Here is a case of incorrect checking of sensors and not checking the basics. My wife invited her friend over for a catch up one week-end and she brought her husband. They drove up in a new 1988 Daihatsu and once the intro was done Dave and I went off in our own little corner and had a few beers and watched the telle. It was not long before he wanted to know what I thought was wrong with his new car. He went through about how many times he had already taken it back to the dealer and that they could not find the cause. The dealer admitted there was a problem and that they have never struck one like it before.
The problem. In the morning the engine was a little difficult to start and once started did not idle properly (idled very slowly). After more questions I got a clear picture of what the problem was and already had a good idea of where to look. This symptom was classic and indicated that the auto choke was not operating. Technically it is not called auto choke on EFI, this term is related to carburettors, but most people understand this terminology better. Cold enrichment is the correct term.
A quick recap on EFI cold starting - The auto choke (cold enrichment) is built into the system. The system by whatever means, injects more fuel when the engine is cold to aide in easier starting. Along with that the idle speed is increased. I went out to have a quick look under the bonnet to ascertain the type of system fitted. I determined that the enrichment for cold starting is instigated by the coolant temperature sensor as it is with most EFI engines. I told him I thought it was the Coolant Temperature sensor at fault and he came back with the response that the dealers had already tested it and it was OK. If this was true then his problems were more serious than I thought. No matter, I thought, just to be safe I had at my disposal at home a resistance wheel I used for electronic work that I dabbled in.
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So I lent him this wheel and showed him how to substitute the temperature sensor and what value resistance to set the wheel at in the morning just before he was ready to start the engine. If the engine started and ran at higher than idle speed, the only conclusion to be drawn is that the coolant temp sensor was faulty. But how can this be, the dealers checked it! The answer is simple, they did not check it correctly. All they checked was the resistance between the two terminals of the sensor. They failed to test leakage to earth through the body of the sensor. When the sensor is screwed back into the engine, one of the terminals is earthed through the body of the sensor and this sends the wrong signal back to the ECU. The ECU thinks the engine is hot and accordingly stops cold start injection, idles the engine at normal hot idle speed and injects less fuel to maintain cold operation. |
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He rang me the next morning and told me the car started and ran perfectly. Diagnosis over. He went back to the dealer, had a heated argument with the mechanic and finally got the temp sensor replaced and all was good.