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2004 Cadillac DeVille - New Alternator, Now Problems

I have a 2004 Cadillac Deville. It has 175k never had any problems. Then recently the alternator went out. Had it replaced and bam. All kind of stuff happening. Car cranks fine when cold. Has a rough idle but drives out smooth. At stops it doesn’t cut off but again idles roughly. After driving 15 minutes and the engine gets hot when you switch off it will not crank. As soon as it’s cool. It will crank up. I just took this car to my mechanic friend who replaced the crankshaft sensor, camshaft sensor, plugs, wiring harness and coil pack! All of which showed up with codes none of it fixed the problem!!

Maybe crankshaft position sensor is bad. Best answer I can give you given you posted almost nothing about the car.

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Thanks. Sorry I had to rewrite it when it kicked me off and I lost my connection. I had previously stated the last work I had done but forgot to include it when I rewrote it. I have since updated the post!! We are out of ideas!!

Can you list the codes the car showed? Is the check engine light still on?

The no-crank when hot should not be difficult to diagnose the problem. I’d suspect the starter, especially if it has never been replaced, but replacing a starter on this car is expensive and a headache - it is in the vee under the intake manifold.

Rough idle sounds like failed Idle Air Control Valve (IAC). I had to look to see if this car had one, it does. If there is no check engine light, I’d suspect the IAC. They are cheap and easy to replace.

When I have a problem like this I address the last thing changed which in your case is the alternator. Have the alternator diodes checked to make sure the output is smooth. Put an oscilliscope on the alternator output to see if the voltage is constant. If not replace the new alternator.

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When you say the engine “won’t crank” that means to me that the starter motor doesn’t crank the engine over. Is that what you are saying? If so, the starter system needs to be looked over for a problem. If you mean the engine turns over but won’t start then that could mean a number of issues could be at fault. The first thing to check is the ignition system. If that is good then the fuel system needs to be checked out.

It turns over it just won’t restart. It makes noise like it’s going to but doesn’t ever actually start.

I revised it. That part unfortunately has been replaced.

What part? I referred to the starter and the IAC, which has been replaced?

Ask your shop to do the basic charging system/battery test. A diy’er could do something similar: Before first start of the day the battery should measure about 12.6 volts. Immediately after starting the engine, 13.5-15.5 volts. What do you get?

Also be aware on modern cars that whenever the battery is disconnected (which was probably done to replace the alternator) the engine computer may lose its learned parameters, and that can cause rough idle, hard to start, etc when the battery is re-connected. Usually that problem will resolve itself after a few drive cycles. But sometimes the car has to be kick-started back at the dealership with an idle-learn procedure.