06 cobalt ls, 2.2 liter, 85k miles. *SO FAR I DON'T HAVE A CHECK ENGINE LIGHT*

I'm not really sure when it first started, maybe 6-12 months ago, but it seems like its slowly progressing and getting worse. I remember months ago noticing a slight vibration during idle - maybe once every few seconds - kinda felt like a misfire. Otherwise the car ran fine, no stalling, no loss of power, and accelerated just fine, ran normal, etc.

Back in May I noticed a 'jerk' while coming to a stop. Then I noticed it more and more. I just figured it was my rear brakes grabbing a rough spot on the drum (my rear drums have a rust lip and makes a squeaking noise/ shakey feeling). Around early August I was coming to a stop sign and stopped, the car jerked and stalled. Then later on that month I was pulling into a parking spot at a store near my house and the car stalled. I monitor the RPM's all the time now, and notice this jerk is associated with the engine - almost as if it's a stumble. Over the last month the car has stalled about 6 times - all while coming to a stop/or stopped for a second or two. I did have the transmission serviced in early July because solenoid B was bad and they replaced both shift solenoids - fixed my 3-4 gear issue. The engine vibrations have gotten worse and I can definitely feel it in my seat while idling now - vibrations are worse with the AC on. All the engine mounts look good. The RPM gauges stays steady around 700 and 900ish with ac on. I have no issues accelerating and theres no loss of power. This jerk/stall seems to mostly occur at the first start of the day, although it's happened on a hot engine too. When I let it idle and rev it, the rpms slowly fall to normal idle so that seems normal.

I've taken the TB off 3 times and cleaned it real good with carb cleaner, brush, and rag. I've blew out the air filter. I've sprayed carb cleaner on all the hoses, around intake manifold, around fuel injectors, purge solenoid, around brake booster and hose, etc and no change in idle. I even sprayed a little water instead of carb cleaner. I cleaned the MAP and MAF with electronics cleaner. I replaced the plugs with ac delco double platinums gapped at 45, and still no change. A few days ago I bought a bottle of techron fuel system cleaner and have driven 50 miles and still no change really. I've heard brake boosters having an internal vacuum leak but my brake pedal feels normal, no hardness, no spongeyness, etc.

What else could it be? Maybe somethings acting up on cold engine, and the computer adjusts something so thats why it doesn't act up as bad on a hot engine? I'm thinking of getting a harbor freight scanner or something that can monitor the engine a bit more, maybe I can narrow it down?

*SO FAR I DON'T HAVE A CHECK ENGINE LIGHT*

 

So today first start of the day:

Got in car, started with AC on in defrost mode, let it idle in driveway for 1-2 mins. Backed out, took off, got 2 blocks away at a stop sign and stopped, car stalled a second or 2 after stopping. Car started back up and I drove away. I got 3 more blocks away to a dollar tree, pulled into parking spot, and the car jerked/rpm's dropped acted like it was going to stall but didn't. I came out of store, let the car idle for about 5 mins in parking lot and then left. Drove around 10 miles and did a bunch of stops, not once did it jerk or stall.

Later on today I drove around some more and not once did it jerk or stall.

Seems like this issue happens on a cold engine? What could it possibly be? The vibrations occur all the time at idle no matter what. My exhaust sounds normal, I don't hear a misfire.

 

If you've done all that, it could be a fuel problem itself. I would start with draining the current tank of gas and filling it with a known brand name. Next check your fuel filter, has that been changed at all?
Start with those 2 (as they are the easiest) and report back with results.

As for the OBD2 reader, I would highly recommend having somebody watch your air/fuel ratio during your drive. This will let you know if you're running too lean or too rich and when.

 

I did run it to quarter of a tank today and then refilled it to full.




Today before the first start/drive I disconnected the electrical connector at the evap purge solenoid. Then I drove around 10 mins and it jerked 3 times at various stop signs. jerks happened maybe 1-3 seconds after being stopped. After 10 minutes of driving and driving it a few other times today within 2 hours of each time, this jerk or stall didn't occur, although the idle still vibrates the seat. Seems like after it warms up, jerk/stall doesn't happen. With the evap purge wire disconnected the idle was still vibrating. I took off the evap and blew through it and couldn't get no air past it - so that tells me it's good. I plugged back in the wire to the evap purge. Also sprayed around engine with carb cleaner again and no RPM changes.

I was revving it in the driveway and the vibrations went away in higher RPM's. As soon as the RPM's drop to idle it vibrates.

The car cranks and starts normal every time. It accelerates just fine too. Shifts fine etc. It's just this vibrating idle/jerking stalling issue at stops. If it wasn't for me cleaning the TB 3 times I would swear that that's what the issue would be!

Can I find a cheap scanner somewhere that shows air/fuel ratio? I was thinking of getting a fuel pressure tester and checking fuel pressure at fuel rail. My TB is clean as a whistle... is it possible to have a tb issue cause this? i still have no check engine light. i dont know if fuel filter has been changed. i bought car 2 years ago. the carfax has doesnt mention it being replaced and previous owner didnt say.

 

You can buy a Bluetooth OBD2 scanner from Amazon for $25. Most of them work - I use Torque Pro as my app on my phone.

 

sorry for long delay... been busy lol.

Thursday the car stalled again at a stop light and this was 2 mins after first start of the day lol. The rest of the day Thursday it was fine. No jerks/ stalls, but the vibration is still always there at idle.

Today at first start of the day, I let the car idle in driveway for 10 minutes before leaving and not once did it jerk or stall at stops.

Can a bad exhaust valve cause this? Cold engine jerks/stalls? Still have no check engine light.. lol. What is the normal compression for these 2.2 engines?

 

How many miles on your car? Have you checked structural integrity(engine mounts) because unless rpms are associated with vibrations it sounds mechanical..clogged catayltic converter because as it warms up it clears some? If tuned, are you able to load pcm back to stock? Alot of your symptoms were like mine, progressivly worse until diagnosed bad pcm. Not saying that's it or anything much. Just throwing out some basic troubleshooting thoughts on the information given.

 

Car is completely stock. Just hit 85k. No tune, no aftermarket cold air intake.

I rented a compression tester at auto zone today. Haven’t really done this test too much. Excuse any ignorance lol. Depending on how how long I cranked it, the compression seemed even across, 180ish give or take either a little less or more by maybe 5psi. Sometimes I could get 160 all across if I cranked less. None were crazy low or crazy high compared to the others. I did this test maybe after the car ran maybe 3-4 mins total on cold engine. Coolant temp might’ve been 150ish. It was about 60f out here in Michigan.

I also did the dollar bill test in front of tailpipe. Didn’t really tell if anything was abnormal or not lol.

Im hoping I ruled out a bad valve as it seems most expensive to fix lol. The mounts look good. Nothing looks cracked. Still no check engine light.

what were your pcm symptoms and how did you diagnose?

 

Has anyone ever did the dollar bill test over their tailpipe?

Are there any exhaust pulses that would cause the dollar bill to flap around?

i just tried it again and the dollar bill seems to flop around back and forth. Some people say this is normal other says this is indication of a bad valve... what about on cobalts though?