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Runs bad when engine warms up

Tiny
DIRTDIVER1970
  • MEMBER
  • 1991 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 230,000 MILES
When the engine temperature gets around 160 degrees the truck will shift gears at very low speed. It will go into overdrive at 45 mph and have a tremendous loss in power. It will not down shift when climbing a steep incline at low speed unless you stomp the gas pedal or down shift manually. It will spark knock and smoke when accelerating. I have changed out the following parts:
Map Sensor
EGR Solenoid
EGR Valve
Oxygen Sensor
Coolant temperature Switch
Coolant Temperature Sensor
Ignition Coil
Ignition coil module
Throttle position sensor
Distributor Cap
Rotary button
Fuel injectors
Throttle body rebuild kit
Transmission filter
If I take the thermostat out it runs and shifts good. Any help will be greatly appreciated.
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Saturday, November 9th, 2019 AT 4:03 PM

7 Replies

Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
Lets see if we can help you out.
First have you plugged a scanner into it to see if there are any codes stored?

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Rich
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Saturday, November 9th, 2019 AT 8:58 PM
Tiny
DIRTDIVER1970
  • MEMBER
  • 4 POSTS
I have not taken it to a mechanic for that. I did try the local auto parts stores but they can not hook into one this old. Also my service engine bulb is not working so I cannot check the flash codes.
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Sunday, November 10th, 2019 AT 7:43 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,758 POSTS
No scanner needed on this truck. look at the guide all you need is a small piece of wire to activate the code reading. It sounds like an engine coolant temperature sensor could be shorting out when warm.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Please run down this guide and report back.
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Wednesday, November 13th, 2019 AT 10:03 AM
Tiny
DIRTDIVER1970
  • MEMBER
  • 4 POSTS
I have replaced both, the coolant temperature sensor and coolant temperature switch. I'm going to replace the bulbs in the instrument panel tomorrow so that I can check the flash codes.
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Thursday, November 14th, 2019 AT 12:51 PM
Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
Okay, let us know.
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Thursday, November 14th, 2019 AT 5:37 PM
Tiny
DIRTDIVER1970
  • MEMBER
  • 4 POSTS
I got the engine light working. I get code 32- EGR circuit error. I have a new EGR solenoid and valve and vacuum lines. I found a few test on YouTube for these parts and they appear to be good. The engine light is not staying on all the time and on some trips it does not come on at all, but it still runs bad when the engine warms up. I read that this could mean the computer is going bad.
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Sunday, November 17th, 2019 AT 3:02 PM
Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
EGR wouldn't give you the issues you are having.

Let me ask. Do you have a 4l60 or a 700R4 transmission? Because if you have a 700R4 a detent cable issue would cause all this. Either broken or mal-adjusted.

Rich
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Sunday, November 17th, 2019 AT 7:40 PM

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