The dreaded bucking/stumbling problem....2008

[gone.h2]

Hi everyone....I'll try not to ramble because I know this has been discussed A TON yet there always seems to be different solutions. I'm fairly experienced with working on cars, but no pro by any stretch. I'm using an OBDLink MX scanner/transmitter that let's me log individual sensors in real time as well as all the basic code reader functions (well worth the relatively tiny investment IMO).

2008 Wave, 4dr, LS, 200k, automatic...my son's car and he's not mechanical so I don't know if this problem came on gradually or all of a sudden (he and his mother tend to drive with a problem for a while before speaking up).

When the car is cold it runs and shifts perfect. As it starts to warm up it IMMEDIATELY starts bucking and surging and stumbling. When I test drive it I can predict exactly when it's going to start failing just by watching the temp on my scanner.

First thing I found was a code for the cam position sensor, so I replaced that. No change. No more codes.

I read about the valve cover gasket problems and sure enough the gasket was leaking oil and the plugs were drowning. Replaced the gasket and replaced the plugs (good old OEM style copper). No more oil leaks but it still runs terrible once warmed up. I think the plug wires are ok based on inspection, and the fact that it runs great when cold.

O2 sensors are reading as they should (upstream reading hi/lo and downstream reading fairly steady (high) after warm up), but I had a spare so I changed the upstream one anyway. No change. I'm not bothering with the downstream sensor right now because that shouldn't affect drivability. The converter seems to be ok based on the readings I'm seeing.

I haven't done a deep dive to look for vacuum leaks, but checking the easily accessible lines revealed no splits or cracks.

I read about the intake hose rotting problem and sure enough, the previous owner had wrapped the hose in THREE different layers of tape (I didn't notice this after first because he used black tape on the last layer). I removed the tape and the hose basically fell apart. I'm waiting on a new hose that should arrive tomorrow from Amazon. Delightfully cheap ($25CDN).

IMO the car behaves like it's starving for fuel intermittently, as though the ECM is sending erratic signals to the injectors. Running fine when cold makes sense to me since the ECM is going to make it run rich until it warms up, masking the problem.


So as I wait for the intake hose to arrive tomorrow, I'm trying to be proactive and ask anyone here who will offer an opinion:

a) could leaks in the intake hose cause it to run THAT poorly? It seems too good to be true that a new intake hose would fix such a terrible drivability issue, but with the air flow sensor being right there perhaps a small leak after the sensor is all it takes?

b) if the new hose doesn't fix the issue, and given what I've already checked, what should be my next move....and the next.....and the next LOL


Thanks VERY much to anyone who takes the time to offer advice. I really appreciate it. I belong to several sites like this for other cars I own/have owned and was pleasantly surprised to find the same type of site and support for the humble Aveo/Wave.

Cheers

Kevin

[Deano]

Hi Kevin,

In my opinion, your comment: "IMO the car behaves like it's starving for fuel intermittently, as though the ECM is sending erratic signals to the injectors. Running fine when cold makes sense to me since the ECM is going to make it run rich until it warms up, masking the problem." - that is probably a very good clue. From similar experience, I suspect you may have a poor ground connection somewhere, that is sending erratic signals to the ECM. Have you checked the condition of the main wiring harness to chassis ground connection right in front of the ECM? I think it's G103 (on a 2009 Aveo). Mine looked fine, until I tried to unbolt it. The bolt snapped off because it was so rusty underneath. I cleaned-up the rusted surfaces of that ground connection, bolted it back up, and my bucking and poor driveability problems disappeared.

Might be worth checking.

Deano

[gone.h2]

Thanks very much Deano...I'll definitely have a look at that.

Can I ask if, when you had the drivability problems, you noticed whether it was better or worse when the engine was cold?

[Deano]

When I had similar driveability problems, it first showed up within a few minutes of driving after starting from a cold engine. The bucking and driveability was so bad, the car was basically undriveable. Managed to nurse the car back to the garage, and didn't try driving it again until i had fixed the problem. So i didn't get a chance to compare the problem with both cold and hot engines.

Deano

[avguy]

I recommend replacing the plug wires, because it's very common for them to permanently degrade when subjected to oil. Static resistance testing of the wires doesn't mean much because the problem only shows up under load. And no, the intake hose is not likely to be the cause of the problem. I had my '05 intake taped up for years, and it ran like a top right up until rust overtook the body and sent it to the boneyard.

[gone.h2]

Update:

As predicted by avguy the new air intake hose did not help.

I have not yet looked at the ground for the ECM but I plan to...thanks Deano (behind passenger kick panel?).

I am going to replace the spark plug wires and see if that helps. Something occurred to me after avguy's suggestion: This driveability problem was not an issue before a recent valve job (yep...broken timing belt) nor did it appear immediately afterward. So this issue came on some time after the valve job and somewhat gradually...which coincides roughly with the new, incorrectly installed, leaking-like-sieve, valve cover gasket filling the plug wells with oil. I thought the wires seemed to be in good shape but avguy rightly points out that you can't SEE problems with spark plug wires especially if they work fine at idle but not under load. Fingers crossed.

[Deano]

The wiring harness ground connection that I was referring to is under the hood, right in front of the ECM. The wiring harness might also have a ground connection near the starter. But i recommend you first check the ground connection by the ECM.

Deano

[gone.h2]

Update: spark plug wires didn't help

I'm going to go have a look at the ECM ground right now.

Another thing I've been thinking about is the catalytic converter. I've seen lots of discussion about the cats on these cars plugging up.
Can someone please tell me what the signs of a plugged converter would be? I think I have a general idea, but if anyone can share experiences or information specific to this car I'd appreciate it. The car won't rev past 3000rpm now...even in neutral. Does that sound consistent with a converter problem?

I was also starting to wonder about the coil pack...would a failing coil pack cause the engine to not rev even in neutral?

Still no codes except today I started getting misfire codes ie 300, 303...

I'm starting to think about starting a new thread because I don't know that 'bucking' and 'stumbling' is an accurate description anymore. The gist of it is that the engine won't produce power and won't rev past 3000...the 'surging' is more of a dying/running/dying/running cycle. Does that make sense?

Frustrating...

[avguy]

You can test for a plugged cat by removing the upstream O2 sensor and running for a short time, to see if performace improves noticably with an another way out for the exhaust. It's naturally a bit loud, so doing this test might depend on what your neighbors are like. If that doesn't change anything, then I'd recommend a compression/leak down test.

[gone.h2]

Thanks very much....great idea. I wouldn't have thought that such a small hole would make much difference but I guess there only only needs to be a small difference to prove the theory. I just changed the upstream sensor so it will be easy to remove.

Is there any way to tell if the cat is plugged from the downstream O2 sensor readings? I can watch the reading in real time with my scanner.

Really don't feel like taking the exhaust apart for nothing.