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No Start On Cold StartPosted 7/2/1997By Craig Ahlman
1993-95 Chrysler Concord, New Yorker, LHS, Dodge Intrepid and Eagle Vision.
Recently I matched wits with a 1993 Sonoma with a 4.3 CMFI (Vortec) engine that would not start in the morning without a helping hand from a can of ether. But when it was shut down 10 seconds later and cranked again to see if it would repeat the symptom, it started and ran fine the rest of the day. Coolant and MAP sensors checked normal on the scanner and there were no vacuum leaks found. The IAC motor worked fine and TPS was within range. Injector resistance and injector patterns also appeared normal. A carbon treatment didn't help. Fuel pressure was within spec when the key was turned on at 54 PSI and registered 72 PSI with the return line clamped off running. Nothing appeared to have changed between the first start and the second start but without fail, the engine would crank through eternity on the first try and start on the second. So where was the problem? Here's what I found: The problem was a combination of fuel pressure and combustion chamber temperature. Fuel pressure on these engines needs to be above 58 PSI when the key is turned on and when cranking - more like 60 to 62 PSI. That may not seem like much difference but on the Vortec engine, those four pounds will make the difference between start and no-start when it is cold. This engine uses poppet nozzles, similar to a diesel injector in the way it opens. It takes 40 PSI to get the poppet open and begin spraying. As a result, every pound is important so there is enough volume and a good spray pattern. Even a few pounds below the spec for a given injector will cause the spray pattern to go bad, sending globs of fuel into the cylinder head instead of spraying it evenly. Since the fuel can't stay suspended, it will sit on top of the piston and go along for the ride, or blow out the tailpipe without getting burned. (Remember, liquids and solids don't burn, only gases). After cranking awhile, there is enough liquid fuel in the cylinder to foul out the plug, but it still may not start or act flooded. The reason the engine started on its own after the initial start with ether was the combustion chamber temperature had warmed up a bit from running even a few seconds. The warmer cylinder allowed the mixture to atomize better and the engine to start for the rest of the day. Why wasn't the fuel pump strong enough? Deadhead pressure was high enough with the engine running, so why wasn't it high enough when the key was turned on or cranking? Battery voltage! It is common for these pumps to get loose bushings. When that happens, they can lock up against their own roller bearing surfaces which are loaded with the fuel pressure. Then they will unlock when fuel pressure gets low enough and start working again.
"Excited" SentraPosted 7/2/1997By Paul Kujawa
You encounter a 1988 Nissan Sentra that drives fine but runs very rich - around 5 percent to 8 percent CO. All the sensor voltages at the ECU check out perfectly. Fuel pressure is perfect at 14 psi. You disable the canister and PCV systems and the vehicle is still too rich. OK, it must be a defective fuel injector. So you replace the injector and it is still too rich. Trying an ECU offers no help either. You know you must have missed something, but a recheck of all the sensor voltages turns up nothing. The lab scope shows the injector is operating at 50 volts! Wait! There must be something wrong with your lab scope, so you get out your DVOM and check the injector voltage. It also shows 50 volts. You unplug the injector connector and check the L (blue) wire at the connector for voltage. It has 0 volts with the key off. The L wire is the battery voltage supply wire to the fuel injector and the ECU (terminal 114) from a fusible link off the positive post of the battery, so it should have battery voltage, key on or off. The B/R (black/red) wire at the injector connector (ground side of the injector to the ECU) has 0 volts on it as well. The vehicle starts and runs rich - and there is no battery voltage to the injector or the ECU on their battery voltage supply wires from the fusible link? That's right! The ECU is obviously generating the 50 volts to the fuel injector. To learn more, I brought one of these vehicles into our test lab. I disconnected the fusible link that powered up the fuel injector and the ECU, and checked the L wire at the injector connector for voltage with the key off. It had .18 volts. The B/R wire at the injector connector had 0 volts. When I turned the key on, the L wire jumped to 54.8 volts. The B/R wire remained at 0 volts. Since the L wire powers up the injector and the ECU on terminal 114, it was obvious that terminal 114 of the ECU was supplying the 54 volts to the injector. I left the injector connector unplugged and jumped 12 volts to the positive terminal of the fuel injector from the battery. I then used a jumper wire and jumped from the negative terminal of the injector to the B/R wire in the injector harness connector to put 12 volts on the ground side of the injector at the ECU. The L wire at the injector connector remained at 54 volts. Losing voltage on terminal 114 of the ECU results in the ECU sending 54 volts out on that wire to the injector, even if the injector itself never lost voltage. Putting the injector circuit on a lab scope showed the peak/hold injector pattern - except the voltage started at 54 volts and it never spiked over that. If you come across a rich running Sentra, don't forget to check the battery voltage supply to the fuel injector and don't be surprised if it reads 50 volts! Give me a call if you need any help.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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