 | |||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||
|
Where to Start on a No Start?Posted 4/14/2005By Jeff Bach
underhood intruders can create.
He was trying to get his car running again after having a used engine installed. The car had been sitting idle for seven or eight months after he and his friends had given up on it as "possessed." I have been seeing a few of these project cars lately, maybe as a sign of the "recovering economy." This one had that smell of old fuel "cologne," and the paint had taken on an oxidized look with the familiar outlines of fallen oak leaves and baked-in, greasy handprints. I suspect the owner had finally succumbed to the pressure to get something done with it after exhausting all the possibilities he could think of. When I get one of these "story cars" in, I like to take mental notes of the history but still treat it as a basic "no start" and begin from scratch. A good jumping off point for me is a current probe reading of the fuel pump drive current during a five-second crank test (Figure 1).
Next, I pulled the dipstick and found the oil level on the high side with a strong smell of fuel in it. At this point I have it boiled down in my mind to one of several basic possibilities: either too much fuel, not enough spark, or a "can't breathe" condition. I pulled the ignition fuse and connected the current probe around an adapter in its place. Figure 2 was the result during cranking. Both coils show the same result. The primary waveforms tell the story. That steep vertical beginning of the primary current ramp is characteristic of a coil winding that is partially shorted. It doesn't look like much of a telltale, but I've seen quite a few of these on cars that run fine most of the time but may cause problems on a hot soak or even extreme cold when the coil needs to produce a high energy spark. This particular problem seems to be one of the ones that separates the diagnosticians from the parts swappers.
As many of you know, just because a spark can jump the gap on a plug in open air, that doesn't mean it has the energy to get the job done under the pressure of compression. An ST 125 (or equivalent) is a much better indicator of the coil's ability to perform. I put a couple of Northstar coils on it that I had laying around and changed the oil while I had it in the air. I was able to obtain the waveform in Figure 3 while cranking, just before the engine fired up for the first time since its last birthday. I figured I better give it a bit of a cleaning since the converter was probably loaded with fuel. A quick, wide-open throttle run produced an amber cloud of rust with a nice blue hue.
The sound of ticking lifters and rust scraping from the rotors fills the air as I drive this one once around the block, ever vigilant of the local authorities as the numbers written on the long-expired, cardboard temporary tag had illegibly faded into its "raisiny" background. I picked up a malfunction indicator light as I was coming back, which turned out to be a corroded coolant temp sensor. I informed the customer of the progress I had made thus far and he, being a mechanic of sorts, decided to replace the coolant sensor himself. I showed him the pictures of the waveforms when he got here with his mother to pick up the car. He was obviously amazed by this technology. And when I showed him the cut and dry difference between the good and bad coils, he seemed relieved to know that it took more tools and technology to find the problem than he possessed.
I told him "I'd be lost without my scope," even though I got by without it for a long time. It's just that it's more fun doing the job when you can "see" instead of imagining what's happening inside of the circuit. I gladly fulfilled his request for copies of the before and after waveforms (he wanted to be able to show his buddies). He went on his way, smiling.
|
||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
 | |||||||||||||||||||||||||||||||||
I got a call from an old friend recently, asking for my help in trying to point her nephew in some direction he hasn't already been.
This car, by the way, is a '95 Pontiac Sunfire with a 2.2 engine. The engine has a healthy sounding cranking compression, so valve timing doesn't seem to be the issue, and the fuel pump relay drive signal current coming from the power control module lasts the full screen of four seconds. I know from this test that it's not brain dead and I have a reference pulse. This is a good preliminary indication of the crank sensor's health.
Somewhere during the conversation with the owner about this car, I distinctly remember hearing the words, "I know it's getting spark." I'm guessing he either stuck an old plug in a wire and laid it on the rocker cover and visually witnessed an arc, or held a screwdriver in the end of the wire and felt a jolt. That would explain the goose egg in the coil column on the chart.
I like to blow out of the converter as much of the liquid fuel as I can before it reaches "light-off" temperature. Sometimes all it takes is one good flooded load of fuel to melt the bricks in an otherwise perfectly good cat.
