2008 Aveo 5 speed engine bucks at low RPM

[OG-Lou]

Problem Solved.
Back when the valve cover gasket leaked oil into the spark plug wells, the oil must have carbonized-created a weak track to ground so the spark plug only fired on the exhaust stroke. On the intake stroke the internal cylinder pressure is higher and requires more voltage to fire the spark plug=the higher voltage went to ground.

There was a black line down the porcelain insulator and a faint line inside the spark plug boot.

It took 1 new (used) spark plug and one spark plug wire but I installed a wire set.


Yes, more compression (compression stroke) requires more voltage for the spark plug to fire. I had a spark plug tester that when plugs were tested under air pressure, it took more voltage for the spark plug to fire the higher the air pressure was.

2008 Aveo 5 speed, no A/C, no Cruise control, no electric door locks, just a radio, heater, and rear window heater.

Current Problem The engine bucks under very light load from idle up to 1200/1400 RPM when accelerating. Under moderate load when starting from a stop, the engine bucks from the beginning speed up to about 1600/1800 RPM. If the throttle is opened almost all the way, the engine bucks up to about 2100/2200 RPM.
I had my Zurich ZR-13 OBD-II scanned connected in graph mode and set on various live data points, such as short fuel trim, RPM and timing advance that seem like it might give me some clues as to what is happening when the engine bucks.

1.RPM was just to see in what range the bucking was happening.
2.The short term fuel trim I can't say if it was normal or not but my guess it wasn't out of range.
3.However, the timing advance graph correlated almost perfectly with the bucking. Sometimes the timing was advanced say +10 degrees and then went to -5 degrees, back to +15 degrees, back to 0 degrees and when the bucking stopped, the timing went to over +20 to 35 degrees advanced.

In other words, when the timing advanced changed back to around zero and advanced a little bit, the engine bucked. When the timing went and stayed over +15 degrees, the engine ran smooth.
I have a 2009 Aveo-5 similarly equipped and the timing advance does not bounce around much and the engine runs smooth.

Two years ago I replaced the timing belt, water pump, pulleys, spark plugs (Iridium), valve cover gasket (oil in the spark plug wells) and did some other maintenance. This is when I first noticed some bucking. I also had a crankshaft -camshaft error code. I re-installed new copper core spark plugs and replaced the camshaft position sensor. The wiring and spark plug wires all tested good. The bucking decreased to a point it only happened on wide-open-throttle starts from a complete stop. I have another thread on this forum about that repair and issues. Shortly after the timing kit was installed I got a throttle body code and replaced the throttle body and cleaned the EGR crud from the intake manifold. I also rechecked the timing belt synchronization 4 times and know for an absolute fact it is correct.

Recently the bucking got worse as I said above. there have been NO recent codes since the timing belt kit was installed except the MIL/check engine light never came on when you start the engine, like it is supposed to work. The solder joints in the center cluster were cracked so I re-soldered the resistors and cleared the code for the MIL and seatbelt lights not working. BTW, thanks to the other Aveo members for their auxiliary information center repair information. It was an easy fix.

I sprayed carb cleaner on all of the vacuum joints and hoses with the engine idling, looking for a vacuum leak. No change in engine RPM when I sprayed everything I could get to. If there was a vacuum leak, the RPM should change if carb spray entered the leak. I inspected the large diameter rubber air tube from the air cleaner to the throttle body and didn't find any leaks or holes. I re-tested the sparkplug wires and they test 5K ohms.

Next I filled the tank with no ethanol premium because maybe the engine knock sensor was hearing a knock I couldn't hear. The engine still bucked.

I changed the Knock sensor and still had premium fuel in the tank. It still bucked.

I didn't have any codes but I decided to change the camshaft position sensor again just because it seemed to decrease the bucking 2 years ago. It still bucked.

I have a Tech-2 replacement diagnostic device** that connects the OBD-2 port to a laptop computer and it can do a "re-learn" procedure for the crankshaft position sensor. I did the "re-learn" and it still bucks.

**VXDIAG VCX NANO OBD2 Support GM/OPEL GDS2 & Tech2win Diagnostic Tool Code Scanner
We need a whole area dedicated to this tool. It replaces a very expensive diagnostic tool from the past. It isn't a scope but it is reasonable cost wise.

Here is a link describing what the VXDIAG VCX NAN is used for.
https://vxdiag.blogspot.com/2018/12/...x-nano-gm.html

I had a spare crankshaft position sensor so I installed it. The engine still bucked.

I had a used ECM-computer from another 2008 Aveo similarly equipped, installed it and the engine still bucked. I tried a coil from another Aveo and the engine still bucked.

There is no change after the engine warms up. With a cold engine the bucking is less noticeable.

I did a search on the Aveo forum for bucking and manual transmission surges and it seems no one fixed their bucking or power losses.

[avguy]

.... The short term fuel trim I can't say if it was normal or not but my guess it wasn't out of range....

What are the actual ST an LT trim valves when running ok, and also when it's bucking? Any deposits on the plugs, or anything else abnormal about their condition? Have you done a compression and fuel pressure test?

[Eg.h2o]

Would an IAC motor give you these issues?

[OG-Lou]

Eg.H20 Would an IAC motor give you these issues?

At idle the engine runs smoothly. Off idle and with a load is when the bucking happens. The throttle body is new. The bucking is the same with the old throttle body and the new throttle body. I changed the throttle body because it caused a code. It has two sensors in it and the values were not in sync. To me it worked well but a code came up so I changed it.

The sensors can be changed in Chevy V8 engines but the kit is almost as money as the whole Aveo throttle body. It also looked like too small of work without a lighted magnifying glass so I just paid a little more for the whole throttle body.

The typical issues with a defective IAC are:
1. Irregular idle speed.
2. Check Engine Light (MIL) comes on.
3. Engine stalling.
I don't have any of those problems.

[OG-Lou]

Avguy

I have been sidelined for a few days because of a bone-spur on my knee. Going down stairs was a "B". The pain is decreasing so I can do more in a few more days.

The spark plugs are almost new (2K miles) but I haven't checked them in a while. I did check the compression 2K miles ago and app cylinders were within 5# of each other. I have NOT checked the fuel pressure because this bucking is at low RPM. At hwy speed the engine runs perfect. I will do a fuel PSI and Short term / long tern fuel trim check-test soon. I did some of these tests briefly and nothing correlated with the bucking. I know STF trim normally jumps around within a range so it is difficult for me to tell what is too much change.


My current problem is mostly an extension of an earlier set of posts.
https://www.aveoforum.com/forum/f82/2008-p0340-22978/

I am working on having a Chevy garage employee do a few tests if I can't get this bucking figured out. I think I can get a deal.

[slayerized6]

When was the last time you replaced the coil pack? It might be time for another tune up. Bucking at low RPM is usually an ignition problem

[OG-Lou]

The coil was replaced and no changes happened.

Today I did a compression test, 175 on all cylinders.

I did notice 2 black lines on the spark plug insulator from the #3 cylinder. Sure enough 2 lines were on the inside of the spark plug boot from where the ignition wire clip that grips the top of the spark plug all the way to the steel body of the spark plug. The wire itself is good, it is just grounding out the spark sometimes according to the 2 black traces on the spark plug insulator.

I cut off the bottom end of the damaged rubber boot to eliminate the spark's path to ground. I rechecked the spark plugs gap. .044. I have a new set of spark plug wires ordered. They should be here in a week.

The road test revealed a slight improvement but nothing major.

I also did another crankshaft sensor relearn. No improvements other than the slight improvement with the fix of #3 cylinder spark to ground.

Why the grounding of the spark on #3, I suppose the original valve cover oil leak that filled up the spark plug wells with oil had something to do with the 2 black traces on the spark plug insulator.

Since I installed a new valve cover gasket, no more oil has built up in the spark plug wells.

On my clock and MIL, seat belt warning light, ETC supplementary instrument panel (part number 96836870 that is no longer available), that I repaired by re-soldering the 2 resistors last week. I went to the auto recycler and bought the same thing from a 2007 Aveo without cruise control and installed it on my 2008 Aveo. It worked OK for the 4 days I had it installed so maybe a part (96413710) from a 2007 W/O cruise control, will solve the "no longer available" problem. Just to be on the safe side after using the 2007 clock for 4 days, I took it apart and added solder to those 2 resistor attachment at 4 points.

There are a few places that have a "new old stock" clock for a 2008 but a $10 used one was quicker and cheaper than the $60 or $70 for the on-line units..

The supposedly excellent local mechanic I called has not contacted me so I need to call him again.

Before the bucking my fuel MPG was around 35 - 36 in town but now it is down to 31.6 MPG AND the way it functions it similar to someone that is totally new to a stick shift with the bucking and jerking. It is probably hard on several of the parts on the car.

Tomorrow I will do a fuel pressure check but at high speeds (over 2,000 RPM) the engine is smooth as silk so I do not expect to find low fuel pressure at idle or off idle.

[OG-Lou]

Fuel pressure test today,results 56#PSI at idle and maybe a 2# gain upon opening the throttle,then back to 56#.

No codes and the engine still bucks at low RPM under light throttle. The more I open the throttle,the more bucking I get up to around 2,500 RPM. At hwy speeds the engine runs smooth.


Yesterday I did another crankshaft relearn procedure again using the VXDIAG VCX NANO "TECH-2" clone. Maybe the tool isn't working and maybe it is. I have read a few stories where a shop did the re-learn procedure but it didn't take.

The VXDIAG VCX NANO menu is not the easiest to navigate.

Here is a link describing what the VXDIAG VCX NAN is used for.
https://vxdiag.blogspot.com/2018/12/...x-nano-gm.html

Prior to 1986 I had an "ACE Master Mechanic" certification but medical issues forced me to change career's so I didn't re-new my certification. Yes, things are a bit different now compared to 1986 and I appreciate that the fuel mileage has increased.

[avguy]

... Today I did a compression test, 175 on all cylinders. ....

The compression on both of our first gen Aveos was right at 200, so I'm wondering if this lower compression is related to the bucking your reporting. Can you do a leak down test, in order to try to zero in on what's going on? Or, perhaps first try another compression tester, to see if the first one is off by a bit? I'm assuming you can rev the engine parked, with never a stumble, correct?

[OG-Lou]

Problem Solved.
Back when the valve cover gasket leaked oil into the spark plug wells, the oil must have carbonized-created a weak track to ground so the spark plug only fired on the exhaust stroke. On the intake stroke the internal cylinder pressure is higher and requires more voltage to fire the spark plug=the higher voltage went to ground.

There was a black line down the porcelain insulator and a faint line inside the spark plug boot. I cut off the bottom end of the spark plug boot to eliminate the ground path but that didn't eliminate the problem. The engine still bucked. I finally got a code indicating which cylinder was acting up and with the visual carbon track on the spark plug insulator I decided the wire needed to be replaced.

Plug wires have about 5K OHMs of resistance per foot and the wire's resistance was within specs. I fixed a lot of high resistance spark plug wires in my day so I know what to look for and change or modify.

It took 1 new (used) spark plug and one spark plug wire but I installed a wire set for less than $15 (black) from eBay. My wire set was made by HERKO Group and is their HERLUX line.

Now my car runs as well as it did when it was new.

I am hoping I can get back to 35 to 36 MPG in town and 39 on the Hwy if i drive 65 MPH.

It is certainly much easier and more satisfying to drive the car once the engine bucking was eliminated. It used to feel like I didn't know how to drive a stick shift car. Now smooth as glass.