04 Suburban 2500 stalling, stumbling, no codes... need help

**95z28man** · 08-23-2018, 05:04 PM

Guys I've had my 6.0 Suburban in the shop multiple times for this random issue that has proven difficult to diagnose. I don't want to keep running in and out of the shop so I need to start digging into this myself like I used to but I need help from anyone with experience. This truck starts and runs like a top. Good battery, new fuel pump assembly, new air filter, thoroughly cleaned throttle body and MAF, new crank position sensor, even a new PCM. The shop has tested for vacuum leaks, did what they called a wiggle test on all the wiring harness connections to see if something would act up from shaking the connections, nothing happened for them. This issue has only presented itself to me while driving. It has happened with a cool engine within a mile of driving, a heat soaked engine after long trips, with and without the AC on. The truck will be driving fine and then suddenly begins stumbling, running rough, shuts off at low speed and low rpms coming to a stop. I can sometimes keep it alive by shifting to neutral and revving it. I have become good at throwing it in neutral and restarting on the fly. It always starts right back up but sometimes will immediately act up again, sometimes it is fine for the rest of the trip. I have noticed that playing with the AC controls will cause it to act worse WHEN it already has started to stumble. On one occasion I was in cruise at 65 with AC on cold at notch 3, turned it down to notch 2 and it immediately surged and kept surging down the highway, almost slightly enough for the tach to not even pickup the rpm drops. No codes are being thrown but I can tell you one thing for sure from my blue driver scan tool, as soon as the problem hits, the short term fuel trims will drop and raise dramatically down to as low as -35 on both banks simultaneously. The charts just go crazy. Long terms not affected much. Charts will smooth out as the stumbling stops. Rinse, repeat. I'm at a loss as to what else to look at. It doesn't seem to do it for my shop so I really don't want to leave it there and pay them to drive it around all day using my fuel as well hoping it will act up and even then may not be able to nail it down. I can do a good bit of work myself and I intend to in order to stop spending my time and money at the shop. I will post again if I can think of anymore details, but hopefully you guys can give me some more ideas to run with. I need to pull the plugs and check them out as well, I have new ACdelco plugs and wires ready to go in but I was hoping to fix the problem first so I don't risk fouling any new plugs right away if these fuel trims are really that off.

Thanks guys, hopefully someone can help me out here.

**james07moore** · 08-23-2018, 05:14 PM

Can we cover a few basic things first....

When did this start happening? was the truck worked on prior to this issue? if so do you know what was done? I will definitely say the the fuel trims going crazy tells me it is an AIR/FUEL ratio issue. Have you had the intake off ? Are the intake gaskets ORANGE or BLUE?? Also, has anyone changed any of the oxygen sensors ? There is a possibility that the pre and post catalytic converter oxygen sensors are connected backwards....

James

**harner** · 08-23-2018, 05:15 PM

I am surprised there are no codes. I'd take a look at those plugs and see if you can narrow the problem down to a bank or even a cylinder. Maybe an injector is not working properly? Do you get gas from the same station all the time? Maybe bad gas? I'd inspect those plug wires as well.

My 06 Silverado used to have a weird stumble around 1500rpm. It never stalled, but would stumble when cruising at say 55mph in OD. Put a set of NGK's in and it's been running like a top aside from it needing valve seals and a small oil leak from somewhere. 237k miles and it keeps going.

**95z28man** · 08-23-2018, 06:35 PM

Bought the truck in Oct. Immediately had a new intake manifold and gasket installed due to a code that is escaping my memory now. So the manifold and gasket should be good to go. The first I noticed this issue was in late December. Going down a bumpy stone road up to pretty steep incline driveway, rolled to a stop and the truck shut off. Started right back up and was fine. Leaving the same road it was ok until i pulled out on to smooth pavement and it started to stumble but never died. Fast forward a few months and it hasn't been shutting off at all but will occasionally stumble after hitting a big dip or something in the road. Then a couple times I had to slam the brakes on the highway and it would start to act up after that. One day in heavy traffic it was hot and kept stalling frequently but could be started again in neutral no problem. That's when I had enough and figured it was fuel pump or filter and the truck has 138k on it with an intank filter and no service record of it being done so I had it done for peace of mind. Problem came back again. Swapped the crank sensor myself and took it back in to be relearned. Problem came back so now I leave it with the shop for a few days to actually try to diagnose it. Of course they can't get it to act up but that's when they did all the standard testing vacuum, fuel pressure etc. His gut was telling him it was the drive by wire TPS throttle body. Between myself and the shop we both had the intake tubing off and cleaned the front and back side of the throttle body and the IAC. Cleaned MAF, replaced filter. I also checked and cleaned all sensors and plugs in front of the throttle body. I got my blue driver scanner for this very issue now. This is when it started surging on the highway after turning down the AC a notch and I noticed the fuel trims going crazy. As far as i know nothing has been touched with the O2s. The fact that both simultaneously act up and the graphs for each look almost identical makes me think the sensors themselves are good. Remember only the short term trims are reacting this way, the long terms may be a little shaky but remain relatively steady and in acceptable ranges. Talked to my mechanic on the phone as this was happening and described it to him. He sat on it overnight and him and his brother could only think the PCM was going out because when I shut the truck off in my driveway and started it back up the fuel trims had reset, idle went back to normal and it seemed fine again. I had them do the computer and reprogram it. Seemed fine for a couple days and now just today as I was rounding a right curve it stumbled and the trim readings went super low again before seemingly fixing itself mid drive the rest of the way home. My original thought is maybe there was a large piece of debris in the tank getting sucked up then dropping when fuel pressure dropped. But now it seems more electrical to me.

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**95z28man** · 08-23-2018, 06:39 PM

And I always fill up at the same Exxon station with 93 fuel. I did run a small bottle of gumout in one tank prior to the issue seeming to get worse. Ill have to check the plugs and see what they look like as well. Bad gas would have been out of the system by now with how long it been going on.

Also, when the intake manifold was done I believe he mentioned something about the gasket had issues so they came up with a new one and thats what they put on my truck, haven't checked what color it is yet. Also should mention that all the parts my shop put in are GM parts so AC Delco and the like. I also have AC Delco plugs and wires ready to go in when I get some free time. I may end up swapping the TPS on the throttle body as well for shits and giggles and because it was my mechanics first gut feeling. I also read about a wire that goes from the AC somewhere, under the engine, up next to the power steering pump and to the battery I think. Someone had issues like mine and that wire ended up being slightly damaged enough to cause intermittent issues with no codes. But since the AC controls seem to play a part in adding to the severity of the issue when it pops up, that may be something to look at. Hopefully someone has been in my shoes before with one of these trucks and can pitch in.

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**Injuneer** · 08-23-2018, 07:53 PM

The electronic throttle control system integrates the demands of pedal position, cruise control, traction control, idle speed control, etc.. Anything that upsets one of those systems could throw the throttle control off. Responding to changes in engine load, e.g. changing A/C demand would affect it. And you indicate that even though the engine has the electronic throttle body, it still has an IAC valve? The systems I am familiar with simply use the throttle position servo to control idle. But then I haven't seen your specific setup.

Can your scan tool produce a data log in .csv or .xls format?

When your short terms go very “negative” (-35%), the PCM is reacting to the O2 sensors reading very rich. It won’t change the long terms unless the short terms stay that low for a period of time.

**james07moore** · 08-23-2018, 07:54 PM

its not bad gas nor is it a plug, plug wire or an injector issue. From the description that you just gave has me thinking that it is a wiring issue simply because of the condition that make it happen. I have had plenty of experience with drivability issues that do NOT set codes. The code that sets due to intake gasket failure is a lean fuel trim code p0170/p0171.

what are your long term fuel trim numbers??
Can you monitor the O2 sensors with your scanner ??

**james07moore** · 08-23-2018, 07:56 PM

Hey Injuneer.. LONG TIME no see! I hope all has been well for you SIR!! I still remember all of your HELP and you are very much appreciated !!!

**95z28man** · 08-23-2018, 08:59 PM

Sorry I may have used the wrong term but my throttle body has a small hole in it above the blade that I assumed was used to regulate the idle when the blade is closed. It is a drive by wire throttle body with no physical cable attaching to it. The long term trims in both banks fluctuate around in the low negatives ie 0 to maybe -7 or 8%. The short terms rapidly jump up and down so quickly I can't keep up. One second -30s next second could be -10 or 15... The blue driver scanner does save my logs but I was having trouble figuring out how to play them back. I will post a pic of the highway trip that you will see a major sudden dip in the graph when it started. I too, feel like this has to be electrical. I need to get under it and check every ground I can see as well. I did check the ground from the body to the back of the block and it looked ok and felt solid. I know that doesn't rule out corrosion under the connection but it's so hard to get to it will be the last one I try.
Notice in the screenshot about 3/4 through each short term trim graph there is a major spike and it gets very erratic from there on out.

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**james07moore** · 08-24-2018, 06:27 AM

Those numbers tell me that you are running RICH and the computer is trying to lean things out (REMOVE some of the fuel). Have you inspected your MASS airflow sensor, has your air box been modified? Have you verified your fuel pressures?? This is NO vacuum leak for sure, a vacuum leak on the intake side would cause a lean condition and your trim numbers would be all in the POSITIVE. This is why you have no codes, the computer has NOT identified an issue, its simply trying to get the air/fuel ratio correct.