Hello! I am hoping someone here has experienced this before...

Two days ago I bought my 16yr old his first car, a very clean 2003 base model Deville. $2800! He is in love. The sellers seemed super cool and the woman who owned it is a local real estate agent, so it's tough to think she was trying to slip one over on me, but, you never know. I'll probably see her around town in the next week or two and may ask her if she experienced this before.....

Anyways, yesterday we got plates on the car and my kid and I go out for the first real drive. We get 3mi from the house on a 40MPH road, and there is a huge BAM. The car bucked. Like it slammed into gear. He had barely touched the gas, basically to keep it at 40. This was followed by 5 more huge slams. I am 99% certain this was the car shifting from 4th to 3rd when he gently touched the gas, then 4th to 3rd when he let off.. Then 3rd to 4th when he touched the gas again, etc.

We pulled over.. I couldn't see any issues, so we fired it back up and drove home in 3rd with no issues.

Tonight we turned the car on and went for drive #2. I drove so that I could feel the problem for myself.. I threw it in Drive, and the car drove beautifully. We took turns driving it around town and on the highway and drove around for 45 - 60min with no problems whatsoever. He babied it. I didn't beat it, but did accelerate and try several driving styles in an attempt to duplicate the problem, and couldn't. I manually shifted it from 3rd to 4th and got a good feel for the transmission. 4th most definitely works. Today, I was unable to duplicate the issue. The thing ran like new.

I doubt this was a fluke, so I am kinda nervous. I don't mind fixing it for him, that isn't an issue. I just don't want there to be a mystery problem that he encounters when he starts taking it out on his own and would like it to be properly functioning. I am not expecting a new car or anything, it's just an around-town first-car. Heck, if it only does that once in a great while, I am fine with that, too. lol.. I'd just rather fix it now than later.....

My first thought was engine mounts. With the hood open, when you shift from P to D the engine does a pretty significant movement. If I hold the brake and hit the gas, it does not bounce around excessively.

It's a 2003.. Automatic.. 150k miles.. Trans fluid looked good, car runs great, cool, and shifts wonderfully.. There are no messages or engine lights on. I did not check for codes but I could. Could there be codes if the service engine light is not on?

What would you check next?

Thank you for any assistance or ideas...

 

Harsh shifting is sometimes the result of the computer trying to protect the transmission when it suspects there's a fault in the system. The computer runs the trans pressures to maximum so the clutches and bands CAN'T slip and burn-up.

Downside it that the harsh shifting is really hard on other parts of the trans--leading to destroyed splined joints, maybe failed one-way clutches, etc.

Connect a scan tool, find out what the computer wants you to know. Some cheap "consumer-grade" scan tools can't communicate with anything but the engine (computer); and they do a poor job of that. You'll need a REAL scan tool. "Codes" can be useful, but the real diagnostic power is in the data stream and the bi-directional control.

 

Thanks! It also had a long crank time, just got a Fuel Pressure Regulator in today which fixed that problem. Had it out again tonight with no issues. lol..

As for a scan tool... Please forgive my ignorance- can I just swing by Autozone and have them check the codes? Or would a decent one be found at a local repair shop? Or a Cadillac dealership?

 

As said..."codes" are the BEGINNING of diagnosis, the live data is far more useful.

Don't be confused between "code readers" and "scan tools". A scan tool does everything a code reader does, and FAR MORE.

I don't know of any parts store that will be willing/able to perform a proper scan of your vehicle. They may read codes, and--even worse--they may use code readers so primitive that they LIE about what the code is. "Generic" codes get substited for manufacturer-specific codes. Among the most-common of this sort is where a real scan tool will display--for example--code P0304--where the last digit is the cylinder number of the misfire, a cheap-junk generic tool would substitute P0300, "random" misfire. Because you're not given the specific cylinder that's misbehaving, you've got to check ALL the cylinders instead of just the problem child.

Used "professional grade" scan tools with a reasonable accessory package can be had for under $500. If they cover the newest vehicle you'd be using it on, they're a tremendous bargain. Other folks like the software-and-adapter-cable-plus-laptop-computer solution, but I've never used one so I can't help you there.

MY scan tool is a Snap-On Solus Pro with 8.2 software, good for 2007 and older. Enormously obsolete for professional use, but suits me just fine as my newest vehicle is a 2003.