My engine likes to rev between shifts intermittently. Under normal driving conditions (not "spirited"), when the clutch is depressed RPMs will jump 300-500 RPM like you neglected to take your foot off the gas. It also takes a long time to drop back down to a normal level.

I posted a more complete description on it a while back:

https://www.chevyhhr.net/forums/prob...s-14468/page6/

(Look for my post about half-way down the page)

BTW, awesome reason for editing your last post...

 

Had a couple of other GM manuals that did that. Dealer mechanic said it's deliberate to prevent a high-vacuum condition that's somehow bad for emissions or something. I just lived with it after that.

 

There used to be an after market thingy for aircooled VW's that disengaged the A/C clutch via a vacuum switch. It did that sort of thing.

Does this happen with the A/C on or both ways.

Then of course, the turbo may have something to do with it. Dump valve maybe? Don't know nothin' bout that!

 

It does it with and without the A/C.

One of the ideas they're floating IS an intermittent failure of the "recirculation valve" (GM's version of the blow-off valve) as Donbrew is suggesting.

They're thinking there's a possibility that it's not releasing properly and forcing the pressure back into the engine past the throttle plate.

The biggest issue is getting it to duplicate.

I've figured out how to make it stop when it's acting up, but I can't make it act up on command. I can set up the conditions that it acts up the most, (hot, especially after a restart with some heatsoak) but it's still a crap shoot as to whether it acts up or not.

 

Well I belive there are 2 sensors to control the recirculation valve. The bypass valve solenoid is mounted to the intake (next to the MAP,has 3 vacuum lines). Then there is the boost contol solenoid (mounted to the turbo). Could be a vacuum issue, have you got a real(mechanical) vacuum gauge?

I would start at those 2 and work your way around the system, there's also the square vac canister under the intake manifold, I would try bypassing it. I no longer have it on my engine.

If that doesn't yield any results we could try out my k04 on your car if you're interested. I know we're close and I don't need it at the moment. I'm rebuilding the gen 2 motor and looking for a donor car to drop it in and sell.

Just a thought!

 

I already tried bypassing the vacuum canister (pulled the two lines off and ran a short jumper direct).

No dice.

I also tried disconnecting the evap system as the GM engineers also had a thought that I was getting fuel backfeeding through the evap system.

No dice there either.

I do have a real vacuum gauge, but at this point, I'm not sure where or what I'd be looking for as I'm not sure what I SHOULD see. If it was a carbureted V8, I'd be golden!!

...then again, I probably wouldn't have this issue to begin with...

 

I don't know if you have the ability to log it or not, if not you may want to consider HPTuners or wait for Dbeluscak to get his package and get together. You have a stock inlet and maf loction, correct? If so from the sounds of it Air is getting in somewhere, the mix goes lean the controller adds fuel to keep it at stoich.... Logging will give you idea of what is going on and while I know the upfront cost of the tuning package is high that will be outweighed by the diagnostics ability you will have for you HHR and any other car/truck you want to use it on. Right now is sounds like a guessing game.

Just my 2 cents

 

I seem to remember that the turbo has a PCV valve or maybe 2. I wonder If that could be the source.

 

Donbrew, I am new enough to this drivetrain, that I haven't found any PCV valves yet!! It would be worth a look though if I can figure out where they are.

I did figure out a new detail last night that could put a different spin on the issue. Let me explain...


Whenever the issue is occurring, if you hold the clutch in and free rev the engine over 3K, the RPMS will just hang wherever you come off the gas, then slowly start to fall a second or so later (not a long time, but an eternity when your expecting RPM's to drop immediately, like they normally do when it's not acting up).

Here's where I found the change. If you keep doing it while slowing down, it will stop acting up and start reving normally shortly before you come to a complete stop (still with the clutch depressed). If you then accelerate (normally) up through the gears, it will start acting up again as soon as you're rolling.

This new information leads me to believe there MIGHT be something in the programming (or the ECM) itself as the physical speed of the car should not have an effect on anything unless the reading from the speed sensor is causing the ECM to change its settings.

I know these engines do modify the way the throttle operates when you're shifting gears. I'm wondering if there could be a glitch in the programming that's causing it to make TOO MUCH of the changes at certain times.

I still have been unable to determine what actually sets it off (no ability to duplicate on demand). I can only do experiments to test things when it decides to act up.

I am planning on buying an HP Tuners unit, but instead of tuning (at first), I may use it for some data logging while it's acting up as Drloch suggested. I may also try to tune out the changes they made for emissions purposes that were a part of the effect referenced above (just to see if it has an effect).

 

There aren't any tables available that you can access to alter any emissions related items that could be causing your issue. If it happens during logging then you should be able to zero in on whats the cause is.

HPT will be less expensive than throwing parts at what you think the cause may be..... Think of it as an educational expense.... isn't that tax deductible?