Stutter under Part Load
Thread Starter
6th Gear
Joined: Jun 2013
Posts: 6
Likes: 0
From: Wien
Stutter under Part Load
Hi everyone!
I recently bought a 1988 Corvette Coupé with the 4+3 manual transmission. It's in lovely condition and only has 33,000 miles on it, but it has one serious issue:
When cruising at constant speeds, especially around 1000-1500 rpm, a strange stutter occurs. As soon as I start accelerating gently, it stops instantly. After cold start everything is perfect. But
the warmer the engine becomes, the heavier the jolts are.
Until now practically everything related to the engine has been replaced: New spark plugs, wires and capacitor, new temperature sensor, new injector nozzles, new ECM ... nothing worked.
American cars are very rare in Europe and therefore it's difficult to find a practiced mechanic for a Corvette. So you really are my last hope!
Thanks in advance for any ideas!!
I recently bought a 1988 Corvette Coupé with the 4+3 manual transmission. It's in lovely condition and only has 33,000 miles on it, but it has one serious issue:
When cruising at constant speeds, especially around 1000-1500 rpm, a strange stutter occurs. As soon as I start accelerating gently, it stops instantly. After cold start everything is perfect. But
the warmer the engine becomes, the heavier the jolts are.
Until now practically everything related to the engine has been replaced: New spark plugs, wires and capacitor, new temperature sensor, new injector nozzles, new ECM ... nothing worked.
American cars are very rare in Europe and therefore it's difficult to find a practiced mechanic for a Corvette. So you really are my last hope!
Thanks in advance for any ideas!!
Race Director
Joined: Dec 2006
Posts: 10,383
Likes: 4
My 2¢
Any fluctuation of the volt meter during this stuttering?
Does your brake power booster make a hiss noise when you apply brakes?
Have you cleaned the EGR valve and its associated plumbing?
Remove the power tube (flex air intake hose) from the throttle body and work the throttle from closed to open. Does it move smoothly? are the round holes that hold the axel of the throttle plate still round or are they oval?
Have you cleaned the idle air controller?
In general it sounds like to me you've got some un metered air entering the system.
Any fluctuation of the volt meter during this stuttering?
Does your brake power booster make a hiss noise when you apply brakes?
Have you cleaned the EGR valve and its associated plumbing?
Remove the power tube (flex air intake hose) from the throttle body and work the throttle from closed to open. Does it move smoothly? are the round holes that hold the axel of the throttle plate still round or are they oval?
Have you cleaned the idle air controller?
In general it sounds like to me you've got some un metered air entering the system.
Thread Starter
6th Gear
Joined: Jun 2013
Posts: 6
Likes: 0
From: Wien
Thanks for the quick answer! The power brake booster is quiet and the EGR valve is fine. I already cleaned the IAC valve and when doing that the throttle seemed perfectly alright. But additional air could be the reason.
Is it enough just to replace the gaskets on the throttle body or do you think that a major leak is necessary for this problem? Is there a way for me to verify your assumption?
Is it enough just to replace the gaskets on the throttle body or do you think that a major leak is necessary for this problem? Is there a way for me to verify your assumption?
Race Director
Joined: Dec 2006
Posts: 10,383
Likes: 4
Thanks for the quick answer! The power brake booster is quiet and the EGR valve is fine. I already cleaned the IAC valve and when doing that the throttle seemed perfectly alright. But additional air could be the reason.
Is it enough just to replace the gaskets on the throttle body or do you think that a major leak is necessary for this problem? Is there a way for me to verify your assumption?
Is it enough just to replace the gaskets on the throttle body or do you think that a major leak is necessary for this problem? Is there a way for me to verify your assumption?
Safety Car
Joined: Aug 2009
Posts: 3,745
Likes: 275
From: Melbourne, Victoria, Australia
Hi everyone!
I recently bought a 1988 Corvette Coupé with the 4+3 manual transmission. It's in lovely condition and only has 33,000 miles on it, but it has one serious issue:
When cruising at constant speeds, especially around 1000-1500 rpm, a strange stutter occurs. As soon as I start accelerating gently, it stops instantly. After cold start everything is perfect. But
the warmer the engine becomes, the heavier the jolts are.
Until now practically everything related to the engine has been replaced: New spark plugs, wires and capacitor, new temperature sensor, new injector nozzles, new ECM ... nothing worked.
American cars are very rare in Europe and therefore it's difficult to find a practiced mechanic for a Corvette. So you really are my last hope!
Thanks in advance for any ideas!!
I recently bought a 1988 Corvette Coupé with the 4+3 manual transmission. It's in lovely condition and only has 33,000 miles on it, but it has one serious issue:
When cruising at constant speeds, especially around 1000-1500 rpm, a strange stutter occurs. As soon as I start accelerating gently, it stops instantly. After cold start everything is perfect. But
the warmer the engine becomes, the heavier the jolts are.
Until now practically everything related to the engine has been replaced: New spark plugs, wires and capacitor, new temperature sensor, new injector nozzles, new ECM ... nothing worked.
American cars are very rare in Europe and therefore it's difficult to find a practiced mechanic for a Corvette. So you really are my last hope!
Thanks in advance for any ideas!!
Also check the voltage goes to at least 4.5 volts with full throttle .
Thread Starter
6th Gear
Joined: Jun 2013
Posts: 6
Likes: 0
From: Wien
Check your throttle position sensor, with a multimeter check with engine on or just ignition on the top and middle wires should show 0.54 volts d/c if it is not set correctly it may cause your issues.
Also check the voltage goes to at least 4.5 volts with full throttle .
Also check the voltage goes to at least 4.5 volts with full throttle .
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Thread Starter
6th Gear
Joined: Jun 2013
Posts: 6
Likes: 0
From: Wien
Finally it turned out that there was an error in the ECM, as it "thought" that the engine coolant had reached temperatures of around 250° F after reaching real 120° F, therefore the gas mixture was incorrect.
After changing the ECM, things are much better, but I'm still not content with it, as there still are little jolts when cruising. Probably I wouldn't feel it if it was an automatic, but I want to stick with the original manual box. Apart from that, I just can't believe that a car that was once called on of the best cars in the world behaves like that.
After changing the ECM, things are much better, but I'm still not content with it, as there still are little jolts when cruising. Probably I wouldn't feel it if it was an automatic, but I want to stick with the original manual box. Apart from that, I just can't believe that a car that was once called on of the best cars in the world behaves like that.