IAC Problem?

good luck to you.

 

I'm glad you asked.


Where number 1 is, there are two lines. Those represent the short term corrections.

2 and 4 are the long term fuel trims. I can't answer why the graph is skewed off the screen though. I pushed a button I shouldn't have or something. But 2 and 4 should mirror each other in this graph at least. More on both Long Term and Short Term Fuel Trims (LTFT and STFT) in a minute.

Number 3 is the RPM of the engine climbing. The numbers in the list in the top center of the capture are the key, but the colors didn't come out as well as I had hoped in the downsizing of the pic. This was taken with a Tech 2, and then using the TIS 2000 software to turn it into this picture.

The vertical line running through the graph is what those numbers in the top center were measuring. The other graph like it, the vertical line is further to the right in the graph, and that is where it is accelerating.

OK. Long Term Fuel Trim is the actual learned value of the corrections needed to maintain the proper fuel ratio. 0 is perfect. That means everyone is happy. The right fuel, the right air, and just as important, all at the right time.

Short Term Fuel Trim is the "right now" corrections based on the Oxygen Sensors findings. STFT is a dynamic reading that changes as the O2 changes. O2 goes rich? The STFT goes opposite. So, negative numbers in either LTFT or STFT means the PCM is subtracting fuel. Positive numbers in turn mean the PCM is adding fuel. Expect the STFT to cycle a bit from around +3 to -3 on a happy engine in good fuel control.

Next, are the learned values I have spoken of. When the STFT stays in an area that the PCM decides is too far out, then the LTFT will start ratcheting up. So, say the STFT stays at +25 for a long enough time, the PCM will start increasing LTFT accordingly. LTFT will continue to increase, meaning the injector on time will continue to increase in millisecond increments until the STFT starts maintaining the +3 to -3 that was mentioned earlier.

That being said, the next time the vehicle is in those same conditions (RPM, Throttle Position, Vehicle Speed, etc), the PCM know to put the LTFT at the same correction it had as last time.

STFT are the grades from the pop quizzes, and LTFT is the final grade.



This is a great illustration in numerical terms as far as how it works. As you can see, the LTFT is adding fuel at a high rate, but since I repaired the issue, the STFT is taking fuel away at almost the same rate in percentages.

 

Right! Good eye, and good thought process.

When a MAF gets numb, it over-reports the air entering the engine. So, the PCM in turn adds more fuel, then the O2 says it is too rich, then the trims cut back.

As RPM increases, then the MAF then switches to under-reporting the air entering the engine, the O2 sees that, then the PCM starts adding fuel.

 

A quick question.

If a 4 cylinder car had one injector spraying no fuel at all, what would fuel trim be?

 

25%? just a guess!

 

Nailed it.

If a 4 cylinder car came in with a dead miss and around +25% fuel trim, I would be heading straight to the injector.

Good job!

 

heres a question for u guys that may be related, my mom has an '89

oldsmobile delta 88 royale 3800(3.8liter). the engine light has been on for a

while but shows no reason except when you first start the car and let it idle

for a lil while it seems like it dips maybe 100-200rpm randomly. I checked all

the plug wires and gave it an oil change but its still on. Im tempted to take

the throttle body apart and see if it needs cleaned. basically similar to

rangers situation. keep in mind it has a new water pump and alternator and

the engine has 150,000miles on it. Also while im driving or my mom is driving

the engine never shows any signs of messing up. it revs up perfectly.

*edit - a friend of mine may be able to get a hold of an obd1 scanner from his autotech class

i also used to think it might of had something to do with the electrical due to me playin around with the crappy stock gm side posts. so i

overhauled the battery connections and with the car running at the battery i get 14.4v and 14.3 at the amp.

 

Scanner will be a help, as always.

 

well yesterday we didnt get a chance to take off the throttle body due to rain

but we got all 6 plugs replaced and all 6 wires and it runs a hell of a lot

better. really needed a tune up! i cleared out the computer then started the

car up and the light stayed off for a few seconds then came back on so now

im just waiting on that scanner.

 

well when i had my 87 gmc i used a paper clip to check the engine light.

on mine anyways.the top 2 terminals to the right needed to be wired together.

your local parts store should have something cheap to minimize meesing it up.
mine was a little keychain with 2 prongs to plug it in.

ALDL OBD1 OBD cable OBDI OBD2 engine codes
Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical

 

hope that helps.^_^