way.

[FB71]

Tim, all you need to retreive DTCs is a jumper wire. Ground the STI terminal, turn the key on, and count the flashes of the CE lamp.

I have to concur with your conclusion of the ECT (Engine Coolant Temp sensor) being at fault. If you have a Harbor Freight store nearby, you can aquire a DVOM for less than $8, to evaluate the ECT

[Tim]

Thanks to all of you for the help! The advice here is first rate and friendly too.

I replaced the coolant sensor last night, and that seems to have fixed it. The light hasn’t come back on yet, and I noticed a few other things as well.

The engine, which normally takes about 3 seconds of cranking to start, fires right up in half the time. I initially wrote the long cranking off as a quirk of the early EFI, but it seems like the sensor was the cause. In addition, it idles properly now, and seems to use less gas when driving in the city.

Just as a side note, the radiator fan sensor was bad when I bought the car, and now this coolant sensor is defective. Is this a coincidence, or would lack of proper maintenance cause both sensors to fail?


-Tim