TPS and IAC setting problem

[pdeason]

TPS and IAC setting problem (maybe)

I have been working through the "rough, surging idle" condition with a 1991 Corvette that I purchased recently. Car has 76,000 miles.

I had replaced the plugs, wires, distributor cap, rotor and two vacuum lines that were dried out. I also removed the throttle body and cleaned it and the IAC. With each thing I did the performance got a little better. However, the surge never went away and when I would let off the accelerator it would buck a little and when I stopped the car would sometimes idle down so far it would shut off.

I checked the TPS and it read about .36 and I got the code 22 a couple of times so I figure it must be bad as well and I replaced it today along with the pcv valve. Without any adjustment the car ran much better and the idle seemed pretty good. When warm there was a slight lope but not enough to change the rpms.

However, I could not leave well enough alone and I tried to set the minimum air flow and TPS. I put the car in diagnostic mode, waited the 30 seconds and then disconnected the IAC. I forgot to take the paper clip out before I started it and that may have something to do with what follows (or it may not). I could not keep the car idling long enough to set the minimum air flow. This the same thing that happened before the new TPS only it was much worse. There was a very distinct rich smell. After several attempts at restarting it and it dying I abandoned that and reconnected the IAC. I checked the TPS and .45 volts was as high as I could get. I could elongate the holes and gain more adjustment to get the .54 volts but I should not have to do that.

I restarted the car and drove it for about 15 minutes. Nearly every time I stopped it shut off and the rough idle is back. I checked for codes and got a 22 and a 45. I assume the 45 was related to the rich condition when I was trying to set the minimum air flow and the 22 was me fiddling with the TPS.

I need some guidance. Help!!

 

[Hib Halverson]

Did you follow the Service Manual procedure for setting minimum air and adjusting the TPS? If not, I'd suggest it.

In re: min. air, if it has been previously done incorrectly or the throttle body has been changed you might have to make a preliminary adjustment just to keep the engine running. Also, if the PCV valve is not the correct part the min. air adjustment might be affected.

Lastly, to work though your "rough, surging idle" are you using the diagnostic info in the FSM?

On the TPS is the most voltage you can read is below the required setting, I'd check to make sure you have the right TPS sensor and that the TPS sensor's mounts are not damaged.

 

[KANE]

...I checked the TPS and .45 volts was as high as I could get. I could elongate the holes and gain more adjustment to get the .54 volts but I should not have to do that...

Welcome to the CAC!!!

I've done a couple of TPI swaps over the years and have had some experience dealing with TPI. In fact, Hib and I even traded emails over a self-test fail that created a code 43 from base timing being set too low (took forever to figure out!). So, like Hib said- did you set the minimum air speed to service manual specs?

If so- I'll ask some basic questions-

• Did you disconnect the battery after you made the repairs? If so- GOOD. You want the ECM to wipe itself clean of the way it had learned how to set itself. Otherwise- your TPS could have been bad and it is still applying the old way it had to calculate its fuel & spark.
• What is your timing set to... when the brown wire is disconnected (to prevent computer controlled advance)?
• Is it set to the correct timing for your year (IIHRC- it is 12 degrees... but varies by year)?
• Are all your ground points good?

Getting a code 22 and 45 (together) can point towards the TPS system- but you've replaced the sensor... right? Could be shorts in wires or the ECM.

A code 45 (O2 sensor circuit) could set when the voltage is erratic from the TPS. However- it could also be the ECM due to the 5 volt signal it supplies to the TPS- being erratic.

Voltage should be between .4 and 4 volts on the TPS.

• Blue- Reference
• Gray- 5 volt signal
• Black- Ground

The Gray wire is the 5 volt reference wire. I'd check to make sure you don't have a short somewhere in that wire & curcuit. If you had over 5 volts- I'd say check the blue wire.

Since you have less than 4 volts, grab a volt meter and make sure you are getting 5 volts from the gray wire itself. If it is NOT cranking out 5 volts and there are no shorts- could be the ECM. You'll need to make sure.

You want to check the voltage and wires in two places- at the sensor and then at the ECM. If the reading are different at the sensor versus the ECM- then you have a short in the wires. If the readings are low on both the sensor and the ECM- then your ECM is at fault.

But make sure your ECM is bad before you go out and buy one!

 

[JAKE]

First, I'd disconnect the battery for 30 seconds or so to clear any stored code. I'd then use the IAC resetting procedure to ZERO it but this time remove the paper clip/wire from the ALDL before starting the engine again.

It''s often difficult to get the engine to idle with the IAC disconnected so you'll have to manually hold/play with the TB lever until the engine can finally can idle on it's on. At some point it should.

I chose to set mine at 550 rpms, which 100 RPMs higher than what you find in the manual. As I read long ago, GM supposedly issued a Tech Bulletin recommending the higher RPM setting. I never actually read the bulletin, but 550 had no down-side on a stock engine. Of course, a modified engine with a longer duration cam, etc. will call for a higher RPM setting.

I'd then shut down the engine and reconnect the IAC BUT before restarting it, I'd adjust the TPS voltage - engine OFF ignition ON. If yours is adjustable, the TPS voltage should be .54 +/- .075/ .61 worked best for me; some set theirs' higher. However, if you go too high, say above .67, you may get a "voltage too high" code.

If your IAC is a free-mover, and all else is working properly, you should be good to go. Of course, if you have something like a vacuum leak, well the engine won't idle well until it's fixed.

Jake

West Point ROCKS!

 

[pdeason]

I did follow the fsm instructions (with the exception of removing the clip before starting the engine). I just checked the reference voltage and it was 5.09. I disconnected the battery and let it reset. Then I decided to try something different. I put the voltmeter on the tps and then adjusted the minimum air screw until it got to .54 volts. This was the only way I could get it to that setting since it was already adjusted as high as it would go and it was only .46volts. I know this is bassakwards but IT WORKED. I drove the car for about 20 minutes and it ran and idled as good as it has since I got it. The closed loop idle with the a/c on and the transmission in drive is about 650. I think I will leave well enough alone for now.

It still hiccups when you are going about 40 to 45 mph with the car in overdrive and you let up on the accelerator a little. If you coast completely it does not do it. Any ideas??

 

[vetteboy86]

Not sure I have heard of anyone changing the idle screw to acquire the proper voltage from the TPS. I have put a new TPS on my car, and also helped an install on another car, and both parts were easily adjusted per the FSM.

 

[pdeason]

I know it sounds weird and I am sure it is not the correct way. However, it may have been that the minimum air flow was set too low for the tps to even be adjusted high enough. The screw was essentially all the way in so that the throttle plates were completely closed. That may have been why it would not idle at all when I was trying to set it the first time. I may go back and try to set the minimum air flow the correct way at some point and then set the tps again. For now I am going to leave well enough alone and enjoy the car for a while.

I appreciate the help. This site is loaded with valuable info.

 

[KANE]

I know it sounds weird and I am sure it is not the correct way. However, it may have been that the minimum air flow was set too low for the tps to even be adjusted high enough. The screw was essentially all the way in so that the throttle plates were completely closed. That may have been why it would not idle at all when I was trying to set it the first time. I may go back and try to set the minimum air flow the correct way at some point and then set the tps again. For now I am going to leave well enough alone and enjoy the car for a while.

I appreciate the help. This site is loaded with valuable info.

You are probably close to getting it set! I think all of us may have taken the fact the idle air was set correclty from the get go.



Actually, it sounds like you may have had the Throttle Body idle screw backed in too far. :duh

That is completely possible. Although not a scientific answer- the idle screw should have some threads protruding towards the throttle stop and thus push the throttle blades open a very small amount.

So- yes... if you didn't have enough of the threads showing (meaning your idle was set incorrectly)- you'd have the problems you spoke of.:W

Is your code 22 gone? Hopefully it is.

FWIW- When I was running TPI w/SLP runners on a crate motor with an L82 cam and 1.6 rockers- I ended up with a lot of that screw's thread showing (turned forward) to set the blades forward enough to keep it from stalling out and dying during the minimum idle air set procedure. 8.5:1 compression and the L82 cam (plus 1.6 rr) created a need for a lot of advance in the base timing and a need for a lot of mimimum idle air. My combo was set at what I called a "combat" idle- about 750-800 rpms.:L

 

[pdeason]

I will drive it to church today and see how it does. If a code was set, how long would it take after I had checked the codes for that code to disappear. In other words, if I check and get a code 22, I then drive the car some more, will the same 22 code be there until I clear it or does it clear each time I check it?

 

[vetteboy86]

I'm pretty sure that the code will be set until you reset it. The only way I know of doing that is by disconnecting the battery.

 

[Schrade]

I have been working through the "rough, surging idle"

Vac leak. START HERE:

No one got the proper technique for vacuum leak detection... :confused

FIRST; block the pedal, or block the throttle linkage. YOU HAVE TO DO THIS TO GET A STEADY IDLE , around 1,250 rpms, or a little higher. If you don't, you can fish a propane hose, or WD40, or Krylon, or hair spray, or pee, anything, until you need a sweater in Hades, but if the motor is lopin' already from a vacuum leak, you ain't gonna' find any vacuum leak, unless it's so big that Little Jack Horner can stick his thumb in it and pull out a pineapple NOPE.

AFTER you get a steady idle, THEN, you can spray starting fluid everywhere - that's the best detector, because it's vapor pressure is very high - a LITTLE at a time, until you get the SURGE. Follow the vacuum lines to the EVAP cannister too. You might even have to follow the EVAP lines from the EVAP cannister to the gas tank too, but that vac leak will show up as a DTC 32 EGR fault, and won't normally show as loping idle, unless the EVAP cannister has been by-passed (been there, saw that uh-huh).

If you get a surge in a tight spot, and can't tell exactly where it is, light a book of matches, blow out the match heads while they're still burnin', and feed the smoke to the tight spot.

Fear not the starting fluid; you cannot put the can down fast enough and light the matches fast enough to catch the starting fluid, because the vapor pressure is so high.

Bookmark this post YUP.

Wrench Wizard OUT!

A new car on the lot or a 30 year old lawnmower will lope idle with a vacuum leak uh huh.

 

[larry bud]

I did follow the fsm instructions (with the exception of removing the clip before starting the engine). I just checked the reference voltage and it was 5.09. I disconnected the battery and let it reset. Then I decided to try something different. I put the voltmeter on the tps and then adjusted the minimum air screw until it got to .54 volts. This was the only way I could get it to that setting since it was already adjusted as high as it would go and it was only .46volts.

Doesn't make any sense. Are you sure the little arm on the TPS is making contact with the correct point on the throttle body? All you're doing when you crack open the min air screw is opening the throttle manually, which in turn opens the TPS.

All the TPS is is a variable resistor.

Just another quick thought: You're loosening BOTH bolts that hold the TPS on, right? I mean, you can rotate that thing probably through 30 degrees of travel.

 

[pdeason]

Doesn't make any sense. Are you sure the little arm on the TPS is making contact with the correct point on the throttle body? All you're doing when you crack open the min air screw is opening the throttle manually, which in turn opens the TPS.

All the TPS is is a variable resistor.

Just another quick thought: You're loosening BOTH bolts that hold the TPS on, right? I mean, you can rotate that thing probably through 30 degrees of travel.

I was loosening both screws. The holes allow for very little movement. However, by advancing the minimum air flow screw I was able to move throttle enough to get it to .54 volts. The screw was not even making contact with the throttle before so it is very possible that the setting was so low that letting the throttle close completely was choking off the engine. With the car in diagnostic mode and the IAC completely closed, as the fsm instructs, this would result in a car that was very difficult to keep running with no throttle which is what I was experiencing. If I did the procedure over from the new setting I could probably do it the correct way and the reset the TPS. However, as I posted in another post I checked my injectors and the results were not good.

17.8, 8.7, 17.1, 5.4, 8.5, 5.3, 17.5, 6.4

 

[larry bud]

Something is wrong if the holes allow for "very little movement". I don't have one in front of me, but the slots that the bolts ride through are about 1/4" long each in the TPS.

 

[pdeason]

Something is wrong if the holes allow for "very little movement". I don't have one in front of me, but the slots that the bolts ride through are about 1/4" long each in the TPS.

The screws are about 1/8" in diameter and the holes are approximately 3/16". That is not much play. However, that should be sufficient if all you need to do is change it a few hundreths of a volt. Mine needed over 1/10th of a volt adjustment and there just was not enough twist room to get that much change in my experience. As I said, the minimum air flow screw was set such that at closed throttle the throttle plates were completely closed and that was probably not right. Where I adjusted it to is probably pretty close to correct. I will wait until the injectors are replaced to go through the procedure again.