Hi everyone, first just wanted to say how much i appreciate this forum its helped me so much.

90 l98 auto
OK here's my problem. Very high idle 1800-2200 depending on engine temp/slight surging at idle

I have a tech 1 and the only codes the ecm are throwing are 21,22(tps voltage high/low)

I have replaced -injectors, fuel pump, tps, and iac.
used the procedure from this forum to reset iac. The car will idle perfectly the first time firing it up after reset. If I shut it down however the car somehow forgets or somethingand the next time i fire it up its back roaring again.
I have not adjusted throttle blades and the car has a non adjustable tps(which i have checked with a volt meter and i believe the voltage is way to high .89V)and i'm not sure what the next step is?? any ideas?

Thanks

 

I think that the TPS voltage is supposed to be .54

 

It should be .54 according to the FSM for my 89 I believe your 90 should be the same.

Here are detailed instructions to setting your idle I was given by a forum member last week after i changed my IAC.

http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm

 

Do a base idle for sure, however follow this as the other one forgets to tell you to disconnect timing bypass.....

first thing to do is disconnect and re-connect your battery or ecm connector as the ecm will detect faults and make adjustments to try and rectify them. ie retard timing.

Set your car to a base idle (as per my shop manual)
when hot.

Connect a paper clip, to your diagnostic terminal under your dash connect to terminal A and B (the top right two pins).
Turn on ignition but don't run the engine.
wait at least 30 seconds and then remove the idle air valve connector (IAC) (the square plug) then turn of ignition and unplug the paperclip.
Now remove the timing connector a single wire near the fuel pump relay.
Start your vette and set the idle speed to 400 rpm in neutral (auto trans) 450 rpm (manual trans) then check your throttle position sensor,
the top two wires it should be set to 0.54volts dc, also confirm with engine off that the tps rises to 4.5 volts at full throttle.
Turn of the ignition and reconnect the iac valve connector and timing connecter and start your vette.
it should go to 1200 rpm then drop slowly to 600 rpm, take the car for a 15 minute drive with the usual stop lights.
It is normal for a little hunting while the ecm relearns, also do a few spirited take offs.
All should settle after 15 to 30 mins normal driving and have a stable idle. However will get better with many more miles.
Of course any vacuum leaks fouled plugs can affect the correct base idle.

Good luck

 

I would not disconnect the timing connector, that will make the car idle too low and make the minimum idle speed setting too high. Once the timing is added back in idle speed will be higher then what you want.

Are you sure your throttle is not sticking causing an inconsistant idle reading .89 volts at idle is too high.

 

I appreciate the replies very much

ok here's the situation,
I first reset the iac using the (disconnect est connector) method. disconnected battery etc..
before firing it up, after reset IAC, I went to tackle the TPS. Being my 90 has a non adjustable tps, I decided that since it was made of plastic slotting the holes would be a rather simple way to make it adjustable.. I'll be &@mned if its not .54V.
Anywho, reconnected est and fired it up. it idled fine still slight surging. Drove around town 40mph for 35min and came back to the house and shut it down.. Then fired it back up. It went straight back to 2000rpm and surging at idle. ecm shows an est bypass code and still a TPS voltage low code.

Being that it idled down (even though slight surging) im assuming no vacum leak..
Would MAP sensor have anything to do with it? Im not sure what factory parameters are on it. tech 1 shows MAP sensor at idle 1.24V. Not sure if that helps or not

Next on ides? Whats causing the ecm reset?
If any figures from the scan tool will help let me know.

Thanks

 

You have a sticking throttle or failed tps. The tps low voltage signals to me that sometime in the drive the tps went under. 44 volts to set the low tps code. Either the tps has a bad spot or the throttle sometimes sticks high and sometimes snaps shut and then reads low.

 

I appreciate the inquiry, but for certain I can say that the throttle is not sticking. when i replaced the iac i pulled the throttle body and cleaned it throughout. it opens and closes smoothly.
I cleared the codes before the dirve and as soon as I fired it up the TPS low voltage code was there. I took a volt meter to the tps before and after the drive. It showed .54 before and after the trip.

However, tech 1 shows TPS voltage at .04V before and after I adjusted it. no idea on that one.

I still have no clue why the ecm resets the iac back to a 2000rpm idle after shutting it down the first time after IAC reset procedure

 

Does the tps voltage move when you open and close the throttle as read by the scanner? If the scanner is not reading the correct tps voltage I would suppect a wiring issue. Check the signal wire back to the computer for continuity. The car will not idle correctly with a bad TPS signal.

 

Measuring the TPS voltage at the TPS tells you that the TPS is OK. That voltage needs to get back to the ECM, which apparently is not happening. Check the wiring/connectors.

 

took voltage of tps sensor at ecm. it read .04 volts. searched for about 2 hours for a bad wire until i simply cut it and ran a new strand. checked the tech 1 and what do you know. tps voltage .57V. Fired it up and all is well. idled perfectly (no surging)!!

Thanks guys!!!

now i can see what i can do with other problems on the car (mainly cosmetic issues)