moneypitvette

Hi everyone I'm in trouble again with my 90 C-4. It starts up ok every time but stumbles and stalls. My wife said something she did this morning set off the alarm and she said it was her fault not the car. It started the stalling stuff yesterday on her way home from work. It would stall at idle but if you kept the RPM's up it would go but when coming to a stop it would stall and always started right back up.So today I thought I'd see about trouble codes and I got a 46 (VATs) but I thought it was because she had the alarm go off on her and maybe not a prob with VATS. I tried running the R's at around 11 hundred for a bit and then it would idle at around 600 but in short order it started to stumble then stall. I disconnected the batt to erase codes and see if it would help but no good.I tried to run codes again and the cooling fans would come on then you'd hear a click they would go off then back and forth erratically and the only codes that came up was 12, 12, 12, 12, 12,and on and on. So here I am lost in space again, I hope you people can bail me out of another jam. Please

MrWillys

Once the car is started VATS will not shut it off. VATS only prevents it from starting. There's a myriad of things that could be wrong, and all comments that follow will only be a guess without any diagnostics. Dying upon deceleration could be the VSS signal is not making it to the ecm, or bad fuel system, or ignition, head gasket, or all of the above. do you have a scanner? does the ecm see speed? have you ohmed the injectors, checked fuel pressure, or even pulled a plug or two to check for color?

joe paco

you can reset the idle air control, although this is generally to reset after it had been disconnected.

-key off, depress accelerator slightly, above idle, 1200 or so
- start and run for 5 seconds.
-shut off for 10 seconds.

the PCV, ventilation valve, affects idle and is figured into the bypass air equation. check to see that it has a reaction when you pull it out, engine running. also place finger over the end to see that it pulls a vacuum, than snaps back when released.

you can also disconnect the TPS to see if it improves or reacts in some way. MAP sensor can be unplugged same way, to get a reaction.

check for fuel in the regulator vac line. if any fuel is leaking, it enriches the idle. EGR valve should not be open at idle.

if you have the FSM, 6e3-b has a shopping list of things to be aware of.

IF it runs normal except at idle it is probably not injectors. it should idle pretty well with one injector out of the circuit.

just some ideas that are easy to eliminate.

added: forgot, vac leaks in hoses, etc. these generally cause higher idle I think. hoses that are routed close to battery can crack from the acid fumes.

DUB

Code 12 is SUPPOSED to be there...it is GOOD that is it. CODE 12 means nothing other than being a way for you to tell that the ECM is operating correctly...at least to that point that is. The codes when manually read come in 3's....so code 12 will flash three times. IF you have any other code...it would flash 3 times...then code 12 will flash again. SO ...basically code 12 is a beginning and exit code to let you know that ANY code in between these 12's is something to notice. SO...knowing that the computer displays the codes 3 times...and YES...the cooling fans do RUN...if you get 4 code 12's...then there are NO CODES to be displayed...because the computer displays all codes 3 times.....make sense????

You REALLY need a scanner...along with checking your injectors and fuel pressure. There are many sensors that need to be checked...especially the Throttle Position Sensor (TPS)...watching your O2 sensor cross-counts and the lean/rich flags..block learn values and the integrator.

DUB

7d8

Could it be Idle control valve.? I had an 86 Iroc that would stall after warm up. Would have to brake / gas at stop lights. Car would start right up when cold, but once warmed up would not idle. I even cleaned it, but I think the plunger itself was not moving. This part did not throw a code on the car. Car drove solid otherwise.

moneypitvette

The plunger and the IAC is working. It won't stop spitting out 12 codes even tho I know there is a hard code, because the engine stumbles and the service engine soon light starts flashing, which means there is a problem. but when I read codes, all I get is 12's forever. we pulled positive wire off battery while running and engine stayed running, so I think it's not alternator. plus alternator is new and battery is brand new. some times when manhandling the ecm , the engine did stumble but don't know if it was going to do that anyway or not. the rpms were very erratic. pulled on all individual wires and I couldn't find a problem with them.cleaned the throttle body and IAC. even replaced a plug wire that might or might not have been rubbing. ..thank you for your time and help. Neil

pcolt94

Years ago you could disconnect a battery lead and have the engine keep running (no problem, no damage). But back then ( 50s - mid 80s) cars did not have the computers, microprocessors and eproms we have in the cars today since the 90s. The battery keeps the voltage at a steady level. There is a relationship between the voltage regulator and the battery. Some regulators work differently and a large voltage can occur of the battery is disconnected. Some regulator see no load, think the battery is not charged and pumps up the voltage.

You cannot push a good charged battery to high in voltage for what happens it the current goes up. So in an extreme case if the voltage regulator ran wide open, system voltage will not go that much higher but current will be high and you will cook the battery. The battery acts as a ballast.

But basically you might be able to take the battery cable off, but you might be gambling with a lot of rare and expensive modules. (I know I ran off on this but it may save somebody electronics).


I have had IAC units test good, go in and out or even work for a while plus other symptoms. I have cleaned them but never lasted. My outlook and on a problem like yours when there is any question it might be the IAC, I just replace it with a new one. That’s how I do it now and it has saved me time and effort over the years.

moneypitvette

ok, I checked on purchasing a reman ECM and it is stated that I MAY have to transfer data to the PROM, from the old unit to the new unit.. has anyone done that and if so, can I do that from home?
thank you for your time. Neil

DUB

*****SPEAKING FROM EXPERIENCE*****

I deal with bad computers numbers of times each year...so this is not my "first rodeo".

UNLESS this is a GM REBUILD...from the GM dealer/AC Delco distributor IN a AC Delco box...DO NOT BUY IT....just speaking from experience.

You can send your computer to http://gmautocomputers.com/

K & B Special Products out of Georgia (770)777-1034.

This is where I send mine out to and the turn around time is rather good. BUT...I do not care how fast I get it back...I just want it RIGHT...and NEVER had a problem.

So if I read your comment correctly. When the engine is running the "Service Engine Soon" light is flashing while it is running.

Have you obtained a scanner yet? I know it has been written..and you have commented...but before you go and spend money...I would highly advise again that you connect it and see what is going on. This is important..as I know you must know....and should not be overlooked or side-stepped.

DUB

MrWillys

The memcal, or PROM must be removed from the old ecm, and placed in the replacement. The ECM may have been used in many different models, but the memcal is specific to your application. Pull 2 1/4" screws, and push down on tabs memcal pops up. Reinstall in replacement.

moneypitvette

Update on my C4 I don't have a lot of the tools at home to check all the things that were suggested to check out so I thought and thought about what it was doing and what could cause all the things that were happening. I said lets go to NAPA and get a reman ECM. I installed it about an hour ago and that baby is running soooo sweeeeet.All the problems are fixed! I was having nightmares about fuel pumps, injectors etc.,etc. but all is good now. Thank you all for your help . Neil