Hi everyone. New here and hoping you all can help me with my newly purchased 94 convertible with 174 k on it. I got a good deal on it but it has some issues. Mainly starting with the issue in the title. I've read countless threads on here about this same issue and I still feel I have no idea whats going on with it. When started it runs between 1200 and 1400 rpms and then starts surging between those two once it warms up a bit. (a minute or two) Driving it, it runs good, has alot of power and doesn't smoke or anything. It sounds like it may have a small leak on the pass side exhaust as it makes a like a loping woop woop sound.(maybe where the exhaust manifold connects to the pipe0.
Things I've tested: I've one at a time blocked off pvc, brake booster, egr and egr tubes going to exhaust, charcoal cannister and all other vacuum lines attached to the intake. Nothing made any difference. I've replaced the tps, it was cracked, and the iac. TPS values were fine. iac is another story. Oh, and no codes reading from the dash. Throttle body was cleaned and I found no excessive wear on the throttle shaft and the plates seemed to close nice and tight. Did propane test and carb cleaner test around the TB and intake etc and no idle change.
Now on to the iac. I ohmed the old iac and the new one and both tested fine. However after messing with this for awhile I've determined that the iac does nothing. Either one of them. I've moved them around and started ran the car and they never move. I've left one in and the other out with the plug in the one that was out and all it does is pulse in my hand and vassilate maybe .001 back and for. Thats also with just the key on when its supposed to park itself. I've taken one of them and pulled out the pintle and carefully placed it in so that it retract and block off the pintle orifice. It did not change the idle nor did it move position when running or not. Nothing changes this idle problem and nothing seems to move the iac. Heres one question. If the idle is so high like mine is, does the iac even come in to play or is it just inoperable because idle is to high? I read the voltages from the iac plug with just ignition key on.. A= 10.7 v B,C,D all go from 11 volts to no volts reading back and forth every second or so. I don't have any other scanning tools or software except for the onboard and as I said, no codes show up. It seems like a massive vacuum leak but I cannot find it anywhere. I also checked the bolts on the intake and they were fine. No coolant leaks and rad pressure is fine. Could it just be a bad ecm? It really doesn't seem to respond to anything be it unplugging sensors or blocking off vacuum ports. It just keeps idling high and surging. Any ideas?
Thanks in advance, Billy

 

Take the old IAC and extend it all the way out by hand and then install it in the throttle body. Then plug the harness into the new one. You might have to have someone inside the car and hold the gas down a little to keep it running. But you should be able to get the RPM down to 700-800 or so with the peddle. Also while this is going on take a look at the IAC that is plugged into the harness and see if it's moving. If it's not then you need to ohm the wiring going from the IAC plug to the PCM. This would be a lot easier if you had a good scan tool. Even if you rent one from Auto Zone it can tell you the demanded IAC %. Being able to see the IAC in your hand and seeing on the scan tool what it's calling for will shed some light on the problem. If the IAC is moving or if you can't get a low idle with a dummy IAC in the throttle body then you have a vacuum leak. Keep in mind it could be one of the small lines for HVAC inside the car. You can always unplug them and cap them off to eliminate that as a possibility.

Until you figure it out you can always plug off the IAC and leave it unplugged. Just adjust the throttle blade to allow it to idle. It's not perfect but will at least let you drive the car while your figuring it out. I never bothered with idle tuning in my LS swapped 84. I've been running a dummy IAC for a year with no issue. It just doesn't start in cold weather very well.

 

just wondering in all your tests and look at things have looked at O-2 Sensor Voltages you might be having a problem but not enough to throw a code

 

I did most of what you said but instead of ohming the wires from iac to ecm I checked for voltage which as I stated was 10.7v on A and b,c,d fluctuated from 11v to no volts. I guess I could ohm them but I think they are Ok since they do show volts and are consistant even though I'm sure those voltages are wrong. I did not know auto zone rented out scanners. I will call them up and see what they have . Thanks.

 

I've thought of that. Would a bad 02 sensor cause the surging as well? I'm trying to fix this without buying test equipment and just throwing parts at it. After all of the high idle/surge threads I've read, it could be anything from fuel pump ,pressure regulator, injectors, 02's, throtttle body, egr, intake gasket, opti, ecm, maf, tps, iac, map, etc.. Its a real head scratcher. Thanks...

 

The O2 sensor can make car run overly Rich then over compensate Lean and possibly cause the car to run funny. As for the test equipment I picked up a Snap On 2500 for $125.00 on Let Go money well spent so give a look for one.

. If you don't have a Factory service Manual get 1 will save a lot of hair loss

 

Yeah the fact that Auto Zone will rent you really nice top end scan tool and then give you all of your money back when you return it makes it real hard to justify buying one.

 

As for autozone and the rest of the common auto parts stores. They do not rent out obd1 scanners. Only obd2's so no luck for me.
I messed around with it a bit today. Unplugged the Maf while it was running. Almost stalled and then idle increased by about 200 rpm and fan kicked in. Seemed like it might have been idling better but hard to tell as the fan is so loud. Got a code h 48 out of it which is expected. Does this mean the Maf is Ok?

Unplugged fuel injectors one at a time, didn't seem to make much of a difference at idle. Did get a code h 18 so I assume the computer is somewhat working. Pulled a couple of plugs. They looked pretty old but were burning nice. (light tan color) Cleaned some grounds and checked wire harnesses and all seemed OK.
I'm still concerned about the iac tho. Reading the diag code list, their doesn't seem to be a code for faulty iac. Can anyone look at what I posted before and comment on it ? I'll post it here again. Remember, my problem is high idle and a 100 -200 rpm surge at idle.

"I ohmed the old iac and the new one and both tested fine. However after messing with this for awhile I've determined that the iac does nothing. Either one of them. I've moved them around and started ran the car and they never move. I've left one in and the other out with the plug in the one that was out and all it does is pulse in my hand and vassilate maybe .001 back and for. Thats also with just the key on when its supposed to park itself. I've taken one of them and pulled out the pintle and carefully placed it in so that it retract and block off the pintle orifice. It did not change the idle nor did it move position when running or not. Nothing changes this idle problem and nothing seems to move the iac. Heres one question. If the idle is so high like mine is, does the iac even come in to play or is it just inoperable because idle is to high? I read the voltages from the iac plug with just ignition key on.. A= 10.7 v B,C,D all go from 11 volts to no volts reading back and forth every second or so."

I know I should just buy a scanner but I am trying to keep this fix low budget. I just want it to get to run normal and I want to spend the money on other issues I have on the car. Its a beautiful car. Aqua metallic paint job is flawless but the car has issues. Needs a new vert top and seals, front tires, alignment, shocks, has a bent rim or two, needs a rear right wheel bearing which previous owner gave me and I'm sure it will need more than that too. Its a long project but right now I'm just interested in getting the engine right. I will keep this thread going till this problem(s) with the engine are resolved. Then I'm sure I'll start others as I get to other areas of the car.
Thanks for looking, Billy

 

If you have a computer, you're in luck--the 1994 and 1995 Corvette have excellent support in terms of computers. No need to buy expensive scan tools!

Buy this cable: http://aldlcable.com/products/aldlobd2u.asp
Download this software: http://fbodytech.com/eehack-2/download-eehack/

There you go. Less than $100 and you have a "scan tool" that is in fact more powerful than any scan tool on the market save for maybe a real GM Tech II. The best part is that you can use the software to save a datalog which you can then upload here on the forums so that helpful members can analyze it and look for issues.

Good luck!

 

Is it fairly easy to use? Does the software come with instructions? I have a couple of old laptops but they both have XP on them. The eehack requires win 7 or higher so I'ld have to upgrade one of them to make it work. Which means more work...lol...
Sounds like it might be the way though. thanks for the info..

 

It's fairly easy to use; you'd open the software, and in most cases, just click the "connect" button in the upper-left to start datalogging. It logs automatically as soon as it's connected. The most you should ever have to do is if it doesn't connect, you may have to click "Settings" and make sure the correct COM port is selected--but the software should automatically choose the correct port for you in most cases.

Then just drive around collecting data, and when you're done, click the "Save" button to save it. That simple!

It's capable of a lot more than that in case you ever want to get into learning how to analyze the data and start tuning your own car, but since you're looking to get some diagnostic help, just getting a datalog is a fine start.

 

I don't know about a 94 but IIRC, on the L98 ones, you are not supposed to pull or push the IAC pintle.

Just to be clear, IDK if the Autozone scanner will work. They are OBD2. IIRC, 96 was full OBD and before that, it was OBD2 plug but OBD1 ECM?

http://www.1320electronics.com/products.html

 

I usually toss my O2 sensors out after 50K or 5 years especially if it is the upstream one. From what I read, they don't fail instantly always. They will get old and lazy so it might Nickle and dime you to death on the fuel mileage.

 

The fact that I can rent a car for so little also could be said that it is hard to justify buying one. Personally, I'd rather buy one than get one that has gone through a bunch of rough hands. Kinda like buying a rental car. You know it has been rode hard and put away wet. I prefer to buy my own when it comes to electronics as opposed to using a community one. Kinda like a sloppy seconds. Sure, some people like it. No judgment on that. My old AutoXray, if you can get one is good. The MT2500 I have with different cartridges for different stuff isn't bad. My 1320 Electronics has one that works well with my Android phone. Not sure about the fruit phones.

 

Sure, I don't blame you for trying to save a buck but what is your time worth? With a scanner, you can see what the CTS, IAT, TPS and the IAC counts are that might be affecting this. I don't buy "one off" tools myself unless they are so cheap that I can afford to throw them away. I tend to throw stuff away that has sat for a while so if it is a $20 tool and I am trying to make space, I can toss it. A $500 tool, I prefer not to do that. Kinda why I don't want to do "one off" jobs. The one I would suggest if you want to keep the car and not spend too much money is the 1320 Electronics one.

 

$60 and work or you can get what I recommended if you have an Android phone and $90 with far less fiddling than buying an OS upgrade.

 

The Autozone scanner will not work. 1996 is OBD2 PCM and OBD1 everything else. Everything before 1996 is OBD1, though as you state, the physical ALDL connector on the 1994-1995 is the OBD2 design.

The 1320 tool will of course work just as well as the OBD Diagnostics one, though I don't have enough experience with the ALDLDroid to say if it's as effective a tool as EEHack, not to mention I wouldn't know how to interpret its datalogs if it does provide that ability.

But if OP has an Android phone and doesn't want to toss a copy of a later OS onto a computer, then yeah, that's another perfectly viable option.

Fruit phones are a non-starter for two reasons. One, Apple is a little more strict with how the Bluetooth stack works on a stock phone, and two, to my knowledge no one has ported ALDLDroid to iOS. It's kinda in the name--ALDLDroid.

 

The screen is pretty simple to understand. I just get live data from the screen with the 1320 electronics unit. Using the free software, it can log although I don't believe it is going to be as nice wit graphs as others on the market.

 

Update: Ordered cable, new windows and a battery for laptop. $75. bucks total. I will update when I get it all set up and get some data. Heres a couple pics of the car. I got it for $3500. I'm looking to put less than a grand into it to get it to a daily driver. Previous owner gave me 5 g's worth of receipts he put into it. He wasn't a mechanic and paid ALOT for services done to it. It still needs substantial work done.

 

Ok, I used scan 94/95 as recommended by Gary Doug. Here is what I captured when the engine was warmed up and idling. Can anyone take a look and comment on this? Thanks....