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So, after putting the A/C-delete belt on to take my POS A/C compressor out of play, everything works great:

  • No frightening rattle / clanging from the engine bay
  • Idles smooth
  • Sounds like a brand new engine. Which it should, since I've only put about 8k miles on it since I rebuilt the thing.
  • CEL for "System too lean bank 1," which has been haunting me for a year, is finally gone. Very weird!

Note that I also replaced the oil dipstick tube when I changed the belt. I had the one from the Malibu this engine came from, but it's a different length and shape than the Solstice tube, and wouldn't bolt to the intake manifold correctly. shrug A side effect is that the dipstick doesn't seat firmly in the tube now - must be different diameters - so I'm waiting on a replacement dipstick to arrive.

(Tangent: could a loose-fitting dipstick cause a vacuum leak?)

The Other Shoe When I first start the car, everything runs very, very smoothly. Sounds great, lots of power, etc. When it gets up to temp, however, the engine starts getting the hiccups. Somewhat at idle, but very noticeable when accelerating. If I keep throttle below ~20%, it runs well enough to drive, but is very disconcerting. If I let the car sit overnight, everything is exactly the same the next day (runs great until it warms up).

Suggestions? There's also a weird issue with the coolant temp sensor. It occasionally reads "---". I've ordered a new one, will put it on this weekend.

Update

Earlier today, I decided to drive it lunch. After lunch, I made it about 50 feet before it went into "Reduced Power" mode. I pulled the negative battery cable for a few seconds, then everything was fine for the subsequent short drive home.

That's happened before, though I forgot to mention it.

Update the 2nd

Title updated to reflect new information

So, it's not directly related to the engine temperature or closed-loop status.

I've been driving the wife's car for the last few days. A little while ago, I started up my car, and it ran fine - until I hit the gas (in neutral). It revved fine, but when I let it go back to idle, it started trying to die, then revved up, went back down, sounded like it was having a seizure. The "reduced engine power" light came on instantly. I shut it down as I don't have time to look at it right now.

I'm wondering if there is a problem with the throttle body. There's no CEL so I assume the TPS is fine. I was running a compression test a few weeks ago and propped the butterfly valve open with a wrench, may have messed something up in the process. I checked the harness connector to the throttle body and it looks fine, will pull the intake hose off when I get some time and look for binding, etc. There is a significant amount of oil in the intake on the turbo side (near where the hose connecting the intake to the valve cover goes), maybe the butterfly valve is getting gummed up or something.

The weirdest part is that this just started immediately after I replaced the engine oil dipstick tube and put the A/C-delete belt on. (The only other thing driven by that belt is the alternator, which now has a dedicated belt.)

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    I think what's happening is that the car runs fine under cold-start enrichment because there's enough fuel to counter the leaner-than-expected AFR. When the engine reaches operating temps it switches over to closed-loop, at which point the excess fuel isn't there to bail you out (I should really add this to the answer)
    – Zaid
    Commented Feb 3, 2017 at 20:41

2 Answers 2

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Regarding the tangent, yes, an ill-fitting dipstick can be the cause of a vacuum leak.

Pulling out the dipstick is a way mechanics can figure out if the car is suffering from unmetered air/vacuum leaks. If there's no immediate change in RPM, there's a leak somewhere.

I think what's happening is that the car runs fine under cold-start enrichment because there's enough fuel to counter the leaner-than-expected AFR. When the engine reaches operating temps it switches over to closed-loop operation, at which point the excess fuel isn't there to bail you out.

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  • I've never heard of that! I'll try plugging the tube with something that won't get sucked in and see what happens.
    – 3Dave
    Commented Feb 3, 2017 at 20:38
  • @DavidLively The tube isn't submerged so you pull unmetered air through the PCV system. if you pull the oil pan or dipstick tube you'll see the dip stick extends past the bottom of the tube. I'll add though that usually this isn't enough to cause rough running.
    – Ben
    Commented Feb 3, 2017 at 22:47
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So, here's the difference between the Solstice and Malibu dipsticks (both 2.4L LE5)

enter image description here enter image description here

The shorter one with the larger diameter, bright yellow handle is from the Solstice. Fortunately it was still in the shop on one of the old engines.

But, in the process of chasing this, I found that I hadn't tightened two of the four bolts holding the throttle body to the intake manifold. When the car isn't under boost, the other two did a sufficient job of sealing the TB against the manifold but, as soon as the turbo kicked in - which I don't do until the car is warmed up - a significant leak was created.

Another problem also appeared: the MAP connector had been replaced on the injector harness, and was not properly soldered. I'm sure that was also contributing to the problems I described.

Thanks to everyone. (I am posting this as an answer since it was the actual problem, though the other answer sent me in the direction that led me to the solution. )

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