Engine vibrating and revving high

[avaholic]

Hey all, I just bought my first DSM (93 Talon TSI/AWD) a little over two months ago. It needed a ton of work and I've steadily been working on it and fixing things.

One of the problems I can't figure out about it is that when I'm driving it and I take it over 2500 rpms and let go of the gas, the engine vibrates and makes a really loud noise. Also, when I push in the clutch to switch gears (only when accelerating) it will rev about 500 rpms higher and then drop sorta slow.

I've looked through all sorts of threads all over the web and can't find anything that really helps. Can anybody help me out with these two problems??

 

[soulefood]

i would check the throttle body butterfly valve and see if its got a lot of buildup. that would explain the slight rev when the clutch is disengaged. not the deceleration noise however.

 

[avaholic]

I gave the throttle body and elbow a thorough cleaning about a week ago. Where exactly is the throttle body butterfly valve? It's not the throttle plate itself is it?

 

[AlanK]

It sounds like a manifold leak to me.

Carefully spray a bit of 'start ya bastard', butane, LPG, or similar flammable product around the manifold gasket, and also the plenum chamber. If the revvs increase, you have a leak somewhere.

I was able to buy the manifold gasket, but I had to make a plenum gasket. Use gasket paper that is suitable for oil/petrol.

Check your vacuum hoses, and replace any that may be warn or leaking.

Cheers,
Alan.

 

[avaholic]

Great, I'll check that out! Can anyone help with the engine vibrations though? It doesn't shake enough to make the car feel like it's going to explode or anything, it's just really loud and I can't figure out why the heck it would do that

 

[AlanK]

Check for the leak, that may be causing your problem.

What is the idle speed set at? Is it idling too low?

Do the vibrations get worse or better as the car warms up?

Also check the usual things, such as spark plugs and leads.

Remove one ignition lead at a time, while the engine is idling, and see if you can isolate one cylinder that is not firing properly.

The problem may be mechanical, such as warn bearings, but I would check the easy things first.

Cheers,
Alan.

 

[avaholic]

The idle is set at the lowest it can go (the BISS is screwed in all the way minus about a half turn). I wish it would idle too low haha. The vibrations are exactly the same whether the engine is hot or cold. If i'm giving it gas the vibrations aren't too bad (except for above 65 or 70 mph) but once I let off the gas and the rpms are above 2500 the vibrations start.

 

[falcon8r]

Are you sure the noise on deceleration is engine and not drivetrain. The rearend can get some bad backlash gear or bearing noise and will put an evil vibe in the car. It is kind of a harmonics thing, hence RPM dependant. Tres...

 

[Jets303]

If your ever in the denver metro are Im a great mechanic and I wouldnt mind looking at it for ya!

 

[avaholic]

That would great! I will actually be in Denver this Thursday through Saturday.

And falcon, I'm 99% positive it's not the drive train cause the noise comes from the front of the car and not the back. And it's not solely rpm based seeing as how, if I'm giving it gas it's fine.

 

[njdaewoo]

Would a motor mount cause this problem with the vibrations?

 

[avaholic]

That sounds like a possibility. I'm completely new to dsm's and pretty much cars in general. How exactly would I check to see if my motor mounts aren't cracked or anything?

 

[Calan]

The idle is set at the lowest it can go (the BISS is screwed in all the way minus about a half turn).

The BISS is not an "idle speed adjustment". The idle speed is controlled by the ECU. The purpose of the BISS is to adjust the idle airflow so that the ICS is in the center of it's adjustment range, so that the ECU can maintain the proper idle speed across varying loads and conditions.

The only way to properly change the target idle speed is by reprogramming the ECU through tuning software. If you can drastically change the idle speed with the BISS, then you are probably maxing out the ICS or there is a leak somewhere, or base timing may be off (the ECU also uses timing to maintain idle speed).

To properly set the BISS, you need to ground the diagnostic plug and timing plug to prevent the ECU from trying to adjust the idle with the ICS and timing, and then set the idle to 750rpms with the BISS.

1G BISS Adjustment

As for the vibration, do some tests to see exactly when it happens. For example, coast the car in neutral, engine off, clutch in, clutch out (engine decelerating), etc. This will help narrow it down.

There may be something in this article that can help you pinpoint it as well:

http://www.dsmtuners.com/forums/articles-miscellaneous/337788-what-causing-irritating-noise.html

 

[avaholic]

That makes sense. I've already been to that exact same page for adjusting the BISS and did it already. If that didn't change anything, does that mean then that it's something to do with the ECU?

 

[Calan]

If that didn't change anything, does that mean then that it's something to do with the ECU?

You mean adjusting the BISS didn't change the idle? If the car is idling around 750 +/- 50 rpms to begin with, it shouldn't change it...at least not much. If the idle is higher or lower than that and properly adjusting the BISS doesn't bring it back to around 750 rpm, then there is most likely an air leak, problems or misadjustment with throttle body-related components (ISC, idle switch, TPS, etc), or tuning issues.

EDIT:

I meant to say "it shouldn't change it...at least not much, when the timing and diagnostic plugs are not grounded". In other words, adjusting the BISS under normal operating conditions shouldn't have much of an effect, because the ECU will just compensate for it until it runs out of adjustment with the ICS.