03 sierra 5.3l missing on 2468

Asked by Dec 19, 2017 at 11:45 AM about the GMC Sierra C/K 1500

Question type: Maintenance & Repair

driving home going 115 kph. dash flashed "reduced engine power" could not get over 80 kph then dropped down to 30 kph once I got off next off ramp then pulled over on side of road to check under hood and wouldn't pull itself. noticed rich fuel smell from exhaust. towed and hooked scanner no codes no mil .misfire on scanner shows 2 4 6 8 .used c.o.p. tester 35000 v showing on all 8 pulled plugs and checked resistance with multi meter and all were equal (replaced with new ac delco iridium plugs since I had them out and have 261000 km on them) dry compression test  #1-123 #3-125 #5-124 #7 124 #2-123 #4-124 #6-125 #8-126. fuel pressure at 55psi. injector balance test showed drop of 8 psi for all injectors. did noid light test on all injectors, all flashed just before its plug fired with c.o.p. tester. pulled fuel rail on right side of engine checked for restriction - none. also checked fuel jumper line from test port down and no restriction. pulled throttle assembly and ran cam into intake to check for intake port blockage- clear. cleaned throttle assembly while I had it off. swapped coil packs l to r and checked voltage ground and reference signals at main plugs on l and r - all within spec. removed valve cover on right side and checked valve opening at .464 intake and .455 exhaust average .oem spec seems to be .466 and .457 . barometric pressure is 99kpa (same as atmospheric ) at engine off and 28 with engine running at idle . maf sensor is cleaned and tested ok. fuel trims left to right on graph are equal and o2 sensor readings once heated are equal on sensors 1 and 2 both sides. replaced bank 2 sensor 1 just to try with no change. I can unplug main plug to coils for 2 4 6 8 with no change in engine idle .if I unplug main plug for 1357 coils engine quits. did cylinder cut out 2 4 6 and 8 no change in idle 1 3 5 and 7 each made engine pretty much quit. not sure where to go now, I have spark compession air and fuel on 2 4 6 8 but no fire. any HELPFULL suggestions?

12 Answers


Cam and or crank sensor pulse erratic, wiring from ECM damaged, or even the ECM itself not recognizing those cylinders. I did experience that one time on a Ford, the guy backwards jumped a vehicle and that was what happened to his, replaced the ECM and fixed the problem. Don't know if that would fix your but it is something to look for if you have exhausted all other diagnostics.

ruled out cam and crank sensor since it would have to skip even numbered lobes and only read odd numbered lobes, wiring from ecm was checked with noid light and reference signal test to coils was also normal.

figured it out- jacked up and checked exhaust temp with infrared. right catalytic converter plugged pulled pipe and missing went away


WOW usually you start feeling a loss of power long before the cats go away. Bad fuel maybe, how does the other one check out?

thats what surprised me no loss of power until the display said reduced engine power. I never checked the other side they are a set with the y pipe. the other confusing thing is bank 2 sensor 2 showed it was working starting to think I had back flow to sensor 2 to give it that reading that was close to bank 1sensor 2


If one is plugged so no flow then it would feed back from the other side through the y pipe so both would be very close to the same downstream readings, it's the upstream that should have been way off. Anyway once the cats are fixed and the exhaust is flowing and you have reset the ECM and done the drive time protocol then see if it comes up with anything else, still got me bugged of why only one catalyst, you might want to check for fuel pressure leak down you could have a dripping injector on that bank, I know you did a balance test but that won't show a pressure leak after shut down, excess fuel on that side could explain why the cat melted down. Did you smell anything like a hot metallic smell at anytime during intial start up? Hope that's all it is, sometimes they just go bad.

fuel pressure stayed above 50psi for an hour after shutting off .I am going to rechck compression after cat to make sure and double check muffler as well

repairs complete-after driving rechecked compression fuel pressure and fuel pressure dropped to 52 psi after 1.5 hrs. rechecked o2 sensor readings bank 2 sensor 1 showed a lean condition so switched l to r and lean condition went to l replaced o2 sensor as well. im thinking the o2 sensor saw a non existent lean condition on r bank and overfueled r bank and with the excesive fuel in r bank with o2 sensor reading lean normally, showed a normal reading when overfueled . I cut the y pipe apart because I found chunks of honeycomb in exhaust partially blocking pipe to muffler. once I had y pipe cut apart I found the catalytic on r side melted and twisted sideways in the tube totally blocking it off. the other thing that is confusing is that at no time diid the mil light come on and no code were recorded

above was me don't know why its under a guru nme


This sounds like MMT was used at some time in the fuel, newer vehicles don't like the stuff, did you notice any kind of coating on the O2 sensors? See if you can shake your muffler some, if it rattles then you have catalyst debris in the muffler, that might cause a problem later on id it's not restricting flow now. It is odd that you didn't get at least an O2 downstream code with the cats melted like that, and the upstream should have showed at least slow to almost no switching cause it could not breathe. WIERD

when I saw the piece of honeycomb in the pipe I removed the muffler and pipe and dumped and banged it to get debris out then blew air back through with shop vac on inlet end. o2 sensors were fairly clean and visually looked good. I have owned it since new and never put any octane boost or fuel additive in. other than a broken wire to the bank 1 sensor 2 I have had no ignition or emissions problems. only other repairs other than normal maintenance have been both front wheel bearings ,upper left ball joint, tail gate and left rear outer door handles. definitely weird with no codes and the ecm new something was up since it went to reduced power mode but never threw a code.

Well its been 9 months and 27,000 km and no problems so far, other then a rotted brake line. Still no clue as to why catalyst plugged so quick and why o2 sensor didn't throw a code. If t ever does fail again I will repost

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