2001 Cadillac DeVille with Multiple Misfires

This engine was running flawlessly. On a fairly long highway trip, it started to misfire. By the end of the trip I am pretty sure that there were two or more
cylinders misfiring. Please note that I had never experienced *any* misfires before this event. I want to make this clear because I think this diminishes the likelihood that plugs are involved, although the plugs have not been changed in some time. Plugs are very expensive to change in this car and only the dealer will change them. As the car has 150K miles on it and has other non-engine problems, I am trying to avoid spending money needlessly. If possible, I would like to examine all the less expensive possibilities first, ruling out everything but the plugs before I "bite the bullet." (Around $400.) Could someone list all the other possibilities that might be making this engine misfire on multiple cylinders, and that could cause this to occur suddenly? Many thanks for your help! Frank
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Bad fuel, dirty fuel filter, water in the fuel, faulty coils, faulty wires, lots of other faulty things. Have you had the engine scanned for fault codes yet? Did the Check Engine light come on?
You are far better off getting the car scanned for codes before you go wandering down the paths of everything else that could cause a misfire. You'll just be throwing good money after bad if you start shotgunning this problem. A lot of aftermarket auto parts stores will scan a car for no charge. Go to Auto Zone or another local store and see if they'll scan it and give you the codes. At that point you have something substantial to start with.
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Here's what the AutoZone got: Code 300- Random Cylinder Misfire Code 440- Evaporative Emission Control (Known problem- Gas Cap) Codes 1359, 1360- Ignition system
The AZ guy said probably the coil or the ignition module since misfires are random.
The "Service Engine Soon" light is always on.
Your thoughts?
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I'm sorry that was "random MULTIPLE misfires."
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Not likely a coil pack then. More likely a fuel restriction.
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First off - is the gas cap on properly - cranked for two or three clicks? The emissions system watches for proper pressure in the gas tank and sets the CEL if the cap is not proplerly sealed.
It can take several (a dozen or more) engine starts/stops to clear the CEL if it was caused by a gas cap that was not properly sealed. Or, it may take 50 miles of driving. Check that and see what happens.
If it's an ignition problem you are better off getting it scoped. You can try pulling a plug wire, but that is not a good indication of whether a coil pack is working or not. In short, you're not likely to find a difinitive quick fix.
I will leave it to some of the other guys who may have seen some of these codes and determined definitive fixes to comment beyond this. The codes are a good starting point and someone may well be able to speak to what you have.
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Son in law had a caddy that did the same thing. He changed the plugs and wires but still miss fired. He had AutoZone did a scan and said fuel injectors. he then took it to a shop and had new injectors installed, that did not help. Someone said it was the chip in the computer.
He never did fix it because of the cost he sold the car.

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The cap issue may go away after more "cycles" but I'm not driving the car much these days. This is now a brand new cap with certainly at least 3-4 clicks. I will call the dealer in the morning. In the past they have been pretty honest about the wisdom of doing expensive repairs. I will report back here on what they say (or do.) Thanks to you both for your helpful comments. Frank

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It was the coil, according to the dealership. They are replacing it at about $465 + $100 diagnostic. Thanks again for eveyone's help! Frank
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