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1994 Caprice Classic: a check engine light..run rough..stalls

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I have a 1994 Caprice...
I have a 1994 Caprice Classic (5.7). After running fro between 25 to 45 minutes the car get a check engine light and begins to run rough and finally stalls. I have changed the Distributor, Cap and Rotor, MAP Sensor, Ignition Module, and Coil, The plugs and wires are also new. When I allow the car to cool down it runs fine again. Any ideas?
Submitted: 6 years ago.Category: Chevy
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Answered in 10 minutes by:
4/28/2011
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
Dave Nova
Dave Nova, Chevrolet Technician
Category: Chevy
Satisfied Customers: 19,452
Experience: ASE Master Certification. GM World Class Certification
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Hello and thank you for using JustAnswer. My name is XXXXX XXXXX I will do my best to help you with your Chevy problem.

Have you checked the computer for trouble codes?
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Customer reply replied 6 years ago
I am unable to retreive and trouble codes. I have the appropriate code reader but it does not flash any codes with the ignition switch turned on but not started.
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
When the engine stalls, will it not restart until it cools down?
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Customer reply replied 6 years ago
The Engine will restart but the Check Engine Light is still on and it runs rough. If I allow the car to cool completely again it runs fine. If I keep the engine RPM elevated I can drive the car to get where I need to go but once I stop it will stall.
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
After the engine is warmed up and having problems, test fuel pressure with fuel pressure gauge. You should have 41-47 psi. Let me know what kind of reading you get.
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Customer reply replied 6 years ago
OK I will check the fuel pressure once the car is warmed and get back with you
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
Sounds good.
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Customer reply replied 6 years ago
Dave,

I there is a huge thunderstorm here now. I will have to wait until it is over. I will get back with you with the fuel pressure readings. How long are you available today?
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
I will be here all day even into the evening hours some. Just let me know when you can.
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Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
One more thing I'd like you to check when you get a chance is to unplug the vacuum hose from the EGR valve at the back of the manifold to see if this makes a difference.

Let me know what you find.
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Customer reply replied 6 years ago
Hi Dave,

OK I finally got a chance to check the Fuel Pressure. Key on Not srtated it reads 43 psi. Engine warmed up and Check Engine Light on it drops to 37 PSI and remains steady. From what I have read in the ALL DATA manual that is normal.

Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
You should have 41-47 psi with the key on,engine off. 37 psi with the engine running is okay. Did you unplug the vacuum hose at the egr valve on the back side of the manifold?
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Customer reply replied 6 years ago
Not Yet. What should happen once the vacuum hose it unplugged?

Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
Hopefully the engine runs better. The problem could be due to an egr valve that is opening prematurely or is sticking open. If this is the case then disconnecting the egr valve would cause the engine to run properly. Try it and let me know what you find.
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Customer reply replied 6 years ago
Currently the Check Engine Light is not on. I am waiting for it to come on and start running rough again. I did unplug the vacuum line anyway and thereseemed to be a very slight increase in engine RPM.
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
Yes, the vacuum leak will increase engine rpm. Let me know what happens after you have run the engine a while.
Dave Nova
Dave Nova, Chevrolet Technician
Category: Chevy
Satisfied Customers: 19,452
Experience: ASE Master Certification. GM World Class Certification
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Customer reply replied 6 years ago
OK here is what happened. THe service Engine light came on and engine waqs starting to run a bit rough. I disconnected the Vacuum Line at the EGR Valve. NO notable difference. However I did notice that if I tried to advance the Throttle quickly the Engine woudl stumble badly and try to stall. If I did a slow steady increase in the Throttle then the Engine only stumbled slightly and was able to keep running.


Customer reply replied 6 years ago
OK here is what happened. THe service Engine light came on and engine waqs starting to run a bit rough. I disconnected the Vacuum Line at the EGR Valve. NO notable difference. However I did notice that if I tried to advance the Throttle quickly the Engine woudl stumble badly and try to stall. If I did a slow steady increase in the Throttle then the Engine only stumbled slightly and was able to keep running.

Customer reply replied 6 years ago
OK here is what happened. THe service Engine light came on and engine waqs starting to run a bit rough. I disconnected the Vacuum Line at the EGR Valve. NO notable difference. However I did notice that if I tried to advance the Throttle quickly the Engine woudl stumble badly and try to stall. If I did a slow steady increase in the Throttle then the Engine only stumbled slightly and was able to keep running. I accidently clicked on the wrong button. Please continue to assist me

Customer reply replied 6 years ago
Hi Dave,

I am not sure if your still getting these messages. I accidently clicked on the wrong button. I am still in need of your assistance. Please continue to assist me


Thanks
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
Yes, I am getting your messages, sorry for the delay. Do you still have the fuel pressure gauge connected? See if the fuel pressure drops when the throttle is opened.

Also If you can get your hand back to the egr valve, see if the diaphragm is moving when the throttle is opened.

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Customer reply replied 6 years ago

Yes I still have the Fuel Pressure Guage on. The Pressure drops to 34 psiupon acceleration and then returns to 37 psi

Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
That is your problem. Fuel pressure should actually increase when the throttle is opened. Your fuel pump is not supplying enough fuel to keep up with the demand of the engine. You will need to replace the fuel pump.
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Customer reply replied 6 years ago
I see. So you suggest the Fuel Pump rather than the Pressure Regulator?
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
Yes, When you tested fuel pressure earlier it looks like the fuel pressure regulator is working to regulate the fuel pressure. The regulator will typically not fail and cause a loss of fuel pressure. 99% of the time it is the fuel pump. You can locate the rubber section of the fuel lines and crimp shut the return line (smaller diameter line) and repeat the test. If the fuel pressure increases greatly with the line crimped off then replace the regulator. If the pressure and the symptoms stay the same, replace the fuel pump.
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Customer reply replied 6 years ago
Hi Dave,

Well it looks like it tested the way you thought it would. I appreciate your assistance. I will be dropping the Fuel Tank later next week. Thanks again
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 6 years ago
You are welcome. Thanks for using justanswer.
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