Diagnose 1st Replace parts second

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Ricko1966

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This isn't aimed at anyone so nobody take it personal. It's just good advice. So a post goes up. Getting bad milage, runs horrible I changed my carbureator twice,put on a FP regulator, 1st one leaked so installed another one, still runs bad, I replaced my cap,rotor,wires,now runs like crap,searched for vacuum leaks, pulled off all my smog equipment,still runs bad. Now let's jump to the end results. Well turns out the fuel level and fuel pressure were wrong on the carbureator, I found I crossed 2 plug wires and 1 cracked a plug insulator, deleting the egr and esc, caused detonation problems,on and on and on. Yeah finally figures out my distributor wasn't advancing. Now if this were a real situation,this guy did all kinds of extra work,spent lots of extra time and money,and created all kinds of new problems to diagnose,because he didn't diagnose in the first place. If you don't know how to diagnose,ask. Just because you threw a code that doesn't mean that part has failed,it is possibly a symptom of another problem. Replacing an O2 sensor that has a break in the car side of the harness doesn't fix anything. Putting a 5lb fuel pressure regulator on a car that only has 3 lbs of fuel pressure doesn't do anything,changing good plug wires,doesn't do anything. You'd sue your doctor for removing a healthy,arm,leg,lung,kidney,or your mechanic for changing a good ecm, transmission,or speed sensor, but you think nothing about doing it to your car.
 
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fast 99

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Agree it seems to happen a lot. Vehicle needs basic items checked before tossing parts at it. From years of doing diagnostics, it's rarely one failure.

Something as simple as a vacuum advance leaking could cause poor idle, power and fuel economy. Checking vacuum is another missed item. There are codes that point directly to the problem, 02 heater [still needs to be confirmed] and those that do not lean code.
 

DrvnDrvr

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One of the first things I installed in my square was a vacuum gauge to tell me if a leak was occurring. Took out a lot of guessing.

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GTX63

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Gas mileage sucks and it runs like a dog down the road? Check centrifugal advance. It is easy to check if it is rusted, binding or the springs are loosey goosey.
It wasn't a problem on a small block Chevy for the first twenty years or so, but with all of these barn finds and "WillIt Start?" YouTube videos, it is now.
 

RustyPile

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This isn't aimed at anyone so nobody take it personal. It's just good advice. So a post goes up. Getting bad milage, runs horrible I changed my carbureator twice, put on a FP regulator, 1st one leaked so installed another one, still runs bad, I replaced my cap, rotor, wires, now runs like crap, searched for vacuum leaks, pulled off all my smog equipment, still runs bad. Now let's jump to the end results. Well turns out the fuel level and fuel pressure were wrong on the carbureator, I found I crossed 2 plug wires and 1 cracked a plug insulator, deleting the egr and esc, caused detonation problems, on and on and on. Yeah finally figures out my distributor wasn't advancing. Now if this were a real situation, this guy did all kinds of extra work,spent lots of extra time and money,and created all kinds of new problems to diagnose,because he didn't diagnose in the first place. If you don't know how to diagnose,ask. Just because you threw a code that doesn't mean that part has failed,it is possibly a symptom of another problem. Replacing an O2 sensor that has a break in the car side of the harness doesn't fix anything. Putting a 5lb fuel pressure regulator on a car that only has 3 lbs of fuel pressure doesn't do anything,changing good plug wires,doesn't do anything. You'd sue your doctor for removing a healthy, arm, leg, lung, kidney, or your mechanic for changing a good ecm, transmission, or speed sensor, but you think nothing about doing it to your car.
All well and good, Rick.. I agree with that sage advice wholeheartedly.. So many people just start throwing parts at a problem, hoping something will stick.. All us experienced mechanics know that's the last thing to do.. But the advantage those professional mechanics, both active and retired, have is: We KNOW how the various systems actually work.. Therefore we know WHEN AND WHICH system/component isn't working as it should.. I can't count the number of times I see someone tackle a carburetor when the actual problem is a wet distributor cap, or something else in the ignition system is the actual problem..

The worst advice an inexperienced person can get is bad and erroneous advice from a similarly inexperienced person.. Neither doesn't know it's bad advice, so down the rabbit hole he goes.. Throw in the fact that very often good and accurate advice goes totally unheeded, and the person with the problem has now compounded that problem..........
 

Radiohead

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Hence my apprehension when it comes to answering questions over the internet. Too easy to get f'ed off in the noise.
 

viaminveniam1939

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Hi Guys -

Per your suggestion…

1986 Chevy K20, 5.7L 350, 3-Speed. Holley Carb. FYI - I am new to this website (not quite sure where to post this?). New to my Squarebody. I consider myself handy, but my understanding of automotive systems -in general- is still very much evolving.

I had her idling for about 7 mins (no issues for ~3/4 of a year now) and then suddenly, she just shut off (did not notice any struggle from empty fuel tank). Now she won’t light. She cranks & the battery is good. Initially, I thought I MAY have run out of gas (in the middle of swapping out a fuel tank). Added some fuel and changed the fuel filter (was going to do it next week regardless). I also checked to see if the mechanical fuel pump is pumping (appears to be doing so). Assuming its a spark/electrical matter now?

Not quite sure what the next most logical diagnosis step(s) are (though I have some thoughts). Any guidance would be greatly appreciated.

Thank you :)

Nick
 

Ricko1966

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Hi Guys -

Per your suggestion…

1986 Chevy K20, 5.7L 350, 3-Speed. Holley Carb. FYI - I am new to this website (not quite sure where to post this?). New to my Squarebody. I consider myself handy, but my understanding of automotive systems -in general- is still very much evolving.

I had her idling for about 7 mins (no issues for ~3/4 of a year now) and then suddenly, she just shut off (did not notice any struggle from empty fuel tank). Now she won’t light. She cranks & the battery is good. Initially, I thought I MAY have run out of gas (in the middle of swapping out a fuel tank). Added some fuel and changed the fuel filter (was going to do it next week regardless). I also checked to see if the mechanical fuel pump is pumping (appears to be doing so). Assuming its a spark/electrical matter now?

Not quite sure what the next most logical diagnosis step(s) are (though I have some thoughts). Any guidance would be greatly appreciated.

Thank you :)

Nick
Nick you should start a thread specific to your truck,we can devote to just your truck. Have a great day.
 

CheemsK1500

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The internet is a curse and a blessing for mechanic advice. For example, anytime a Q jet related question arises, some will respond in a helpful manner, but many self-proclaimed "geniuses" will tell you to throw it in the trash and a get a Holley because they refuse to believe a Q jet is a perfectly good carburetor when set up correctly.
 

RustyPile

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Nick you should start a thread specific to your truck, we can devote to just your truck. Have a great day.
I second that motion.. What you're doing is referred to "hyjacking a thread".. Sort of like butting into an ongoing conversation and changing the subject. You're new here and this is your first post, so as for me, you're forgiven.

@viaminveniam, go to the 'INTRODUCTION" section and start a new post and introduce yourself, you'll receive lots of "howdys" there.. Then go to the TECH DISCUSSION forum and start a new thread, post your questions there and you'll receive lots of help.
 

viaminveniam1939

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Roger that! Thank you all.
 

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