SwissHHR

For the first time ever, I had a stuttering engine today (2008 LT 2.4 manual, 167'000km):

- after about an hour's drive straight on and then going uphill, the engine was slightly stuttering three or four times, then was ok again.

- parked the car for a couple of hours and drove back. First the same route downhill without any issue, then on to the highway, where the engine started stuttering quite violently. Could sometimes step on the throttle without any response and then one or two seconds later, the engine would respond. Also for a short period of time I couldn't almost reach 80kmh/50mph. The car felt as if it had no power. ESC and traction control lights came on during the stuttering. Engine light didn't light up.

- drove off the highway because I felt uneasy. Drove on on the main roads with still pretty heavy stuttering.

- parked the car for two hours. Noticed a quite pronounced fuel smell (not gasoline smell per se, but the kind of smell I think you get with bad combustion)

- drove home the remainder of the route without any stuttering and everything perfect. Also the ESC and traction control lights were gone.

- looking into the engine bay, I didn't notice anything unusual

Engine codes (according to Haynes HHR repair manual):
P171: System too lean
P300: Random/multiple cylinder misfire detected

What could be possible causes for this kind of behaviour?

RJ_RS_SS_350

Lot of good suggestions in here.
https://www.chevyhhr.net/forums/prob...2/p0171-48761/

Oldblue

Remove the exhaust manifold heat shield inspect the manifold for cracks, inspect the flex pipe for leaks.

donbrew

P0300 is not the cause it is the effect.
The definitions are misleading. it should be "something like a misfire detected". No random, no multiple cylinder.
P0171 should be "fuel trim system too lean". It means that the O2 sensor is sensing too much unburned fuel (caused by a misfire) so the computer is leaning the A/F mixture.
is there a CEL or not?
Without being there, it is hard to tell why.
coolant temp
coolant in combustion chamber
fouled plugs
bad coils
dirty throttle body
loose wire
and a lot more

greg8453

What does the manual have to say about these codes? Much easier to read the manual and you can trust they know what they are talking about.

Oldblue

Post #31

https://www.chevyhhr.net/forums/gene...e-65607/page4/

WoodysMobile

Start with the basics first.
and circle in from there after verifying the simple mechanical and connections for failures.

You know your history and maintenance work. (or possibly lack of) maybe the easy fix.

Clear codes, road test and try to re-create.
Happy New Year to all.

SwissHHR

Thanks to all for your input and Happy New Year to everyone!

Drove to work yesterday and today without any issue.
When it occured the day before yesterday, there was no CEL. Also coolant temp was fine.

I have an old laptop with HP Tuners on it. I will record my drives for the time being and when it happens again, maybe from the output there is more information about the cause.
I will ultimately take the car to the shop for inspection. But I guess it would be good if they are able to reproduce the issue, which at the moment doesn't seem to be possible. Will report back once the issue is solved.

Oldblue

Okie dokie, let us know.

donbrew

There were some internal changes made to the LE5, that should not make a difference to the computer.