: Unknow Problem

04-18-07, 08:37 PM
I have a 76 Coupe Deville with the 500 and have been having some problems with the engine cutting out around 30 miles an hour. It sounds like the engine is being choked out. It revs up in neutral and in park, and experiences the problem in drive. We replaced the fuel pump and it felt like there may have been some help then i drove it again and same problem. car sits a lot because i drive a motorcycle to school. The cat has been replaced by a previous owner and seems to be the newer style. If anyone can give me some ideas it would be appreciated i am trying to sell.

Old Fleetwood
04-18-07, 10:39 PM
Check out your fuel filter.

04-18-07, 10:52 PM
If not fuel filter, look at coil. Suggestion if car still has working a/c: Drive car at low speed, 20 to 25 mph. If engine is running smoothly, turn on a/c. It engine begins to stumble while driving at that speed, it is ignition coil. $45 for parts.


04-18-07, 11:29 PM
Still sounds like it's the Catalytic Converter to me.
Are you absolutely sure you can trust the previous owners word that it was replaced?
Those are kind of expensive.

04-19-07, 12:37 AM
I would suspect the catalytic converter as well. I just replaced the original OEM converter for my 81 CDV last month. I had to give the car half throttle to do 100kph on the freeway... now the car is normal again.

Converters can be tested to see if they are plugged. They operate somewhat like a furnace, and the back of the converter should be 100 degrees or so hotter than the front. My exhaust shop tested it for free and showed me the results of his laser temperature gauge when my car was running. Make sure the catalytic converter is fully up to operating temperature before bringing it in.

By the way, driving around with a plugged converter can actually burn your valves amongst other things - rule it out ASAP.

04-19-07, 01:21 AM
yea im not quite sure if it was replaced one of my buddies said it looked like a newer type and that exhaust flow seemed to be normal but he also thought cat at first. maybe ill get it out and see if it changes anything.

04-20-07, 12:20 AM
I thought the exhaust flow on mine was fine, and I was wrong. Just because it seems to rev freely doesn't mean it's not plugged... ;)

The Ape Man
04-21-07, 10:58 AM
The way to check for a clogged cat is to attach a vacuum gauge to the manifold vacuum and see if vacuum drops off with steady increasing throttle. The engine usually has to be warm for this to show up. Your description isn't completely clear to me. If the loss of power is instant like someone shutting the key off then look for a broken distributor pickup wire at the pickup side. These wires break from constant flexing when the vacuum advance moves things around. They make a good connection and then not when vacuum changes with load. A gentle tug on the wire with 1 finger will let you know if this is your problem.