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91 Deville DTS Rough Idle


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My 91 Deville DTS with 72k miles has a rough Idle. My MPG started started declining so I changed the plugs. Did I forget/do something wrong? No Check Engine Light and the MPG has improved. I am using 91 octane. However , my parents may have used 87 octane when the price of gas was high. Rough Idle shakes the steering wheel/car

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My 91 Deville DTS with 72k miles has a rough Idle. My MPG started started declining so I changed the plugs. Did I forget/do something wrong? No Check Engine Light and the MPG has improved. I am using 91 octane. However , my parents may have used 87 octane when the price of gas was high. Rough Idle shakes the steering wheel/car

What engine 4.9? Check the distributor cap, rotor, check the ignition wires

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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  • 3 weeks later...
DTS? The DTS didn't come about until 2000.

Recheck the firing order. You may have crossed a wire.

Sorry- Deville Touring Sedan with 4.9V8........I have replaced plugs, Ignition Wires,Cap,Rotor ,and Fuel Filter.I'm sure I don't have a wire crossed. Car still has a rough idle. Almost feels like it hesitates when you first get on the gas. Could it be a sensor or catalytic converter ? still no check engine light... maybe the coil?

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DTS? The DTS didn't come about until 2000.

Recheck the firing order. You may have crossed a wire.

Sorry- Deville Touring Sedan with 4.9V8........I have replaced plugs, Ignition Wires,Cap,Rotor ,and Fuel Filter.I'm sure I don't have a wire crossed. Car still has a rough idle. Almost feels like it hesitates when you first get on the gas. Could it be a sensor or catalytic converter ? still no check engine light... maybe the coil?

I recall our old Guru once mention that a bad TPS would act like that. Somewhat like a bad accelerator pump on a carburetor.

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DTS? The DTS didn't come about until 2000.

Recheck the firing order. You may have crossed a wire.

Sorry- Deville Touring Sedan with 4.9V8........I have replaced plugs, Ignition Wires,Cap,Rotor ,and Fuel Filter.I'm sure I don't have a wire crossed. Car still has a rough idle. Almost feels like it hesitates when you first get on the gas. Could it be a sensor or catalytic converter ? still no check engine light... maybe the coil?

I recall our old Guru once mention that a bad TPS would act like that. Somewhat like a bad accelerator pump on a carburetor.

My mechanic has discovered #3 cylinder not firing.No voltage from the ECM for that cylinder(injector). Suggest new ECM. Can anyone relate?

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