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Thread: need ideas on a missfire problem
12-29-2010, 09:25 AM #1
- Join Date
- Nov 2010
need ideas on a missfire problem
I got a 87 1/2 with the tbi 350. A couple weeks ago i put on a different set of heads I bought off a friend and the truck ran just fine. on Xmas eve on my way to work I noticed the truck started to develop a missfire. As the day went on the missfire became worse. After work I put on new wires and a coil but it didnt make a difference. The truck has a rough idle and when driving down the road it struggles to run. While it was sitting and idling I noticed the every few minutes it was making a load popping noise and spitting fire from between the manifold and the exhaust pipe on the drivers side which leadds me to believe the missfire is on that side. I just went and pulled the valve cover off to look at the pushrods to see if maybe one was bent but they look fine. Later today I am going to start it up and watch the rockers as it runs to see if I have a collapsed lifter or even worse, sheared a lobe off the cam. If you have anymore thoughts on the matter please let me know.
12-29-2010, 10:37 AM #2
Check the exhaust manifold gasket on that side. Also check to see if any bolts are loose on the new install. Open the hood and press the throttle and listen closely for ticking or clicking sound when you accelerate. I always say go back to the last thing you did and retrace your tracks first. Since you installed new headers, your engine might be running too rich now if you significantly opened airflow.
12-29-2010, 02:20 PM #3
May be a vacume leak. Hopefuly just a loose hose at best and at the worst an intake gasket. Try some spray cleaner around the intake and TBI and see if there is a change in how it runs, this would indicate a leak. Is this a steady or consistant miss?2000 GMC YUKON SLT, 5.3L tow pkg, G80 rear/w 3.73 gear
12-29-2010, 02:59 PM #4
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- May 2010
- Central IL
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this sounds to me more like the timing has slipped some. Make sure your dist is still tight and double check your timing. Also check the obvious, vacuum lines, etc. If no leaks and timing is good, you possibly have a dist going bad or computer. When you start getting backfires and missfires, it's usually timing related.1986 Chevy G20 Tow Rig - 5.7 TBI conversion
1978 Mustang II King Cobra Road Racer - Holley 650, MSD 6AL, 5 speed, 10 point Roll Cage, 9" posi, fuel cell, custom interior.
01-02-2011, 08:02 AM #5
Could also be a head gasket. Check for any signs of coolant in the crankcase. It will show up on the dipstick and the underside of the rocker covers as a white milky substance. Did you use a non re-torquing head gasket, or a steel shim head gasket? Either way, if you can re-torque the head bolts it would't hurt.
01-02-2011, 10:42 AM #6
Last edited by 2COR517; 01-02-2011 at 10:46 AM.When you hear hoofbeats, look for horses not zebras.
01-02-2011, 03:36 PM #7
- Join Date
- Dec 2010
Is the vehicle blowing out white smoke upon starting the vehicle? If so it is a pretty good indicator you have a head gasket leak and are burning antifreeze and water. I would probably agree more with DualDJ on this one though... It sounds like a timing/ distributor problem to me as well. When you said you replaced your coil, are you talking about the pick-up coil in your distributor? Also would try a cap and rotor as well if so. My '90 350 tbi did this to me and the cap and rotor fixed the problem.
01-02-2011, 03:58 PM #8
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- Sep 2010
- Far West, Oregon USA,
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Hey BD, check that your #5 & #7 wires are not running too close alongside each other. They can crossfire when they are close together & will cause it to fire when the exhaust valve is still open on #5 & light off the unburned fuel in the exhaust pipe. You also need to keep #8 & #4 far apart on the other side.1988 Chevy C-3500 2wd (no pic)
350 c.i. 5.7 L Stock Block, 4 Bolt Mains
L-31 Vortec Heads, Edelbrock Cam & Intake,
Holley 650, Flowtech Headers, Magnaflow exh.
Jet Trans 700R4, B&M Ratchet, 4:10 gears,
3" susp. lift kit "shadetree"
No rev limiter, No speed limiter lol
01-04-2011, 12:04 AM #9
what condition were the heads in, it could possibly be a bad valve. I do agree with dpeter though, i would ckeck for intake leaks, and bad vaccume lines and rule out all other simple solutions before i pulled the heads.Keith Webb Sr.:mdr:
1999 silverado 2500
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