Shaking at Idle

[924Carrera]

I did some searching but didn't come up with much that seemed to fit my issues. My S4 (B6, 2005) has a strange "shaking" feeling when at low idle. It will run perfectly smoothly, then every 3 or 4 seconds will kind of shudder almost like it is misfiring. However, actively logging in VCDS while it is doing it shows 0 misfire count on all cylinders. Also, it has brand new coils, several new coil connectors, new plugs, etc as I just recently did a bunch of work on it.

As soon as you touch the gas, the shaking stops. When you first start the car cold and it sits at high idle around 1000 RPM, it is butter smooth. Only when the idle settles down to 750 or whatever it is, does it begin the odd, long-interval shuddering. The car pulls hard in all gears without vibration, and pushing in the clutch makes 0 difference, so flywheel issues don't seem logical either. However, the shaking is enough to concern me, and after all this car has done to me, I'm nearly paranoid driving it half the time.

I occasionally notice a similar but more fast-paced shudder when loading up the engine (such as accelerating on a hill in too high a gear around 2500 RPM) but I'm suspicious of a bad CV on that as one of my CV's somehow expelled all its grease without tearing the boot, halfway through a 2000 mile road trip, so it went quite a while nearly dry. The fact that it doesn't howl is testament to the quality of the OE axles. In any case, this has nothing to do with issues at idle.

Any ideas? Could motor mount problems manifest themselves in this fashion? The mounts are likely original, although my snub is new.

Thanks,

Alex

[Aok303]

id also check to make sure your oil cap is on tight

[924Carrera]

id also check to make sure your oil cap is on tight

Yeah, it definitely is as I have been checking and topping off the oil after doing the valley pan gasket. It's actually tighter than in the past because I put a new cap gasket on as well. Thanks for the advice.

[badger.]

Mine would do something very, very similar (never threw a misfire CEL, don't know if anything was in VCDS), but I changed the spark plugs and it went away. But you said you got new ones. What kind did you get?

Other potential ideas: Check IM for any loosening. Also vacuum lines. Also, did you take off the throttle body when you pulled the IM? If so, did you use a new gasket? If the gasket isn't seated quite right or something you can be pulling in air the MAF sensor isn't accounting for.

Not sure if any of the above are necessarily your problem. But they are easy to check :)

[924Carrera]

Mine would do something very, very similar (never threw a misfire CEL, don't know if anything was in VCDS), but I changed the spark plugs and it went away. But you said you got new ones. What kind did you get?

Other potential ideas: Check IM for any loosening. Also vacuum lines. Also, did you take off the throttle body when you pulled the IM? If so, did you use a new gasket? If the gasket isn't seated quite right or something you can be pulling in air the MAF sensor isn't accounting for.

Not sure if any of the above are necessarily your problem. But they are easy to check :)

Good tips. I used NGK BKR6EQUP 4-electrode ones, AKA the correct ones as far as I know. IM shouldn't be loose as I torqued to spec after applying blue loc-tite, but I can check again easily enough. I took the IM and TB off as one unit, didn't touch that connection at all, and used new IM gaskets of course. Vacuum lines could be a valid issue. One of the stupid SAI ones broke in the process and I replaced that section with some of the Contitech 3.5mm hose, but there's a good chance the one of the other parts of the line or another line could be cracked or leaking, considering the brittle state they were in. I'll have to investigate that more carefully. Thanks.

[VinnysS4]

Open VCDS and navigate to the short term and long fuel trims. What are the values while the engine is up to operating temperature at idle? Bring lap top under hood. Spray carb cleaner or electrical cleaner around the intake manifold and watch the short term fuel trims. If they go into the negative, (rich) when sprayed. ... you have a intake manifold leak. If it only happens at idle...I suspect a vacuum leak.

[JimmyBones]

Open VCDS and navigate to the short term and long fuel trims. What are the values while the engine is up to operating temperature at idle? Bring lap top under hood. Spray carb cleaner or electrical cleaner around the intake manifold and watch the short term fuel trims. If they go into the negative, (rich) when sprayed. ... you have a intake manifold leak. If it only happens at idle...I suspect a vacuum leak.

This is good advice. Starting fluid works well for that test too. The measuring value blocks that you want to look at are 32 for long term fuel trims and 33 for short fuel trims.

This issue sounds like vacuum leak to me. The first thing that I like to do with any VW or Audi is to check the PCV at idle. Make sure that the area around the oil filler cap is clean. So don't dust your engine in metal shavings like some of the RS4 guys do (just kidding because there is a recent joke about that in the RS4 section). Then with the engine running at idle, try to take the engine oil filler cap off. It should come off nice and easy just like if the engine was off. The engine should shutter for a second and then smooth out. If the cap will not come off because it is being sucked into place or the engine dies then your PCV valve is bad.

[924Carrera]

This is good advice. Starting fluid works well for that test too. The measuring value blocks that you want to look at are 32 for long term fuel trims and 33 for short fuel trims.

This issue sounds like vacuum leak to me. The first thing that I like to do with any VW or Audi is to check the PCV at idle. Make sure that the area around the oil filler cap is clean. So don't dust your engine in metal shavings like some of the RS4 guys do (just kidding because there is a recent joke about that in the RS4 section). Then with the engine running at idle, try to take the engine oil filler cap off. It should come off nice and easy just like if the engine was off. The engine should shutter for a second and then smooth out. If the cap will not come off because it is being sucked into place or the engine dies then your PCV valve is bad.

Yeah, I'll try these things as soon as I can. Hopefully the PCV valve isn't bad since I also recently replaced it. However, there are so many places for a vacuum leak that I could have exacerbated with the intake pull, that I wouldn't be at all surprised. Like I said, I definitely snapped off an SAI line and repaired it, so who knows if I cracked another without realizing it.

[Terry.Reese]

http://www.ebay.com/itm/131699669138...%3AMEBIDX%3AIT

hope that helps if you have to replace.

[VinnysS4]

http://www.ebay.com/itm/131699669138...%3AMEBIDX%3AIT

hope that helps if you have to replace.

http://www.ebay.com/itm/VW-BUG-GHIA-...FUhIcn&vxp=mtr
this works too i believe.
When i broke mine, i went down to O'Rileys and picked up some hard emissions line for like 4 dollars and it worked great! kept it local too

[924Carrera]

http://www.ebay.com/itm/VW-BUG-GHIA-...FUhIcn&vxp=mtr
this works too i believe.
When i broke mine, i went down to O'Rileys and picked up some hard emissions line for like 4 dollars and it worked great! kept it local too

Yeah, I got some of the Continental version of that with one of my recent parts orders, so I still have quite a bit left over for patches if needed. I believe that line that Terry posted is the one that connects near the ABS module to the vacuum reservoir. It would be useful if that fitting was broken, for example. I've dealt with Wolf Auto a few times and have had decent results.

[Terry.Reese]

Yeah, I got some of the Continental version of that with one of my recent parts orders, so I still have quite a bit left over for patches if needed. I believe that line that Terry posted is the one that connects near the ABS module to the vacuum reservoir. It would be useful if that fitting was broken, for example. I've dealt with Wolf Auto a few times and have had decent results.

most all of the parts I have replaced so far was from Wolf, good people.

Hope you get it fixed, I think I fixed my shimmy, discovered my + battery cable was really loose.

[924Carrera]

Finally managed to try something out. The vacuum jet pump that connects to the brake booster hose was shot, so I replaced it. The idle shake doesn't SEEM to be there any more when I sat for a few minutes, but I'll have to drive it for a while and see. I'll report back if it ends up not being fixed.

[924Carrera]

The idle shake continued to persist even through an engine rebuild with all vacuum connections checked. I finally just split the intake manifold into its top and bottom halves and noticed that some of the factory sealant looked like it had deteriorated. Resealed with Loctite 5970 and the idle shake seems REDUCED, although still slightly noticeable. However, what really changed was the other vacuum leak behavior, which was the car feeling "jerky" when holding the throttle just barely cracked to where the fuel is cutting on and off (around 2200 RPM typically when cruising). That has smoothed out a lot, so the fact that there even is still idle shake perplexes me because it did seem fixed on my first test drive but then seems to have come back at least a little bit.

Edit: After driving it some more, it does seem that the idle shake is pretty much gone. I am pretty sensitive to these issues after trying to fix them for so long, but I'm guessing that the small bit of shaking I can still feel is just normal engine behavior as the ECU adjusts fuel trims and so forth. This normal behavior was being amplified by the vacuum leak. So, if anyone has been chasing down a vacuum leak with no luck in finding the source, the intake manifold's internal seal is a good bet. Although the large groove where sealant is applied and can be peeled out is very obvious when you split the intake in half, less noticeable is the small amount of sealant on the INBOARD side of each intake port, which keeps air from making its way past the (in theory perfectly flat, but obviously not really) mating surfaces of the upper and lower intake sections. There is no groove for this sealant bead, so much less is needed and it's much harder to notice the residual amount from the factory.