Even with All Our Resources, Fixing Cars Can Be ChallengingPosted 1/16/2004
By Jeff Bach
My shop offers years of experience, meters, scopes, scan tools, countless hours of training and the latest technical information resource library. In addition, we have the Internet technical expertise of fellow "iATNers." One would think that with all that, the challenge of fixing cars today would be left to those who turn a blind eye to those resources, and the rest of us would be just doing what seems like routine maintenance.
I say this because a fair amount of what used to be real problem jobs have been solved before and have been documented, either through a bulletin or in the technical archives of the iATN network. With this in mind, I must admit that a fair amount of the problem descriptions I get to hear do invoke a bit of tongue biting, while trying to restrain myself from saying something that may offend either the owner or one of his well-meaning, "half-fast" mechanic friends. Particularly when I get a car in my shop on which someone has thrown all the parts they could think of with the collective reasoning at the coffee shop - including the ones that fixed their friends' cars with the same problem (you know, "doesn't run right").
Being thought of as a last resort has its advantages and disadvantages. One disadvantage is that it's difficult dealing with someone when they feel they are over a barrel. One advantage is that they are ... over the barrel.
I had just such an instance recently with a '96 Cadillac Deville that came in with complaints of "rough running, hard to start cold" and a Service Engine Soon light on. The code was a PO300 code for a misfire. Somehow the owner managed to find that much out. I suspect it was through a free code scan that some of the automotive parts chain stores do to get people in their stores. What a great marketing tool! I mean, to tell someone they have a misfire code and then get to sell them all the parts the store can think of that could cause a misfire ... then send them home with the same problem and have no liability or responsibility for fixing it.
Well, with everything under the hood new ... where do you start?
Sitting there, running warm, the car ran great. Knowing these cars the way I do, I figured I'd do some basic "scope-n-probin" and hook a fuel pressure gage up before parking it outside to get it to the desired condition for having the problem occur.
Figure 1 is a quick shot of the primary ignition system current waveform. I use it to check for high-resistance secondary faults such as bad wires.
Even though I know they're new, I look at them so as not to be fooled again. Figure 2 is a shot of another '96 Sedan DeVille with a few bad plug wires for comparison. The downward dip at the end of the primary circuit shows up with a current probe, and is a good indicator of this high resistance in the secondary usually attributed to bad plug wires.
I set the trigger to capture current changes below the zero line and blow up this portion of the primary circuit to get a good view.
This particular Northstar powertrain is well-known for hard-start cold problems due to fuel leakage from injectors and pressure regulators, as well as weak fuel pumps and leaking flex lines. I connected a current probe to the fuel pump circuit using a fuse adapter and secured it so I could park the car outside and check it in the morning. I also hooked a voltage lead across the same circuit to get a look at supply voltage during cold cranking.
The next morning I checked the gage, which had bled down to about 10 PSI. Figure 4 is a good shot of the cold cranking voltage and current for the fuel pump as it slowly built the pressure up during an extended crank.
Once the car started, the fuel pressure went to normal. The running fuel pump current waveform is shown in Figure 5.
These cars use a roller vane type pump with 10 commutator bars. To get the rpm you need to count 10 humps and measure the time between them. Divide 60 by this time and you have the rpm. As this type of pump wears the rpm increase to maintain the same pressure. These engines use a permanent magnet starter motor that will crank, sounding fine, even with a weak battery. The low voltage will wreak all kinds of havoc when you're trying to diagnose a cold-start condition.
There are many variables that can affect the readings for this system. But if the basics check out and you have a decent battery, what I've been seeing is that the cranking rpm on a pump that is causing a hard-start cold complaint usually land on a grid between 4,500 and 4,800 rpm with a current draw of 7.8 amps. The difference between the cranking rpm and the running rpm is usually around 800, while the rpm difference on a good running car is closer to 1,500 or better.
The pump waveform shown in Figure 6 depicts the current for a new pump both with the engine cranking and again after it starts.
The fuel pump solved the hard-to-start cold problem but not the rough running when first started. The fuel pressure regulator was also leaking fuel through the diaphragm and allowing one of the cylinders to misfire until it warmed up a little.
Getting the OK for the fuel pump was easier than I thought it would be, given the skeptical attitude I encountered when this car first came to me. The customer happened to be in the area and dropped by to check on our progress. After I explained the misfire code and how it would require a new pressure regulator to keep the light from coming on, he asked how I knew the pressure regulator wouldn't solve the hard-starting problem. I gladly explained, using a couple of props and a little research data. I obtained the OK for both parts and sold him an oil change also, due to the fact that his oil (even though it had been fairly recently changed) had the smell of raw gas in it.
I devised a simple chart, which I found to be useful in looking at fuel pump trends for these Northstar powertrains. With all these easy cars to fix, I had to do something to keep from feeling like the "Maytag" man.
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