Erratic running

steve_964 · Aug 4, 2007

Anyone have any ideas on this problem?

The idle and general engine running has gone from always lively and smooth at all speeds including idling to sometimes flat and although not sluggish ,it doesn't always seem lively.Also in a steady cruise at any speed every so often it seems like the petrol supply stops ,accelerating a bit seems to overcome that problem though.
However when I start the car sometimes it is very smooth at tickover and on other occasions it seems a little rough and almost seems like its out of balance,the engine also tends to slow down past idle speed and sometimes recovers back to about 800 rpm instead of approx 950,but on other occasions it stalls.
I have taken it back to the specialists that i use ,but the diagnostic tester shows nothing wrong ,i'm booked to take it back for further investigation ,but with 160 mile round trip every time wondered if there was any advice as to if i could check anything before i make another trip.

Thanks in advance,

Steve

p.s the car is a 1990 964 C2 Coupe

chrism964 · Aug 4, 2007

Have you tried changing the DME relay?
They cost about £20 from the OPC and is just a pull out and plug in job.
They are frequently a cause of starting/fueling/idling problems.

steve_964 · Aug 4, 2007

Hi Chris

Thanks for your reply ,have already tried that one though !!
Had no effect on my current problem ,but at least I know I have a new DME relay fitted

Steve

Gordon Attar · Aug 6, 2007

These type of problems tend to be electrical and very difficult to locate, usually your specialist should have a good idea, but presumably yours is not comiung up with any ideas.

Next cheap check is to replace the air flow sensor/check that its fitted correctly. You can check the distrubutor cap and rotors for wear. After that you need to start checking the electrics, the coils are quite often a source of issues, unfortunately the best way to do this is usually by using a doner car thats in perfect working order and to start swopping parts over. Otherwise you can end up replacing parts one by one and that can be a very expensive route.

I had a slight surge in revs once and the caps and rotors were worn, replaced them it helped but didnt solve, then replaced the coils that helped again, these parts are not to expensive. Northways then started using parts from a sales car and it turned out in the end after many hours labour, to be that a spindle in the distributor was slightly loose! Fortunately Northways didnt charge the whole cost.

Fred Hindle · Aug 6, 2007

Check the battery condition. A poor battery is often the cause of poor running at tickover. Engine soumds as if it is 'hunting'

Cheers,

Steve Brookes · Aug 8, 2007

Obviously you have an issue to solve with the car but don't worry about the occasional hesitation problem you have. I get this now again and have heard a lot of reports about it on Rennlist. It happens when the engine is not under load, particularly when cruising along at no more than 50 and going down hill.

Like others I'd suggest that you solve the idling problem first.

steve_964 · Aug 14, 2007

Update on the erratic running,

In my concerned state about the problem with the car I completely overlooked the fact that my brother had spent several years troubleshooting and improving the performance of quite a selection of BMW's.His diagnostic knowledge has been very good over the years.
I managed to pin him down for a few minutes to get his opinion which he duly gave .
He looked in the engine bay and said "no problem,just remove air filter box,airflow meter,throttle body,the 3 branch air inlets between throttle body and cylinders and any breather pipes that you can". He said "they'll be full of burnt oil deposits from the breathers and other deposits and filth, you need to clean all the parts out with carb cleaner so they are all spotless and put it all back together,it will be fine trust me " .so off i went carefully stripped it all out making plenty of notes about how it all came apart so I didn't miss anything when putting it all back.Sure enough he was right,although they didn't seem too dirty to me I got him to look and he said it wasn't bad considering but the work would make a difference.So I cleaned everything meticulously and reassembled it all renewing a rubber connector that someone had carelessly split at some time previously.after checking everything was secure and going over my notes ,I started it up and guess what ?,it idles so sweetly ,I don't think it has idled so nicely since i've owned it,I still have to see how it behaves on a run but hopefully i've solved that issue too.
Thanks to anyone who replied to this thread,and any questions about the job I undertook,don't hesitate to ask,

Regards

Steve

skendrick · Aug 15, 2007

I been having the same problem but my car has been cutting out when cold so I quickly cleaned the air filter yesterday lunchtime and sure enough she started up this morning without cutting out although still trying to find idle.
So it looks like I will be doing the same this weekend.

Are the K&N filters worth buying or should I stick with the standard?

Thanks.

steve_964 · Aug 15, 2007

Re air filters,

My car is running in completely standard trim,I have resisted the urge to get more horsepower or more noise for a car that my chosen service specialists can service and set up without any variation from the normal set up.
I cannot say whether after market filters are better but no doubt some 964 owners will be able give all the answers to that.

Steve

smallspeed · Aug 16, 2007

more efficent filters are generally more restrictive.. I say generally as there have been great advances in filter material technology in the last 5yrs however Ive only seen it applied to fluid filtration so far (hydraulics)

performance filters (as a general rule) have bigger holes in so let more air into the engine (therefore creating marginally more power). If you go the performance filter route get a decent one and keep it in good nick (ie oil it and clean it regularly).. also prepare to fall in love with the noise.. I have a cone filter and a cat bypass and it sounds superb (although Im currently considering a cup pipe too)

also i saw something recently that showed a cone filter to increase inlet temp's quite a bit over the standard air-box arrangement. What was interesting was that the drilled airbox mod was somewhere in the middle.. personally I feel the inlet tract of the standard airbox is a bit constrictive, and you dont get much cold air ducting benefit from it due to the pick-up point.. I would expect a heat shield to help with a cone filter, however Im working on something a bit cooler (litterally) and will post some info when its done!

Steve Brookes · Aug 16, 2007

I've seen some dyno stuff posted by Ninemeister on Rennlist that suggests that there's nothing to be gained power wise by switching to performance filters. Having ditched by square K&N for a standard filter, the seat of my pants, confirms these measurements[8D].

Peformance filters DO give more induction noise though.

smallspeed · Aug 16, 2007

i think the problem is although margainally more air can be flowed the extra intake temp cancels out any benefit.. Youre probably better with something like a K&N pannel in the standard airbox for performance, but it wont do much for the noise!.. Problem with the 911 layout is there isnt much scope for drawing cold air in (unlike a front engine car) but atleast the standard airbox draws air from away from the back of the engine (where a cone filter sits) which is probably the hottest bit [&:]