Hesitancy/Stumbling

[3050rpm]

Since getting my 325i new in '95 there's been a quirk with the car I haven't been able to chase down. Upon easy acceleration I encounter, with varying severities, a hesitation or stumbling, occasionally even a bucking, in the 1800-2000 rpm range. On either side of that everything runs smoothly, but if I set the cruise control in that range there's almost a guarantee of rougher going, and when accelerating through that rpm range I get a distinct feeling of back pressure coming through the gas pedal, almost as if the car is resisting me. Weird, for sure.

Early on, those many years ago when the car was still under warranty, I took it to the dealer, but didn't get much other than "these cars are meant to be driven hard," that and a couple bottles of Techron. Changes of spark plugs, air filter, fuel filter have some, but not an entirely satisfactory, result. The injectors have not been touched. Then again, there have been times when the phenomenon has completely, and mysteriously, disappeared and the engine pulls cleanly and smoothly through the 1800-2000 range, but it's there more often than not.

My driving style is very much on the easy side, mostly highway with the cruise control set between 70 and 75 (well, okay, there have been a few stretches of 100+, but only when road and traffic conditions permit), always use brand name 93 octane (occasional use of 91), oil and filter changes at 5000 mile intervals, service intervals are followed. I haven't come across any similar mentions of this occurence, and since I've had this since day one, and it seems to defy the more plausible explanations, as well as the passage of time (and miles), maybe it's just a sometimes annoying idiosyncracy, akin to a birthmark. But any thoughts, suggestions, similar experiences would be appreciated.

[c1apton]

A couple of things: Run a good injector cleaner like STP gas treatment through it with 91 octane fuel & a different brand of fuel (if you drive "easy" as yor mentioned 93 is overkill) Also consider a new fuel filter and check all air intake hoses for proper fit. You might also switch to a K&N air filter or a lower restrictive air filter set up. In the early to mid '90s BMW did alot of experimenting with air filters (running changes in almost all models)

[Dudesky]

Originally Posted by 3050rpm

. . maybe it's just a sometimes annoying idiosyncracy, akin to a birthmark.

Yes.

To the contrary, this is not unusual at all, particularly with the M50 engine. Additionally, earlier versions of VANOS were fairly crude at best. Complaints vary widely, from rough idling, rocking from side to side, to near stalling in some cases. Power and stability at the lower end of the rpm band is not one of the best accolades of the E36/46; they really are much happier at higher revs. Their design is focused on a normal operational range of about 2k - 3.5k, and beyond for full delivery of stated horsepower. The 3 series can easily be not the most fun car to drive in the city, and it can be a little tougher than the average car to keep it flowing smoothly. And this is something many "3" drivers hate to admit about their car, so you don't hear it too often. But you'll note that its personality abruptly changes once you have some more space and open road- this is the car's hallmark. In short, don't be afraid to maintain higher revs, and your car will be happier.

Of course, something could really be wrong with the car beyond its design limitations. Maybe check for any vacuum leaks. Or have the dealer check the DME/ECU to see if it's running the most current software upgrade(s). You might also consider getting an aftermarket chip. Aside from increased HP, they also make corrections and adjustments for a smoother response curve.

[3050rpm]

Gents,

Thanks for your responses, the tips and background information are quite helpful. Just to bring things up to date a bit, I changed the spark plugs, fuel filter, oil and filter, and air filter last month, and from what I could tell the hoses looked in good shape. Radiator and engine block also flushed. I think I will get some injector cleaner though (which I can purchase "free" with the money I'll be saving on the next tank of gas seeing I don't need a diet of 93).

The mention of the VANOS caused me to recall an article in a car magazine or the automotive section of the newspaper some years ago about some runnability problem that had cropped up with that, and I think the '95 model year, or shortly before, is when the first generation of VANOS was introduced. I can't recall the technical details though, but I guess if it had been severe enough I would have had a more serious problem by now. The car is running quite well now, probably the items mentioned above individually contributed their 2 cents to the cause.

As to the "personality" of the car, I very much agree, although I really don't have much of a problem in the city (except on those hot days when I get that bothersome clutch shudder), but then again I really don't encounter all that much stop-and-go, creeping along city traffic (thank goodness). But at 3500+ rpm in top gear on a nice stretch of highway, and the car really comes into its element, very much designed for Autobahn driving. I remember coming across a comment by someone new to BMWs, wondering if there was something wrong with his car as it ran about 1000 rpm higher on the highway than he was accustomed to in other cars (Japanese, I believe). Someone reassured him all was well, that the higher revs were there on purpose, to give the torque and horsepower needed for solid high speed driving, rather than having a drivetrain more fitting for city/suburban driving panting for breath on the highway. I don't know how reasonable the answer was, but it suits me!

[c1apton]

You also might try cleaning you MAF (mass air flow meter) Dust and dirt accumulate in and around inside of it. Just pull your air intake hose off (beteen air box & MAF) spray carb cleaner in it, let sit a few mins, then wipe clean around large "butterfly flap". NOTE be careful of screen mesh or on some models it's a fine wire that contains a heating element - you don't want to damage or disconnect it. This will help the driveability & smoothness on acceleration.

[3050rpm]

Many thanks for that, I appreciate your very practical tips (and it came just in time, no need to make two trips to the auto store now, will just add to the fuel injector cleaner on the list!)