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engine misses but why???

Discussion in 'Chevy Trucks' started by TTBoy77, Nov 15, 2007.

  1. TTBoy77

    TTBoy77 Member
    Messages: 87

    I have an 89 full-size blazer, 350tbi, 4l60e trans, np208 transfer-case, blah blah blah. Here is the problem. I gave this DD a tune-up about 2 months ago including plugs, wires, cap, rotor, PCV valve, and fuel filter. The truck runs great at idle. I can rev it up to red line and it won't miss a beat. However, when I put it into gear, the truck starts to miss. What do you guys think? A bum vacuum line? Did the timing somehow get thrown off? The trans is at the full mark (checked properly of course) so I am stumped.
  2. sweetk30

    sweetk30 PlowSite.com Addict
    Messages: 1,588

    ac delco plugs i hope? i work as a mechanic and its been found and proven time and time again. install what the factory did for best perfomance. lets start there and see what you did.
  3. TTBoy77

    TTBoy77 Member
    Messages: 87

    yes sir...they are delco
  4. William B.

    William B. Senior Member
    from S.E. IA
    Messages: 979

    Sounds like vacuum to me.
  5. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Did this problem just start recently or did it do it as soon as you did the tune up?
  6. CHPL

    CHPL Member
    Messages: 82

    Un plug the timing connector (tan and black wire) See if it still does it. If that solves the problem you have a weak pick up coil. This was a somewhat common problem on the chevy trucks and full sized caprice with the 5.0 and 5.7 liter engines from 87 through the mid 90's.
  7. TTBoy77

    TTBoy77 Member
    Messages: 87

    It started about two weeks before the tune-up. I don't know all that much about vacuum so if that is it, I don't know how even begin checking.
  8. TTBoy77

    TTBoy77 Member
    Messages: 87

    Just thinking, why would the brand of plugs matter? The truck runs great as long as I am not trying to drive it. Like idling in the driveway, runs great.
  9. drumbo

    drumbo Member
    Messages: 44

    I had a similar problem with my 454. It had some cylinders that were down/not firing consistently. They had to replace all the fuel feeds (I think they said it was kinda like a 'spider' that feeds the fuel into the cylinders). The cause, they believe, is substandard fuel that we have to use these days. I run Sea Foam or Lucas through it every now & then for prevention, per their advice.

    Have you tried those fuel treatments yet? If so, did they help?
  10. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    You said it idles fine but misses in gear, how does it run down the road? Does it miss real bad while under load? Does it "pop" through the throttle body under load?
  11. Duncan90si

    Duncan90si Senior Member
    Messages: 602

    Get a can of carb cleaner and with the truck running spray all the vacuum lines one by one and around the throttle body gasket. If you have a leak, the carb cleaner will change the idle of the motor. Then you have pin pointed the culprit. Thats how I do it.

    Also, if it is popping out through the TB, then you may have a flat cam shaft.
    Last edited: Nov 16, 2007
  12. TTBoy77

    TTBoy77 Member
    Messages: 87

    It misses while running down the road. No popping or backfiring. I did a compression test and had between 150 and 175 psi on all cylinders so I am assuming there isn't a bad cylinder. :confused: :dizzy: :confused: :dizzy:
  13. TTBoy77

    TTBoy77 Member
    Messages: 87

    Yes, I do run a can of Sea Foam through the fuel system every now and then and I don't notice any drastic changes, good or bad.
  14. Dave50mod

    Dave50mod Junior Member
    Messages: 4

    Does it se antifreeze? could have cracked a head Could hae worn a cam u wont here popping until really high wot just some suggetsions but worse case try the easy stuff first
  15. TTBoy77

    TTBoy77 Member
    Messages: 87

    No..it doesn't use any antifreeze, just oil. No white smoke out of the exhaust either. I'd like to try the easy stuff first, but I already did that by doing the tune-up (or so I thought) 'cause it ran great for a couple of days after doing all that stuff I mentioned earlier.:confused:
  16. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Does the miss seem exactly the same now as before the tune-up? You said it ran good for a couple of day after the tune-up but is the miss now the same as before?
  17. TTBoy77

    TTBoy77 Member
    Messages: 87

    Right now it's better than before, but it is slowly getting worse.
  18. sechracer

    sechracer Senior Member
    Messages: 478

    Did you get your parts from the dealer or the parts store? If you got them from the parts store, I would recommend trying the dealer for a new cap. The way that they run the leads in the cap, they tend to have internal shorts. I put a autozone top of the line on my truck, it ran great for 1 week and died. Turned out to be the cap, had got a new one from the dealer. No probs since.....
  19. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    That's a very common problem with the aftermarket caps on the 96-up Vortec engines there sechracer. OEM is the only way to go on the 96-up Vortecs..

    TTboy, since it did change when you did the tune-up and it is getting worse, I'd still be looking at a ignition problem. Like a defective cap or rotor or a bad plug or wire. Could be a wire arching to the exhaust manifold...seen it many times. A failing pick up coil or ignition coil will cause a miss under load too...

    Since it misses in gear you can check the wires to see if there arching off anything by looking closely for any arching while a helper holds the brake on while in gear and holds the throttle a little. Get the truck somewhere totally dark, (like a garage with the light out). Watch real close around the plug boots for any arching or you may see a quick glow.. Look over the whole system..coil boot, around the cap and between adjacent wires...
  20. javaman70

    javaman70 Junior Member
    Messages: 29

    From my experience I would try wires. Get top of the line wires, cheaper ones tend to short out(some are just garbage out of the box). Make sure you use the dielectric grease.