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Cadillac Repair/1992 Cadillac Seville STS weird problem

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Question
The problem started 3 months after I bought the car used from a private owner that got it at an auction. The speedometer starts jumping and the car idles crazy while wanting to stall out. Sometimes it will idle high or low, depending on how it feels. It acts so random, It jerks and rpms jump while driving, overdrive gets stuck and doesn't want to shift sometimes, It wants to idle at 500 wanting to stall or it'll idle at 1500 and the IAC is set at 750. If it does stall it'll start right back up, it will mess up at random times while driving but every time only when the speedometer is jumping and/or reading 0mph or 120mph. I thought it was an intake problem because it was burning weird too. I attacked every thing that has to do with Gas and Intake. Here's a list...EGR, IAC, TPS, O2 sensors, Plugs and wires, Dis.cap, Fuel pump & filter, Map sensor, multiple relays and fuses, ran a redundant ground, cleaned fuel injectors, FPR is fine... I think it is the ECM, but what else could it be? Instrument cluster? Whatever part this is it has to be electrical and the car only runs messed up while the speedometer is going. I just don't know what it is. Could you possibly help me? Thanks.

Answer
Hello,

I would love o help you however the biggest issue you have that will cause you to chase ghosts as you already have to no avail is your description "intermittent." Unless the problem is a constant, you won't be able to fix it unless you are very, very lucky in one of your guesses. It could be as simple as a defective sped sensor, be a problem in the IP, the wiring or the ECM.
I would suggest you do one thing that won't cost you anything. Get under he dash and look for ground points. You will find 3 or 4 under the dash. There will be a number of wires with eyes that will be bolted to steel. Usually a 10 mm head bolt. Remove the bolt and wires, scrape the steel background to bare metal. Reattach and tighten.

See if that fixes the problem. The next part I would consider is the speed sensor. Some are mounted behind the IP and others in the transmission. The speed sensor can create all sorts of weird issues.

Had an Olds that "intermittently" would cause engine to race, transmission to not shift properly when the speedo did not work. When working, no problem. It too, went from 0-140.

I suggest highly to those repairing older cars to get a factory service manual for the car. You can probably find on Ebay or Craigs. Diagnostic flow charts are great for a problem like yours. Otherwise you are working in the blind which is costly and frustrating.

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Rob Painter

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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?

Experience

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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