vibration/shaking at idle

Westee · Sep 12, 2005

Getting a vibration/shaking when rpms fall back to idle. Shaking usually starts at about 1200 rpm on way down. Sometimes after a few seconds at idle, shaking stops. No engine light on, but havent tried to pull codes. Just cleaned MAF and installed new IAC but problem still persists.

I know the EGR is not supposed to pull vacuum at idle, but isnt the EGR supposed to pull vacuum at acceleration? Because the vacuum source on EGR valve isnt pulling vacuum at engine rev....Could this be my problem or am I way off base?

Thanks!

urban96 · Sep 12, 2005

check your balencer and see if theres any rubber sticking out anywhere

skylovia · Sep 12, 2005

i believe egr only operates under load, when you just sit there revving, your not under any load. one way to check your egr to see if it is working is to actually apply vaccum to the egr solenoid, just take the vaccum hose off it, and link it to another source of vaccum. if you car bogs, then it is working properly, if not, then you may have a stuck open egr, by the way, does it run krappy just at idle, or at cruise too?

Westee · Sep 12, 2005

it really only does this when the rpms are falling back to idle. the car runs fine at cruising speeds...What would be symptoms of my EGR stuck, cuz thats what I think it might be....

Westee · Sep 12, 2005

and would this cause the car to vibrate and shake like that??

urban96 · Sep 12, 2005

also have u checked for any vaccume leaks?

Westee · Sep 12, 2005

Havent checked for vacuum leaks. What is the best way to do that?

mo_dingo · Sep 12, 2005

The car doesn't have much vacuum when accelerating, so that previous argument doesn't make sense.

The EGR works ONLY when all 4 of these are true;

1. The ECT sensor is telling the computer that the engine is at operating temperature.

2. The TPS indicates that the throttle is partly open. If closed, or fully open, the EGR is disabled.

3. The MAF sensor indicates a load higher than idle.

4. The EGR control module timer has completed it's countdown.

The reason the EGR isn't used at idle and warmup is b/c there isn't much output of nitrogen oxides and it also causes rough idle. And it's also shut off at WOT b/c it causes a power loss, and this state isn't considered the normal operating state.

Easy way to find a vacuum leak. Get a vacuum gauge and test it. Stock motors usually put out 22inches of vacuum and it's very steady, without fluxuation.

You should check your harmonic balancer, it may be loosing it's guts.
Scott

1PissedOffCobra · Sep 12, 2005

Been There!

Dude! I got your cure! Just like Urby said, its probably your balancer. Mine was doing the EXACT same thing. I picked up a 1994 Cobra(#1882 with 75k miles) for for $5,000 last week because it was doing the same $#%! and the guy thought the car had serious problems. I swapped out the balancer ( and plugs and alternator while I was gettin my hands dirty) and is running like it did in 94'. I was able to pick up a Dorman Balancer from o'Reily for $69. Remember to rent the puller while your there too.

Westee · Sep 13, 2005

Thanks fellas...I'll look into the harmonic balancer!

a50sn95 · Sep 13, 2005

urban96 said:
check your balencer and see if theres any rubber sticking out anywhere

Another vote. Just change it. If it's original, change it now. Money in the bank....
It's like a grenade with the pin pulled. Sooner or later, something will slip, and you don't want to be near it..

CManT1914 · Sep 13, 2005

I know it's already been said but..............

.........balancer!

Methodical · Sep 13, 2005

Now!!!!!

Westee · Sep 13, 2005

My car has 126k on it and the balancer has never been changed. I'll get a new one this week. Any particular brand?

CManT1914 · Sep 13, 2005

Westee said:
My car has 126k on it and the balancer has never been changed. I'll get a new one this week. Any particular brand?

I got an FRPP one from summit, it cost $70. A lot of guys like the PP ones that 50resto carries, I think they cost about the same. Then others will say only Romac, but not everybody has the $$ for a nice Romac.

a50sn95 · Sep 13, 2005

No reason for big buck on a stock shortblock. Stock replacement should be fine.
I have the Pioneer SFI, but they make a non-sfi, too.
Dennis

yellowstang1994 · Sep 13, 2005

Damn a '94 Cobra 75k miles for 5 grand? YOur making me feel like **** now. I bught my '94 GT last year with 94k for 6 grand.

1PissedOffCobra · Sep 13, 2005

Ya, it actually belonged to my Brother In Law. He kinda needed the money and wasn't driving it cus he thought it had engine problems. Turns out it was a $70 fix with the balancer. So now now I got some cash to exploit this 94' cobra's stockness. I went ahead and called www.bieberfever.com and ordered AFR 165's a Comp X-Treme energy 215/224 .533 bumpstick (built to perform with SC) , rockers and BBK headers. I also have a Procharger P1SC, but its set @ 2 O'Clock, so I need to 12 have it turned by P/C. I also need the mounting brackets, FMU and tubing. Any ideas of where I can pick this stuff up used, or for pretty cheap? Maybe I should ask that somewhere else......?

"I was so broke growing up, If I wasn't a boy, I wouldn't have had anything to play with"
-Rodney Dangerfield-

CManT1914 · Sep 13, 2005

1PissedOffCobra said:
...but its set @ 2 O'Clock, so I need to 12 have it turned by P/C.

Huh? What do you mean it's set to 2? I've never heard of this before.

1PissedOffCobra said:
"I was so broke growing up, If I wasn't a boy, I wouldn't have had anything to play with"
-Rodney Dangerfield-

that's wrong...

danimal_08 · Sep 13, 2005

Your problem

Well, i had the same problem with my '94 and it ended up being the balancer. It would rattle at 1200 rpm all the way through about 4500 and the shifter rattled bad when you got on it. Check the rubber wedge and make sure the outer ring of the balancer hasn't slipped. Good luck

vibration/shaking at idle

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