Engine Revving Problem

Deacon Jones · June 23, 2017

I have a 1997 LS-400 that was previously owned by my Grandmother, so while it's pretty old it has very few miles on it (~45000) and saw regular maintenance at the local dealership throughout its entire lifetime.

Recently, I started having an issue with the engine where, while the car was in neutral, the engine would rev and then idle cyclically. It will rev up to ~2300 rpm, then drop back down to ~1200 rpm, then repeat in about a 2 second cycle. At the same time, when the car is in drive, it feels like the acceleration assist is several times its usual intensity (it is harder to brake; with the brake fully depressed at a stop the engine is at a higher rpm than typical; when the car is driving but the gas pedal is not depressed, it travels much faster than usual). I took it in to the dealer and was told that his best guess was some sort of carbon buildup that needed to cleared out. His tone basically LED me to believe that he has no idea what was wrong, but felt compelled to offer a suggestion.

This issue does not occur every time I turn the car on. More often than not, I start the car and it operates as it should without any of the symptoms described above. But when it does manifest the issue, it persists through turning the car off and on again, cycling gears, etc. The only reliable way I've found to get away from the problem once it starts is to leave the car turned off for 15+ minutes before turning the ignition on again.

Does anyone have any idea what the problem might be, and / or any way to resolve it? I would greatly appreciate it.

Thanks.

Malc · June 23, 2017

It might just need a good burn up the Motorway to clear out years of carbon build-up. Maintenance is no substitute for use .............

I remember having, both very lethargic, a F reg Volvo estate and a BMW 318i coupe from 15+ years ago when i had my car sales business. I then drove both cars exceptionally briskly and through the gears, resulting in several loud bangs and plumes of black smoke trailing behind me.............. and the cars then driving with a new lease of life and like " normal "

Too much molly coddling and not enough use, these are after all 4 ltr power machines, might have resulted in the situation you now have.

But it's your risk whatever you do with it ..... others on here might have a different possible view

Malc

Deacon Jones · June 23, 2017

I appreciate that suggestion. I probably should have added this. I drove the car from Dallas, TX to Oregon (~2200 miles) over a few days after taking possession of it. The same issue manifested itself both before I left Texas, and at the end of the trip.

Surebet · June 25, 2017

Many years ago I had a 1997 Vauxhall vectra automatic with the same kind of fault. The culprit was a sticking idle speed control valve, it is an electrocal-mechanical valve approx. 6 inches long and 1.25 inches in diameter. This was a yearly issue for me and I tried everything, by stripping it down and washing mechinical part out, however it always resulted in me buying a new unit,( there was never any visible sign of any carbon build up) at the time approx. £100.As soon as the new valve fitted problem was resolved, Vauxhall knew at the time this was a built in fault as the parts department personnel stated they were selling hundreds each year. This is the only car I have experienced with this issue( idle RPM fluctuating erratically between 800-2500, when foot off gas pedal). I don't know if this car you have even has one of these valves, the symptoms you are describing seems very similar. Might be worth checking, If this is the problem, then don't know how much it will cost now, but it was straight forward job to do, 2 bolts holding unit on and an electrical plug in to valve. Al the best with your fault finding..

Deacon Jones · June 26, 2017

Thanks for this suggestion Surebet. It gives me some place to start. I'll check it out.

Newbie1 · June 28, 2017

It might also be worthwhile pulling out the ECU from the passenger foot well and opening it up to inspect the capacitors (take usual precautions-anti static bracelet etc,) Degrading and leaking capacitors can cause all sorts of odd running issues. Even if it is not the cause of the problem, it's a worthwhile check to do for any old ECU to prevent more of the PCB and components being damaged due to capacitor leakage. There are several places in the US that can undertake remedial work if that is a problem.