Thread: Misfire under load, increase with engine warmth and run time

Yeah and that CRC maf cleaner is a little pricey, but it will literally last you forever...not literally. But yeah, there's enough to clean like 200 mafs. Only problem I have is grabbing the CRC when I should have grabbed brake cleaner because the brake cleaner was 10ft away and I'm lazy like that.

Nothing is missing in your pictures. Pull up a vacuum diagram of your engine and you'll better understand what you are looking at.

The boost bypass solenoid would originally have a piece of foam over that open nipple. That's how it vents to work the bypass valve.
The manifold on top of the SC is capped because it's not needed. Back in the early years of the H/C/G body some of the HVAC doors etc worked via vacuum and had a connection there. Once GM went with only servo actuators that connection point was not necessary.

I don't think your problem is map/maf related.

There are a lot of people on the forum who are well versed with solving these kinds of issues. Oddly as many give you advice, you are either not able to follow it financially or would prefer to spend a lot of time going in a different direction. It's a bit frustrating trying to help you out on this issue.

When you unplug a sensor on a running engine it's about the same as you closing your eyes when you are running in the woods. You'll stumble and try to catch your balance, as will the engine. By unhooking sensors you have to expect that the pcm is missing an input that it's used to seeing. So it'll compensate the best it can.

Free for you to check:
Pull the spark plugs, do any look significantly different than the others?
Look at your spark plug wires, do any have a mark on them where they are laying on/touching metal?
Start motor at night and mist the engine with water, see any sparks jumping around?

Originally Posted by BillBoost37

Nothing is missing in your pictures. Pull up a vacuum diagram of your engine and you'll better understand what you are looking at.

The manifold on top of the SC is capped because it's not needed. Back in the early years of the H/C/G body some of the HVAC doors etc worked via vacuum and had a connection there. Once GM went with only servo actuators that connection point was not necessary.

My bad on that one. mine has the vacuum line at that 3rd port , and I couldn't confirm for sure if it still applied on the L32's until well after I posted that.

**Those responsible have been sacked**

I wonder if its as simple as a clogged cat. the symptoms are all there. sluggish, misfires, lack of power.

Originally Posted by mguzzo

I wonder if its as simple as a clogged cat. the symptoms are all there. sluggish, misfires, lack of power.

I've had that happen before too, but I don't recall it starting only when warmed up, as initially indicated. Maybe that was just circumstantial timing.

I still think it could be the MAF. if OP can get a replacement at the J/Y for around $7, that's about the price of a can of CRC (here anyway), so might as well swap it out & see. If it makes no difference, then they didn't need the cleaner anyway, and there will be a backup MAF for down the road.

I know its late, and I'm not (or ever was) expert, but the open-loop control jumps out to me on this.

I am now big on being very sure about vacuum leaks (because of age), but this is not how it reacts when it is vacuum. Two vacuum spots you may have missed - the tee beneath the S/C snorkel and the Evap solenoid near the EGR valve (it has an O-ring inside).

I only continued the hidden vacuum leak spots to complete the thought - I know vacuum leaks well now, and this doesn't sound like it (small = random intermittent misfires; large = shaking engine).

What about O2 sensors? (here's a video on open-closed loop closed loop control (starts at about 1/3 into the video)).
Understanding Open loop / Closed loop Basics - YouTube

Originally Posted by Wakingcrow

You should easily be able to tell if the rubber tee is bad - maybe try water at the joints. The Evap solenoid leak will be slight and only give more slight misfires - not like what you described. (And I certainly don't thing it needs PCM reprogramming).

2. I sprayed brake fluid around the sc and engine top according to a video and engine didn't try to stall.
3. It doesn't misfire under idle or even when revving at idle, but sometimes does backfire after rev.
4. Seems to only happen at the same time boost engages, but I don't know if that's a chicken or egg.
5. It's def not the cat, because there is none, just a pipe. Been like that for a long time before all these problems.

2. Was that "engine top" for checking vacuum at the S/C bypass valve (solenoid) lines? This is located on top of the S/C toward the front of the engine.
3. That backfire sounds normal - I always get 2 misfires
4. (redundant) Check vacuum lines at that boost bypass valve. (I recall it being easy to mess those lines up.)
5. Maybe the O2s have gotten dirty. (for saving money and not remove-and-replace method of fixing, O2s supposedly can be cleaned with something like SeaFoam - of course, there are completely conflicting opinions).

Here's another basic explanation on fuel trims (and relation to O2 sensors).
Understanding Open loop / Closed loop Basics - YouTube

I know you don't have a scanner, but they're very helpful.

first off you need to reconnect the front O2 sensor. Without it the car is stuck in open loop. so you have no fuel trims and it's running like crap.

next you need to pull plug #3 (middle plug on front bank) and check its condition. Make sure the plug is not broken or bent. and the wire is in good shape and you have dielectric grease in the boot.