First let me start by saying I really feel the HHR was built for me. I am a backyard mechanic and can't crack this one though. Any help would be appreciated.

My problem is as follows:
Idles rough with a misfire. P0301 code.
When I start it and start diving the light flashes and it rides rough. I usually don't push through and just turn it off and turn it back on. It still idles rough but then rides fine.
Muffler rattling during idle, but this could have been when I backed it into a curb.

Things I've Tried:
-knock sensor
-new spark plugs
-new cylinder coil
-fuel injector swap
-decarborizer spray into the intake
-Check compression and cylinder 1 was reading 90 vs 120 for the rest.
-Injection cleaner
-Oil additive
-Sea foam
-I remove the plug on cylinder 1 and had a light gray debris.
-In blind troubleshooting, I disconnect the purge valve hose and it sounded fine. Then I realized what I was doing. Just raising the RPMs.

Things I think it might be, but not sure:
-Bad catalytic converter?
-Bad wiring harness (voltage was going to it, but I was unsure how to check the control signal)
-Some sort of vacuum thing?
-Denial on bent valve or leaking valve?

Anyone here familiar with the problem? It is a 2008 LS with 130k mi

 

Hi Davvid2, to the Forums! Sorry to say, I'm leaning towards the low compression in cylinder #1. Could be rings/cylinder wall wear, valves bent/burnt/otherwise not sealing, possibly even head gasket. You should perform a cylinder leak down test to find out what's causing the low compression. Here's a link:

https://www.chevyhhr.net/forums/how-...-engine-56053/

 

One problem might be the flex pipe.

If is was a bad cat you would have P0420 and horrible running.

Hows the coolant consumption?

Do a leakdown to narrow the bad compression down. I am assuming the low one was #1.
However 120 is pretty low should be more like 180PSI, so maybe operator error.

Why replace a knock sensor? You give no indicator for that.

 

Welcome to the site, I need more information

Which engine do you have 2.2 or 2.4

Is there oil and ooze in the hose from the valve cover to the elbow at the throttle body?

Have you cleaned the MAF sensor and the throttle body?

 

Don,

You are right. I did the compression the quick and dirty way.

The coolant consumption is fine. Replacing the knock sensor was just from something I read online and figured it wouldn't hurt from buying a AC delco one on ebay.

 

Oldblue,

I did see some light oil in the throttle body. I did rough cleaning, but I might take it off and give it a good cleaning. I'll try more seafoam. It all started when I let my tank go low with some bad gas. When I get out of this only midgrade for me. Not worth the headaches.

RJ,

Thanks for the welcome. I'll likely be trying that in a few weeks I guess. I found some refabed head replacements from cali. Any recommendations, I am on a budget.

 

Well, you don't yet know what the issue is, so I'm not sure what to recommend. If you think your compression test is inaccurate, you should do it again.

If you own the compression tester, you may be able to convert it into a leak down tester for very cheap, videos on youtube. Or, if auto parts stores don't have a leak down tester to loan, you can get one from harbor freight or any other tool discount store. Check the link I gave you above, you'll need to modify the harbor freight one to work properly. Then you'll need access to an air compressor capable of producing over 100 PSI.

If your leak down results indicate excessive leakage into the crankcase, then it will be cheaper to replace with a used engine. This may also be true if the problem is valves. But we're getting ahead of ourselves, you've got to diagnose the problem first.

Oh, and regarding midgrade gas, from what I've learned here, it is mixed at the pump, so you're not avoiding any junk that may be in the underground tank of regular. And the octane rating isn't going to cause any problems with your engine. If you are concerned with "bad gas", look for Top Tier fuel, I use the GasBuddy app or website every time I need to fuel up.

 

Use throttle body spray not Seafoam

 

??? I've never heard of "mixed at the pump". Maybe they do things different out in California.
I've always thought it was mixed at the distributor site when the delivery driver was filling his tanker truck. Was always sent to us in a multi compartment tanker that the driver would unload into our 3 separate underground tanks.

 

Well, that's what I get for not doing my own research. So some stations have 3 tanks, and some only have 2. The 2-tank stations mix their midgrade at the pump. Thanks for the correction, FMG!

So, if the station you're filling up at has 3 tanks, you could avoid contaminated regular by using midgrade. But if they've allowed the regular to get contaminated, why would you think any grade is safe? I've heard advice to not buy gas if the truck is there, come back later after the tanks have settled. Maybe that's how OP got bad gas.