Rough Idle When Hot

Hi everyone , new member here :) will try to make this brief.

I own a 2000 silverado with a 4.8 liter and it has approx 88000 km's on it . at about 85000 i decided to do a bit of a tune up , change the spark plugs and fuel filter. About a month later after the truck has warmed up real good from driving and i am sitting at a stop light, after the truck idles down (approx 30 seconds) i get what feels like an engine miss fire. Took it into a garage and nothing came up on the scanner, GM said the same thing except that they did say that it was the #5 & 6 cylinders that were stumbling a bit , but that # 6 was really bad but not bad enough to cause a code , so they do not know if it is an injector or a spark problem. So i put 2 bottles of injector cleaner in on a half a tank of gas....... problem seemed to be dissapearing.....untill i filled up with gas again......which really made it seem like an injector problem. Took it back to the garage and had the injectors purged and combustion cleaner run through it.......... still does it....... checked the resistance of the plug wires with an ohm meter and they are all within the same range approx 420-520 give or take a little. My next option is to let it cool down and have a look at the #6 spark plug but i doubt that i will be able to see anything so i think i will just replace the plug and go from there. Bought the truck used last year, Ran great up till this started. Any suggestions would be appreciated

oops forgot to add that when i had it at GM the guy there said that they see ALOT of trucks with rough idle problems and his explanation was that they are set to run so lean to be fuel efficiant that at times when they idle they sometimes do not get quite enouigh fuel....... Mine is an exception cause it just started doing it. So far from what i have read in the forum this does seem to be a common problem too.

hi, i am just posting in this forum to try and help you out, i have an 87 toyota pickupm and mine used to idle rough quite a bit, try taking your throttle body out and give it a thorough cleaning. this may be the problem, but not sure, just giving my two cents. One more thing to add, you should only use one bottle witha full tank of gas up to about 80 litres or so.. two bottles of cleaner with half a tank is waayyyy to much. :nono: another thing that may be causing it is a clogged fuel filter.

Hope it works out, let us know what the outcome is.

Jay

I knew that 2 bottles was too much :D but i figured give it a good go. also the garage does not think that it is the throttle body being dirty cause it is specifically 2 cylinders that are stumbling. I am going to pick up 2 new spark plugs today and change them and see if that helps. I had a look at them last night and they look fine , nice light gray color and i also checked the gap which was also fine. :confused: This is really getting frustrating, it almost makes you want to just start buying parts and replace them but i have wasted too much money on what should be a simple problem to find and fix. it has to be either a spark or a fuel problem. Will get those two plugs tonight after work and try it and get an update here.

i have a 200 5.3l and same thing i changed my fuel filter then my plug'd i put bosch plus 4 18.00 a pr. canadian also put new bosch wire's oh it has 110km on it and guess what still have the problem so i changed the fuel filter again just incase cause io\'ve heard of new filter's being restrictive so anyway still same problem also had injector's cleaned and de-carbon the combustion chamber still rough idle????????? next iam goin to remove the intake and clean it out,,be sure to check the intake base for a slight air leak arount the 5 and 6 intake port's maybe spay some wd-40 see if the idle up's a bit you could have a very small leak worth a try?

As wel all know aluminum expands when it gets warm and since the heads and the intake are aluminum any small leak would go away as the engine warmed up. As for replacing the plugs on the 5 + 6 cylinders have not done that yet for one reason. I have seen alot on this forum about disconnecting your battery and leaving it for 15-30 mins, so at work the other day i did so and left it for 7 hrs. The problem has not gone away entirely BUT it does seem to not be as bad as it was, i would say it is almost gone..........almost. Will try it this way for a while and see what happens. One thing i should clarify , it idles like a charm , you can barley even feel it running , it is after it warms up that this starts and it feels more like a bad engine miss but it does not do it all the time at every stop light , sometimes it is hardly there and other times it feels like someone walked up to the truck and hauls off and kicks the bumper, that is pretty much how it feels. Will keep you guys updated.

Had Similar Problem (2002hd, 8.1) - Check #5 Wire. Mine Was Rubbing Against The Steering Shaft. Only Grounded When The Shaft Was In The Exact Position. Dealer Discovered When He Did A Roade Test With The Scanner On Board. Problem Was Interminttent Due To The Steering Shaft Always Turning.

Most likely it was the #6 cylinder.....# 5 is on the passenger side :biggrin:. This alas is not the same problem that i am having.......this "enging stumble" i will call it only happens after the engine has warmed up and it is idling in gear. GM tech had scanner hooked up at the time that it was doing this..... nothing conclusive though.... does not do it bad enough to cause a fault code although i disagree, i described what it feels like in an earlier post. Have not gone anywhere since i got home from work friday so nothing new to reprot at this time.

Well it has been about a week since i disconnected the battery and let it sit all day to reset the computer and the symptoms still are not back........not sure if that is a good thing lol........ I guess time will tell one the computer has relearned everything..... so far so good though.

Well to make a long story short, the problem came back after about 3 weeks and resetting the computer did not help any this time. I am going the bite the bullet on this one cause it is really starting to piss me off as well it is too damn cold outside now to be working on it ..... -23c . So i am taking it to the dealership tonight after work. I hope i won't need the muscle relaxants and a tub of vaseline lol. jk . They have been pretty good to me there in the past, hope the kindness continues. Will update after i get it back hopefully WITH the problem fixed.

Keep me posted, the solutions to stupid problems like this are worth their weight in gold.

My truck is running good to date. Do you like the air intake mod. any fuel savings. how much power increase.

I say there is a considerable power increase especially with the dual exhaust. As for the fuel savings part , not too sure yet i have only filled up once and the tank ful that was in there went pretty quick with my heavy foot playing with that gained power :biggrin: There is definetly a big improvement in low end throttle response and when you put your foot into it approx half throttle it has this impressive roar that was never there before.... Well worth the 350 in my opinion.

Well this problem seems to be solved. It was a combination of a couple of things. First off the plugs i replaced about 3 months ago now were AC double Platnium (old stock). Next, the gap on those spark plugs was at .60 (old as well) The Plugs that GM now sells are Iridium (spelling) and the gap is only .40. 3 of the 8 spark plugs were bad as well as three plug wires. You should see the gouges in the porcelin on the plugs , i was amazed that this did not cause a misfire code. With new spark plugs that were gapped @ .60 and old wires (85,500 km's), they could not handle the resistance of such a gap and caused severe carbon arcing. I am still amazed that this did not cause a fault code. I said right from the begining that it felt like a engine miss but they said nope. I say , if it walks like a duck,looks like a duck...............IT'S A FRIGGEN DUCK. So all i can really think to add at this time is this ;.........If you have rough idle , do not always depend on there being a code. This which I have expierenced is proof. If you have rough idle , change your plugs and get the AC iridium AND change your plug wires even if they look good....... i still can not believe with the damaga caused by the carbon arcing that there was no code....................

Glad to hear that your are good to go. Is your truck is back to 100%??

Hard to believe that the source of the problem was the plugs and that no codes got flagged.

Have not changed my plugs yet (8.1 - 58K mi), only wires. Your saying stick with gm (a/c) plugs or just make sure the gap is right.

With all my past vehicles, I have had the best perfomance (inlcuding bikes) with autolight plugs. Don't know why.

Anyway, good luck...

My baby go tthe all clear today. Took it by the dealership to have the codes cleared because after the GM tech checked it over and found the bad plugs/wires, it made things bad enough to throw a code(misfire). The problem was not with the plugs but with the wires, when i changed the plugs it is possible that some of the boots that go over the plugs got cracked and this is where the spark would at times go, thus damaging the plugs as well. As we all know electricity will follow the path of least resistance. As for which plugs you should change to, it is just my opinion but i say stick with AC's. GM has discontinued using the double platnium with the .60 gap in their trucks from factory and started using the AC iridium plugs with the .40 gap for a reason(not like they would ever tell). In the past i have used other types of plugs as well but the increase in performance just did not last all that long. By all means if you are having luck with Autolites try them out, but i advise not using a .60 gap. I would close them up to .40 or even .45. The GM tech told me today that the .60 gap is more for fuel efficiancy and also makes for harder starts in the cold (which we get alot of here, -55c no windchill last feb)