Fuel System Can Cause Problems on '76-'79 SevillesPosted 7/17/2006
By Jeff Bach
By the time one of these cars gets to my shop, pretty much all of the information given to us is an attempt to fill us in on what's been tried and what the car is doing now versus what it was doing. In our diagnostics, we try to get the car to duplicate its original symptom. To begin with, this usually entails a fresh set of spark plugs and an oil change. The majority of the problems that these cars end up with seem to result in fuel dumping in the engine to the point that they flood the crankcase and foul the plugs beyond any hope of clearing the flooded condition.
I see two to three quarts of extra oil with a strong fuel smell on the majority of these Sevilles that get towed in for no-start conditions. There are no scan tools or onboard diagnostic provisions built in to the electronic control units (ECUs) on these cars so no codes can be pulled or any parameters accessed to give us hints of where the problem might be. I've seen plenty of them that I could start two or three times with a fresh oil change and a new set of plugs, and then on the next crank cycle be fouled to the point of no start again. These engines will crank and never hit a lick. You'll swear there's no spark but it will test nice and hot. They will keep doing that once they foul until you replace or clean the plugs again.
It's not atypical for technicians to get one of these cars towed in and initially check for spark and verify good ignition. They may do a fuel pressure test or if you're an old schooler, you carefully use an external hydrocarbon source as a substitute for the vehicle's fuel injection system. Upon seeing a limited or no attempt to start result, technicians may pull the plugs and realize the fouling problem. Once a new set of plugs has been installed, the thing may start too rich, go through a spell of decent running and then die as a result of fuel starvation. This diagnostic scenario is more common on this fuel injection system than you would think. One good thing I'll say about this system is that it has a tendency to separate diagnosticians from parts throwers. I've seen a few poor stubborn fellows just wear out their throwing arm and never get one of these cars running. Generally, their logic goes that you throw parts at it until it's running and then develop the story around why it needed all the parts. It's hard using that "parts trying" logic on a customer when you don't get their car running.
Step 1: Disable the fuel pumps by unplugging the two-wire harness next to the fuel tank filler neck to keep from worsening the fuel contamination problem.
Step 2: Verify spark with an ST 125 or equivalent spark checker. There's nothing worse than having a complex fuel injection system diagnostic conclusion get overturned by a wise guy with a rotor button.
Next, I get a lab scope reading of the injector pulse width and frequency during the first few cranking pulses before disturbing any connections to determine the basic state of health of the ECU and the injection system circuit integrity, noting the ambient temperature.
Step 3: Disconnect the coolant and air intake temperature sensors and get another shot of the first few cranking pulses. I now have enough information to establish a baseline and get a comparison to known good readings.
The image in Figure 1 is the current waveform of the first injector pulse from a cranking '79 Seville that was towed to the shop with the usual no start, fouled plugs, crankcase full of fuel scenario.
This brought the injector pulse down to 54 milliseconds with still 20 percent more fuel than it needs to run. That doesn't sound like a lot but as anyone who's ever turned a mixture screw trying to dial in stoichiometry knows, the margin isn't that wide.
Some checking with a volt ohmeter revealed that power and ground circuit integrity are good. I can now say with confidence that the ECU needs replacing.
Laying in the trunk of the car were the "so-far-tried" parts. The parts included a used ECU, a spark decoder, a rail fuel pump, a fuel pressure regulator and a coolant sensor. I opted to give the owner's used ECU a shot with the scope hooked up. Figure 3 is the first cranking pulse with the sensors still disconnected.
This one, it turns out, is worse than the first - and had I a little less experience with these jewels I might ask myself the question, "What are the odds of having two bad ECUs failing at almost the same too-rich fuel rate?" Overwhelming in favor of, would be my answer.
After showing him the evidence of the previous waveforms, the customer eagerly went for the whole repair: tuneup, oil change, new ECU, etc.
Figure 4 reflects a more normal first cranking injector pulse when I can safely reconnect the fuel pump.
The one thing that sticks in my mind about this adventure is what the elderly gentleman said to me when I was breaking the bad news to him about the cost of the whole job. Taking the news rather well, he said: "The way you approach finding the trouble with these things reminds me of myself in my philosophy at work ... I believe and light a candle rather than curse the darkness."
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