Benny771

Hi:
I was wondering and hoping for some input on a TPS.
Looking for the things that tell me when it is going/inaccurate.
It is set right and i do not have a code from it but what i have read, the TPS may still be my issue.
What normally "acts-up" when this is bad?

This is what i have:
Irratice idle
Surging at idle (more cold then warm)
Hard to start when cold.
loss of power at higher rpm's
Overdrive seems to kick in early
Seems to run rich.
Possible, heats up faster

I have never changed this. 150,000 KM, auto.

pcolt94

Hesitation can be a symptom of a bad TPS. However with all your other problems have you ever cleaned the passages in the throttle body? It may not fix all but you may have multiple problems.

Some of the problems also sound related to leaky fuel injectors.

bill mcdonald

mine generally ran really bad. it would run, but it was just bad. stuttering a lot. felt like it was loading up.
Threw a volt meter on it and moved it by hand, I could see the voltage jump around. it was not smooth at all. my idle voltage was right, it was when I moved it it would jump all over the place.
Replaced it and all was fine.

However, some of your other symptoms do point to the AIC motor. it could have gone bad. it could be coked up with carbon.

best thing to do is test the tps sensor. check for smooth voltage increase as you open the throttle.

next would be to remove the AIC motor, clean out the TB with TB cleaner. get in the AIC passage, and in the blades etc...

decide if you want to buy a new AIC, or clean the tip up a little and reinstall. do the idle reset process, and see what you have.

bill mcdonald

what year vette are we talking about?

pcolt94

I believe you mean the IAC motor (idle air control) which is in the throttle body.

bill mcdonald

yeah........... mondays........... you know.

Benny771

Woops, sorry. 1987 Vette.

SaxyVette

I've gone pretty much thru my '86. Car has 174K..
My mech put new plugs/wires/points/cap/rotor/IAC..cleaned the intake timed, and probably more I missed..
my service engine light is still on..I did not replace the TPS.
Motor runs smooth and strong once it starts to rev.
The idle is still rough tho.
Can my mech just clean and adjust the TPS ?? or is replacement necessary (how much) ??

The adjustment part I don't understand.
I tried to read the manual but I'm really not a mech myself.
My local shadetree is a great mech but doesn't have a lot of experience with the TPS on a corvette.
Plus he's a Ford guy anyway..oh well

I need an idiots guide to TPS to show my guy..!!

Oh yeah..the idiot part is for ME not Jimmy

Benny771

Hay, no stealing my thread.

What i have done since.
Installed a IAC that i had from years ago (thought it was a issue years ago) but it wasnt.
Had a hard time starting the engine, turned the idle screw on the Holly TB to full just to get it to 600 rpm (still ran like poop).
Disconnected the TPS and it made no difference in idle or responce ,but the "check engine" light did come on. WOW!
Ordered the TPS for 80$ at the dealer because my meter shows it as bouncing all over the place. It may just be my meter (digital) slow to respond.

Am i on the right track?

rodj

If he can't follow this , get a new mechanic

http://www.corvetteactioncenter.com/...and+Idle+Speed

bill mcdonald

The TPS is just a sensor that reads the throttle posistion by voltage.
at idle the sensor should be in the area of .54-.58 volts. as you push it wide open, it should reach 5.0v or close to it. Increasing along the way.

I dont know anyone who has cleaned one. usually replace since they are a sealed unit.
If you washed the engine, it is usually a good way to ruin one quick.

The IAC motor completly different from the TPS sensor, this is just a motor with a pointed tip that seals off a port in the throttle body for air to pass through at idle. as the tip moves away from the port, the idle goes up because air is allowed in.

Usually the port, and the tip get gummed up with carbon and the computer adjustment no longer works. this carbon needs to be cleaned out. once done, the idle may go higher then it is suppose to, so one would do the IAC reset process to get the idle set right. It depends on who has done what to it, such as tried to set the idle by turning the throttle body screw in or out without knowing what they are doing.

I found after doing the idle reset, I got the best idle the car has ever had out of it.

As for you check engine light. we need to know what the code is to move forward. this could be leading to a bigger problem.

bill mcdonald

at idle, what TPS voltage did you get?
did you do the idle reset that was posted in the link?
did you see any carbon on the old IAC tip, or in the throttle body? Any attempt to clean it out?

SaxyVette

Benny my apologies..was late, wasn't thinking, I'm not a real hijaker by nature. I have simply not found any good reference to my problem until I read your thread and I got excited.
Good luck with your '87
Rod Thx that's my answer !!!
Bill also Thx, it's showing code 23
I'm gonna exit stage left, start a thread and find the fix.

Cheers gang

Benny771

Well, set everything the way i was instructed. 400 RPM reached.
Could not get .54 volts. .46 Max
Put the old TPS back on and got .54 volts easily.



Crap new TPS.....figures!
3 hours time $100 for TPS as well.......argh!
Stuck with the old TPS

Idle was smooth, will go for a test drive tomorrow and post.

90WhiteLightning

I basically have the same symptoms Benny771 has. I have a 1990 with only 24K miles on it. I'm trying to decide what to do. I first ohm'd out the IAC...and got about 51 ohms. My Chilton manual says it should be between 40-80 ohms (something like that). So, I'm thinking my IAC is OK. I then went to the TPS. Using a needle poking into the wires, I turned the ignition to the ON position and got < .5 volts. As I turned the throttle, I could see the voltage go up to 5 volts (full throttle). So, I'm thinking the TPS is OK. Could the IAC still be my problem even though it ohm'd out OK? Carbon build up, etc?....I was thinking of doing 1 of 2 things...
1) Remove the entire throttle body, replace the IAC (there is no way I can get the IAC out without removing the TB...no way you can get a 32mm open end wrench in that space), clean the throttle body real good, etc. I could also dump $100 for a new TPS.....put it all back together and see what happens...
2) Take it to a vette INDY shop to work it.....they may try the same things??

Thoughts? Thanks!

SunCr

TPS gets a 5 volt reference from the ECM and the return voltage is representative of where the throttle blade is. The parameters are: Idle: .48 to .62 volts (fixed at .62 sometime around or after '90); .63 to about 4 volts, acceleration/cruise; 4 to 5 volts, WOT. Idle and WOT have ranges to compensate for voltage drop due to resistance, temperature, etc. Reference is fixed at 5 volts which means it will get a decent signal even if the Battery is down to 11.5 volts.

An idle surge can be caused if the return voltage rises above above .62 volts. It's a 3 wire harness: Reference, Return and Ground. Should the Return intermittently come into contact with Ground - (and it only takes a nanosecond as anything much longer than that will set a Code), voltage signal will rise and cause the Surge - ECM thinks its accelerating. So, examine the Connector and make sure it's in good shape - wriggle it around - check harness for bare wires.

SunCr

PS - Not a great idea to puncture the insulation on any device that uses resistance to control voltage as exposed wiring will deterioate (faster) creating resistance which will skew the signal. Get some weatherpak terminals and simply use them to jumper the harness to the device, then stick your meter on the bare terminals. Or use a scanner so you can see what the ECM is seeing; on 20 plus year old wiring, I've seen a point or two between the Sensor and what's at the ECM.

90WhiteLightning

Thanks for the info SunCr.....I'll check the wiring and connector a little more closely. I'm just wondering if I should just go ahead and take the TB off and replace both the TPS and ICA. About 1yr ago I had the injectors replaced because the OEM injectors started to short out due to the ethenol in the gas, etc...something like that....I got the Bosch injectors from the main POC here in the forum (guy down in GA I think?). The car
has been running pretty well since then...... until recently.....each day it has gotten worse and worse....

SunCr

Your money might be better spent on some means to check fuel delivery and sensor input; ie, a scanner or software and a cable to use with a laptop. If it shows, lean, that would give you a list of other things to look at. If it's a high mileage unit, you might try directing some propane around the outside of throttle shaft, though the earlier (model years) throttle bodys seem to be more prone to leakage in that area. You might also check wiring at the Coolant Temp Sensor which acts as a choke and shares it's ground with the TPS and Manifold Air Temp Sensor. A crappy connector or intermittent open would tell the ECM that it's cold, so it dumps fuel. Simple check of the IAC is to ohm it out between PINS A & B and C & D. Specs are 20 ohms. The TPS can be checked as previously posted - use an analog meter and make sure the needle stays steady when you open and close it, though if you find that condition, it mucks up acceleration, not idle. Your '90 uses a MAP instead of a MAF - make sure that connection is clean and tight, though when it loses that signal, they usually just quit. If a scan shows rich - or just another thing to look at - clamp off the Evaporative Emissions Hose from the Canister to the throttle body. If the problem goes away, troubleshoot that system.

aklim

To do the IAC and TPS, best thing to get is a scantool.

I set the min idle by adjusting the set screw till the IAC counts are about 15-25 give or take with all the ascessories off. After that, I set the TPS. The thing about the scantool is you can see TPS voltage. It MUST be a smooth climb as you gently hit the gas. Any hiccups or spikes or drops and you need a new TPS.