Our shop Supra started to exhibit a strange problem one day after running flawlessly for over a year and a half. When started, it would idle very rough for several minutes like it was only running on 2 or 3 cylinders. After 2-6 minutes it would start cleaning up and finally run properly.
We tried various things to try to narrow down the problem. The symptoms were inconsistent at first. Sometimes the engine would start and run fine. Other times, it had the problem several times in a row. Engine temperature appeared to have nothing to do with it. Cold or hot, the engine would have the problem sometimes. Day by day, the engine was running rougher and it was taking up to 10 minutes for the engine to clean up. Finally, the problem was so bad, the car would hardly get off the line even with lots of clutch slipping. In neutral after starting, the engine would only rev to 3000 rpm with full throttle. Every time, the engine would eventually clean up and run fine.
We checked the coil pack to make sure that the problem was not ignition related. Everything there was fine. We scoped the injector outputs. We installed a fuel pressure gauge to make sure that we had proper pressure. We pulled off the return line to verify that we had good fuel volume. We did a compression check, checked the ignition wires, unhooked the cold start injector entirely, checked the injector ballast resistors, injector resistance, injector leakage and finally changed the ECU. (It's never the ECU). None of these produced any change. All exhaust ports were hot, indicating that all cylinders were firing sometimes but the mixture appeared to be incorrect.
Finally we unplugged the injector harness to one set of 3 injectors and the engine ran much smoother even though only 3 cylinders were firing. We then plugged in one injector from the other bank of 3 and the engine ran rougher but picked up about 100 rpm. We plugged in one more and the engine picked up another 50 rpm but got even rougher. We then plugged the last one in and the engine lost 400 rpm and shook like mad. With the injector output plugged into the other 3 injectors, the engine ran OK on 3 cylinders again. Something was dragging down the injectors when the second output was connected. This would cause the injectors to be lazy in opening, making the engine run lean.
After a bit of head scratching, we decided that the only possible explanation was a poor injector ground. On the EM-3, the injector ground is connected to the white plug on the ECU. We checked the pins and connector and saw no problem. We disconnected the injector ground wire from the chassis connection which was a bolt coming through the firewall, near the transmission tunnel. We re-grounded this wire with an alligator clip to bare metal. The problem cleared up immediately. The ground bolt in question was the same one used for the last 18 months on 2 different systems. When checked with an ohmmeter, the resistance varied between 4-5 ohms. It should have read 0 ohms.
The bolt held part of the heater assembly on to the chassis and looked like a good ground. It WAS a good ground for a while. The failure appeared to be caused by salt water corrosion under the bolt washer. The roads are heavily salted in winter here. I have seen a lot problems in 20 years of working on cars, but never one with these inconsistent symptoms. I cannot explain why the ground would always eventually become good enough to make the engine run after a while, yet 10 minutes later, would break down again and cause the problem. This took a lot of time to solve. There is a good lesson here. Grounds are easy things to check if you have a problem. Even it it was working and suddenly you have a problem, grounds should be one of the first things checked.