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It's loosing it's mind! Help C4C5Specialist! or anyone else!

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Tanzanite3

Guest
We are still having a problem with the Vette which is annoying but not serious. We take it out and drive for a while 1/2 to 3/4 of an hour. Shut the car off for a few moments. Zip into a store for something and return about 5 minutes later. Start the car and it will not idle.. stalls .. will restart but just stalls again instead of idling. Once running, if I hold the idle speed with the throttle at about 1000 rpm and shut the engine off with the key, then re-start all's ok. No codes and no check engine light. Very intermittent and definately not all the time. Maybe once or twice a week. Like I said, just enough to be annoying. I mentioned this once before on here and this is where something has to "relearn" settings was suggested and I tried that .. it worked. However, what is causing the car to loose it's mind in the first place? The problem is more liveable now .. but, I'd like to fix it for good! Where do I start?

Thanks in advance
Glenn
 

69MyWay

Well-known member
Joined
Jul 3, 2001
Messages
4,364
Location
Auburndale, Florida
Corvette
1969 Killer Shark
Our 90 has been doing something like this.

Once in a while, she will start it up and the engine will run really rough. It is a stick, but I think if it was an auto that it would try to stall.

If you drive it like this the speedo will not register above zero no matter how fast the car is going, the a/c will not engage, and it breaks up around 3,500 RPM.

Shut it down, wait a few moments and everything is fine.

We pulled the CCM diagnostic historic and current codes. Can't remember the code number, but it told us that it is having a history of loosing connection with the computer.

Possible problems include poor wire connections from the i/p to the computer, bad computer, bad ground, and or shorts in several other wires. It does not make a check engine light.

Since ours is so touch and go, it makes it very hard to find the exact problem. I don't want to just start changing parts. Since it will clear up on the re-fire I am at a loss as well. I would prefer it just quit all together or keep doing the problem so I could fix it.

Next time it does it to you, keep it running and see if SYS flashes on the i/p. If so, follow the G.M. manual for the diagnostic procedure of retreiving the CCM (not just the ecm) codes. You will have to ground out two connections and use the D.I.C. to punch in the command and read the output.

If you have never done this before, you will get a real kick out of it. In fact, it will allow you to cycle the horn, interior lights, etc. from the D.I.C. button selection. Good diagnostic tool.
 

JT ZR-Won

Well-known member
Joined
Dec 6, 2000
Messages
1,273
Location
Bradenton, FL
Corvette
'90 ZR-1 #365 (SOLD...but Dad's got it now)
Glenn, definately not sure if this is the problem or not, but it's worth a shot. I know that when the "idle-air-control" sensor underneath the throttle body sticks, this will cause the car to stall. What happens is the little spring seizes in the "outward" position, thus blocking off the only hole to let air in the plenum if the butterflies are closed......(at an idle). Since no air gets to the plenum, the car stalls.
This is just a possibility. There is a way to test the IAC sensor. It's been quite a while since I've done it, but I believe that after you remove it from the plenum, leave it plugged in. Turn the key just till you hear the fuel pump........at this point, I believe the IAC is supposed to contract inward. If it doesn't the unit might possibly be seized. Hope this might help.....

Jay
 

c4c5specialist

Technical Advisor
Joined
May 29, 2001
Messages
3,682
Location
New Haven, Ct. USA
Corvette
Nope, but someday.
I Agree, sort of, :)

Hi there,
While JT is on his game, another consideration along the IAC line of thinking, is that on either end of the IAC is a small switch, to let the computer home itself to the correct setting.
If for some reason, this switch is bad, or is NOT working at all, the computer will NOT rehome the IAC. Therefore, it cannot adjust accordingly to computer commands.
This is the first course of action, as if the computer is commanding 150 counts, and the car still stalls, this is your starting point.
Best to all, c4c5
 
T

Tanzanite3

Guest
Thanks Guys

Thank you C4C5 and Jay.. I'll get the manual out tomorrow (if it isn't too damn hot like today!) and troubleshoot the IAC.
 
S

Scott E

Guest
Go ahead and replace the IAC,I just put one on my 92 LT1.Its got 90,000 miles on it.Mine would stall,run rough,low idle rpms,worse with a/c on,had to hold the accel pedal partially down when starting.sometimes it would start and run fine,but not always and the hotter it has gotten outside it seems to have gotten worse.But,now that I have replaced this IAC,its not acted up ever since.I bought the IAC thru a distributor for about 49.00$ Good luck,hope this helps!
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
IAC blues too!

So here's my story,
I took the throttle body off.
I cleaned out the IAC cavity.
Very carefully cleaned the pintle with some steel wool(upside down so no stuff got back into the IAC)
put it all back together ( with new gaskets for TB & Plenum) the car wouldn't idle at all! So I guessed I somehow dammaged the IAC motor and ran down & bought a new one from Standard.

This has an adjustable pintle!!!! which I set for
1 & 1/8 inches from base of IAC ( Threaded part) to tip of pintle. Now setting them side by side
the OLD GM one is about 1/4 inch shorter in that dimension, Hmmmmmmmm?!

Now the car runs like CA-CA @ idle (cold) but dosen't stall. When the cars up to temp it idle's OK between 6-700 rpm with or without A/C.

That the deal!

Mike
 
T

Tanzanite3

Guest
Ah Hah !!!!

I think that I've cured it and it only cost me time!!

Removed the throttle body and checked out all the air passages.. no blockages. Removed the IAC Valve to discover that there was a lot of carbon-like buildup on the "pintle". I read where this was a sensitive little thingie so I was careful and cleaned it off with some 000 steel wool. That took most of the junk off but still left a shiny colored surface kinda like a varnish. So, I cleaned that all off with a very fine nail polisher.. worked great and cleaned right down to bare metal! Then, I cleaned the seat in the throttle body where the pintle makes contact to shut off the air when the throttle is depressed. Reassembled it all and Voila.... no more stalling. I think what was happening is the pintle was sticking to the seat and not withdrawing when the throttle was placed in an idle position.

If I had one of the special tools, I could have checked the pulses from the puter and made sure that the pintle was moving in and out correctly, but, I didn't have one! So, I just used a bit of old-fashioned common sense (Got lotsa that!! LOL) if somethin is dirty and it used to be clean then, it should be cleaned! So far, we've taken it out three or four times since and all's well!!

Wish us luck and maybe this is the problem with some of the others with similar trouble!

My '98 Caddy Eldorado had a sticky throttle just at the start of it's motion, it too was caused by a build up of carbon-like crud around the butterflies where they seat with the throttle body.. once again, cleaned them and all's ok. Two throttle body dirty problems in two weeks!! Too much!!

Question: How does that carbon-like crud get way up there?? The gas is injected much further down, plus this is the intake side!
 

69MyWay

Well-known member
Joined
Jul 3, 2001
Messages
4,364
Location
Auburndale, Florida
Corvette
1969 Killer Shark
Re: Ah Hah !!!!

Tanzanite3 said:
!!

Question: How does that carbon-like crud get way up there?? The gas is injected much further down, plus this is the intake side!

Like Ken said, it is in the air. Also, if you look at the engine you will notice a tube coming off the non PCV valve cover and routing into the throttle body on the passenger side.

This tube venting works in conjunction with the PCV. There are oil vapors that constantly recirculate through this plumbing. Back in the day of carbs, you usually saw a PCV crank case breather filter on the inside edge of the air cleaner. That would keep some of that crud from getting into the inner workings of the carb. The fuel injected engine has no filter between the breather and intake so all that goo just flows right on in.


I imagine you could get one of those crank case breather elements and chop it to size and shove it into the breather tube. I don't know of anybody that has ever done this, but it sure would keep a lot of the goo out of the intake. You would just have to remember to change it at each oil change or service interval.

I am glad to hear your problem has been solved. I am even happier it was not a computer related problem like our little gremlen. I guess the best clue is that you were not getting the SYS flash on the i/p.
 
T

Tanzanite3

Guest
Well Now!!

That makes a ton of sense.. and to think that I thought Ken was kidding and being a smartie... all along, you were right on the mark Ken.. My humble apologies..

Also, if you look back over the thread, Jay was damn close to the final verdict too .. if not right on! And yes it's still working perfectly since the cleaning!

Thank you all for your assistance, this is a super place to get help and ideas and I really appreciate it!

Beer for everyone.. next round is on me!!
:beer :beer :beer :beer
:beer :beer :beer :beer
 

Ken

Gone but not forgotten
Joined
Jan 30, 2001
Messages
8,237
Location
Hermosa Beach, CA
Corvette
1987 Z51 Silver Coupe
Re: Well Now!!

Tanzanite3 said:
...and to think that I thought Ken was kidding and being a smartie.

I'm sorry about that Glenn, sometimes I just don't feel like going into detail. ;)
 

Bonnell

Well-known member
Joined
Nov 15, 2002
Messages
599
Location
Lake Mary, Florida
Corvette
2008 Convertible
Hi,

I am new to this forum but I recently had the same problem. Unfortunately, I took my '88 to the dealer (before I found CAC) and they replaced the ECM ($325!). I then had idle problems which I was presented a valid fix for here in this forum.

Just in case I have the problem again, what did you do to reset the system as you mentioned?

Thanks,

J Bonnell
 
E

Eagle85C4

Guest
Dissconnect the negative battery for minimum 60 seconds to 5 minutes....I would go for the longer time frame just to be sure. it allows the ECM to die and loose all the settings. Upon restart it relearns the settings and off you go....
 

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