Rough idle all of a sudden.....I'm stumped. Ideas?

[IQraceworks]

For some reason, out of the blue, one day last week I noticed that my car developed a slightly rough idle. I was at a stop light and noticed I could feel it through the seat. As soon as I get the motor off idle, it smooths out and feels like normal. Car still runs like a top....just that rough idle. If y9u put your hand on the motor..you can feel a slight bobble/miss. It's usulally as smooth as butter at idle.

I started with the basics....pulled all the plugs and found a wet one. Great, leaky injector...that must be it. Replaced the injector, no change in the idle.

I then reset all the motor adaptations.....no change in the idle.

Scanned for codes just for the heck of it...nothing.

I ran a log trying to see if I could see anything...nothing. ARF's, STFTs, LPFP,HPFP, everything looks fine....so signs of anything odd.

I went ahead and put in all new plugs and coils (thanks FCP!).....still no change in the rough idle.

Checked for any leaking vacuum lines, or anything else odd under the hood. nothing. Valve cover is a new OEM one with around 6K miles on it. Valves were cleaned around 15K miles ago.

What's some other things that could cause a slightly rough idle? I'm out of ideas.

 

[I am Jacks N54]

Injectors

 

[carabuser]

With anything other than a full set of index 11 or 12 injectors I'd say that would be your problem.

Just because an injector doesn't leak, doesn't automatically mean that it's healthy. Spray pattern and flow rate would need to be looked at for each injector to truly gauge health.

You can also try logging VANOS actual and target position. I've recently replaced both units on my vehicle because I noticed they were deviating from target and fluctuating quite a lot. I'll put a post up on the forum once I've had a chance to log the results.

 

[carabuser]

Also spark plugs make a difference. My idle was never smooth running anything other than the stock bosch 3 prong plugs.

There's probably tuning changes to be made to increase idle quality. I did read the functional description of idle control and it's quite complex. Not just a simple PID that attempts to maintain idle speed.

The ECU generates 2 torque requirements. One is used the control the throttle plate and the other is for the spark timing. At idle the car will use multi spark and multi injection which also adds to the complexity.

The minimum requirement of that will be a smooth idle that customers will be happy with, once that is achieved they will go for minimal emissions. There's probably plenty of room to improve idle quality at the expense of a little more CO2.

 

[IQraceworks]

Well, I had a little more time to do some troubleshooting yesterday. I logged STFTS, LTFTs, VANOS target and actual, AFR........all the data is reading just like it's supposed to. I would have thought that if it was an injector/fuleing issue...something would have shown up in the data. But there is nothing out of the ordinary that would show any issue.

I think the next thing to do is to go through all of the injectors, swapping out out at a time with a known good one and see if I can find the one that might have a funky spray pattern or something (if it is in fact an injector issue). If that all checks out, then on to a compression and leak down test.

It's so strange.....I've been running the same setup for almost two years...and it's always idled smooth as butter. Then one day as I'm slowing down at a light....it started feeling rough. It's not like I just made a pass down the track or anything....just cruising down the street.

 

[Torgus]

While this might just mask you problem try raising your idle? It doesn't hurt anything.

 

[IQraceworks]

While this might just mask you problem try raising your idle? It doesn't hurt anything.

I'm already running a slightly higher than stock idle...around 750. Even if I hold my foot on the gas just a little, enough to raise the rpms to 1000 or so, I can still feel some sort of little bobble in the idle. Now if I raise it up past 1200 or so it completely smooths out....but I'm going to bandaid the problem by having a 1200rpm idle

 

[mang.vang]

Have you tried a compression test? I'm in the same boat just havent had the time to do a compression test.

 

[IQraceworks]

Have you tried a compression test? I'm in the same boat just havent had the time to do a compression test.

No, not yet. I've been working on my diesel truck all weekend (new injectors and turbo) and haven't done any more troubleshooting on my 335i yet this week...hopefully this weekend.

 

[Torgus]

No, not yet. I've been working on my diesel truck all weekend (new injectors and turbo) and haven't done any more troubleshooting on my 335i yet this week...hopefully this weekend.

Tell me more about the truck!

 

[IQraceworks]

Tell me more about the truck!

It's an 05' F250 with the 6.0 Powerstroke in it. It's got 260K miles on it, and one of the injectors was acting up. I figured I might as well replace all 8 of them.....and then I figured I might as well go a little bigger on the injectors......and then I figured if I do that, might as well put a bigger turbo on it. It kind of spiraled out of control......but when it's all said and done, it should be good for a long, long time. Still cheaper than payments on a new $70K+ diesel truck

 

[martymil]

You could have cracked a ring land or have a vacuum leak by the sounds of it since you done everything else.

Time to do a leak down test, are you still running a plastic manifold as they do partially split from heat and boost and cause these sorts of issues.

Has your WGDC gone up by any chance comparing to old logs ?

 

[Pete]

Do not disount the possibility of a vacuum leak without a proper smoke test with a smoke test machine. If you dont have access to one, most shops will be able to do it for you. A visual inspection alone is not good enough to rule out that possibity.

 

[SlowE93]

Do not disount the possibility of a vacuum leak without a proper smoke test with a smoke test machine. If you dont have access to one, most shops will be able to do it for you. A visual inspection alone is not good enough to rule out that possibity.

No need anymore. He doin the flinstone with it for now.