08 HHR 2.2L.
All I did was replace the timing tensioner. Now the car is hard starting, bogs, lost brakes, engine light is on.
How did I screw up my car? Tensioner is easy replacement.

 

Did you use the new style tensioner? If not, the tensioner may not have engaged. Did you reconnect the MAF sensor when you put things back together? Has the battery ever been replaced? Were any vacuum lines left disconnected?

If you don't have a code reader, you need to borrow one, get a friend to come over and use his, or putt putt very slowly(because no brakes) to a shop(or auto parts store) that will read your codes. Get just the codes, not the description they will try to give. Come back and report the codes in a new thread with a title that is more specific(so the right people will read it).

You probably just are without the power assist to your brakes, so they should work, just more effort is required. Test this theory before you drive it.

How many miles on the car, auto or manual trans, Post this info in your new thread.

 

Wait. I just realized... If your new tensioner didn't engage, your timing has probably slipped. You shouldn't drive the car or even try to crank the engine. You may have already bent your valves trying to start it.

 

Ditto to both of RJ's posts above, more info, and it could have "jumped time" which is potentially fatal to the engine.

 

New one engaged. New style. I have the cover off.
No vacuum lines were disconnected.
I first started it with MAF disconnected. Then installed it. Ran fine at idle. Hour later I went to move car and wouldn't start. After lots of cranking it started. Check engine light on.
Disconnected battery, light out. Starts fine now. Idle is good. Quite engine. Test drive shows a bog. If feather through it I can get it to run well from say 4000-redline. Brakes came back.
Got home. Still a low rpm bog. Engine light came back on with restart.
Pulled cover again. Chain is tight.
270000km (168000 miles)
Battery is good.
Ran fine till I changed tensioner. Except rattle.
Throttle body is oily inside. Should clean it.
30 miles from the city.
Rough getting to 40mph but after 50 it woke up. Took it to 75.
Still sounds good.
Automatic.

What's a good title for this?

Oh, black smoke from exhaust when revving in park while engine bogging. Won't go over 5000 rpm. Sounds like it's under water.

 

OK that's good news, sounds like no valve damage! Now you can get the codes read.

 

Man I tell ya. Be F'd if I bent valves. Love the beast. Wife's car. It's here for life. With the miles I wouldn't pull the head. Just either rebuilt or preferably a performance engine.
She saw a Google about a stroker somewhere and she's all OH YA. Lol.
Sunday tomorrow. Thinking I'll put it back together and drive to the city. Get a can of throttle body spray, some Seafoam and hopefully a scan. Battery is disconnected for the night.

 

First step MUST be to get the codes read. That alone will determine what is causing the current problem.