5.7 liter Vortec Misfiring badly

Discussion in 'GM Powertrain' started by SuperMatt, Jul 7, 2009.

  1. SuperMatt

    SuperMatt New Member

    Hello. I have a 2000 GMC Savanah 1500 conversion van with a 5.7 liter Vortec with approximately 80,000 miles. About 3000 miles ago it started dropping A LOT of coolant through the tail pipe. I bought this van used and noticed when i flushed the coolant out that it looked like it had a stop leak added to it as there was a rubbery film all over the inside of the radiator. i had heard that these engines had a bad history of needing the intake manifold gaskets replaced so i replaced the gasket, flushed and put the engine back together including a tune up with new filters, plugs, cap/rotor, wires, fuel filter, pcv valve, etc. i installed ngk iridium plugs to proper gap. the engine ran well but it still was leaking coolant, so i decided to just buy some more radiator stop leak (Barsleak brand) as apparently the intake manifold gasket was not the problem. Adding the stop leak DID CORRECT THE PROBLEM and the vehicle no longer was leaking coolant through the exhaust. I have just given you some history of what has been going on with the vehicle up until this time. We have put about 3000 miles on the van since then and it was running fine (apparently) until last week when it started running VERY rough. i could tell it was misfiring badly. I put my code reader on it and it was throwing two codes. a random misfire (P0300) and an O2 heater failure. My code reader has the ability to watch real time diagnostics. it showed the #4 cylinder misfiring on almost every stroke(it registered well over 10,000), and the number 3 cylinder misfiring intermittently. On this motor the #4 cylinder is on the passenger side second from the front, and the #3 cylinder is on the driver's side 2nd from the front. I figured with 80,000 miles it couldn't hurt to change out the o2 sensors and maybe that would fix the problem. 2 new (before cats) sensors were installed and it did NOT fix the problem, however the o2 code did go away (so far). I searched for vacuum leaks and could find none. I next hooked up a vacuum gauge and the readings were:

    at idle vehicle in park:
    irregular needle drops from 21 to 19

    at idle in gear:
    irregular drops from 18 to 16

    The vehicle surges and sputters when in gear. you can here an irregular pattern to the misfires. It feels like it wants to stall out but it has not. In gear and under load (normal driving) it has a decrease in power and you can feel the van surging when it misfires.

    I next pulled the #4 spark plug wire off the distributor. the engine ran noticeably more smoother but with an audible clicking sound). In gear at idle the vacuum stayed at a pretty steady 21. once i put the wire back on the engine again started misfiring.

    I next pulled the front passenger side tire, and pulled the plug. it appeared ok and was not fouled. i checked the compression on the #4 cylinder only (so far) during 4 compression strokes it rose steadily and topped out at 150. i did not see an issue with the compression.

    Does anyone have any ideas as to what can be causing this? I dont have the money to get it to a shop, and i dont have the money to start randomly replacing engine components and praying it will fix the problem. i can try doing a leak down test or simply checking compression in other cylinders? This engine has only one coil. can it possibly be the fuel injection? it has no fuel rails, but rather a set of spyder injectors. egr valve?? cam position sensor?? any help would be greatly appreciated??


  2. werty

    werty Rockstar 100 Posts

    id say do the leak down...must be a pita in a van?:happy:
    unkess you can find that misfire somewhere else:grrrrrr:
  3. L0sts0ul

    L0sts0ul Rockstar 100 Posts

    check your compression, sounds to me like a head gasket leak on possibly both sides or even a crack in the block... could be a craked head too...
    coolant in the exhaust (in my experience) has always come from cracked heads, or head gaskets.
  4. werty

    werty Rockstar 100 Posts

    $the head is cracked and the valves are recessed.............:neutral:
  5. 2ToNe04

    2ToNe04 Epic Member 5+ Years 1000 Posts

    He would easily tell though, i remember on my old camaro i had a cracked head and coolant was being burned and each time i would step on the gas white smoke would come out and it would smell like hell.
  6. werty

    werty Rockstar 100 Posts

    its a van.it doesnt smell like a camaro.:great:
  7. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    Well I just had heads rebuilt on my 99 5.7 it was $2k after all the reblacing of bolts, full gaskets, labor to finding the fried valve and then tune up. I was getting constant misfire, fried exhaust valve wasnt closing so full rebuild on heads.
  8. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    You said your plugs are at the "proper" gap. Do you mean .060? If so, I would pull them and replace with AC Delco platinums gapped at .040. Your truck will like the ACs best.Those big gaps puts a tremendous load on the entire ignition system. Coil, cap, rotor. It's possible that when you pulled that one wire, you gave the coil enough of a break to fire the other plugs with more power. GM issued a bulletin some years ago to crank the gaps back down to .040. I did it in my truck, made a huge difference. It's 65 bucks, and when you have all the plugs out, you can test compression on all eight cylinders too. I would clean the cap and rotor also.
    Last edited: Jul 8, 2009
  9. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    Why such a difference for the AC plugs for GAPPING
  10. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    Whatever brand you use, you should pull the gap down to .040. As for AC Delco plugs specifically, I don't know the physics of why they are better, but many guys, with much more experience than me, say to only run AC Delco plugs. It's what the General (Government now LOL) designed for their ignition system. My personal experience is this: I had Bosch+4 in my truck. It was running very poorly. Sloppy idle, pinging, overheating. Put the regular single tip platinums in at .040 and it was like a new truck. It was the ONLY change I made. I was sold!

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