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L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

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Old 07-15-2008, 10:24 AM   #1
c4cruiser
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Default Stumble while decelerating

I've noticed lately that my 87 has a bad stumble when decelerating at low RPM's. For example, if I start to slow down heading towards a traffic light or stop sign, I take my foot off the gas and let the engine slow the car. Once I get to around 1100-1200 RPM, there is a sudden loss of engine braking as though the throttle plate was forcibly closed to the point whre it feels like the engine stops.

It doesn't stop, but it's more like a immediate drop from that 1200 RPM to about 400 RPM. The engine doesn't run rough, but just this big bog or stumble.

Ideas?
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Old 07-16-2008, 08:23 PM   #2
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ttt.

Anyone have any ideas??
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Old 07-16-2008, 11:27 PM   #3
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I have an 85 4+3 with the same condition. It never stalls but it does help if I don't wait so long to push the clutch in when coming to a stop. I'll be glad to hear any suggestive hints also.
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Old 07-17-2008, 01:16 AM   #4
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Default bog

sounds like the EGR valve is sticking open.
Wait till the car is cold, see if you can feel underneath the manifold plenum where the egr valve is, and see if you can move the diaphragm with your finger tips.
You have to use both hands, get close, with your legs on each side of the suspension control arms and kinda sit on the tire. If it moves easy, it's probably all right. If it's sticky or ratchety it might just need cleaning.
You might have to move the plenum out of the way to get a better feel for it.
1 hour to remove and replace the plenum if the gaskets let go easy. When installing a new gasket, spray the gasket with a graphite spray, or Pam cooking spray or a copper based gasket spray
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Old 07-17-2008, 09:02 AM   #5
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Quote:
Originally Posted by c4cruiser View Post
I've noticed lately that my 87 has a bad stumble when decelerating at low RPM's. For example, if I start to slow down heading towards a traffic light or stop sign, I take my foot off the gas and let the engine slow the car. Once I get to around 1100-1200 RPM, there is a sudden loss of engine braking as though the throttle plate was forcibly closed to the point whre it feels like the engine stops.

It doesn't stop, but it's more like a immediate drop from that 1200 RPM to about 400 RPM. The engine doesn't run rough, but just this big bog or stumble.

Ideas?
actually the IAC works as a dashpot on deceleration. When you release the gas pedal the throttle blade slams shut. The IAC opens to let air in the engine so it can decelerate smoothly. Check the IAC.
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Old 07-17-2008, 09:49 AM   #6
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Thanks!!

Would a bad IAC also cause a cold idle surging? When I start the car when it's cold, the idle will be fine for the first 20 seconds or so and then the idle RPM's begin to surge. The idle speeds will change from 700 RPM to 900 RPM and back for about a minute before the idle smooths out and the engine warms up. This only happens when cold.
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Old 07-17-2008, 10:27 AM   #7
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Thanks!!

Would a bad IAC also cause a cold idle surging? When I start the car when it's cold, the idle will be fine for the first 20 seconds or so and then the idle RPM's begin to surge. The idle speeds will change from 700 RPM to 900 RPM and back for about a minute before the idle smooths out and the engine warms up. This only happens when cold.
it could, when the engine is cold the IAC needs to work to help with mixing of air and fuel especially when its in the enrichment mode.. when warm the IAC still is needed at Idle but a slow or inop IAC will be more noticeable on decel and under small loads at Idle. Check the IAC per FSM
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Old 07-17-2008, 01:29 PM   #8
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Quote:
Originally Posted by FICINJECTORS View Post
actually the IAC works as a dashpot on deceleration. When you release the gas pedal the throttle blade slams shut. The IAC opens to let air in the engine so it can decelerate smoothly. Check the IAC.
Check that magazine article on TPI Tunin'... There was a reference to acceleration enrichment, AND, 'DEceleration enLEANMENT'. I don't remember the process exactly, but maybe it's gone awry on his car (injector problem???)?
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Old 07-17-2008, 01:29 PM
 
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