Idle problems, no CEL, but turns up "knock sensor"

[Texasbimmerfest]

A knock sensor cyl 3-4, shows up on my Peake tool, but it is not tripping the CEL (check engine light). I start it cold, and it starts fine and runs fine for a while. Then when sitting at stop lights the idle will slowly bump down to roughly 500-600 rpm and it sounds like it's misfiring. It sounds the same when taking off from that stoplight, and i have no power.

I'll put it in nuetral when this problem shows up and try to keep the idle at about 1000 rpm to hear for misfires, etc and now and again it sounds like it'll fix itself for a second and the rpms will jump back up. Same when starting off from the stop light. It'll be sluggish and sounding like crap then BAM it's like it gets turbo charged and i take off.

I also thought bad gas, so I put about 5 gallons of premium, and 1 bottle of each: gas treatment & octane booster. This didnt help 15 miles later.

Recently, I sprayed engine degreaser on my cold engine bay and rinsed it off with my garden hose.

It has the symptoms of misfiring. I replaced all the sparkplugs about 2000 miles ago or less. I gapped them right. I checked them before i put them in. It was running great after i put them in, this is very recent.... only started a couple days ago.

[Texasbimmerfest]

p.s. this is a '95 ti, M42

[dave45056]

Have you checked the rubber boot between the airbox and the thottle body?
In general any vacuum leak can cause the problems you described, so look for any cracked hoses. You cannot check the corrugated boot while in place. Take it off and bend it around.
It wouldn't hurt to double check the plugs either.

[tiFreak]

Originally Posted by Texasbimmerfest

Recently, I sprayed engine degreaser on my cold engine bay and rinsed it off with my garden hose.

not a great idea, you probably got some water in an electrical connector, my boss likes to do that too, the cars typically run like crap after too

I'm honestly not sure where the knock sensors are but that would be the first place I'd look, pop the connector off and spray some WD-40 in it, let it dry, than reinstall and drive it

[Junk]

knock sensor, I thought the strategy was to bump timing until there was knock and then back it off is why you should see better performance when putting in 91 vs 87 or whatever.. I am guessing even a bad knock sensor shouldn't cause the computer to back timing so far that it runs rough.. but I could be wrong..

Does it only happen at/off idle?

[Texasbimmerfest]

It (so far) always starts when it is at idol, in gear (automatic tranny) at a stoplight for example. Mostly, after I've been sitting at a stoplight for a minute or so. Then, it will continue when I take off being really sluggish and hearing the puttering in the exhaust system. So far it's only happening after it warms up after driving a few miles.

[b.u.ti-ful]

Check the spark plug wells for water.

[Junk]

hmmm.. only after warmup.. Well Since I wasn't sure about the knock sensor I looked at the electrical diagrams, it seems like it shares the ground with the tps, and temp sensors.. so maybe tifreak in the right track, I just don't see why it only happens when warm and at idle..

did it only start after the engine cleaning?

Maybe the charcoal canister evap purge? I think that opens when warm only and maybe at idle only..

[Texasbimmerfest]

Originally Posted by Junk

hmmm.. only after warmup.. Well Since I wasn't sure about the knock sensor I looked at the electrical diagrams, it seems like it shares the ground with the tps, and temp sensors.. so maybe tifreak in the right track, I just don't see why it only happens when warm and at idle..

did it only start after the engine cleaning?

Maybe the charcoal canister evap purge? I think that opens when warm only and maybe at idle only..

I cleaned the spark plug wires, no moisture in there, I unplugged the MAF and connected a "cold air" intake, it didn't help. I used the ear-to-screwdriver trick and heard the ICV ticking. I guess my next step will be to change the 02 sensor per
http://stlbmwcca.org/wwwboard/viewto...6b3617c2a42706

It sounds like he has the same symptoms, but his CEL is lighting up. If only I could be that lucky. Yes, it only started after the cleaning. At first it only started at warm and after idle, but I got it once yesterday at cold start-up. It must be getting worse, 25 miles later...

[Texasbimmerfest]

UPDATE: Clarification is key. When the problem starts, it will persist through whatever RPMs I can get it up to. It sounds and shakes more like a harley although not as loud when idleing or giving it gas. It randomly fixes itself then seems to randomly start again, although it is gradually leaning more twards the problem, and less towards fixing itself. I just checked for vacume leaks, there are none.

The DISA valve moves freely when the car is turned off. When at idle (with or without the problem existing at time of test) it is fully extended like when the engine is off, but when i manually press it in it will stay. I read when the car is idling it is sopposed to be sucked in (like pushing it in) right away. can anybody confirm this?

[Junk]

Hope so! I am not too optimistic because in the link you posted, he still has a rough idle when warm, just that his cel went away. They did mention the crankcase vent.. and ICV valve.. I was thinking that earlier, but threw myself off thinking it should do it more when cold, but maybe the heat affects the ICV motor and it's finally burning out? but that's my new pick of the day..! Next step, put a picture of the engine bay on the wall and start throwing darts!

[b.u.ti-ful]

This thread might help with the breather valve:

http://www.318ti.org/forum/showthrea...kcase+breather

[Junk]

Now it happens at all RPMS? NOT the ICV then..

Not when you're only on the brake right? If it happens at high RPMS as well, I don't think any little vacuum leak would do it, maybe a leak at the intake boot as Dave45056 pointed out ..

Not sure about DISA, but I would just unplug all un-necessary vacuum lines and plug them at the manifold. I don't think a difference in intake runner length/volume will make it run rough really.. maybe a leak in the diaphragm but that would just be a little leak..

Sounds like it is running real rough.. Bad coil? Maybe let it run rough for awhile and check the plugs? or probe the coils looking for one with unusual resistance? (Since it's intermittent it sucks, but if it gets bad enough to the point it does it all the time, it should be easier to troubleshoot )

Am I hurting more than I am helping?
Good luck!

[Texasbimmerfest]

After no help with the air intake boot, vaccum testing, and bypassing the MAF I decided to do it the right way, start at the plugs. I got no difference in idol when i unplugged #3. BAM, now I'm in business. The spark plug was way too easy to back out, and way too dirty for being so new. That's probably for a new thread though. So I cleaned it up with the a sheet of the 20 dollar/roll microfiber towels, wrapped it around the spark plug wrench, and wiped down there as much as possible. After I reinserted the spark plug (snugly this time) BAM! I blew up my engine. Kidding. BAM! It fixed the roughness. Took it for a testdrive and no previous symptoms came about, verified that with MPG.

I consider myself lucky because Ive read about people spending a LOT more trying to fix this same problem. I'm still wondering why my CEL didn't throw a misfire code. And I plan on tackleing the ICV pretty soon since I now know more about it. Thanks everybody!

[b.u.ti-ful]

Done.