90 Chevy Silverado K2500 Runnability problem

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by michaepr, Sep 5, 2011.

  1. michaepr

    michaepr New Member

    Hello, Name is Pete and I am new to this forum, having looked at some of the posts I wondered if anyone can assist me with a problem I seem to be having with my truck. First, for some background, I had the 90's engine replaced with a 1996 at 96k miles when the old engine threw a rod. The replacement engine had 60k miles on it and it is solid, no oil use, high oil pressure and it has been running well. However, about a month ago something strange has started happening, when I take off from a stop if I floor it it back fires, then coughs a few times and runs fine after that. Sometimes for no apparent reason after the truck has been on the freeway running for an hour or so, when I slow down on city streets it feels like I develop a miss and it coughs and chokes, once it cools down it runs fine. Last week I changed the distributor cap, rotor, and voltage regulator, and also the fuel filter. It seems to have more power but the dreaded backfire is still there and the miss after the engine is hot. When cold.. its fine. At the time the new engine was installed, I had the shop put in new plug wires and plugs, but it has been 75k miles since that.. I am getting ready to change those next. However any assistance on what else could be causing this would be appreciated. Some people are telling me to change the CAT..( its original.). others the O2 sensor and Knock Sensor.. before I go this route I would like some guidance.. Thanks in advance for the help
  2. billnorman

    billnorman Rockstar 100 Posts

    What size engine do you have and what kind of fuel injection or carburetor? Older models were not supposed to go 100,000 miles on spark plugs. Backfiring (actually frontfiring if in the intake) can be caused by a bunch of things usually lean mixture, but may be as simple as too low an idle.
  3. michaepr

    michaepr New Member

    Chevy problem

    It has a 350, with Throttle body injection.
  4. LSU truckman

    LSU truckman Rockstar 100 Posts

    Another possibility and with higher miles on an engine is a stretched timing chain. It can cause those symtems as it changes your valve timing allowing combustion gasses to blow past your intake valves and up through your intake manifold.
  5. michaepr

    michaepr New Member

    Thanks to all of you for the help. I think I solved the problem, I changed the plugs, two were fouled with brown deposits pretty badly in the body of the plug and electrodes, probably caused by insufficient spark on them, I noticed the wires to those cylinders had been jammed against the rear of the valve cover and the firewall and had essentially been worn half through and the spark was probably arcing to the valve covers. I changed all of the wires and put high temperature Belkins with new wire looms and completely re routed them. For good measure I changed the O2 sensor as well.
    Engine runs as smooth as if it was brand new. Really was glad it was not the timing chain, that would have been a bit annoying since I just rebuilt the engine :gasp:
    Again thanks to all for the help, I hope this answer helps someone else who might be having a similar problem
    Last edited: Sep 12, 2011

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