Why does my grand national backfire while driving 55 miles per hour on the highway?

Asked by May 30, 2012 at 08:59 PM about the 1984 Buick Grand National

Question type: Maintenance & Repair

1 Answer

13,525

Backfiring is usually a timing issue. Run this vacuum diagnostic on your engine. This will tell you much about your engine and it's systems. You should get between 17-22 in. hg. depending on your elevation above 2000 feet MSL or mean sea level. To perform this test hook vacuum gauge directly to manifold vacuum. 1. Low steady needle usually indicates leaky intake, a leaky vacuum hose, late timing, or incorrect camshaft timing. 2. If reading is 3-8 inches low and fluctuates at that low reading suspect an intake manifold gasket leak at intake port or faulty injector. 3. If the needle has regular drops of about 2-4 in hg. at a steady rate the valves are probably leaking. (Perform a compression check). 4. An irregular drop or down flick of the needle can be caused by a sticking valve or an ignition misfire. 5. A rapid vibration of about four in hg. indicates worn valve guides. 6. Slight fluctuation of one inch or so indicates ignition problems. 7. Large fluctuation perform a compression check. 8. If needle moves slowly through a wide range check for PCV issues. Hope this gives you a good place to start and helps you figure it out.

Your Answer

Buick Grand National Experts

  • #1
    Joey Warren
    Reputation
    140
  • #2
    kenet12
    Reputation
    130
  • #3
    spideybee
    Reputation
    100
View All

Related Models For Sale

Used Buick Regal
138 Great Deals out of 6,554 listings starting at $3,995
Used Chevrolet Corvette
180 Great Deals out of 16,275 listings starting at $3,000
Used Ford Mustang
457 Great Deals out of 42,833 listings starting at $1,488
Used Chevrolet Camaro
502 Great Deals out of 31,149 listings starting at $2,495

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use.