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Cadillac Repair/1990 Deville running rough - where to start?


I just bought a 1990 Sedan Deville with 90k miles from a family friend.  It had been sitting over five years - started about twice a year - and the battery was stone dead.  We swapped in a new battery, changed the oil and filter and started it right up.

We drove about a mile and bought four new tires (the old ones were SIXTEEN years old!!).  Drove another mile and filled up the gas tank with 91 octane (old gas about 1/2 and new gas about 1/2).  We also added a can of Seafoam to clean it up.

Seemed to run well, but idled a little rough.  I had never driven a 4.5 before so I don't know what the power level should be, but it seemed OK.   Maybe a little low on power.  Drove it about 400 miles to get back home (from LA to outside Las Vegas) and got just under 20 mpg.  Again, I have no prior knowledge of what it should get.

We ran the self-diagnostic and the only code is for disconnected battery.  Nothing else.

Here's my question:  It idles rough in gear and almost sounds like an intermittent miss in the tailpipe.  Seems like it runs a little better in the mid-RPM-range than at or just off idle.  I still haven't used higher RPMs or full throttle.

Where should I start?  I see postings about fouled spark plugs, bad wires, dirty throttle bodies and more.

Thank you for any assistance you can offer.


You should be using 93 octaine. Car calls for premium and unfortunately, like my Mercedes, you simply need to use the highest octaine available. I doght if you hurt anything, but I would suggest you go by the manufacturer's requirements.

Since this car has been sitting so long, this is where I would start. Change the fuel filter located under he car by he fuel tank.
Change the anti freeze/coolant. Have the transmission flushed and change the differential fluid. Might be a good idea to flush the brake system as well. This is not a big deal to do, but condensation builds inside the master cylinder and you don't want to find one day you don't have brakes!

These are issues no one thinks about and they are all crucial when a vehicle has been sitting parked for over a year. The life span of coolant is 3-5 years before it turns acidic and sarts destroying the cooling system.

I am nt familiar with Seafoam. I would put a bottle of heat into the fuel tank to suck up any water that may be in the tank. After you have gone through this tank of gas, put a bottle of STP or any other brand fuel injector cleaner.
See if it runs better.

These engines are aluminum and can never be overheated a little bit! It doesn't matter where you are, if the temperature gauge reaches 225 or the hot light comes on, the engine must be shut down immediately or the engine is literally melting!

When you have the coolant changed, have a coolant system pressure test. These engines, like all Cadillac aluminum engines are notorious for having been over heated and blown head gaskets are the result and not the cause since the 4.1 was introduced in 1982.

In fact, make the cooling system test the first priority, because that could be your running issue.

Don't get me wrong, the 4.1, 4.5, 4.9 and Northstar were great engines for performance and economy. On the highway, I have pulled anywhere from 25-30 mpgs wit these engines. My Allante with the 4.1 pulled 28 mpg when I wasn't driving rough with the little sports car. Then I would drop under 20-lol

Now, it could be that the engine has bad wires and that could account for what you hear from the tail pipe, but on these engines, the first thing that has to be eliminated from an overheating event. You will not find a good engine in the junk yard, because these cars are usually there as a result from ver heating. Factory crate engines if you can find one are about $4k.

I am not saying the engine was over heated, but it is so commn, that I would be eliminating this issue first!

Cadillacs have their issues (I have owned over 25) and I will address them for you.

Intermittent no crank- cause because of that big black resistor chip in your key known as VATS (Vehicle Anti Theft System) which never was designed to prevent theft! It did successfully strand the owner though! The dealer will ht you $800 plus to change the ignition lock cylinder and harness. The cheap fix is to go to an alarm store and have the system bypassed for about $60.00.

Heater-A/C blower quits or runs until battery is dead. BCM (Body train Computer Module) issue requiring computer to be replaced. Depending on the year cut off and I no longer remember, could also be equipped under the hood with a $400 non-servicable resistor module that is no longer available, requiring the system to be bypassed with a toggle giving one the only options of high blower or off.

Trunk pull down quits working. All one needs to do is adjust the latch, disconnect pull down motor and close like a normal trunk.

Ride Control-Commonly compressor under hood will operate, but air lines, fittings or shocks leak. Dealer cost is insane and I recommend checking to see if after market like Monroe is available for replacement.

That is all I can think of off hand over and beyond the common over heating issue. Everyone replaces the thermostat first. Guess what? It is never a thermostat problem!

In the event, you do find a blown head gasket, you need to find the cause. Water pumps fail, small cooling jackets are easily plugged, fans don't run for various reasons, radiators plug.

You have a nice car, but you need to understand the common issues with it. I owned one of these 1990 Devilles new and I loved it! My 1992, which I traded the 1990 for, pulled 30 mpgs going from Wisconsin to Florida.
I have owned just about every model Cadillac has offered numerous times, from the 1983 Eldo Biaritz, Allantes, Sevilles, Devilles, Fleetwoods etc.

I loved my 1990 and my 1992 Devilles. I have now ventured away from Cadillacs and have a Mercedes S, which I love.

Anything you have a question on, feel free to write. I would tell you that if you are keeping the car to buy the factory service manual for the car. You can find one on Ebay or Craigs. With this manual, even for those not extremely mechanically inclined will save money, because of all the repairs that can be done by the owner instead of sending the car to the garage. One other tip-NEVER take the car to the dealer!

Good luck and get back to me after you have the cooling system pressure tested.


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Rob Painter


Alarm system questions cannot be answered on this forum. These systems are not what I can answer. Without being physically at the vehicle and not knowing what kind of electrical service has been done on the vehicle, there is no possible way to give an accurate answer over the internet. My expertise is in Ignition/key based anti-theft systems. These issues include GM VATS (resistor chip in key blade) PASSLOCK (MRD)-ignition lock rotation based, no special ignition key and the PKIII Transponder (computer chip in key) systems. These systems are not alarm based and are integral with the starting of the engine. This is why I cannot diagnose alarm problems without physically looking at the vehicle: Alarm systems are a completely different annimal than ignition key/lock based anti-theft system. Many alarm questions come from vehicles 10 years old, and since older, many hands that had been involved over the years.I am an expert in all GM factory (ignition/key based)systems. Alarm system questions pose to many situations beyond my knowledge as to what has been done to the vehicle over the years. Some guy may have actually wired the stereo into the alarm system. Who knows? Over my past 30 years in vehicle wiring repair, I have seen unbelievable wiring disaters done by guys that consider themselves "mechanics." I have seen stereos and alarms intalled using surgical tape. I have seen modules burn up, un-fused circuits, wiring jambed between the doors and even lamp cord used for a starter kill. To answer alarm questions over the internet without examining the vehicle is like asking; What does it take to remove a dent?


Education/Credentials-ASE certified. 11 years with a GM dealer and 17 years with a repair facility dealing with only the repair of theft recovered vehicles.

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