RJ_RS_SS_350

If you manually moved the butterfly valve, the throttle body may be damaged.

donbrew

On a 2009 (FlexFuel) there is absolutely no reason to reset the ECM between E85 changes. The O2 sensors know how much alcohol in in there, unless you can figure a way to stratify the alcohol so that each ounce has a different content. It is a dynamic measurement, that's why the STFT varies so much.
My fingers are faulty; I meant P030x not P010x.
Resetting the ECM resets all of the OBD data, including misfire count. Misfire count should show up in a freeze frame.
The only way I can think of to have a misfire without a code is if all cylinders mis fire in a cycle. The computer looks for variations in the RPM of the crankshaft; if it slows when cylinder A is supposed to be in power stroke that cylinder can be pinpointed. (it is not that simple). Like if the fuel pump stumbles.

Has the crankshaft sensor or ECM been replaced? If either has been replaced the crankshaft sensor variation has to be relearned.

fastsuv

The OBD II system doesn't set a code for every single misfire. There has to be a repetitive pattern that occurs based on the algorithm used for detection. In some cases, the misfire count is erased after a certain time period if there are not enough misfires fitting the pattern.

The web shows lots of questions from people that feel misfires without a code.

The following web sites may be useful:

Misfires

Misfires, Detection and Diagnosis


Steve

d2mcgill

There is a cover plate On the side of the motor which exposes the primary timing sprocket when removed. I removed the plate and went in with a borescope and long ratchet on the crankshaft bolt to rock the engine back and forth, granted it’s a bit of a Mickey Mouse test, but I didn’t note any play at all.

That’s a valid point, I agree with you that brake cleaner can cause issues if you wash the grease out of the gears on the butterfly valve, but I was careful, and the throttle body was very clean to start with, I just wiped off a minor amount of soot. The misfire also happens on part throttle between 1300-2200 rpm, at which point the valve has begun to open, so if it was sticking I’d expect it to run rich (which would potentially be visible in the live data as the ECU tries to lean the mixture), but I see nothing suspect... However, I suppose I could have a faulty lambda or MAF reading, I’ll double check.

Cat Man HHR

This condition reminds me of a Porsche 914 I worked on, believe it was a 72.
The throttle position sensor was bad.
The thing was a variable resistor and the spring arm was weak pressing up against the stationary grid at low throttle opening.
Above 2000 it ran well.
Could your throttle body sensor be give you a problem ?
I know no codes have been thrown.
You have checked more than anyone on this forum has to pin point your problem.

Picture of the 914 sensor.

d2mcgill

I absolutely agree with you, I only reset the ECM as it then runs in a default calibration while it takes new measurements and re-learns the suitable the trims... The theory (I was getting desperate) was that if the misfire was sensor/mixture related the misfire might temporarily clear while the ECM was re-calibrating... But misfire remained consistent. I also tried disconnecting various sensors one at a time and going for a drive to see if the fault cleared... It did not, but I did get the expected fault codes.

I also agree with your take on having a misfire with no code, but it feels like a single cylinder judder/cut out, not the engine missing a full cycle on all cylinders... This is what’s breaking my brain, it not a mild misfire, it’s a full on lurch under load with both light and heavy throttle inputs (ie. feathering the throttle as well as mashing it to the floor, the misfire remains), then as soon as you hit 2300rpm, the engine runs faultlessly and the power comes back.

Crankshaft sensor and ECM are original, cam sensor was replaced.

At the moment all my tests point to weak or intermittent spark, yet no codes and the coil packs appear to be ok and new plugs had no effect... I’ll start back at zero and change the plugs again today, after that, I’ll focus on the coils...

d2mcgill

That’s a good suggestion, I’ll check the potentiometer in the throttle body.

RJ_RS_SS_350

If memory serves, that inspection cover plate is for the balance shaft chain, not the main engine timing chain. You would remove the valve cover to inspect the timing chain for tightness.

Of course, to set timing, the crankshaft pulley and timing cover must be removed. The crankshaft pulley bolt is a one time use, requiring a new bolt upon reassembly.

Oldblue

My thoughts precisely, RJ , the cover plate is at the water pump and balance chain sprocket , and the back of the timing chain tensioner.

You might see some part of the chain

But removing the valve cover is the best way to be sure


This much slack can be a problem.

RJ_RS_SS_350

Have a look at the upper timing chain guide bolt, it may be broken off. Here is a how-to guide for replacement with a redesigned guide bolt.

If, during the replacement process, you find that the original bolt is broken, replace the timing set also.

https://www.chevyhhr.net/forums/how-...de-bolt-62897/