GM Truck Club
Founded in 2004 ~ We're the #1 Chevy Truck & SUV Forum.
Silverado & Sierra | Tahoe & Yukon | Suburban & Yukon XL | SUV & CROSSOVER
Page 1 of 2 12 LastLast
Results 1 to 10 of 14
  1. #1

    Default 5.3 #4 cylinder misfire, valve carbon buildup

    I've got a 99 gmc sierra with the 5.3, 130,000 miles. The engine idles a little rough and the cel is on. The obd2 code showed a misfire in the #4 cylinder (P0304). So far I've replaced all the plugs, plug wires, new fuel filter, and new coil pack and injector in the number 4 cylinder. Checked the compression in the #2 (130) and 4 (122) cylinders and checked the pcv and air filter.....all are ok. After all that the pickup idled a little better but then the cel started flashing??? I posted to another forum and found it could be valve carbon build up. I ran seafoam through the master cylinder vacuum line, let it sit and then ran it for 5 miles or so in 2nd gear at 3500 rpm. The cel quit flashing (but still on) and it idled a little better so I figure maybe I'm on the right track.... Was wondering if anyone knew of a better way of cleaning up the valves (short of tearing into the motor) or any other ideas of what the problem could be. FYI the engine idles fine until it warms up and seems to run good otherwise...after only 2 weeks the new #4 spark plug is pretty black compared to the #2.

  2. #2


    Go to Seafoams website and they show how to clean the intake manifold, I think this will let the Seafoam not only clean the intake but also all the cylinders. Also I would remove the throttlebody and clean both sides with throttlebody cleaner and a soft toothbrush, DON'T use carb cleaner. Your year throttlebody has a Idle Air Control Valve (IAC) mounted in the side. Remove this and clean the pintle with the t/body cleaner. These have a habbit of getting carboned up. One last thing, when you indroduce the Seafoam into the master cylinder it doesn't get to all the cylinders evenly so some cylinders get a good cleaning and others don't.

    ---------- Post added at 03:41 PM ---------- Previous post was at 03:40 PM ----------

    I forgot, Welcome to the club.

  3. #3


    I looked on their site. You must mean the seafoam spray to clean the intake system... cool, been using seafoam for years but didn't know they had that product.
    Is there any particular vacuum line that I could use to get regular seafoam into the #4 cylinder? ......... Anyway, I'll try all the tips next week.

  4. #4


    The intake system is fairly new. I wounder if you had the throttlebody off and had the Seafoam spray can hose if you could aim it down the #4 cylinder. Be careful not to brake off the tube but I think it would work.

    ---------- Post added at 05:02 PM ---------- Previous post was at 04:41 PM ----------

    Be fore I retired from SnapOn tools they came out with a fuel system decarbonizer machine thats clean fuel systems for 1987 to 2006 trucks and cars. Theres other like cleaning machine on the market now but if you can find a shop that has one they work great. What they do is disable your fuel pump ( pull the fuse), disconnect the trucks fuel line, hook up the machine to the trucks fuel line, then start the truck and let idle for about a hour. The truck runs on the cleaner from the machine. Its a super cleaning agent and because it idles for a hour it cleans everything, fuel injectors, fuel line and the carbon build up in the engine. Make sure you change your oil after treatment because some of the carbon will end up in the oil.

  5. #5


    Do a leak down test. You may have a bad valve.
    When you hear hoofbeats, look for horses not zebras.

  6. #6


    I cleaned the intake with seafoam. Didn't help.
    I took it back to O'Reilly's to check the obd2 codes again and this time it showed a po300 (random misfire), po141 Heater Circuit Malfunction (Bank 1, Sensor 2), po155 02 sensor heater circuit malfunction (bank 2 sensor1), and po157 02 Sensor Circuit Low Voltage (Bank 2 Sensor 2).....these codes may have been there before in addition to the other code but it was the first time I'd ever used an obd2 scanner so I'm not sure.

    Could this mean three out of four 02 sensors are bad? .......gotta be kidding.........
    I unclipped all the 02 connections and cleaned them but it didn't help. FRUSTRATED

  7. #7


    Did you clear the codes? If your not sure if some of the codes were there, than I would clear the codes and than check see which code comes up.

  8. #8


    Don't worry about the sensor 2 codes, at least not at this point. All they do is monitor if the cats are working properly. They have zero impact on how the engine runs.
    When you hear hoofbeats, look for horses not zebras.

  9. #9
    stephan's Avatar
    Join Date
    Sep 2010
    Far West, Oregon USA,
    Blog Entries


    If your #4 plug is blacker than the rest, then that cylinder is running so rich that the plug can't fire it effectively, or the plug or wire is defective. I've not heard of carbon build up causing a rough idle unless it was so bad on the valve seats that it was holding the valves open, but your compression check didn't show this.
    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

  10. #10

    Join Date
    Jun 2006
    River Ridge Louisiana-4 miles W of New Orleans-didn't flood-water stopped 800 yards away.


    I would probably check compression on all 8 cyl.-Just curious like Stephan says 122 psi is a plenty good enough seal to fire
    See if 4 stands out in some way.
    Like Stephan says- black means richer, or poor firing.Not oily, right, just more sooty.
    Why don't you start there not firing right, or too rich..
    Not firing
    1)Bad spark -wires coils plugs- I think you might have covered this but the sooty plug might be causing a soot track so it shorts instead of sparks.Is there some sort of separate electrical connection plug- 8 prongs- for each of the 8 colis( does it have 8 coils??- think it does)
    2) Bad mixture- too rich - I'm not sure how your more advanced FI work- is there a separate injector for each cylinder.If there is the injector could be bad, or the electrical connection to the injector could be bad.

    I had a bad spider connection-8 prongs- to one of my cylinders once. The shade tree mechanic worked too fast and JAMMED it together bending one downward- stupid SOB-immediately implied there must have been something wrong BEFORE he started working on it.He also crossthreaded the bolt that holds the harmonic balancer to the crank, oil drain bolt, several oil pan screws, crushed the timing chain cover too tight- FREAKIN MESS. Some folks should not be allowed near power tools- his were electrical , not air driven.
    Yeah,I was lazy, didn't want to spend 8 hours doing me intake manifold gaskets- didn't want to buy puller or reinstaller for harmonic balancer.
    I PAID FOR BEING LAZY!! Spent 20 hours redoing, tapping usual crap. I left his intake manifold gaskets in place- I suspect he screwed them up also.

    Oh well- do what Stephan suggests.Bug him for specific step by step advice to troubleshoot.
    1998 suburban-
    1/2 ton

    199500 miles

Page 1 of 2 12 LastLast

Similar Threads

  1. Cylinder 5 Misfire
    By philosofy1 in forum GM Powertrain
    Replies: 14
    Last Post: 04-20-2011, 09:25 AM
  2. Cylinder #6 misfire code
    By AZ Suburban in forum General Chevy & GM Tech Questions
    Replies: 0
    Last Post: 06-30-2010, 07:19 AM
  3. Random Cylinder Misfire...Help!
    By wanna69 in forum General Chevy & GM Tech Questions
    Replies: 3
    Last Post: 07-27-2008, 09:04 PM
  4. 99 1500 No power & cylinder 3 misfire
    By jhc in forum Chevy Suburban Forum (GMC Yukon XL)
    Replies: 10
    Last Post: 07-03-2008, 08:05 PM
  5. Cylinder 3 misfire code
    By Crawdaddy in forum Chevy C/K Truck Forum
    Replies: 5
    Last Post: 07-15-2007, 02:53 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts