Vette stumbles on acceleration
Thread Starter
Racer
Joined: Sep 2005
Posts: 297
Likes: 0
From: Wauseon Ohio
Vette stumbles on acceleration
My 85 corvette is all stock except for the entirely new ignition and fuel filter. When I take off or try to pass someone, it'll chug and stumble and even somtimes pop. I defiently notice a pattern though. It stumbles either when its cold, or it stumbles because it has been running for a bit. I dont know which one. It straightens out after its been running a while. When it chugs, it shifts at high hard RPM's. After the engine has been running and warms up, it'll shift at low and smooth RPM's, and runs great. I spose this could be my idle air control, vaccum leak, o2 sensor, maybe my mass airflow sensor, fuel delivery... I dont know. I just adjusted my timing and my TPS. I hooked a fuel pressure gauge up to it at cold idle, its at about 26 psi. Any advice is appreciated.
Melting Slicks
Joined: Jun 2004
Posts: 3,032
Likes: 4
Isn't fuel PSI supposed to be 40 when you first turn the key?
http://forums.corvetteforum.com/show...re&forum_id=48
http://forums.corvetteforum.com/show...re&forum_id=48
Thread Starter
Racer
Joined: Sep 2005
Posts: 297
Likes: 0
From: Wauseon Ohio
Thats what it says in my haynes book. This weekend im changing the fuel pressure regulator. I know the fuel pumps are easy to change in a C4, but id hate to bust the 20 year old fuel level float or somthing like that while yanking it out.
Racer
Joined: Jun 2001
Posts: 377
Likes: 10
From: Chicago IL
my 85 chugged a bit on acceleration before when cold (open loop I believe) fuel pressure was 28-29 while running. I intalled a holley adjustable fuel pressure regulator from jegs, increased the pressure to 38 and this performance problem went away.
Thread Starter
Racer
Joined: Sep 2005
Posts: 297
Likes: 0
From: Wauseon Ohio
so i was changing my trans filter and fluid and i had my hood up when i noticed a little rubber hose just limply laying on my valve covers. Turned out it was a vaccum line that went under my throttle body. 
. It cleaned up a bit, still chigs and stumbles though. Im gunna go ahead and change my vaccum valves and lines anyways.
. It cleaned up a bit, still chigs and stumbles though. Im gunna go ahead and change my vaccum valves and lines anyways.
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Burning Brakes
Joined: Dec 2003
Posts: 770
Likes: 0
From: Madison Mississippi
Have you checked the voltage on your TPS as you move the throttle? Look for a steady increase in voltage. When the TPS fails, the voltage will jump around and will send an erratic signal to the ECM. This will show up as a stumble, particularily off idle. Ask me how I know 
Burning Brakes
Joined: Dec 2003
Posts: 770
Likes: 0
From: Madison Mississippi
What voltage did you set it at? and did you check to see if the voltage increases at a constant rate or does it jump around? When mine failed, it would show an erratic reading from high to low as you increased the throttle. Oh well, just a thought, it helped my problem as you described. 
Thread Starter
Racer
Joined: Sep 2005
Posts: 297
Likes: 0
From: Wauseon Ohio
My sensor is steady. I just set my timing too. I had a problem like this before, turned out the drain hole by my gas cap was plugged. Water would set in there and seep into my gas tank. Flushed it out with a garden hose. 
Thread Starter
Racer
Joined: Sep 2005
Posts: 297
Likes: 0
From: Wauseon Ohio
A few days I noticed that when i got to fill my gas tank, when I take my cap off i can hear a hisssing (equalization of pressure). Problem solved when I unscrew my gas cap. It runs great when i do. I cant tell if its positive or negative pressure. Anyone know what the problem is? Im gunna take that big round thing that sits in my front driver side corner of the engine compartment under the light kinda, and clean that out.