[FIXED] Aveo 2007 Rough Idle and Low Vacuum

I'm working on fixing a 2007 Chevrolet Aveo for a relative.

The idle is rough and manifold vacuum is only 14" hg. The timing belt was changed a while ago and the driver thinks that it started after the timing belt change. No misfire codes at this point. I liked the example in PD's book and videos of comparing peak compression to cylinder spark event to determine if the timing is off and did that test. I have a couple of waveform .bmps I would like opinions on. In each, the top trace is a current probe around the battery cable during cranking and the second trace is a capacitive ignition probe around the cylinder 1 plug wire.

Hey RH6400!

At first glance, it looks to me like spark timing is OK. A little bit retarded of TDC, sure, but that may be the default timing strategy on this engine. I'm eager to hear what others think of your captures.

What actually caught my eye was the relative compression waveform itself. The large U-shaped valleys in between the compression humps looks unusual... Kinda reminds me of other captures I've seen on engines with jumped timing. Not saying that's your issue, just thinking out loud.

If you can get to the cam and crank sensors, we can try a cam/crank waveform compared to a known good, and see if there's anything obviously out of alignment.

how does it run at higher rpms?? might be an egr valve issue stuck open

Hi Tyler,

I like the cam and crank sensor idea and I will probably try that tomorrow. I didn't try that first because I could not find a known good one.

Hi cheryl hartkorn,

It runs OK at higher RPMs. I wanted to eliminate all possibility of it being the EGR valve so I used a piece of shim stock to temporaily block the flow and that made no improvement in the idle.

any misfires on the counters?

No, no misfire count activity when last checked.

Hi Tyler,

Attached are captures of the crank and cam sensors.

here is a known good.

Hi gav09,

Thank you for the waveform. Is it from an Aveo 2005-2007? Also, it appears to me that mine is off a little. Do you have any thoughts on it?

Bob

The cam/crank relationship seems OK from what I can see. It's tough to get an exact tooth count, but I saw 26 teeth from the sync notch to the cam transition. As this is a DOHC engine, it's still possible that the other cam is out of time, but you'd have to pull the top timing cover to verify.

Does a power balance test indicate if one cylinder is weaker than the others?

I do have the upper cover off. The lower cover is more involved to remove. I used a dowel to find TDC and the cam marks line up at TDC compression of cylinder four. I thought they should line up for that condition of cylinder one but looking at the Chilton manual with a magnifying glass that appears to be correct. I have not done a power balance test yet. My scan tool does not do it and I have not tried grounding the plug wires at this point. Does the 14" hg of vacuum, fairly steady point in any direction?

That waveform was from a 2008. Its hard for me to see the amount of pulses on your waveform. In mitchell it doesn't say whether the crank mark is TDC #1. The cam gears are marked with a "I" for intake and a "E" for exhaust. .

Thanks gav09. A 2008 should be the same. The engine in the 2007 I'm working on is a L91. The sprocket position you posted is the way mine line up with Cylinder Four at TDC compression. I'm not sure what direction to go in with this car but I know the owner is very sure the rough idle started after the timing belt replacement.

So the crank lines up with its mark, and the two cam marks are level with each other? I don't suppose someone swapped the cams side to side? i.e. Intake on the exhaust cam, like that.

I suppose it's possible that one of the cam keyway a sheared off, but the only way to know that would be to remove the cam sprockets...

Only other thing I can think of is that they got the timing wrong the first time around, tweaked the valves on the pistons, and now they're not sealing correctly.

I have not removed the lower cover so I'm not sure the crank mark is correct but using TDC found with a dowel looks like it is OK. The exhaust cam is unique in that it has two marks and a pin that should be at 11:00 indicates which mark is the one to use so the cam sprockets are in the correct spots. The tweaked valves are a possibility but the leakage was < 10% on all cylinders with a leakdown test. So far there does not seem to be anything that really stands out except that 14" hg of vacuum. If I had my wish I would have a known good 2007 Aveo parked next to the bad one to check some things on, but I don't think anyone with one would go along with that.

Did some more work on this today. The attached photos show the crank mark lined up and the resulting position of the cam marks. This does not look close enough to me but I would like input from the forum. The last picture shows where the crank mark is with the cam marks horizontal and next to each other as they should be.

I think you found it! The picture with the crank mark aligned shows the issue better, but either way, timing is out.

It looks like the exhaust cam is one tooth retarded, IMO. You can try the ol' golf tee trick and see if you can pop the cam back into time without taking everything apart.

Thanks for the response Tyler. Can you describe the golf tee trick? I can guess the general idea but I would like to know the details.

RH6400 wrote: Thanks for the response Tyler. Can you describe the golf tee trick? I can guess the general idea but I would like to know the details.

Sure! Basically, you put a golf tee (or the smooth end of a drill bit, whatever) into a valley of the belt sprocket, and rotate the belt so the drill bit/tee ends up under the belt, and gets dropped out the other side. This ends up making the belt jump a tooth on that sprocket.

You insert the bit/tee into the side of the sprocket that you need to rotate towards. In other words, if you want the sprocket to advance, you put the bit on the right side of the sprocket and turn the belt counterclockwise. When the bit comes out the other side, the sprocket should have advanced one tooth from it's previous position. If you want to retard a sprocket, put the bit on the left side and turn the belt clockwise.

It's a real time saver on timing belt jobs when you don't get it right the first time, but don't want to pull everything apart to reset it. :lol: