Can't seem to keep my 2001 Cadillac DeVille from stalling. Starts every time, but starts stalling after running for 10-30 minutes, then stalls more frequently when the engine is warmed up. Had car towed to GMC Dealer. Dealer experienced same problem - stalled when moving it from their service lot into their garage, and had to be pushed into their service bay. No Service Codes were recorded on computer. Dealer wants to replace entire under-the-hood fuse box, including fuses and relays, for approx. $800, saying that the problem is too complex and too time consuming to trouble shoot. Can anyone help?
Sounds like a failing crankshaft sensor. However the first thing to determine is what are you losing when it stalls. Fuel or spark. A quick way to do that would be to spray a bit of starter fluid into the intake and see if it fires or tries to run. If it does then you still have spark and it's more likely the crank sensor. They can fail and not show any codes.
There is another issue with that particular engine but it wouldn't cause the stalling you are seeing. It is the head bolt problem the Northstar has.
As for the dealers idea, you need a better dealer or to go to a much better shop, this is a basic item of troubleshooting not anything very complex.
I would say you are on the money. A friend had a Benz SUV which would stall at odd times. I told him to check the crank sensor and also told him it is the one item on a modern car for which he should keep a spare on hand in the glovebox. Not being a mechanic, he bought the sensor as a spare anyway, fitted it and his issue went away. He kept the faulty one in his glovebox until recently when he offloaded the car. He figured, since it was intermittently faulty, it might just get him out of strife if the new one ever failed completely. It never did.
I really wonder at the efficacy of changing out the fuse box? Good diagnosis tools and methods can ferret out intermittent faults.
Indeed, a crank angle sensor is a first check in such cases given its critical importance in maintaining locomotion. A lot of mechanics seem to be unable to realise that the code scanner, on its own, is not the be all and end all of automotive diagnosis. Sometimes you just have to resort to a deeper understanding of automotive principles and a few more technical tools - like a digital oscilloscope, for instance, where you can *see* the waveforms generated by devices such as crank sensors.
It won't show any codes, but if you can look at the stored data immediately after it's stalled, it should be a good clue about what is going on. You should be able to see dropouts in the TDC signal easily.
I would check the fuel pressure at the rail when the thing is stalling out. If the fuel pressure is good, I'd swap out the crank sensor without even checking the computer. If that doesn't fix it I'd start looking at the computer logs.
I would also check and clean the engine ground strap, just because it will only take a few minutes, it's probably about time for that given the age of the car, and it can cause all manner of peculiar and hard to diagnose problems. It's likely unrelated but it'll save you grief in the future.
Most dealers don't like working on stuff like this, and they likely don't have anyone left who even had training on a 2001 model. Agreed that it is time to find a competent diagnostician who won't just throw parts at the problem.
It's extremely effective at increasing revenue. While you're at it, get the customer to pay for a car wash too.
The nice thing about modern engines is that you can plug in a scanner and see the waveforms. I think with that engine you don't get to see the crank sensor waveform as such, you just get to see a digital signal that is either on or off. Still, that's likely enough information.
I'm amazed at the lack of real diagnostic skills in many shops, especially when you look at the tools available and their prices. When I first started we had the old roll around units with a scope and all the rest. They were helpful but you were pushing a house around. My current grab and go kit is a BOB, a U-Scope and a DiagunV scan tool. They fit in a small lunch box and if you know how to use them you can find pretty much any fault. All three together are less than a lower end Snap-On and far more able on most vehicles. In the shop is a pair of 4 channel Pico-scopes, A handful of scan tools from OEMs down to a small code reader that does generic OBD&Manufacturer specific codes and live data. That U-Scope is REAL handy for things like a failing CKP , backprobe it and watch the signal fade as it warms up. Or checking a CAN signal to see why a module isn't talking.
Young kids who were never taught diagnostic methods. They put it on the computer and they do what the computer says to do, which may or may not have something to do with the actual problem. They don't have a good map in their head about what is going on because nobody told them.
Older guys who were never taught diagnostic methods. They grew up learning that when this happens, you replace that, and they developed a map about what to replace rather than a map about how things work. But closed-loop engine controls changed everything and made it very hard to work that way.
When I was in high school in the seventies, I tried to take auto shop, and I was told that college-track students weren't allowed to take auto shop, that auto shop was only for stupid kids. Now we are reaping the consequences of this sort of educational decision. Auto shop should be for everyone. All students should be required to learn basic diagnostic skills because they'll need them in life no matter what they do.
The first step in diagnostics is understanding the system you are diagnosing, all the individual components, what they do, how they contribute to the system, their interrelationships and, most importantly, what constitutes normal operation.
You cannot diagnose any system unless you have the above covered. That is where most mechanics come unstuck. What's more, these days the above list includes comprehensive awareness of electrical and electronics. The average technician needs to be comfortable using multimeters and oscilloscopes as well as diagnostic scanners because, quite simply, diagnostic scanners cannot tell the whole story.
Many techs I have seen do not understand the *difference* between an open circuit and a short circuit nor do they understand the effect a high resistance can have on a circuit. A perfect example, a friend's Benz had a melted headlight connector which he instantly diagnosed as a short circuit - because it was melted. The fault was actually a corroded connector which created a high resistance path at the connector. High resistance means a voltage drop and that voltage has to go somewhere, in this case turning into heat at the connector. In fact, given the lower voltages and extremely low currents present in modern cars, voltage drops can cause lots of seemingly unrelated issues. If you don't understand basic electrical concepts, including Ohms Law, you will not have a clue about voltage drops, or how to test for it, so will spend lots of time not knowing what you're looking for and be forced to resort to the substitution diagnostic process.
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