So, I like to do things myself, but I am terrible at figuring out the problem. My "mechanic friend" is taking too long, so i thought I would ask the C4 experts

Recently my car decided to change how it runs. In the morning, if left to idle with the defrost on, it idles up high, dips low, then high, and so on. When I drive, if it is below 180ish degrees it will stall terribly when I push the accelerator. At a stop light it will dip down and idle at 500rpm, defrost or not. It seems to run perfectly fine once it is warmed up. Any ideas?

 

Year helps , not all the same

Could start by pulling the codes to see if anything showing
http://www.corvettephotographs.com/c4vettes/ecm.htm

 

Start with the base idle.

first thing to do is disconnect and re-connect your battery 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.
Of course any vacuum leaks fouled plugs can affect the correct base idle.

Good luck

 

mine did the same thing it was the IAC. changes and adjusted it and the idle is not 100rpms on start and then idles right down to 500 regardless of what acessory it on. the engine is also impossible to make stall or change idle now at any operating temp.

Good luck

John

 

yeah, I'd say take the throttle body off and the IAC and clean the passages, use new gaskets and let it reset itself. IAC's get gummed up from normal carbon crud passing thru and either cannot seal the pintle or they just cannot move anymore.

Air leak may be contributing. Pull the sensor plugs and wire brush the eng coolant sensor contacts so the ECM is getting semi accurate info from the coolant sensor in the manifold. A dirty temp sensor plug/contact will cause it to do strange things.

 

sorry about that. it is a 1986 with a TPI motor

 

so, I replaced the IAC and cleaned the throttle body. now it runs 800 rpm on start up, and 1500-1700 when warm. you said to remove the timing connector and adjust the idle...how do i adjust it? is this by retarding and advancing the timing with the distributor?

 

Sorry guy he just told you "how" to set idle...you have exactly to do what Geradvg posted this is the FSM procedure to set idle speed o reset idle speed.to set mecanical idle go on throttle body, driver side,there is a screw tapped with a plug(if still there) turn until you match 400-450 rpm. To ceck the TPS use a digital voltmeter connected to the two upper terminal on the TPS connector (to set this at .54 volts loose upper screw and lightly the lower and turn lightly on the slots untill you match .54 volts on the digital voltmeter)

 

ok. i guess i was not getting it, i wasnt kidding when i said "novice". i dont even have a volt meter lol. i am pretty good at taking things apart and putting them back together, but thats about as far as it has gone in my experience. thank you for clarifying the instructions. i had thought maybe i was missing something.

 

I'm sure you may have other issues but, shut off the defrost and see how it does. Depending on year some cars engage the a/c compressor when you select defrost as it acts as a dryer and clears the glass even before the heat is available. The a/c compressor turning on and off with play with the rpm a bit.

 

It a simple device just two prom to put into the to upper connector terminal of TPS sensor,slide the sensor lightly and you read on display .35,.40,.52...when you match .54 stpo turn TPS sensor and bolt it steady

 

Which sensor is the TPS? where is it located and what does it look like?

I will get a meter, looks like I am going to need it in the future

 

Before you go nuts and before you spend a dime on anything make darn sure that all vacuum hoses are good. Look for even the smallest crack or deterioration. Make sure the manifold has no vacuum leaks and be sure the hose that goes to the power brake booster is not rotten, loose or cracked. A small vacuum leak will cause the computer to richen the mixture because it "thinks" it's running too lean and will cause erractic idle and wierd running.

Once you are sure there are no vacuum leaks then start poking around for codes etc.

Just my 2 cents.

 

i will check for vaccum leaks first. far as i know the old method i use on my '71 truck should still apply, spraying some cleaner around hoses and connections and listening for a reaction from the engine. I can't pull codes on my own. the local parts houses will only do '96 and newer.

 

it's important to ceck all vacuum since a vacuum leak causes high rpms,it's important to clean your throttle body air passage and IAC valve air passage as well.But ,as told,since much calculations of ECM depend on how minimum idle is set,you have to set it correctly as well.You have to buy a FSM for your MY first,it's basic,best 900 pages for your car maintance.You are a NOVICE...it's not a problem,lots of friendly guys here,same passion...

this is the TPS sensor throttle position sensor:


and this is the torx bolts to set mecanic minimum idle



http://www.digitalcorvettes.com/foru...ad.php?t=58873

 

is a Chilton from the local parts house good enough, or is there something better out there?

i certainly do appreciate all the help. This group of guys, minus a few wisenhimers, are sure showing how helpful and friendly they can be. I really think the only dumb question is the one you don't ask. The pics help a lot, Thank you so very much

 

The Chilton's is better than nothing, but not by much. Factory Service Manuals for your year are getting relatively cheap for your model year on e-Bay. If you don't see one at a good price wait a week and look again.

Now, wait a minute Zippy, I am an accomplished Wisenhimer (Oregon term - they just call us smart a$$es down South), but I bat .999 on my advice here, so be prepared to take the good with the pert near good!

Anyhow, this is what caught my eye:You CAN pull your codes. Go back and look at the first response to this thread from Vetteoz. There is a link there that tells you how to pull codes. If your local parts house can't pull codes on a OBD I system, that just tells you they are too dumb to unfold a paperclip!
With that said, just remember, codes are only an indication of what the computer has interpreted as the underlying problem. It is not at all uncommon for a code to indicate that a sensor is bad when in fact there is a mechanical problem that is causing an anomalous sensor reading. THAT's when us smart a$$es come in handy, so go read your codes and report back!

 

Sounds like the temp sensor to me. h

 

could be, once i replaiced the IAC it reversed, runs 800rpm cold, 12-1700 warm

 

i like you already Bob