Tuning! For GM vehicles

[LeanBurninating]

Originally Posted by theunchosen

......The ECU is going to inject roughly a gram of fuel for 15 grams of air. . .and if the throttle is closed it just means you get the same power as you would at idle(it wouldn't rev high) but you inject a ton of fuel most of which gets burned in the cat. If the air isn't getting in somewhere somehow it won't stay revved up like that, you can't burn more fuel than air allows.....

You can't assume a constant air/fuel ratio level. Get any car on a dyno and watch the afr and you'll see it jumps around everywhere. If you put this truck on a dyno and held a constant speed and then jumped out of the throttle, I garuntee you the ratio would not sit still.

We know that when a motor runs closed loop it determines the amount of fuel to add on the fly based on input from other sensors. Its making constant adjustments all the time, it works well but its definitely not good enough to always have the exact same afr in all situations.

The way that the afr changes in this instance when you back out of the throttle is a result of how GM constructed the fuel mapping on the ecu.

[The Atomic Ass]

Originally Posted by theunchosen

Thats interesting.

So you've already looked at the throttle body and it snaps shut after you release the throttle, but it continues to stay revved up. I'm not entirely sure how thats possible. The ECU is going to inject roughly a gram of fuel for 15 grams of air. . .and if the throttle is closed it just means you get the same power as you would at idle(it wouldn't rev high) but you inject a ton of fuel most of which gets burned in the cat. If the air isn't getting in somewhere somehow it won't stay revved up like that, you can't burn more fuel than air allows.

Next suggestion I think Christ already said, watch the throttle body after someone shuts it down and watch to see if it moves after they let up and after it "snaps" shut. If it twitches to a less open position the ECU is telling it to hold open for some reason or another.

From working on the '90 Pontiac I used to own I know, the IAC has enough control to hit redline in neutral. Not like I can find the IAC in this truck, though.

Also, the choke on my Ninja 250 was not a choke at all, but an enriching circuit. With the throttle fully closed it could be revved to 5-6K RPM. Just from dumping fuel into the engine.

I've also tested without the CC cable attached, no change. (the CC module is the only way I am aware that the computer can directly control the TB)

[The Atomic Ass]

Originally Posted by stevey_frac

My cobalt exhibits this float, now that I think about it. I think it's poor mans rev-matching. If you leave it in neutral, but your coasting down a hill on the highway, it'll turn over at about 1700 RPM. But it idles at under 1000 if it's stopped. When you go to change gears, it dosen't fall flat on its face as soon as you take your foot off the gas. Instead it'll slowly fall, and it gives you time to get the next gear in, and take your foot off the clutch. Makes for smoother shifting, and less clutch wear.

Makes for more clutch wear in our case. Both my father and myself know how to shift. Unless we keep the clutch held in until the revs begin to drop, we end up dragging the engine DOWN in an UPSHIFT. Which makes no sense.

[The Atomic Ass]

Originally Posted by LeanBurninating

The way that the afr changes in this instance when you back out of the throttle is a result of how GM constructed the fuel mapping on the ecu.

Seemingly for the maximum amount of fuel that can be wasted. </tinfoilhat>

[theunchosen]

. . .The AFR is pretty irrelevant to what I meant for this discussion.

The AFR is going to stay between 12(this depends on how much power per lb your engine is allowed to output) up to 15-16(if you have lean burn its the latter, former for the rest of us).

The most fuel you can possibly burn in your cylinder is about 1 gram of fuel for 14 grams of air. If you go below 14:1 you don't get more power from the fuel(you get cylinder quenching which keeps the charge from expanding while its in the intake and compression which gives you more power).

Example that this truck is displaying. I shut the throttle valve completely no air gets in. MAP(MAF not sure off the top of my head which it uses) reads I still have 2K rpm worth of air(ish, it fluctuatues with flow patterns in the engine bay) and it dumps that much fuel. I have 0 power. Power is a function of fuel and air and its limited 14.7:1(ignoring lean burn possibilities). for example at 3K rpm I have 300 grams of air the engine wants to burn 20 grams of fuel. If I close the throttle to idle I have about 50 grams of air now, so the most fuel that I can possibly get power from is 3.4 grams of fuel. Even if the engine still tries to dump the other 16.6 grams in I get no extra power than I Would have from just 3.4 grams(realistically with cylinder and piston quenching you might get what you would from burning 5 grams of gasoline total).

If the map sensor reads that I am intaking 5000 RPM worth of air but the throttle is at idle. . .the engine rpms will be at idle.

The test as I stated earlier is just going to be shut off from 3K rpm and watch the throttle body. If it takes a while to wind down and its not due to momentum(would have to be a really heavy crank and flywheel) the throttle body will not snap to idle position. It might jerk down some but i can promise you its slowly actuating towards idle from whatever throttle position it takes to spin 3K rpm freewheel.

It probably is a method for softening shift jerk, but if its really important for your hardware to survive just teach your drivers to double clutch. Its really not that hard.

[The Atomic Ass]

Originally Posted by theunchosen

It probably is a method for softening shift jerk, but if its really important for your hardware to survive just teach your drivers to double clutch. Its really not that hard.

If it was, I would assume it would snap the engine to the next gear RPM quickly. I can still double-clutch before the engine revs fall NOTICEABLY. Whatever GM was on to here, I am apparently not smoking the same grass as they were.

The TB is definitely NOT the culprit here, as the behavior does not change even if I force and hold closed the throttle plate.

[theunchosen]

Well in that case air is getting in some how and its part of your problem(won't affect accel). Either way If you are going to start worrying about sensors O2 sensors are usually what kicks the bucket before anything else and they do induce rich run.

the increased vacuum created when you shut the throttle body to a lower opening induces greater temporary vacuum on your leak allowing more air than normal(at idle the engine is not turning over at 3K rpm and can't maintain the higher vacuum and at 3K rpm there is very little MAP vacuum so it won't influence it there(much)). Christ offered the test for this issue.

[Christ]

The IAC on your S-10, IIRC - is held on by two screws, laterally opposed (in a straight line from each other, opposite sides of the valve solenoid). It's attached to the throttle body. If you look inside your throttle body, you'll find the throttle plate... near that, you'll find a hole which is not covered by a plate... the IAC will be near that hole on the outside of the TB. It will have a 2 or 3 wire plug on it.

The only other wires I can think of that will be in that area are the TPS, IAC, and possibly the IAT, unless you have fly-by-wire, then there will be wires for actuators as well.

[wriley4409]

Just a thought, since you seem to have unexplained airflow into the engine - have you searched for vacuum leaks?

[Christ]

When you check your IACV, just try to clean it with some ether and run a stiff pipe cleaner through the hole it sits in. There is probably dirt and gunk in it, which will hold it partially open.