I will get this blasted Vette figured out yet. My latest problem is the motor idles around 1500-2000 rpms with no codes.

I was having some hard starting (pouring black smoke) and hesitation issues (but the idle was about 1000 rpms) and so I connected my laptop up to the car since my check engine light doesn't work and I don't feel like ripping the instrument cluster out to see if the bulb is shot or removed and using WINALDL, I found that the coolant temp was showing -40 degrees and the TPS sensor was showing 4.8 volts with the throttle closed. I checked into those and found that the coolant temp was physically broken for some time it appears and the TPS sensor had been monkeyed with so I fixed those and the starting is far better and the hesitation when hitting the throttle is gone but now the blasted idle runs from 1500 rpms to nearly 2000 rpms occasionally and WINALDL is showing no codes and the IAC is running something like 130 counts so I don't think its a vacuum leak although I have been over the system quite a bit and found the hose to the cruise control rotted away so I fixed that but it didn't even phase the idle speed. Makes no sense that its idling so high. I take it out and drive it and the BLM numbers on the WINALDL program look really good so I know the fuel isn't an issue and the chip is programmed for at 575 rpm at 200 degrees which is the temp the car runs when warm and what its showing in WINALDL with the new coolant temp sensor. I just can't figure out why the computer is commanding an IAC valve opening that high.

Any ideas before I start cursing at this car yet again this week.

 

I suspect the tps voltage is still too high.

When this occurs the IAC control is no longer used for idle PID control, it instead follows the throttle position in order to provide a dash-pot function.

With the throttle closed you want to see around 0.54v on the tps.

 

Have you done a "base idle set procedure"?

When I got my 86' in 2004, I cleaned the throttle body and the idle jumped to 1100 in gear and 1300 in neutral. The cap was still on the idle adjustment screw. I popped the cap out and did a base idle set and that fixed the issue.

 

Maybe someone (PO) already tried to adjust the TB stop screw without doing the proper proceedure....see if your cap is missing already.

 

I set the TPS sensor to about .6 volts anything less than that and it acts screwy. I don't think that is the problem as that is within factory spec. On the throttle stop the cap is gone and I adjusted it to the blades are fully closed and still idles exactly the same and its going to until I can figure out why the blasted IAC valve is running nearly wide open at idle and not dropping down where it should. I hate to pull that blasted computer out of the bottom of the dash again and try a different chip and computer to see if I can get this idle under control.

 

The TPS sensor has an acceptable range in the "closed" position. Off hand I think it is .40 - .63 if I'm not mistaken.

If you don't have a FSM (Factory Service Manual) I highly recommend you pick one up if your going to hang onto the car.

Read this thread your answer may be contained within it....http://forums.corvetteforum.com/c4-t...lp-please.html
The proceedure only takes a few minutes to complete.....of course it works best with a completely clean throttle body setup.

 

;------------------------------
; PID ENABLE CONDITIONS
;------------------------------
LC543 FCB 3 ; 1%, ENAB T/F IF TPS GT THRESH%
LC544 FCB 0 ; 0, DONT STORE NEW TPS IF ID DIFF VAL < THRESH
LC545 FCB 6 ; 1.8 MPH/3.2, ENAB IAC PID IF Vss < THRESH
LC546 FCB 4 ; 50 RPM/12.5. MIN RPM UNDERSPEED ERR TO ENABLE
; PID DURING TRANSISION DELAY
;----------------------------------------------

If the tps and vss conditions (< 1.8 mph) check out, it could be that you're never exceeding the min underspeed error. This would suggest too much min air keeping the revs high and preventing PID control.

 

Well I did the IAC reset procedure and so far the idle is staying around 700 rpms which I can live with. I set for as close to 450 rpms as the motor would tolerate with the IAC closed jumping the A-B terminal to close the IAC and unplugged and set the 450 rpms after I got the motor up to about 150 degrees just to make sure cold temp (94 humid degrees out today) wasn't going to affect the idle once it got closer to 200 and I wasn't patient enough to wait for it to get to 200 so I set at the 150. We will see how it does but that seems to be all that was wrong with the idle. I am used to the newer stuff learning on its own.

 

What is the command idle from the chip? I like to set it till I get 15-20 counts after it is warmed up and then set the TPS.

 

I have the chip still set for 600 rpm idle by the time it gets to 150 degrees and I haven't driven the car yet for it do its little learning on the idle yet and haven't hooked the laptop back up to see the IAC counts just wanted to note that the idle is at least not 1500 rpms now and so 700 is at least tolerable as long as the computer doesn't screw that up and my tach could be off by 100 rpms as well. I didn't check for a discrepancy with the laptop the last time it was connected. I know my 91 Camaro isn't even remotely accurate. Its off by almost exactly 50%. 1500tach=750 engine, 5000 tach= 2500 engine its that bad. I know the vette isn't off by much if any.

 

Don't overlook the possibility of a worn out throttle body. The shaft will wear to a point where it allows the blades to hang. I dealt with an erratic high idle on my 88 for quite a while. About three months ago, I got a rebuilt throttle body and all my troubles went away.

There is a company that advertises and sells their rebuild services on ebay. They do great work, but have yet to refund my core charge on the old unit that I sent in. They are called XtremEFI. If you buy one from them, don't plan on getting your core charge back. Just plan on adding the $60 to the cost of the project and keep the core yourself. Don't trust them with it.

They machine the bores from 48 to 52MM and use ball bearings on the shaft. Very nice piece. Too bad their reliability as a company does not match their craftsmanship.

 

I don't think the mileage is high enough to be the throttle body but I will have a look at it. I have a lot of customers that deal with xtremefi and have had too many customer complaints to use them. I also deal with rvmorsemachine.com and have had no customers complain about their services which are comparable in price. I have had both do TBI throttle bodies for me in the past and I xtremes job was ok in my instance but rvmorse's was superior in the way the throttle body opened and the look of the unit as well. Xtremes was a little sticky. Just my .02.

 

I wish I would have known of the alternative supplier that you mentioned. I found Xtreme and figured that was better use of funds than buying an aftermarket TB.

Good luck,
Doc

 

Well, I don't get to hang out on the forums like I used to. So I don't get to post things like that much. Just get too many emails and phone calls about stuff to go looking for more questions but I do try to help as many as I can as often as I can.

 

XtremeFI sucks. Core issues and support after the sale is non-existant. Stay away from them.

 

I try not to be quite that hostile but that is the kind of response that I got from my customers about them. In the last 4 years since I found rvmorsemachine.com, I have yet to have a single customer complain about their services.

Brian

 

I just went through this with my 1982, and looked for all kinds of things that might be causing high idle...turns out my IACs (the 1982 uses 2) were evidently sticking. I removed them, cleaned the pintles and all surfaces, carefully lubricated and hand moved the pintles back and forth very gently. Cleaned them off and tried them with the car but not actually installed in the throttle bodies in order to watch and verify pintle movement, which was correct. And then reinstalled both IACs with new gaskets, disconnected and reconnected the battery to reset idle memory, and went for a test drive...high idle problem seems to have been solved.