CHANGED HEADS ON 2.8L motor back fires when cranking engine 4 spark plugs are not burning not sure if firing order is right

Have you checked the spark plug leads are in the correct firing order? If they are set up wrong the plug will fire at the wrong moment in the timing sequence causing issues.
If you still have the heads off turn the crank until you can see that piston 1 is at Top Dead Centre then check the timing mark is correctly positioned. If the Heads are back on you can remove ONLY the spark plug on cylinder 1 and use an extra long screwdriver in the spark plug hole. HAND crank the engine until the screwdriver stops rising. Check you Crank timing mark and Cam timing marks are is where they should be.

theres no distributor and the firing order for your 3.8 is 1-6-5-4-3-2-the following link shows the coil pack
Buick General Motor 3800 V6 Engine Firing Order DIS IDI ignition timing 3.8 Buick Engine Coil Pack Firing Order Spark Plug Wire Location Which way do the spark plug wires go on & what cylinder is what?
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For the Buick V6 231 3.8 liter turbocharged engine:
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Answers!
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Firing order:
1 - 6 - 5 - 4 - 3 - 2.
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Spark plug wire location on coil pack:
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Closest to the back of the car, left to right: 1 - 5 - 3.
Closest to the front of the car, left to right: 4 - 2 - 6.
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Spark plug wire location on engine:
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drivers side, front to back of car: 1 - 3 - 5.
passenger side, front to back of car: 2 - 4 - 6.
.----and this link shows the coil pack to cylinders
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3 8 Buick Turbo Engine Coil Pack Firing Order Spark Plug Wire Location

Won't crank or won't start? If all you did was change the plugs, chances are the plug wires got crossed up. You didn't state the size of your engine. On the 5.2 and 5.9 engine the firing order is 1-8-4-3-6-5-7-2. If this is not your engine size. Just google the firing order for your engine size.

Okay, on the photos I am posting, pick the size of motor that you have....since you didn't post it. When you find the right photo of your motor, the numbers on the engine correspond to the number of the spark plug that goes to the coil...1 to 1, 2 to 2 and so on. This is your firing order and will ensure you have the proper spark plug firing from the correct point:



1FZ-FE engine Firing order: 1–5–3–6–2–4Distributor rotation: counterclockwise



5VZ-FE engine Firing order: 1–2–3–4–5–6Distributorless ignition system



2RZ-FE and 3RZ-FE engines Firing order: 1–3–4–2Distributor rotation: clockwise



2RZ-FE and 3RZ-FE engines Firing orders: 1–3–4–2Distributorless ignition system



2UZ-FE engines Firing order: 1–8–4–3–6–5–7–2Distributorless ignition system

You didn't post your engine size so please pick the applicable motor:






Firing order and spark plug wire installation for the 2.5L engine





Firing order and spark plug wire installation for the 2.0L engine

It is stamped on the top of the engine, in the head. It is usually pretty easy to find, and you wont have to remove any pieces to see it.

3.0L (2V) = 1-4-2-5-3-6

3.0L (4V) = 1-4-2-5-3-6

3.9L V6 Engine. Engine Firing Order: 1-6-5-4-3-2. Distributor Rotation: Clockwise.............did i guess the right motor? let me know/rate me

If it is a 2.5L dohc motor the firing order is 1-4-2-5-3-6  cyls. numbers from left to right 4-5-6 in front and 1-2-3 in rear thats looking at the motor from the front of vehicle.

that your neighborhood mechanic didnt gap the spark plugs if they are not gaped right then the car will not run at all. and also why only change 4 of the 8 you are supose to do them all and a nother thing check the spark plug wires make sure they are connected to the right spark plug the check the dist. cap make sure the cap is not faulty