2005 Express 2500 idling, hesitation, and gear change fluctuations

[ChevyExpressVan203]

I own a 2005 Chevy Express (Extended body) cargo van 4.8L V8 with 115K miles. I have an issue that my mechanic is unable to solve - he referred to it as a gremlin!

The van starts and runs without any great problems at first but if you drive it for a while and come to a stop sign, the van will often struggle to idle - the idle will be rough and choppy and will often result in a stall. But the van will always fire right back up again when it does stall. The second issue (i'm convinced they are all tied together) is that when idling with the AC running, the AC will often stop blowing cold (sometimes when it begins to struggle to idle) and sometimes when it seems to be running ok.

The last component, and the most troublesome is when driving down the road, holding a constant throttle position (on a flat smooth road - no changes in load) at any speed over 30 mph, the truck will either gear down, then up, then down then up - or, the power will begin to fade slightly, prompting me to press harder on the gas, which will gear it down (causing it to race) - but I want to go at the same speed so I need to lift off again - causing it change back up - and the problem persists. If I fully depress the gas pedal the truck will often gear down (as it should), allowing me to build up some speed. As I then let off the gas it will shift up into a higher point in the new gear range but will shortly start to do the same thing if I slowly try to accelerate. It is as though the engine and transmission are having a mis-communication and cant get on the same page. But my gut tells me the transmission is simply responding to an engine that is acting-out. Incidentally, when the truck does shift, the shifts are silky smooth every time. The transmission makes no odd noises and other than the random changes it seems to be fine.

The van doesn't seem to be misfiring, it sounds relatively sharp and crisp when revved in neutral. My mechanic has reached the end of his usefulness and has thrown his hands up. We have changed the fuel pump and fuel filter, one ignition coil (he said it was putting out a code one time when it was in his shop and it pointed to a particular coil bank), changed the plugs, the plug leads, and the throttle position sensor.

I think the last three parts were just thrown at the truck so he could justify his bill for chasing this "gremlin". I need a new avenue to explore as nothing so far has worked and I cannot afford to have someone keep making guesses. My old mechanic offered that the idling issue may be due to a faulty idle control sensor, but I am reluctant to pursue his advice at this point.

Thank you in advance for any assistance you can offer.

[MT-2500]

Originally Posted by ChevyExpressVan203

I own a 2005 Chevy Express (Extended body) cargo van 4.8L V8 with 115K miles. I have an issue that my mechanic is unable to solve - he referred to it as a gremlin!

The van starts and runs without any great problems at first but if you drive it for a while and come to a stop sign, the van will often struggle to idle - the idle will be rough and choppy and will often result in a stall. But the van will always fire right back up again when it does stall. The second issue (i'm convinced they are all tied together) is that when idling with the AC running, the AC will often stop blowing cold (sometimes when it begins to struggle to idle) and sometimes when it seems to be running ok.

The last component, and the most troublesome is when driving down the road, holding a constant throttle position (on a flat smooth road - no changes in load) at any speed over 30 mph, the truck will either gear down, then up, then down then up - or, the power will begin to fade slightly, prompting me to press harder on the gas, which will gear it down (causing it to race) - but I want to go at the same speed so I need to lift off again - causing it change back up - and the problem persists. If I fully depress the gas pedal the truck will often gear down (as it should), allowing me to build up some speed. As I then let off the gas it will shift up into a higher point in the new gear range but will shortly start to do the same thing if I slowly try to accelerate. It is as though the engine and transmission are having a mis-communication and cant get on the same page. But my gut tells me the transmission is simply responding to an engine that is acting-out. Incidentally, when the truck does shift, the shifts are silky smooth every time. The transmission makes no odd noises and other than the random changes it seems to be fine.

The van doesn't seem to be misfiring, it sounds relatively sharp and crisp when revved in neutral. My mechanic has reached the end of his usefulness and has thrown his hands up. We have changed the fuel pump and fuel filter, one ignition coil (he said it was putting out a code one time when it was in his shop and it pointed to a particular coil bank), changed the plugs, the plug leads, and the throttle position sensor.

I think the last three parts were just thrown at the truck so he could justify his bill for chasing this "gremlin". I need a new avenue to explore as nothing so far has worked and I cannot afford to have someone keep making guesses. My old mechanic offered that the idling issue may be due to a faulty idle control sensor, but I am reluctant to pursue his advice at this point.

Thank you in advance for any assistance you can offer.

Why not go for that?

[ChevyExpressVan203]

a) because everything he suggested so far has turned up no improvement, b) as I understand it the ICV will only solve the smallest component of the issues - not that it doesn't need to be done as part of the larger remedy, or c) because I was hoping for independent insight as to what may be causing these issues, not just a "give that a shot then".

[Tech II]

Did your "mechanic", test drive the vehicle with a scan tool attached, to see if the problem was a misfire or possibly a slip condition with the transmission?

If it was a misfire, the scan tool would have narrowed it down to which cylinder was at fault.....if it was a tranny problem, it would have shown rpm slippage or if the TCC was coming in and out of lockup, or if there was slippage in a gear.....

We can only make suggestions.....we are not there physically touching the car....you mention a code, but don't tell us what the code is.....the IAC was a possibility, which can be checked out, and if ok, does not have to be replaced(too many parts have been replaced already).....another possibility is a vac leak that does not rear it's ugly head until the engine is warm, and things start to expand....

There are mechanics and there are technicians......driveability problems should be handled by a technician.....unfortunately, these vans, due to accessibility, make it hard to do proper testing on them....

If you are at your wits end, a GM dealership is the next logical step....ask for their best diagnostician....chances are he has run into this problem before, since he works on these all the time....

[ChevyExpressVan203]

Tech II,

Thank you very much for your input. Unfortunately I do not know if the truck was driven with diagnosis tools attached. But I did reiterate several times to my former mechanic that simply hooking the truck up (statically) to a code reader was not what I was looking for. I needed/wanted someone to drive the vehicle, in traffic, on the highway etc... - to get a complete understanding of the issues. Apparently that was not done because when I was notified it was ready for pick-up and that it was running "great", it almost immediately began exhibiting the very same issues.

I am going to do some research on reliable and trustworthy technicians here in the Lubbock area. I would welcome recommendations should any forum members know of a mechanic they trust. Failing any direct referrals I will certainly consult with a local GM dealership for further exploration. I firmly believe that it is not a transmission issue and that the erratic gear changing is simply a response to varying output torque from the engine.

In researching this issue further I found another forum where someone was describing somewhat similar issues (with a larger emphasis on the idling problem and a GM technician offered the following possible solutions to consider:

Check for air leaks in the Air Induction system (e.g., in the intake manifold)
Check for air leaks in the PCV system and in its related hoses and/or valve
Idle air inlet passage or throttle bore is dirty or full of deposits
IAC valve has failed
MAF sensor is dirty, "skewed" or installed improperly
Throttle plate, throttle shaft or linkage is damaged or sticking
TP sensor is out-of-range or "skewed" high

I would hope its not the last issue as my TP sensor was just changed.

Thank you again for your time and advice. I welcome any additional input you may have.

[Makeluck]

Hi, i know this thread is very old, but did you ever come to a conclusion or find that gremlin? I have been chasing that exact issue for several months now on a 2006 express 4.8l 2500... lots of $$$ has been spent "guessing" and several mechanics as well as a GM dealer involved with still no answer :/ Thanks!

[Blue Bowtie]

Check with your service department to review whether the vehicle has had any TSBs including a re-flash or the PCM for such an issue.

There is at least one TSB, 050604077, which might be related. There are others, depending upon the transmission installed and other variables.

[fromgeorgia]

New guy here but have info that may help. A few years ago I hit a deer with my 2005 Express 3500 (6L). Shop fixed damage and van did great for three days. After much head scratching we found a module underhood in front of steering wheel that had one busted clip, the remaining clip held well enough for intermittent contact. This caused the van to run on 4 cylinders as to prompt me to get it to a shop. Mech showed me a GM bulletin about moisture getting into this connection. May be worth a look.