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Chevrolet Tahoe Misfire

rmkakrmkak Posts: 2
edited September 2015 in Chevrolet
I have a 99 tahoe with a 5.7 vortec that missfires on #4 cyl. under heavy load (pulling a hill) I have replaced the fuel filter, air filter, cap, rotor, plugs and wires no change I also replaced #4 injector no change. I have sprayed the intake looking for a vacum leak no leak detected. I also checked the compression it's good. Anyone have any ideas where to go from here I do have access to a tech 2. When it missfires bad I get a p300 random misfire code

See Also
Analyzing Ignition Misfires


  • O.K. I feal your pain I have a 99 5.7 tahoe 4x4 with the same exact issue only the misfire is sometimes on cyl 3, I bought my tahoe in 2003 from a dealer and they tried to do everything, it was hard to duplicate so I understood why it was so hard to fix it (which they never did), I am a certified Audi technician so I know how to diagnos and work on cars but this is a tuff one, long story short I think my Tahoe is finaly fixed, this weekend my Tahoe broke doen in L.A. during a little road trip, the fuel pump went out, I asked the mechanic if that might have been my misfire concern under load and he said he would bet on it, he said those fuel pumps are very sensitive. I researched this topic months ago and all I found were a bunch of a--holes who think they know everything and none of which helped me, I received my Tahoe back on Monday.
    I have driven it about 300 miles since (some of which was up long hills under load)
    and have had no issues, and my Tahoe feals a little stronger than it ever has, I think this is the fix, if my problem comes back I will update this forum, sorry this is so long I just thought you might want to know, take care and good luck. Oh I don't know if it matters but I currently have 81000 miles on it and the first time I had the issue was at 44000. Good luck.
  • kiawahkiawah Posts: 3,666
    You'd think if you had a fuel pump problem, that it would have an effect on randomn cylinders as opposed to always just one cylinder, wouldn't you? Certainly a miss under load can be a symptom due to fuel supply problem, just hard to imagine that it's always the same cylinder that gets starved. Hope the pump really does fix your problem, but it wouldn't surprise me if it's a head gasket, intake gasket, or some sort of problem like a defect in the head on that cylinder.
  • rmkakrmkak Posts: 2
    The scan tool I used did show a few miss fires on the other cylinders during different times but just a couple compared to the thousands on #4 pulling a hill. Next I think I may change the egr and check the fuel pressure and also try un hooking the cat just for some more ez eliminations before I pull the head and look for a sunk valve or a bad intake or head gasket maybe even a flat cam. I will let you know what I find.
  • danel2danel2 Posts: 3
    My 99 Surburban misses at Idle sometimes. changed wires, plugs twice, cap and rotor. but still does it. any Ideas?
  • dbrown6dbrown6 Posts: 4
    :sick: I have a 98 with 205k miles. I've had the intermittant and #4 show frequently. I have replaced the plugs, wires, cap, rotor, o2 sensors (contemplating Cat converters). Most recently the engine showed signs of clogged cats, but bought a couple bottles of Sea-Foam, one in the tank with 1/2 tank showing and the other through the brake booster. Runs great now! But, when the engine gets warm on a hot day, it starts the misfire routine. Appears to be heat related as it will run well all winter. This has been going on for years. I have ignored the check engine light for the past 5 years with regard to O2, cat and ignition issues and only recently started paying attention when the transmission finally gave out. If I had a bad valve, I would think the power thing would be bad all the time, but I did put in 1 yr parts rather than premium or Delco parts which may make a difference.... I like my Suburban too much to let it go down like this, but what could it be? :sick:
  • kiawahkiawah Posts: 3,666
    Have you thought about a bad intake manifold gasket?
  • arriearrie Posts: 312
    I do not know where crank shaft position sensor is located on a '98 model but if it is the same way as on my '04 tahoe I would have a look at it. In my car crank shaft position sensor is mounted at the side of the engine block at rear of the engine as it reads teeth on a wheel mounted in the back end of crank shaft. The sensor gets very hot as it gets hot engine oil thrown on it and this could make sensor heat sensible, i.e. when it is colder outside the oil pan helps cool engine oil down while on hot days this cooling effect is less.

    I changed my crank shaft sensor once and it did improve idle "jerking" but did not completely fix it though.

    If your sensor is mounted in the front of the engine and reads teeth on a belt pulley like I have seen on some other cars then your issue probably is not with the crank shaft position sensor.

    Another very easy thing to try: Again, I don't know if you have the aluminum heat shields around spark plug boots like my Tahoe did but if you do remove those and drive around a few miles and see if it helps. The shields caused quite bad spark leak to engine ground in my Tahoe and I fixed this by replacing spark plug wires with racing wire set that does not require use of the heat shields. I bought Taylor wire set for $75, which is about the same price as dealer asks for the set of garbage they use, which you can get for $25 at car part stores.

  • you could need a new computer they have bin known to get a glitch which will efect any thing on it aND OR THE FUEL PUMP IS WEEK
  • hey guys, I have been reading the threads and it seems like there is some knowledge here, so this is my issue. 97 chevy silverado, 5.7L I have had a rough idle here for the last year ever since I had the intake manifold gasket replaced by a mechanic because it was leaking coolant into my oil. Have also had a stumble when taking off, seems like the truck misses right once i press on the gas and then drives just fine. Just lately it has started to throw P0131 codes (O2 sensor, bank 1, pre cat). I replaced the sensor, bosch, not the universal one, no change. It then threw a p0300 code and a p0305, with the blinking SES light when driving at 25-30 mph, missing pretty badly. Looked at the plug (#5), it was black and sooty, so i swapped the cyl 5 plug with the cyl 1 plug just to see if it was the plug. Sure enough, next code was a p0301! I replaced the spark plugs with the autolite platinums, after reading other posts i now know i should have done the acdelco, but its too late now. By the time I changed all of the plugs, which was one week after I swapped them, the driver side bank of plugs (1,3,5,7) were all black and sooty while the passenger side plugs were completely normal looking. With new plugs for a week, i checked them again and sure enough, driver side plugs are sooty, passenger side are normal. Still having the rough idle and random misses, usually right on takeoff. It doesn't make sense to me that the O2 sensor giving the code is on the opposite side of the engine that I am having issues with the spark plugs being sooty, which i am presuming is a rich condition. I checked the fuel pressure at the engine and it goes to 63 psi with a turn of the key, after 2 seconds when the pump shuts off the pressure immediately jumps down to 55 psi within a second, and then bleeds down to 30 psi within 3 minutes. When it is running, fuel pressure stays constant at 54 psi, but again once when shut off it bleeds off down to 30 psi. I have read that the fuel pressure needs to stay within 5 psi for 5 minutes or there is a leaky injector or a fuel pump check valve. I am on a budget here in michigan and enjoy working on it, but don't enjoy burning money on parts that don't need to be changed. I read the post about paying for quality parts and agree 100%, have been burned too many times by reman/cheap parts. I am thinking about swapping the injector pack for the mpfi upgrade, but is there any additional diagnosis i can do to pinpoint it further? Have a new fuel filter and had a dealer do their fuel injection system cleaning right after the intake manifold was changed when they also reset the timing. However since then I have played with the timing trying to get it to stop missing after the dealer adjusted it. My dad has a scan tool that gives real time readings on all of the engine vitals, (fuel trim, o2 sensor readings, etc.) is that what i would need to set the timing? Have sprayed starting fluid around intake (while engine is cold) while running, no changes in idle. Thanks for all of your time, advice, and help on reading this long post, i just wanted to give a good background.
  • My 99 Tahoe with 5.7 Liter Vortec has had the same problem (rough idle at low RPM only when warmed up) for the last few months.

    I've done a full tune-up, replaced the fuel filter, cleaned the fuel injectors, replaced O2 sensors, replaced the passenger side catalytic converter (clogged), tested for leaky intake and a few days ago my fuel pump just went out on me so that's new as well. I've taken it to three shops (it's currently in the third shop). I'm pretty good with cars but this one has me at a loss.

    It WAS stored for 3 years in my Dads barn while I was in Iraq and Europe but he started it once a week for me and this problem didn't start until I had put about a thousand miles on it after getting it out of my Dads barn.

    :confuse: I'm getting the random misfire code. It went away after changing the O2 sensors then reappeared. It also went away after replacing the fuel pump but now is back. I'm getting monster amounts of carbon build up on bank 2 (passenger side).

    Please, if anyone has truly solved this exact problem let me know what it took?

  • Hi Fellas,
    I have a 99 tahoe. It has the 5.7L 8cyl engine and is 4X4. I have had problems with it misfiring under load. It seems to only happen when going up a steep hill. When I give it gas and reaches about 50mph, it vibrates. Then, when I slow down to a stop, it bucks like it's going to stall, but then it smooths right out. It was throwing the PO300 code (I had it scanned and am aware that this is a misfire code for all of the cylinders). I had the fuel pump replaced, intake manifold gasket replaced, the valve cover gasket replaced, new wires and plugs, new cap and rotor, fuel injectors cleaned, new fuel filter, checked for vacuum leaks, checked for electrical shorts, and had the timing checked. Still have the same problem. Any ideas???????? Other then the misfire under load, everything else runs great!!!!! It idols smooth and the code resets itself and doesn't come back until I go up a hill! Please help. Thanks.
  • kiawahkiawah Posts: 3,666
    that's the point of maximum fuel need.

    check the fuel pressure, and flow.
  • You have a bad O2 sensor. I guarantee it.
    email me when you verify it [email protected]
  • steverstever Posts: 52,462
    Please post in the forum instead of asking for replies via email. We all benefit when a problem is discussed on the open forum. Thanks.
  • I have the same problem with my 99 Tahoe and have had several repair shops look at it with no luck fixing the problem. Been living with that problem for several years now. If you have any suggestions please help. Thanks
  • Just responding to your message, I found some recent information from a GM employee, 98 and 99 tahoe 5.7l engines have a cylinder head issue due to factory defect, the valve will start to float under heavy load causing a lose of compression. My source is very reliable I called my local chevrolet dealership and they said if I had all my maintenance records GM would be able to offer some assistance with the repair bill, I am a mechanic so I have been changing my own oil ever since I have had it so I obviously don't have receipts.
    New heads are about 1500 dollars and that's just for the parts so even if they "assisted" me the bill is more than what I am going to pay for a random misfire. I am only upset because I had this complaint while my vehicle was under warranty and they just threw parts at my Tahoe until my warranty expired. Live and learn, let me know what you fnd out though. I hoped this helped.
  • You say "the valve will start to float under" so are you saying one particular valve or did you mean to say "valves"? Or, is it random (hence the random misfire)? Additionally, I was getting a random misfire and all the plugs on bank 2 were COVERED in carbon until I replaced the O2 sensor on that side. No more random misfire but still have the service engine soon light on. I never did get the random misfire at load, it was always at idle.
  • Hi Folks,
    I now have the dreaded misfire code p0300 then p0306. Has 160k and always been sweet running until inspection time. The garage which I won't name, attempted to fix this problem. $1200 later only to receive an inspection sticker and the service light to return as I'm driving away, bummer. I've done every possible tuneup part twice also, 2 new highflow magnaflow cats, injector nozzle change#6, two out of the 4 new O2 sensors replaced. The rough idle appears 20 secs after first starting, blinks the service light and stays on. I can shut the truck off clear the codes and drive away, only to return the next time I start it up, same codes. Multi misfire and cyl 6, what do you do next, replace the remaining two o2 sensor. Is it worth it? I'm reading your posts and getting discouraged.
  • Hi everyone, new to the post. Just had a major missfire on my 99 suburban this week, pushed it to work on 6cyl. Had it towed to a new mechanic to get it checkd out. Mind you when i got to work i open the hood and noticed a little smoke coming from the back of the motor, figured it had something to do with the distributor. Being an electrician by trade i could smell burnt wires. After diognostic testing, they found the distributor to be burnt. $350 later i'm back on the road with a rebuilt, only to have my service light come on again with a bank 2 O2 sensor code come in, twice. I'm figuring the motor is running a little different with the new dist. the computor probably has to work out a few bugs i guess. Anyway your missfire, after doing all your tuneup changes could ba your distributor, or the dist. sensor which i had done about 2 yrs. ago. Sounds like its definitley electrical, i would look there first. Good luck.
  • I had a crack in my distributor housing where the screw came in from the cap and had to replace it ($150.00) but still had the misfire so it had nothing to do with it. I replaced the O2 sensor and it ran perfect. Talking with many different mechanics who specialize in Chevy the random misfire is 95% of the time caused by a bad O2 sensor.
  • $250 More
    Ok, just put in 4 brand new O2 Sensors, 3 of them looked like caked on chalk. When started the first time, engine light blinked, misfire P306 and P507 Throttle position sensor. Cleared the code drove about 60 miles hasn't returned yet. I also haven't started it cold yet. I'm also running can after can of seafoam, I still think misfire in 6 has faulty fuel delivery, clogged injector? What do you think next? Grrrrrr!
  • rem222rem222 Posts: 4
    1998 Tahoe LT multiple misfire codes P0300 - P306. I experienced ALL the symptoms mentioned this subject including a dealer telling me my engine is shot. OK - I need a second opinion.
    Took it to my mech Mike Tieg, Tieg Automotive, Greenwood Mo. He had it three days and called me stating he thinks he figured it out, 99% confident, but it isn't cheap. He replaced the fuel distribution spider to the throttle body fuel injection under the intake. He was spot on!
    Put another 100K on it and sold it earlier this month w/ 224K on it.
    Loved my Tahoe for 10 years, now I love my Denali 6.2 L.
  • rem222rem222 Posts: 4
    Johnny - Read my post - last one this string
  • hquickhquick Posts: 5
    I know this may be a bit late...but anyway...
    How do you guys clean your own injectors?
    As far as I'm aware the CSFI 'spider' poppet injectors can't be cleaned without removal. This means the upper half of the intake manifold needs to be removed.
    If you're going to that'll do well to replace the whole injector setup with one of the 'mini' injector units. These are 'proper' injectors....not ball bearing valves at the end of hoses (poppets).
    The other common issue with these trucks/SUV's is the distributor gear wears unevenly and gets 'knife-edged' on one side.
  • rem222rem222 Posts: 4
    Good call, re; the distributor gear, many miles will wear the gear as you describe. That info coupled with the spider info should eliminate much of the guess work about multiple misfires in the many past posts calling for help. When a misfire is localized to a specific cylinder and not random one of the two mentioned above is more than likely the culprit. As far as cleaning the spider - Agreed, you've come this far, replace it (Parts, and Labor it isn't cheap though ). If you want a cheap try - go to your local Yamaha dealer and get a bottle of Yamaha Ring Free and shock treat according to instructions on the bottle. Forget the fuel injector cleaner stuff - Ring Free is designed to destroy carbon. OMC also has a product, just not as concentrated so you use more but they both are effective carbon control. To my knowledge they do not harm O2 sensors.
  • Did you change your O2 sensors on the side with the misfires?
  • momsbusmomsbus Posts: 3
    bought a new 5.7 crate motor for our 97, installed it with new spider injectors, new distributor cap & rotor and fired it up to a misfire problem we can't seem to fix. Cyl.#3 is the one causing the problems, changed plug already and it's still doing it. Going to change O2 sensor next, but any suggestions from anyone would be helpful.
  • I don't know how many people I've already told this to but it is your O2 sensors. Trust me, trust me, trust me. Simply unplug your left upstream sensor and fire it up, if the misfire is gone then that's the one. If not, do the same thing with the right (upstream) sensor and fire it up. How much did the engine cost? :D
  • momsbusmomsbus Posts: 3
    I will go with your suggestion,but I don't understand how a bad O2 will cause only one cylinder to create a misfire? I have changed the plug (has been changed 3 times)-plug wire checked and a injector balance check done on all 8 cylinders
    which all check out working correctly. I have an good scanner and it shows the upstream 02's working (Correctly I'm not sure of the parameters) I have even removed the upper plenuim to check that the new injectors were seated properly. The motor cost a little over 2k plus a new distributor and injector spyder and all filters and fluids and knock sensor and temp sensors and gaskets.
    Thanks for the input and we will run with it, Any other thoughts let us know
  • momsbusmomsbus Posts: 3
    Changed all 4 O2 sensors and I still have #3 and now #5 giving us a misfire......any other suggestions? Anyone? Gonna take the plenuim off again to check these two injectors....but any suggestions are welcome.
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