2008 Suburban Slightly Rough Idle

I have the 5.3L FLEX Fuel with 98K miles. I have purchased this 2nd hand and having a slight idle stumble when in park as well as in drive @ a stop light…etc. If i am under way, the vehicle runs flawlessly. When i am idling with the brake on or in park, there is a slight stumble that is driving me crazy. I have replaced the high dollar IRIDIUM plugs but did not replace the wires…they looked fine. I did clean the throttle body. I get fuel from different places and it does not make a difference. I have not cleaned the fuel system proffessionally but I have used OTC additives sucha as SeaFoam. No codes are exhibited on a diagnostic computer or any updates available for that matter. Can anyone shed some light on this situation and hel me out here!!!

When you cleaned the throttle body did you also clean the MAF sensor?

Wires can “look” fine but not be fine. They really aren’t that expensive so, personally, I would have just replaced them with the plugs. At the very least you can hang out someplace really dark with the hood up & the truck running - look for any signs of a light show associated with the wires. Did you at least pull the other end of the wires to check the connectors at the coil/distributor (whichever).

You also need to put a fuel pressure gauge on it and make sure the pressure is within specs. A new fuel filter can never hurt anything either.

As an at idle problem you also need to check for vacuum leaks. This would have to be a fairly small one (or it would run much worse) which might make it hard to find. A vacuum gauge can be handy for helping to figure out if you might have one. A smoke test is the surest way to find one, though you can also feed some unlit propane around the intake/intake manifold/vacuum connections. A leak will suck in some propane & change the idle.

I did not clean the MAF…but will definitely try that. I will go ahead and do the wires as well this evening. I did not check each connection either, just unplugged and replugged the spark side. I will have to invest in the vaccum gauge and the fuel pressure gauge as well. I do need to change the fuel filter which i will have to locate. Where is it? What are your thoughts on the expensive dealer fuel flush BS?

The expensive flushes normally don’t do a thing. I might eventually give it a shot after I’d exhausted most everything else but mostly expect it to be a wallet flush. Most any shop offers this kind of service too - no need to use a dealer.

Pull the other ends of the plug wires. I once spent almost an entire day looking for a miss at idle. I did all sorts of ridiculous things and was pulling my hair out. I did start out by inspecting plugs & wires, but I was dumb enough to have only pulled a couple of plug wires to inspect them - it was a 6 cylinder & I think I pulled the top 3 off the coil pack (it was dodge caravan). They were all clean and shiny and pretty so I just moved on. Well, after a day of pulling my hair out I finally figured out that just one of those 6 connections was corroded at the coil pack connection. About $20 and 5 minutes and no more miss - after blowing an entire day aggravating myself.

Plugs and wires for this vehicle were over $100 bucks from the box stores. Plugs were $8 EA and the wires were $45. I was planning on taking the wires back today but will inevitably replace them this evening. It is worth it to me to try. Do you know if the fuel filter is an inline or a screw on type?..more than likely a screw on correct?

Its going to be inline. I don’t know exactly where it is on this truck - it will either be in the fuel lines up along the frame someplace or under the hood. A $20 repair manual from an auto parts store would be handy for all questions like this. (If you’re doing all of this you’ll get plenty of use out of one if you buy it).

At the moment I wouldn’t put a high priority on the fuel filter just b/c if it was clogged up or something you’d be having power problems, especially accelerating under load.

When I had a similar problem with my '95 Suburban, it was the plug wires. GM refused to cover it under warranty, saying it looked like ‘someone’ had poked holes in the jackets…right!

You are talking about throwing some money away here (the plug wire replacement, unless there was a fire under your hood your plug wires are fine, the scan confirms this, do you think you can install those wires as nice as they are installed now?) Consider throwing some money at a drivability technican. Don’t feel bad that 1960’s era diagnostic techniques are not working here. See a drivability technican.Are you trained at all in drivability issues for your car? I bet there is something you are trained in, do the old “I will make my money doing what I am trained for and pay you some for what you are trained for” swaparoo.

I’m going to completely disagree with oldschool. The relative youth of the wires and lack of codes tells you nothing. Actually the lack of codes tells me that there is no point in going to a drivability technician who, upon seeing no codes, will charge $100, shrug and revert to a 1960s era question such as “did you check out the plug wires…?” The wires are both free and easy to check out. If one just replaces them this is neither very expensive nor a waste of money.

On the other hand, his sentiment may simply be to not just throw parts at it. On that point I quite agree, but nothing done or suggested so far is throwing parts or wasting money.

There are people that can fix cars without the benifit of codes, quite a novel thought is it not? Get this truck on an O’scope and any problem with the wires will not be hard to see.In reality the OP’s sympton description does not point to a plug wire problem. Anybody check TSB’s? how about a PCM re-program?

OP (the guy writing the post). I suggest you explore SI(service information) 07-06-04-027. GM is saying that a concern that matches your description can be caused by reduced flow in one or more fuel injector. There is an injector flow test procedure detailed in this document. My TSB indicates GM is accumulating data to determine a root cause but it is looking like dirty injectors caused by poor quality fuel. If you want to put plug wires on your truck ,well that is clearly up to you.

A second “rough idle” TSB PIP3146C explains how a valve train issue can cause loss of compression for a very short time. This second TSB came up when I selcted your truck and engine size but it should be confirmed that it applies to your engine,I am not 100% sure.I think the first TSB stands a better chance to at least be a possible cause of your concern, there are 27 drivability TSB’s but some are revisions.

According to your logic oldschool there’s really not much to be done on these boards when someone has a problem with their car. You just say - “take it in to the dealer so an “expert” can figure it out for you. Otherwise you’re just wasting your money.” So maybe just make a cut and paste response like JE Meehan has for the check engine light. Then you can save a lot of time on typing.

The OPs symptom description doesn’t point to a fuel injector problem either - or one could say it points equally well to a wire problem or a fuel injector problem. Given 15 minutes and spending not one dime anyone could tell you about the plug wires. Not so for fuel injectors or TSBs or whatever. There you go from no time & money to lots of money paid out in labor time - and the evidence is ample that the money spent there is often wasted. I have wasted a lot of it myself which is why my last resort is to take it someone else.

I just find this strange - posts about problems with how a car is running come through constantly. The first things to do are always just check the basics. So what’s wrong with saying things like clean the MAF, check the wires, fuel pressure etc. These are not silly and wasteful things. They are simple and cheap things that save many people lots of time, money, and aggravation.

I don’t really see me saying “take it to the Dealer” but not such a big deal.OP I present what I present, you can work with a stratagey in mind or you can run amok,the choice is yours.Give the TSB(s) a read,they fit your concern perfectly (even use some of the same words).

Right up front in line of “checking the basics” is checking for TSB’s. Checking for TSB’s is part of a stratagey based diagnostic approach, this is how I was trained.You can bet the boss is watching how long you are spending on this car and when he hears your plan you can also bet he asks “did you check for TSB’s”?