Engine surging and stalling

[400Magnum]

The current problem is that while cruising down the road at 1500 rpm and 50 mph, the engine speed will bounce up and down 200 rpm. It only seems to happen at this speed/rpm combination. It has been doing this for a little while and we've not had a lot of issue with it. At first it was only once in a while, but lately it's happens more often. Sometimes while driving along if you take your foot off the accelerator to start slowing down, the engine speed will drop a lot, like it's going to stall, and then comes back up to normal. Then the other day while waiting in the drivethru, the engine just quit. It started right up though, no issue there.

I replaced the thermostat about four months ago because it was running cool. While at it, I did a thorough cleaning of the throttle body (it was disgusting). It ran great for a while, and then this issue started.

Before replacing the thermostat, it started throwing the code P0410, for the AIR system. My understanding that this is a minor emissions system that only comes into play during the cold start.

The whole backstory is this. My wife primarily drives the Envoy to work, which takes her about 5-10 minutes at 20-30 mph. Every once in a while (maybe once or twice a week) she'll drive it for about 20-30 minutes at 50-60 mph to meet with some co-workers. Then maybe once every other month she drives it for about an hour at 70-80 mph. So it really doesn't get a lot of chance to run good and hot for an extended period of time. Right now the vehicle has about 135k miles on it.

About eight months ago we had to have the transmission rebuilt. The shop said they had to replace the sun gear and shell, and put in some "better" parts and a shift kit, but didn't give me a detailed list of exactly what they put in that was "better". I do believe that they added a shift kit because the one-two shift is CRISP!! It isn't nearly as sluggish as it was before. The previous owner apparently towed with this vehicle, and I'm wondering if some damage was done from that which is why they got rid of the vehicle.

So I have been doing a lot of research and have come to some idea that it could be one of (or several) of the following, since none of these have ever been replaced as far as I know:
1. Canister purge valve
2. Canister vent valve solenoid
3. Oxygen sensors
4. Fuel filter
5. Fuel pressure regulator

Hooking up the OBD reader shows that the long term fuel trim is right at -3%.

I am going to check to see if I can find any leaks in the vacuum line coming from the brake booster, or any other vacuum line. I read on the old site that this could cause the problem. I've also read that the diaphragm in the brake booster might cause the surging, Is there any way to check the diaphragm other than replacing the brake booster?

I'm also going to look for any leak around the exhaust manifold. Any suggestions on how to do this effectively?

I'm wondering if there might be something acting up in the transmission that wasn't replaced. I've read that the Torque Converter control valve might cause the surging problem, but that wouldn't explain the stalling and near stalling problems, would it?

Is it possible that the problem I'm having is due to the AIR system issue?

Any other things I should look for? Any comments or thoughts? Any ideas or suggestions would be greatly appreciated.

 

[Texan]

First question. What did you do about the PO410 code? Is the CEL on? If so, have you read the codes?
I think that you meant check the "intake" manifold for loose bolts or leaks.

 

[400Magnum]

I haven't done anything about the P0410 code yet, that's why I'm on here asking if that could be cause of the stalling and surging, or should I look elsewhere. My wife is going to be gone for a week soon, so I will be able to work on the car uninterrupted without her needing the car, and I'd like to have as much ready ahead of time as possible.

The CEL is on, that's where I got the P0410 code.

The other stuff that I read regarding stalling and surging said they were checking the exhaust manifold for leaks, that's why I said exhaust. Perhaps they meant to say intake, I don't know. Is leaking intake and/or exhaust manifolds a problem on these engines?

 

[KNBlazer]

Take the truck back to the transmission shop and have them do a diagnosis.... the sooner the better, otherwise if it ends up being your transmission, they can void your warranty because you didn't act on it quicker....

 

[gmcman]

What I think you're experiencing is the torque converter clutch engaging and disengaging under a load since the engine is struggling slightly at 1500 RPM. This isn't exactly a torquey area of the powerband.

Of course like you said that's not the entire problem so why is it doing this?

I agree to document the issues and take it back if under warranty before you start taking things apart.

As far as stalling, it's either losing fuel or air with spark a distant third. If your fuel pressure checks out then I would look at the throttle body again.

When you say that when slowing down with your foot off the accel pedal it acts like it's going to stall....what speeds is this happening at? It's possible the torque converter clutch isn't disengaging and bringing the RPM's down until it forcefully disengages.

While cruising at 50-60 MPH, keep slight pressure on the accel pedal and watch for the lock ( slight drop in RPM). Then without lifting off the gas which would disengage the TCC see if you can very slightly press on the gas and not notice and RPM jump indicating TCC lockup. Then let off the gas and see if the tach jumps up slightly showing TCC disengagement.

If the TCC won't unlock then oviously that's a problem.

Also it's a possibility that your PCM may need a reflash.

 

[400Magnum]

The problem is that we don't feel any surging when its over 50 mph, it only does it between 45 and 50 mph, but I'll try your suggestion and see what happens.

I had the PCM reprogrammed by PCM of NC a few years ago, when the trans was bone stock. The PCM is only mildly modified, aimed more for fuel economy than performance, but I did request for slightly crisper shifting just to try to preserve the transmission a little. Do you think that I may need to get the PCM reflashed to take into account the shift kit installed by the repair shop? Would that have any bearing on what's happening now?