I have been fighting a rough idle on startup for years on my two '97 BMW 328s.
I just read in a 5-series review that a common problem in high mileage BMW 6 cyl engines is that just a trace of sludge in the VANOS will cause them to hang up, causing a rough idle.
This is consistent with the fact that one daughter complained yesterday that her cold idle problem got much worse immediately after we changed from synthetic oil back to 20W-50 GTX, the oil that I have used in all my cars for the past 30 years. (I got a slew of oil leaks when I tried Mobile1 in my BMWs and Volvo).
I know I don’t have much sludge in this engine. When I recently changed the valve cover gasket, I found the valve train to be colored golden brown as you would expect on an engine with 240,000 miles on it, but no visible buildup anywhere.
When I was a kid, conventional wisdom was to add 1 quart of ATF to the oil and drive the car a few miles before an oil change. I don’t know whether that actually worked or not, or whether it hurt the engine. My first vehicle was a '49 Chevy pickup with a road draft tube, so sludge was a bigger problem back then.
After some time on the web, I am seeing that the two approaches seem to be
1) Add a strong solvent like SeaFoam or Berrymans and drive the car just a few miles (some say just let it idle for 30 minutes) before the oil change, or
2) Add a milder additive that is compatible with all seals and metals, like Marvel Mystery Oil or Motor Rx, and run it several thousand miles.
I am leaning toward the Marvel Mystery oil approach due to the cost and my lack of familiarity with Motor Rx.
Any other recommendations or cautions?
You might clarify this rough idle upon startup complaint. This may not be related to VANOS at all and could be a sign of losing residual fuel pressure.
Are you saying the car will run rough, stumble and try to die, etc. for the first so many seconds after starting it?
If the engine is warm, is then shut off and restarted very shortly afterwards, does it still stumble or does it run fine?
If I park these cars for several days, when I start them, they run on 4 or 5 cylinders. This usually clears up by the end of the block, but has been known to persist for up to 5 miles. They don’t stall. This does not happen every time I park them for an extended period, and some times it is more noticeable than others. The lower mileage car sat in the garage for six months last year. I wondered what would happen when I started it - it purred like a kitten that time. The one with 240k miles on it is worse than the one with 150k miles on it.
I doubt that the problem is a leaking or dirty injector. I pulled the injectors from one car and sent them for ultrasonic cleaning. The report said that they were fine, with no leaks and good flow and pattern before and after cleaning. The car behaved exactly the same afterward.
I doubt that the problem is the fuel rail draining down. When I replace fuel filters and the rail is empty, it fills completely and fires on all cylinders within about 3 seconds of when the first cylinder fires. As noted, this intermittent problem sometimes takes miles to clear up.
I thought that it might be a hydraulic lifter leaking down. That would explain why it does not happen every time (depends on the position the engine is in when it stops). However, I was advised that hydraulic lifters in these engines virtually never fail, so the chances of me having two cars with that problem are nil.
The higher mileage car has been doing this for 100k miles and several years, during which it has had several sets of plugs and air filters, the throttle body and idle air control valve cleaned a couple of times, crankcase ventilation system cleaned, etc.
This is a symptom of a common problem with the VANOS systems where the seals on the VANOS pistons will wear and flatten. Officially, you need to replace the VANOS as a whole, which is about $700 plus a couple hundred bucks in labor. You can get a rebuild kit from beisansystems.com but you’ll want to either have a trusted mechanic do it, or know what you’re getting yourself into.
As an aside, I would recommend sticking with the synthetic on such a high-mileage engine. Particularly if it has a high-compression engine (I don’t recall if the e36 did) I’m pretty sure it’s a requirement.
If the VANOS is affected by oil pressure, you might want to try 5W-30 or 10W-30. I can’t imagine that a '97 engine would be designed to run on 20W-50. It could also interfere with proper hydraulic lifter operation when cold. If the bearing clearance is tight, it may even interfere with how well the bearings lubricate. The '60s are long gone. Engines are built with tighter clearances now, and real heavy oil often isn’t a good idea.
The problem is the 20W-50 oil you are putting in the engine.
It was designed to use a MUCH LIGHTER WEIGHT oil, and now you are forcing it to work much much harder with cold, thick oil in the engine, first thing in the morning.
The VANOS system does not work properly with oil that thick when its cold.
Drain out the 20W-50 crap, throw it away, and start using the proper weight oil for the engine.
Just because you have been doing something for 30 years, doesn’t mean that it’s the right thing to be doing today, on a newer, much more sophisticated engine.
You have caused the worse idle problems in your daughters car.
If you are lucky, the problems will vanish when you pour in a proper synthetic oil into her engine. You can choose between 0W-30, 5W-30, 0W-40, 5W-40, or 10W-40. Lots of variety there.
BC.