Brand new to this forum. I finally purchased my 1st corvette at 58 years young. 1989 c4 L98 Idle jumps to 2400 to 2800 at start up and then goes down to 600 to 800 after 20 to 30 seconds. Scary to me as I am old school carburetor guy. After reaching the 600 to 800 range every 15 to 30 seconds it revs to about 1200 rpms and back to normal instantly. The previous owner replaced all 3 cooling temp sensors the TPS and the IAC sensor. Also removed heads and did a valve job and replaced all injectors. Also blocked off vacuum booster which is still blocked due to being bad. My next step is to block all vacuum lines from Plenum and see if anythimg changes. Before I do that just wanted to see if there was any other suggestions.
I also checked timing and its around 8 degrees and vacuum gauge showed 6 to 7 when at normal idle and when it spikes goes to 20 to 25. Any short cuts or suggestions would be much appreciated. Thanks to all

 

I would stop anything you are doing and get yourself a scanner. If you have an Android phone or have access to an old one, I would get something from 1320 Electronics and use that to read data.

When you have that, I would adjust the Min Air rate using the IAC counts.

 

Welcome to the Corvette Forum bbipec!

Don't feel out of place as people of all ages love Corvettes. You will meet a lot of very young Corvette fans here with lots of disposable income and an urge to have a Corvette. I bought my first back in 1991 and I still have it, a 1968 C3 Convertible that started life as an L71. I bought my 1988 C4 coupe a few years later so the whole family of four could go cruising in our Corvettes.

I hate to say it but it appears you have purchased your self a Project versus a road ready Corvette. This is not a bad thing here as most of us know the operating systems of the L98 pretty well.
The suggestion by aklim is a very useful one. Having a scanner lets you see what the ECM sees and this makes finding problems MUCH easier. They vary in price and capabilities. I have an old Snap On MT2500 and it works great for my 1988 C4 Corvette. The Moates adapters with Tuner pro is all you need to do it with your old laptop.

One item you will need is a copy of the Factory Service Manuals and they are available on Helm Publishing website. You can buy a DVD version of the FSM from Rockauto if you don't want to pay $100 for the printed version. You will NEED to have a manual if you are planning on keeping the Corvette and enjoying it on the road. The other device is a Fuel Pressure Gauge that can show you the pressure in the fuel rail. The valve it attaches to is on the passengers side fuel rail towards the rear.

How many miles are on the Corvette as it sits now? What problem was the previous owner trying to fix? The TPS has to be set properly for the car to idle properly. I bought a set of testing rigs for the efi system on our Corvettes at Mid America Corvettes. It included like several test setups allowing you to hook up your multi-meter and set the parts precisely and verify the operation of all the other parts in the EFI system. It is very valuable for the first time Corvette owners to set up their fuel system.

Here is a listing of the parts:

"1985-1989 Emissions Sensors Test Harness Tools 6 Piece Combo Kit

This set of six, 1985-1989 Corvette Specialty Electrical Tools include:

• Throttle Position Sensor
• Manifold Air Temperature (MAT) Sensor
• Oxygen Sensor
• Mass Air Flow (MAF) Sensor
• Idle Air Control (IAC) Sensor
• Coolant Temp and Knock Sensor

Use these tools with any Digital Volt/Ohm Meter. Your Corvette will idle smoother, enjoy increased horsepower, and gas mileage."

These tools REALLY help setting up your EFI system and finding a bad sensor. It cost about $80 at Mid America is Part #: 601-074

The oxygen sensor in these C4's have a life of 24 months or 50k miles. If you want to avoid problems you should always have a good fresh Oxygen sensor in your Corvette for maximum economy and Performance. The oxygen sensors in OBD2 Corvettes are heated units and last twice as long before needing to be replaced.

When you have a moment could you please fill out your "Profile" on this Forum so we don't have to ask you what kind of Corvette you have and a lot of other questions. It saves a lot of questions to have all the information right in front of us.

 

I don't know why you would want to buy the tools from them to get data when you can scan that same data. That harness is for people that don't want to buy a scanner for whatever reason.

 

A paper clip will do the same.
Even though the owner changed the IAC, you will Not get a code from the IAC. If it is being scanned in real time, you will see what the IAC counts are being told to be, whether the IAC can or cannot do it. Common issue on these cars is bad IAC.
You can test it,
IAC testing:

Terminals A to B should be 40 to 80 ohms

Terminals C to D should be 40 to 80 ohms

Terminals A to D should be infinite

Terminals B to C should be infinite

I suggest only using a digital (high impedance) ohm meter for doing these tests.


Usually the IAC does not go bad electrically, but it is common for them to stick and stop moving.

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If the IAC sticks as does not move as it should for each pulse, the IAC count will get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumes it has moved every time it has been sent a pulse.

The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

 

Sure, you need a DVOM to check resistance on the terminals but using a paper clip is cheap and easy. Just like the results. Is everything working as spec? Hard to say with the paper clip. Is the tach correct? See above. I would not bother using the cheap trick and go straight to the scanner.

 

There is no code for IAC being bad.. Thats why you need to manually check the IAC..
I went through this on my 93.. All the real time scans said all was well, but the tuner said there was a vacuum leak somewhere. Then an old dude on Z28 forum said check you IAC. I did it, and it was bad. Put a new one in, and car ran significantly better. And vaccum leak was gone according to tuner..PCM forLess.
Just offering my advice. Been involved in L98/LT1/4s since 91. I just research and try.

 

If you have 0 IAC counts and you closed the TB plates, I would go looking for a leak

 

I see used Snapon MT 2500's used on Ebay. Pricey ,especially if something is missing...

 

Perhaps. But mechanics prices are North of $100 an hour. Without a scanner, it is difficult to begin any sort of diagnostic process and deciding if you are capable of doing it or have to pass it off.

 

Thanks for all the information and positive input and suggestions. I posted a few things but forgot to mention several and also have learned a lot in the last couple of days and still don't know anything LOL. SO the previous owner did provide me with the shop manual 1 & 2 and also a Moates chip burner and Cat-tune software on flash drive which did allow me to look at some items. Above i mentioned the high idle and the back and forth revs. Once engine is warm in runs a lot better and idles at 900 and occasionally drifts up to 1200 or so. Now for the useful information that may help you guys help me and a few things i didnt understand. RPM 900 The TPS is 0.549. M*** air flow is 9.4 Spark advance 21.4 ECT 187 dergrees IAC position was 0 and IAC the count always showed 0 at idle
When i would hit the throttle the IAC would go to -128, 02 sensor was showing 147mv. Target idle showing 725rpm. The prom ID is 0x0DA3
unilim air flow 18. The most puzzling to me was when I shut car off and leave synchronized was the target idle would jump up to 3188. When i restart the idle goes up close to 2800 and then slowly comes back down to 700 to 900. My other concern is previous owner said he only changed the fan to turn on at 200 and off at 185. He also replaced the MAS and relay The IAC the O2 sensor and the knock sensor and I know the coolant sensor between 1 & 3 and I replaced the one under throttle body on intake. He also said he was looking for a vacuum leak and determined that the brake booster was bad and has capped off. I have not purchased software yet so all I can do is monitor right now which is a good thing until I learn more. At least i can see what going on now. I am all ears on next suggestions or what I would need to look at next. I am already thinking because the car has a 162,000 on it and I am 4th owner I may need a stock tune program and start over. Its a 6-speed and its still mainly stock the best I can tell there's no mods. I used my laptop to monitor so good to go there.One other thing the car runs great going down the road other than the occasional high idle when stopping

 

If it is at 0 counts and you turned the screw out as far as it goes, you have a calibration problem or an air leak. Most likely, air leak. This is with everything but the motor off and the system is in closed loop.

Also, isn't the CTS in the manifold at the front and not the side of the head.

 

My 89 does the same . It will come down to 7oo rpm usually within a minute. I just use the time to put the sealtbelt on, make sure mirrors are adjusted and all guages, turn indicators etc are working.

 

Yes Sir. There are 3 sensors for water temp is my understanding. The CTS in the front of manifold is for the ecm the one between 1&3 is the temp gauge inside and the one between 6&8 are for the fan switch.

 

Does it go up to 2800rpm when cold?

 

Mine does when there is an air leak. Massive one.

 

I still say IAC is bad or IAC gasket..
Good luck..

 

Not that high, usually about 11-1200 for about 20 seconds then slowly begins to drop . I have recently changed the ECM, 02 sensors, coil packs, new plugs/wires and the MAF and related relays. I was lucky to find an original Delco MAF . Also did the plenum gasket. When I bought this car last month, it was idling about 1800, would drop to 600, then shoot back up to 1800 and would keep cycling like that. I got the car cheap enough that I didn't mind throwing all the new parts at it. It has 60,000 miles .

 

Is it the same as the Moates laptop capability or the Android? I'll have to get something eventually.

 

I use the AKM cable, looks just like this http://aldlcables.com/ . I then use TTS datamaster to link up to laptop.
Data is real time...
Andrew Mattei made the cables many people used on our cars.
Not sure if he still does it. But, Moates stuff is great.