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Discussion Starter #1
hi guys ,my 85' C4 has an orrible wobbling idle .i set it at factory specs but when it goes in closed loop RPMs go up and down from 600 to 1000 RPMs.Sometimes when idle drops down it goes to 400 rpm and engine almost stall...I've cecked for vacuum leaks and intake manifold gasket leaks.If i disconnect my IAC motor,idle increases at 1200 rpm but becomes steady...Today cecking my timing with my new inductive gun i discover disconnecting EST my idle becomes steady at 600-700rpm,just a little up and down from 600-to 700 .If i reconnect EST idle becomes wobbling again from 600 to 1000 rpm.This is a long story for me but i hate that up and down, specially when you put in Drive at a light stop and 1000rpms make that bang in your transmission...Any suggestions?

 

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I think it a bad IAC valve.

I was fighting to get my 86 to run right for a while now my sounded like your untill I swaped out the $34.00 IAC valve.
 

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Discussion Starter #3
Thanks Marv but my IAC is new,my throttle was cleaned very well, MAF sensor has no more than 3000 miles,and this condition occur in closed loop with a warm engine at 185 F,i've seen disconnecting EST idle becomes steady,around 500-600 rpm,so i realized that is EST retarding and advancing my timing...may be or have sense?
 

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Are you running Headers After I put my full lenght headers on my Vett I had to go with a Heated O2 sender it was getting to cold and making the car surge.

Have you checked the engine temop senders I read if there bad the car can run to rich or to lean also.
 

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Discussion Starter #5
Yes Marv i run Headmann long tube,but my idle was bad before too with the stock exaust manifold,probably i'll go with a 3 wire unit but i think it don't fix...it seems impossible to fix...:huh:
 

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Yes Marv i run Headmann long tube,but my idle was bad before too with the stock exaust manifold,probably i'll go with a 3 wire unit but i think it don't fix...it seems impossible to fix...:huh:
Here's the BOSCH Part # 13077 for there 3 wire O2 I ran one wire to the Fuel pump Relay the other to ground and the O2 sender wire it self.

I also picked up a 3 wire weather pack conector of e-bay for Cheep.
 

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Bona Sera,

I have had the same issue on a 91 L98 and there were two things that on mine have solved a lot of things and it all started with an idle like yours.

- The distributor gear had a lot of play and was a little bit wore out.

- The one that really was bad was my timing chain was completly streched and at idle it makes an awful banging noise when it tries to idle low, then knock sensor detects and triggers idle to go up again and so on and so forth...
So if you have never changed or suspect previous owner hasnt check it because mine was the single pin and it was terribly bad.
 

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The first thing I thought of was a bad TPS sensor then the O2.

I've had a TPS sensor on another car pass an ohm sweep test just fine until I applied 5v to it, then it failed.

Another thing that comes to mind is bad temp sensor.
 

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Are you running rich at all? Noticing any smoke out the tail end? Any Codes?

I am wondering if it is a Engine Coolant Temp Sender like realm stated.

Seems like your ECM is trying to adjust it self when the idle drops.
 

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Discussion Starter #10
Yes i'm sure the up and down is by a timing correction ,indeed if i disconnect EST,running with base timing, my idle becomes steady at 600,so now question is what cause this timing correction...

Juanito i can try to disconnect my knock sensor to ceck if are some noises that cause timing correction...
 

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Well,

it seems like your sitation is similar to what mine was:

New: TPS, MAF, IAC, ECM, No vacuum leaks, no codes, new coil...

All that and i bought the Bosch heated O2 sensor and did nothing... ( I have no headers on though...)

Does the car not run very well above 4000 RPM? because that was another of my symptoms...
 

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Discussion Starter #12
Engine seems to run fine at high rpms,but, after a long storage period (10 months) and a recently installed set of long tube, i feel my engine not very responsive ,i was expecting a big difference with my new headmann set up,but it seems there's a lost of torque at bottom end... i lost my timing tab at dumper, so my inductive light doesn't tell me too much,only thing i can say when i rev engine at 600 rpm and when i give some 1500 throttle i have a 3cm advance of marker,and marker jumps 3 cm from 600 rpm at 1500 rpm...the same when engine is idling in park it jumps up and down from 500 to 1000 rpms...if i put in D with parking brake it revs at 600 rpm steady...i'll take a new vid with my light gun at dumper...
 

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Try disconnecting the knock sensor.

Did you reset the idle according to the book.. there is a sequence for setting base idle.
Car off, set TPS volts to .55.. Make sure you don't tighten it too tight. check that the back tab on TPS releases when you "slap" the butterfly valve around.
Turn key forward to accessory with ALDL ground inplace for 30 seconds to seat the IAC. With key still forward disconnect IAC.

Trun off power, disconnect ground and set base timing with EST wire by firewall disconnected. turn off car, reconnect est green wire.

start car and check idle.. once Good disconnect battery to clear codes.

IF still problems, look into distributor gear being worn as recommended.

start all over again.



Check all the connectors on items you replaced-- double checking the pins for bent broken items. also look at the wiring going into the connector from the back closely.. It may look solid but upclose you can see the wire is stretched or more "wiggly" than the others in the connector. I had several off these on my 86!!

vacuum-- PCV, hoses, vacuum can, brake booster.. etc.

Fuel pressure steady and no bleed down?

EGR working correctly?
 

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Discussion Starter #14
thanks blkvette94,yes i set idle at factory specs using fsm, ideed, as you can see in my video, my digital voltmeter says 0.56 volt...and yes i have some play in my distributor gear,and i think it make me loose some advance...as i told,i've lost my timing tab and, at the moment, i can't set my base timing at 6 degrees and i can't compensate my worn dist gears giving some advance becouse no reference at the dumper...My EGR is disconnected,my vacuum seems to be ok,f i disconnect hose engine starts to rev high...fuell pressure is ok,i ceck it with a pressure gauge.
I think it's a ECM correction,my service belt is a little noisy i don't know if some noises coming from front of the engine can cause a knock sensor feedback and EST retard timing...
 
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