HELP weird rpm!!!

[MS3 GT]

i get bouncing rpm between 500-1000 when back into neutral after high speed(high way speed),it doesnt do that on lower speed (like local speed limited).no cel yet, vacuum leak? i mean my boost gauge is shown 20 vacuum, recently just swapped the turbo(which is bad turbo too), and i did the non fouler trick to avoid that CEL(would this trick cause the CEL not to be shown eventhough if i have a leakage or something bad?) any help will be truly appreciated!!!

 

[MS3 GT]

no one???

 

[MSMS3]

I'm assuming you have a catless downipipe. No other reason for the anti-fouler trick. But that only involves tricking the 02 sensor's concern about catalytic converter presence and temp. That would not have any influence on idle speed variations, IMHO.

You have something else going on. If you did have a vacuum leak, you would probably see fluctuation on the vacuum/boost gauge. A steady -20 mm of mercury would not suggest that. I'd lean toward something on the idle speed controls, a stuck or crudded up throttle body mechanical control or something along those lines. Even mechanical interference with the free opening and return linkage. I'm not an expert there.

On the electrical and electronic side, there are sensors that adjust idle speed in relationship to temperature. That could be a problem. A failed or failing warm idle sensor could think that the engine is cold when it is warmed up and cause idle to jump up.

Just to have piece of mind, I'd even do the simple task of checking to see if the air filter is dirty. Not likely the problem, but if the engine is struggling to pull air through that filter at low rpm, the ECU could be causing a rough erratic idle.

Hopefully others who have actually experience your issue will jump in.

 

[Darth Vader]

I think you mean 20 inches of Hg, not 20 mm, which wouldn't be alot :^)

Anyway, rolling idle not caused by vacuum leaks....MAF sensor dirty, TPS sensor errors, clogged EGR or TB coking, internal engine damage like a blown head gasket. Most of these effects would be constant, not intermittent.

You really need to go over this car for any possible source of leakage, both in the vacuum lines and PCV systems and the intake tract from air filter to throttle body. If all's well, you need to drop codes on this thing first and see if there are any.

Are you just dropping in the clutch at highway speed and letting the engine spin down to idle on its own, rather than running it down on the gear? If so, don't do that, as a practise.

 

[MSMS3]

Yes. 20 inches. Brain lapse. Long week. Good advice to OP.

 

[MS3 GT]

this happaened after i used the seafoam(i put just a lil over half can, recommended 1/3 of can), but there is no CEL? i dont have any money to spend on my car, my businese went down awhile ago, now im totally a broke ass!!!im try to clean the maf sensor 2moro and retight everything, hope its nothing serious!
thanks for the advise!!!!

 

[MS3 GT]

i been doing that all the time, it never happened be4 when i just dropped the clutch and let the engine spin down itself, im newb to car, its this something bad for the engine??

 

[SHURIK]

I have the same issue. Its weird. It slightly bounces in neutral when going faster....

 

[Darth Vader]

It's just not good driving practise to freewheel to lower speeds. What if you suddenly need power for an avoidance maneuver? You got an engine at idle and gear speed way higher so, you jam it in a gear, it speeds up on the synchro, unsettles the chassis and the obstacle is upon you before you can calm everything down.

Why this freaks out the ECM a touch is that it isn't expecting you to go from highest possible vacuum (deceleration) to idle in an instant. Yeah, that could stall or wobble the idle on any engine.

 

[MS3 GT]

CEL came on today,got it scanned at auto zone, p2187 system to lean at idle bank 1......

 

[Darth Vader]

That's a vac leak post maf, virtually certain. Check your intake coupler and TIP and the associated connections first. The system is seeing too much air after it's calculated the amount expected.

 

[MS3 GT]

retright everytihing but still doing the same thing???