TPS Sensitivity Adjustment (fine tuning)

[HenryJ]

TPS Sensitivity Adjustment

Adjust your TPS and I.A.C. on your TBI / MPFI
Authored by: Jay Vessels
Tools needed:

* Torx bits or drivers (T-10, T-15, maybe more depending on the application)
* Voltmeter (digital is best, but a really accurate analog will work)
* Tachometer (the one in the vehicle will work fine if equipped)
* Wrenches and an awl (various sizes, only if the idle speed hasn’t ever been set)


Theory of Operation – (lengthy)

A common myth about fuel injected vehicles is that the idle speed is fixed and cannot be adjusted. This isn’t quite true; there is a setting. It's called minimum air, which is adjustable on TBI and MPFI vehicles. This setting sets the lowest-possible idle speed for the vehicle. The ECM uses the IAC (idle air controller) to raise the idle speed from this adjustment. So, while the exact idle speed isn’t really adjustable, the minimum idle speed is.

Why adjust the idle speed? Isn’t the ECM supposed to do that? Yes it does and it does do a good job, but has to have a starting point. That starting point is called minimum air, or the smallest amount of air allowed to enter the engine with the throttle closed. The ECM can only add air to that minimum setting. If that setting is too high, the ECM can’t slow the engine down to an acceptable idle. If the setting is too low, the ECM may not be able to keep the engine running under certain conditions.

Another reason to adjust minimum air is if there has been some repairs to the fuel system. If the throttle body has been removed (i.e. rebuilt or cleaned) or the TPS (throttle position sensor) has been replaced or otherwise disturbed (i.e. loosened the mounting screws unintentionally -- it happens) then minimum air should be adjusted. Any changes that could affect idle speed or idle quality, like performance upgrades or replacing leaking vacuum lines, should be followed by setting minimum air.

This adjustment, once learned, only takes a few minutes. It rarely has to be adjusted, but it takes so little time to check (and adjust, if needed) that there’s no reason not to do so.



Checking & Adjustment Instructions

To establish minimum air, the idle speed must be set first. The idle speed screw is sealed with a cap from the factory. This should be removed by removing the throttle body and using an awl to pry the plug off. If this seems scary, have it done. It’s not difficult but it’s not worth risking damage to the throttle body or human flesh to remove the plug. Once the plug has been removed, reinstall the throttle body.

Assuming the idle speed screw is accessible and the throttle body is installed, jumper pins A&B on the ALDL (Assembly Line Data Link) connector under the dash. Pins A&B are on the upper-right-hand side. These are the same two pins to jumper to read codes from the ECM. Now turn the key on (the Check Engine light should be lit) and leave the key on for at least 30 seconds. The computer will extend the IAC plunger all the way out to allow adjustment of the idle speed.

After the 30 second wait, unplug the IAC (square 4-pin connector on the throttle body) WHILE THE KEY IS STILL ON. This prevents the ECM from adjusting the idle speed while you make your adjustments.

Block the drive wheels, set the emergency brake, and start the engine. Set the idle speed by adjusting the idle speed screw. The engine should be at operating temperature for this. The exact setting is on the emissions label on the radiator shroud, but in general, the idle speed should be about 500 RPM in Drive, 700 in Park / Neutral, or if you have a manual transmission, somewhere between 600-800 RPM. Remember that the truck is running during this adjustment, so stay clear of the fan, and make sure it can’t roll or otherwise be put into gear while this is done.

Once the minimum idle speed is set, turn the engine off, reconnect the IAC, and remove the jumper from the ALDL connector. The TPS minimum voltage must now be set. Turning the idle-speed screw may have moved the TPS idle voltage away from the specification, so it should be adjusted next.

Connect a voltmeter between pins A (usually dark blue) and B (usually black, or black/pink) of the TPS, and turn the key on. Don’t start the engine. Loosen the two torx screws holding the TPS in place, but don’t remove them. Rotate the TPS until the voltmeter reads between 0.45 and 0.55 volts, with 0.50 being ideal. Tighten the mounting screws (carefully, they thread into soft aluminum) and re-check the voltage to make sure it’s still within range.

That’s it. After the procedure is done once, it’s easy to remember and do. I hope this helps.

A worthwhile adjustment I think.
Mine measured .546 volts at idle and 4.45 volts wide open throttle. The reading for idle should be .8 or less and the reading for WOT should be at least 4.5 volts.

I used a drill to file the holes for counter clockwise adjustment.

I now have .747 volts at idle and 4.62 volts at WOT

Every little bit of fine tuning helps right?

[1337vending]

Funny that you should bring that up...I just acquired the EfiLive tuning/monitoring program and cable, and it just so happens that it works on our trucks! It comes with a full monitoring suite that you can see everything from TPS (% and voltage) to mass airflow sensor reading, manifold vacuum, even the computer calculated engine HP and torque!

But more on topic, I've been doing some testing with removing the hated torque managment and my TPS sensor reads 100% throttle with no adjustment on the sensor itself.

I recall some testing I did on my 87 grand national, that it was more about the change in voltage, since it seemed to 'zero' the sensor when you started it and used the voltage differential to determine WOT. As long as the voltage is above the WOT threshold (4.5v) it shouldn't matter the fine tuning. After all, if it were supposed to be a precision instrument why would it have a chintzy plastic arm sensor*?

*At least my 87 GN did, not sure about our trucks.

[HenryJ]

... As long as the voltage is above the WOT threshold (4.5v) it shouldn't matter the fine tuning. ...

I was surprised to find mine below that peak. It can't hurt to have the timing and fuel maps closer to timed. This should offer better throttle response, as well as maximum performance for WOT.

The rheostat being "cheap plastic" just means that it will not conduct as much heat, be lighter weight and less expensive to produce

These assembly line vehicles leave quite a bit of room for refinement. It makes sense that a sensor would be engineered with over travel so that it would not be damaged. The throttle traveling 90* and the sensor having 105* of movement. Obviously if the idle is too high someone will notice, but if the mapping is not quite full throttle it is not likely anyone will. Better to err to the side of caution and have them timed well below the idle voltage.

Everything makes pretty good sense to me. Now does it make a difference in the real world? I haven't driven it enough to tell yet

[1337vending]

Yeah, definitely adjust it to within the GM specs for the TPS sensor operation, or else it won't be at full throttle. However, getting more than 4.5v at the TPS sensor shouldn't be necessary if the computer already sees 100%. Unless GM has a special 110% mode that gives the truck another 50hp ....if only

[HenryJ]

No , you're right. At 4.5v mapping and timing are set to WOT specs. Mine will now read that just before full throttle, instead of never reaching it. I Casper throttle enhancer "chip" makes it read 4.5 volts at 75% throttle. This has to be better than that.
There also may be an improvement in the lowend from having the voltage closer to the idle spec of .8 volts. Now the mapping and timing will not be delayed for ten degrees of throttle movement off idle to exceed the lower threashold.

[JimmyDiamond]

So Henry, Have you noticed any difference? Did you redrill both holes?


Thanks,
Steve

[HenryJ]

I can't say for sure yet what the difference might be. It should make a difference for sure at WOT, since I was not achieving maximum advance and map.
Every little piece helps complete the whole puzzle.

Yes, I oblonged both holes to allow it to rotate counter clock wise.

I would suggest checking yours first to see if adjustment is needed.
You will need two paperclips and a multimeter. Partially straighten the paperclips so that they can be slipped along side the wire into the connector far enough to test the voltage.
If it reads below 4.5 volts when the throttle is opened fully , some adjustment is needed.

Mine needed not quite another 1/2 hole. Probably closer to 1/4 of the hole diameter is what I oblonged it to.

[Pauleo]

I can't say for sure yet what the difference might be...

I think I'll wait until you figure it out. I would like to do this mod since it is a FREE one!!! But I'm not gonna fiddle with it since I don't have a voltage meter & it's not making any noticable difference for you.

I would think that it would give it a better throttle response at any degree though. Seems like it would make the gas peddle a little more "touchy???"

Keep us posted! (As I know you will!)



Oh how i love those free mods!!!!

[HenryJ]

Pick up a cheap ($9) multimeter at Wally world. Every tool box should have one.

It is not going to affect the throttle response, other than perhaps have it operating more efficiently, since it is not within the specified range.
This is a little like carefully checking the gap on your spark plugs. Do you feel that difference? Not usually, but there may be a little improvement in mileage, or performance.

It is pretty easy to do, and IMO worth doing. I do like a little higher level of tune than most , probably.

[adrenalnjunky]

But do you have the new coversheet on your TPS adjustment?

[HenryJ]

But do you have the new coversheet on your TPS adjustment?

Sorry for being stupid here, but....what?

[adrenalnjunky]

lol - Office Space reference - I forget that not everyone has seen the movie.
(lifted from the web)
"TPS Report" became a term to describe any mindless paperwork after its use in the comedy film Office Space, which followed three software engineers who were fed up with their jobs. The main character, Peter Gibbons (Ron Livingston), is reminded several times by several different people that he had forgotten to put a cover sheet on his TPS Report, which is now standard practice according to a recent memo. The explanation that "We're putting cover sheets on all of our TPS reports now before they go out" is usually followed by "Did you get that memo?"

[HenryJ]

Whew! I thought I had missed something....OH! Wait , I guess I did, thanks for clearing that up

[adrenalnjunky]

If you have ever worked in a corporate environment - Especially a cubicle farm like I have for the last 5 years - this movie is something that you will relate to.

Basically - it's a low-budget film by Mike Judge (Beavis and Butthead fame) that had some unique characters who rekindle the Superman 2 "steal money from the banks" plot and rip their company off...or something along those lines.
It's beome quite the cult classic.

And that's Chris' movie pick for the week!

[Rusty]

Whew! I thought I had missed something....OH! Wait , I guess I did, thanks for clearing that up

That one threw me too - and I have the movie!

[JimmyDiamond]

"we're going to have to move you back to make room for more boxes"

[Rusty]

"we're going to have to move you back to make room for more boxes"

I think I need to dig the DVD out and watch it again!

[JimmyDiamond]

"we're going to have to move you back to make room for more boxes"

I think I need to dig the DVD out and watch it again!

Me too!

[killian96ss]

After doing this mod and having to take my truck to the dealer for another issue they told me that my TPS had to be replaced. Before I took it in my TPS reading was .746 @ idle and 4.62 @ WOT. I did not tell them that I modified the sensor, but it did come up as an error when they scanned my PCM. They told me that normal voltage @ idle is supposed to be .58-.64, and that anything higher can cause damage to the engine because of timing tables coming in too early. Does this sound like BS, or is there some truth in this? I researched the Impala SS forum and found similar information about TPS settings. The SS forum says that anything over .7 @ idle can cause problems and could set the CEL. I was told from some people on this forum that all I should need to do is make sure the TPS sensor reads about 4.55 volts @ WOT and that's all. Any opinions on this?

Steve

[crew cab sonoma]

while you might get an SES light from the base reading being too high, i certainly dont think any engine damage would occur.
you might possibly run into a bit of detonation, but even that certainly wont harm your engine physically.

many dealer mechanics just throw around silly remarks like that. some of them may actually believe what they are saying...

"even the slightest deviation from factory specs could result in total engine destruction. these are ultra precise adjustments, that only highly trained profesionals such as ourselves should ever attempt..."

[HenryJ]

I must have missed this post.

...They told me that normal voltage @ idle is supposed to be .58-.64, and that anything higher can cause damage to the engine because of timing tables coming in too early. Does this sound like BS, or is there some truth in this? ...all I should need to do is make sure the TPS sensor reads about 4.55 volts @ WOT and that's all.

I could find no reference to a specific voltage at idle. Only that it must be less than 1 volt. Engine damage from too much initial advance is a possibility, but it would be long term and in an engine where it is right on the edge of detonation. Detonation can be a serious problem, but usually in high compression engines more than something like our little V-6.

I think I would agree that as long as you meet or exceed 4.5 volts WOT, you should be fine.
I am not sure if that is possible to have 4.5+ and still be in the ".58-.64" range.

Mine has been doing just fine. I have not had one of the local techs examine it (not that I would anyway). I have lost a little more confidence in their expertise recently. I would say use your best judgment.

Remember that the MIL is not really there to tell you about a problem in your engine other than one that may affect your vehicle emissions. The light is not a "Engine damage may occur" light, it is a "emissions standards may be exceeded" light. Setting a light does not get me all that excited anymore. The biggest worry is damaging a very expensive cat.