Low Power on Acceleration

[tomj76]

I have 2009 Cobalt with the 2.2L Ecotec engine and manual transmission.

Earlier I had the code for intake camshaft timing control and changed the intake timing control solenoid with a new one from GM. The symptom (in addition to the code) was poor idling. Prior to the repair I could make it idle OK by removing the electrical connector at the control solenoid.

About 1000 miles after the repair I began to notice lower fuel economy (by 10-20%) and poor acceleration torque above 2000 rpm, especially in 2nd gear or higher. The codes for the camshaft timing error have now self cleared and there are no new codes.

I checked fuel pressure and it shows 60 psi, constant with RPM. I did not measure it during a road test however.

I checked spark plugs and all have 0.040" (1mm) gap.

To me the engine sounds "throaty" when I try to accelerate, especially with the throttle at >70%, as if I'm hearing a noise from the intake.

I checked the TRIM LT & TRIM ST. The short term trim seems fine to me, but the long term trim looks a bit high (sometimes "stuck" around 10%) without much variation with throttle position. I have not found any vacuum lines that are detached or broken, although I'd suspect more issues around idle (rather than during acceleration) and lean codes if I had a vacuum leak.

The MAP sensor reads between 8 and 28 inches HG, depending on throttle position. The MAF reads ~ 0.5 lb/min at idle, and about 2 lb/min with the throttle set for 2000 rpm no load.

Any ideas?

[tomj76]

Not much help on this forum... Oh well.

Eventually a new code was set P0420 (low catalyst efficiency).

So far I've tried a few things: (1) replaced up-stream O2 sensor, (2) replaced air filter (3) clean MAF sensor (4) Check exhaust backpressure at the upstream O2sensor (1-2 psi) (5) Ran the propane enrichment/oxygen storage test and saw a 1-2 second oxygen storage delay.

The scanner reports 0.455 for the catalyst efficiency test (out of 1, with a minimum requirement of 0.5).

The engine output doesn't seem to be much improved with these efforts, maybe slightly.

[Stealthee]

I have a stupid question. Did you recently change your oil when the symptoms started? My brother had a G5 with the 2.2 and shortly after changing the oil it started spitting, sputtering, and misfiring. Long story short when he changed the oil he put a heavier oil in and they apparently don't like that. He did research and found out about the oil weight issue. He changed the oil again and the misfiring went away.

The P0420 is likely a result of the misfires causing excess fuel to get to the cat.

[tomj76]

@Stealthee

Thanks for the suggestions.

I changed the oil before I replaced the CVVT control solenoid, because I had hoped that an oil change would clear that up. I had read something similar to what you're saying, that the screen on the CCVT solenoids can get clogged with sludge if you delay changing the oil. While I had not done that, the Oil Life indicator was not to zero despite a fairly high mileage on the oil from my last change, so I changed it hoping that fresh oil would clear the problem. It turned out that the solenoid was electrically faulty, and the oil was OK.

The owners manual calls for 5W-30, which is what I used when I changed it.

My engine is not misfiring (no misfire code, nor can I feel a misfire vibration), only low power. There is some small amount of intermittent surging that I can feel when I try to accelerate hard, but other than that the engine seems to run smooth.

[tomj76]

I had written a spreadsheet calculation to measure the horsepower to the wheels (ignoring wind resistance) based on the acceleration rate from a start and the spec. gross weight of the car. I found that my car was not delivering more than 40 hp in any gear at 5000 rpm.

I did this with and without the air filter with identical results.

For comparison I checked another Cobalt (my daughter has a 2007 model) and found that it was delivering 90 hp.

I also did another test with the upstream O2 sensor removed (to let the exhaust gas out).... and I got about 60 hp at 4500 rpm.

I re-checked the exhaust back pressure at the upstream O2 bung and found that I would see more than 10 psi when I gunned the engine while in neutral. Then I checked back pressure at the rear O2 bung and found less than 1 psi when gunning the engine as in the first test.

I'm sure I need a new catalytic converter.

[tomj76]

I finally received a replacement and replaced the defective converter. To my surprise it was actually fairly easily removed. I could immediately see that the first catalytic converter cell had melted and disintegrated. The honey comb for the second cell (in the first chamber) was clogged with the debris from the first cell.

I put the new pipe in and measured the acceleration... it came up to 100 hp @ 6000 rpm.

Two lessons I learned in this adventure are:

(1) Diagnosing with backpressure measurements in the shop requires that you create realistic exhaust flow rates. The diagnostic guides that I read instructed to use idle & idle @ 2000 rpm for the test. However, idle and high RPM idle settings don't really create much airflow because the throttle isn't open very far. The engine needs to open to >50% to simulate the air flow seen in the conditions where horsepower is needed. Gunning the engine is a reasonable approximation because the throttle is wide open for enough time to get an indicative test.

(2) I don't know what caused the converter cell to breakdown & melt, but it was obviously very hot. My guess is that the faulty CVVT solenoid on the intake cam was contributing to a large amount of unburnt gasoline to reach the converter. The average MPG was lower since that failure occurred, although a clogged exhaust can be the cause of that symptom. I ran the car for a few months with that condition, which may have lead to the failed converter.