screamin_conure

Can anyone tell me what the function of item #64 in the below diagram is?

http://www.compnine.com/index.php?u=...03&grouptype=B

The 1992 Corvette factory service manual describes the item as an "Air Vent Valve Assembly". The manual gives NO indication as to how the part is to be used or adjusted. Only instructions on how to replace it. It has a threaded part with a hex head that screws into the TB, and then a screw that appears to screw into the center of the hex head. The screw is threaded with a slotted head, so it must be meant to be screwed in or out for some reason.

The reason I ask all of this is that I'm still fighting a fluctuating idle situation on my '92. Sometimes it idles (in gear and at temp) at the very RPM that the ECM is calling for ("Desired Idle" on my scan tool), but other times it will idle 100 - 200 RPM higher than desired idle. When it is idling at normal RPM, the idle is fairly rough. When idling higher, the idle smooths out, but it obviously shouldn't be idling that high. Also, when idling high, the IAC motor steps will drop down to 0, indicating that the ECM is attempting to compensate for additional air getting in SOMEWHERE.

Now before I get all the recommendations to check for vacuum leaks and to clean my IAC, both of those are done. The TB was removed a couple of weeks ago. I removed the bottom portion that the IAC screws into. Cleaned EVERYTHING the point where the metal shined. Installed a brand new IAC (set to 1-1/8" pintle length), and put everything back together with new gaskets. All of the vacuum lines have been checked and are in good shape, and I've gone around the engine while in a "high idle" state disconnecting various vacuum lines on the intake and plugging them with rubber vacuum plugs with no correction of the situation.

The reason I'm asking about the Air Vent Valve, is that I just realized that with the car in a "high idle" condition, in park and with the intake air coupler removed, I can hear a LOT of air being sucked into the TB. If I place my entire hand over the face of the TB while the car is idling, it will litterally suck my hand against the TB with the force of a vacuum cleaner. It made me wonder if that much air should be getting by the TB blades. When it sucks my hand in, the idle starts to come way down. Just wondering if perhaps someone in the past may have tampered with the Air Vent Valve and that is causing the issue. I haven't tried to turn/adjust it. Thought I'd see if anyone could provide any insight on it before I do.

Going out to run through the FSM test procedure for the EGR system, just to be sure that isn't somehow contributing to the situation.

Thanks in advance!!

Ron

STL94LT1

It is a coolant air-bleed screw, it won't affect your idle.

screamin_conure

Thanks! Glad I didn't mess with it!

screamin_conure

Well, after some involved testing with a vacuum guage, hand vacuum pump, and a paper clip, according to the FSM, my EGR system is in excellent working order. Scratch that.

Hmmmm?

aboatguy

Can you scan it to see what your IAC counts are? IF they are stupid low your TB blades may need to be closed some to raise your IAC counts some...(30-45) would probably be a good neighborhood to be in...

screamin_conure

When she's in the "mood" for high idle, IAC counts at a stop, in drive, are at 0. Idle at that point is around 600 - 650, and the ECM is calling for 550. However, there are other times, that IAC counts will be around 15-20, and the ECM's desired idle and actual RPMs are right on. There's definitely false air coming in SOMEWHERE, because regardless of the idle and IAC counts, my BLMs are too high (137 - 142 on the right, 140 - 145 on the left) which means it's running lean and requesting fuel.

It's all very perplexing. Like I mentioned, the car has had the TB removed and cleaned (twice actually) along with a new IAC, new TPS, new 02 sensors, new injectors (Bosch III/FIC), new PCV, new air and fuel filters, shows 40 psi of fuel pressure at idle and 43 - 45 under load, and holds a steady 17" of vacuum at idle. I have ZERO driveability symptoms except the idle. The car cruises like a dream on the highway and screams like a rocket ship at WOT with no hesistation, stalling or stumbling.

I have had an intermittent Code 63 lately (right 02 sensor) that turns on the SES light for a few minutes, and then goes off. Typically during highway cruising. Sometimes it will come back on again after a while, but sometimes it doesn't. Sometimes it never comes on at all. For that code, the manual points to a circuit fault at the 02 sensor. When it sets that code, it will put the ECM back into open loop. When I get this code, I experience zero difference in the car's performance or driveability. The idle issue continues whether I have the 63 or not. I'm guessing the code is due to a flaky connector and/or wire on that sensor, and I do need to get that back to my mechanic to have that checked. But I highly doubt it's a cause or contribution to my issue.

aboatguy

I still believe its your your Throttle blade setting (a little bit to far open and the IAC can't slow the idle down...at zero IAC count its closed and can't remove anymore air)

So first off: Is the throttle sticking? Can you manually close the throttle blades and lower the idle.... If the answer is NO!

Use the adjustment screw to close the throttle blades until your IAC counts are 30-45.

If that doesn't work ...it may be a worn throttle body and the air could be leaking past the bearing surfaces that support the throttlebody shaft.


However, the adjustment screw should fix your problem.


Mike

screamin_conure

I've been thinking about doing this. I know where that screw is, but I've always been told that the screw is set and sealed at the factory, and that messing with it was a no-no. I wouldn't even know how to remove the plug that covers it.

By the way, the throttle is not sticking. I can push down/in on the throttle linkage and/or the throttle blades and there is no change in RPM.

I guess if I can figure out a way to remove that plug, I'll try some very minute adjustments of the screw with the scanner hooked up and see if I can get things adjusted.

screamin_conure

Okay, how's this for a Jekyl and Hyde car:

Took the car to my brother's house this past Saturday night for a 4th of July celebration. As we were leaving, I saw that I was down to 1/4 tank of fuel. Stopped on the way and topped off with premium, and then hit the expressway for the 40 minute drive. When we pulled up in front of his house, the car was idling at 1,200 RPM in park.

Left to head for home several hours later. Idling okay now (lower), but by the time we got home, idle was around 800-900 in park when I pulled it in the garage.

Yesterday, I popped the hood to see about taking aboatguy's advice on adjusting the throttle stop screw. The plug on the back of the throttle body just isn't coming out, so I decided to nix that idea. I pulled the air snorkel off again, and cleaned up a messy duct tape job that I had done to it a couple of months ago. The rubber snorkel has a small tear in it on the bottom lip of the side that connects to the throttle body, and I figured I'd try to seal that with some duct tape. The tape had turned into a melted, gooey mess. After cleaning that, I started the car with the snorkel still off and just kinda poked around a little. I manually worked the throttle linkage several times, and every time the car just settled right back down to a normal idle. I used the blunt end of a screw driver to nudge the butterflies back while the car was idling, just to make sure they were seating properly in the closed position. No change in idle. I shut the car off, grabbed a can of throttle body cleaner, opened the butterflies and gave them and the inside of the TB a few quick shots of cleaner and let them close. I let the car sit like that for a few minutes while I put the air snorkel back on and buttoned everything up. Started it, and it sat there idling very nicely.

Today, I decided to take it out with my scanner hooked up as I wanted to clear the code that was set yesterday when I was running it with the IAT sensor unhooked. I cleared the code, and let the car sit and idle a few minutes before heading out. Idle was a tad high for the first few stop signs, as I could feel the engine/trans working against each other a bit while coming to a stop. WHILE THIS WAS HAPPENING, THE IAC COUNTS WERE AT 0 WHEN STOPPED, IN GEAR AND IDLING.

Now get this; after the first few minutes of stop and go driving, the car suddenly decided to "settle down". The "desired idle" and the actual RPMs were now bang on identical at a stop. The IAC counts were now hovering around 45 - 55 at idle. Even the BLMs had come down significantly (128 on the right, and 134 on the left). I drove around like this for 20 or more minutes, in 88 degree heat, and the car exhibited no, I mean ZERO, high idle symptoms the rest of the drive. I even took it through two full throttle blasts to near redline during the drive, and each time I came to a stop again, the idle was right at "desired idle". It actually even fell UNDER desired idle a few times and I could see the IAC opening beyond 75 steps to compensate. I've owned my scanner since January, and I've spent a lot of time driving the car with it hooked up, and I've NEVER seen these kinds of numbers on the display.

How in the WORLD could this happen?

Jamesnns

Cause im your friend and lent you my horseshoe...lol.

Let's hope it stays this way.

20 minutes and hard driving at 88 outside, you had more then enough time to come up to temp...so...maybe all is good..for now.

See ya ltr today...

tombrammer

screamin_conure how did you come out with your high idle? still working ok? I want to ask what is the scanner that you have? Where did you get it? My 93 is having the same problem since I went with a bored out stock 52mm TB, I am wondering if my IAC is not working, I disconnected it then reset it according to the FSM but no change. My idle is around 1000rpm.

Tom400CFI

Intermittant vacuum leak. Sounds like something that is solinoid activated to me...like the purge canister, or EGR (I know you checked the EGR already).

screamin_conure

Still kinda' wonky. I haven't done anything further with it since checking the EGR. The car right now is actually with my mechanic to check out a SES light/Code 63 that I was getting. Just got off the phone with him and he told me the right 02 sensor appears to be bad. It's an AC Delco unit that was just installed this past March, but I guess stuff happens, even with new parts. Incidentally, the car idled PERFECTLY the whole way to the mechanic's shop! No, nothing wrong with me!

Since I've been more or less running with a funky 02 sensor for a while, I guess we'll see how she is when she comes back. Who knows, maybe that was it.

screamin_conure

Cannister purge already checked. See my last post here: http://forums.corvetteforum.com/c4-t...osch-iiis.html

Yeah, I've been battling this for a while.

screamin_conure

An update on my situation (suggestions welcome):

So, I was getting a code 63 for about the last six weeks, and took it back to my mechanic last week to have it checked out. After extensive troubleshooting, he deemed it to be a faulty 02 sensor, which was just replaced back in March. He replaced it, no charge. The new 02 sensor made no improvement on my idle situation (described in previous posts to this thread), as all of the symptoms remained unchanged, as did the numbers on my scanner.

However, in bending my mechanic's ear about this, he asked me if the throttle position sensor had been changed, as they can often cause issues with idle. I told him yes it was, back in January. This was BEFORE I had changed 02 sensors, IAC, injectors, etc. This got me to wondering what would happen if I put my original TPS back on. So, over the weekend, I disconnected the battery (Jamesnns, are you reading this? ), and swapped out the sensor.

Now get this; The car now idles NOTICEABLY smoother than before. With the replacement TPS, when placed in gear with a cold engine, the idle would get rough and the car would really shake at idle, at least until it got good and warmed up. Now, with the original sensor, starting the car and putting it in gear with the engine still not completely up to temp it is nice and smooth.

In driving it with the scanner connected and the original TPS, I will still get a high idle but it seems to take noticeably longer for it to get high. I will be able to get the coolant temp up to 195-200 degrees and be able to drive around like that with some good numbers showing up on the scanner (i.e., IAC counts at idle 20-25, desired idle and actual idle RPMs right on the money, etc.) with one exception. The new (old) TPS seems to have had a SIGNIFICANT impact on my BLM numbers. Previously, my right bank BLM was around 132-134, and my left was around 145. Now, my right bank is 128, that is until the high idle situation arises, then it goes up to over 140. My left bank, hot or cold, varies between 145 and 160. I'm definitely running lean, but to varying degrees, and not all the time. With my replacement TPS, my voltage was .67 at idle, and with the new (old) one, it is .62. Can a change this small cause such a noticeable change on the way the car runs?

I drove the car into work today (about 24 miles, with a 50/50 mix of city and highway driving), and the high idle appeared about half way through the trip, but by the time I parked the car in the parking lot, idle was normal again. I was watching the BLM numbers at the scanner while idling and the left was now pegged at 160. It got so lean that it actually threw a code 44 (left 02 lean) for a short time on the way in. I guess there's still a on-again/off-again vacuum leak somewhere, but for the life of me, I can't find it. I've pulled every connection that carries air to/from the intake manifold and the throttle body while the car was exhibiting a high idle and plugged the hole, and saw no change in idle. I've also tested the EGR and EVAP solenoids, and according to the FSM, they check out fine, as does the EGR valve itself. The only thing that I haven't messed with is the MAP sensor.

Jamesnns

Ohhh, now you think your a funny man...............