Help with engine diagnosis
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New User
Joined: Nov 2007
Posts: 5
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Help with engine diagnosis
I just bought my 85 and the engine was not hooked up completely. I have now plugged in all computer components and ran all new vaccum lines and the truck is starting to idle a lot better. It is still a little rough at times so I know I still have a little vac leak due to whistle.
Here is my problem. After driving the truck for about 3 miles or more when I shut it off I get a tremendous backfire through the carb. It sounds like a shotgun. What could be causing my problem? I have put in new plugs and changed the wires and cap and rotor. Could it be a cylinder problem or timing?
Any help appreciated.
Here is my problem. After driving the truck for about 3 miles or more when I shut it off I get a tremendous backfire through the carb. It sounds like a shotgun. What could be causing my problem? I have put in new plugs and changed the wires and cap and rotor. Could it be a cylinder problem or timing?
Any help appreciated.
Postmaster
Joined: Dec 2006
Posts: 3,603
Likes: 10
From: Spokane, WA
If you don't have a cheap vacuum gauge, get one and use it too...you can still diagnose a lot of problems with one without a big $$ investment in electronic tools. Get your vacuum leak fixed and then check the timing. Get that good and then you can tune in your carb with the vac gauge to get the highest steady reading at about 650 rpm by going back and forth between the idle mix screw and the idle speed screw.
Senior User
Joined: Jul 2003
Posts: 208
Likes: 1
From: Meggett SC
If your truck is an 85 it probably has the TFI ignition. With this setup the computer sets the idle based on engine temperature. It adjusts the throttle with a little actuator on the side of the Carb. You can tell if its working because you can hear it run everytime you shut the motor off. After you turn the motor off it goes the the home position. The engine has a temp sensor for the computer in the outlet of the thermostat housing. With the engine running ( warmed up)remove the plug from that sensor and you should hear the engine speed up to a very fast idle. Replace the sensor plug and it should return to normal idle speed. If this is not working correctly the truck will idle at too fast of a speed and when you shut it off it will either run backwards of it will backfire.
