pipe

Could someone tell me the symptoms of a ECM going bad. When my car gets to operating temp and as long as i'm on the throttle it's ok. But when I come to a stop the car has a surging idle and the volt meter on the dash takes a dip and the dash lights dim a little every time the turn signal flashes or when I leave off the brakes and re apply them it dims. Could this be the ECM going bad? THANKS FOR THE HELP Pipe

bogus

it sounds like you may have an electrical problem... not so much an ECM problem.

check all the battery and ground connections. corrosion can build up on the main junction behind, and the main grounds below the battery. Check the terminals on both the battery and the main power/ground leads.

From your profile, I see you have a 1990.

This eliminates the optispark...

I would start with getting it scanned... hook a computer up and see what the various sensors are doing... slow O2s can cause this, so can a stuck or poor performing injector.

pipe

Oops sory forgot it's a 90 L98 auto. Also could the O2 sensor be causing this? It's mounted in the Y Pipe after the Header Collector and might not let it run in closed loop after warmup. Is there a way to test this? THANKS AGAIN The car also has some minor bolt on"s Pipe

red L98

i have a ecu of my 1986 vette, its brand new .. im not sure if they are the same for the 90s is it a SD car ? you can try it out if you want just pay for shipping

bogus

it sounds like it's in the right place... do you have headers? or is the exhaust basically stock?

another thought is a clogged pre-cat(s). or even a clogged main cat...

bogus

hold that ECU, a) it won't fit his car and b) you may need it sometime!

pipe

Thanks for the reply Bogus. The car has a New Battery and Alternator. It has the Hypertech hot coil on it and a set of Taylor wires with a new rotor. But I've never changed the Ign. module inside the Dist.

Dan Parker '96

The ECU is pretty robust. Many are replaced unnecessarily (<-- that's a tough word to spell with a cocktail or two). Since it is a computer it can only process its inputs. Garbage in = garbage out. Normally a poor running engine is giving the ECU some bad input from some sensor or parameter.

pipe

Thanks everybody for the replies. It does have hooker long tubes on and the hooker Y pipe thats where the O2 sensor is mounted in the Y pipe right after the collector.

Mike_88Z51

I agree with Bogus's suggestions. Why did you change both battery and alternator recently? Were you experiencing voltage problems before?

My neighbor had a similar problem so he replaced the alternator. Then the battery. Then he asked me to look at it. I took a look and suggested having the "NEW" alternator and battery tested. He didn't think that the new parts could be the problem, so he paid an auto-electrical shop run a complete diagnosis on the entire car and wiring system. They charged him $120 to tell him that the new alternator was not good.

You state that the voltage dips when you use the turn signals. The O2 sensor has nothing to do with that unless you have a SEVERE wiring issue or the idle is so low due to poor mixture that the alternator is unable to put out enough voltage to maintain the system. During the first couple of minutes of operation the ECM stays in open loop and does not use the O2 sensor feedback. Do you have the idle problem when still cold immediatly after startup?

Even with a really poor idle, with a new battery you should easily be able to maintain 12.6+ ~13.1 volts. How much does the voltage dip? I'd check the things Bogus recommended and even the "new" battery and alternator. AutoZone and others will check the battery and alternator for free.

pipe

By the way the car has no cats at all. When I first start the car it idles fine runs smooth. But after a few minutes when it gets up to operating temp then it runs rough but only like I said when I come to a stop then it will surge a little. As the turn signal blinks the lights on the dash will flicker every time the blinker blinks or if I take my foot off the brake pedal and re apply it the lights on the dash will dim slightly. Also the Volts gauge will take a dip every few seconds. Since I have the Hypertech Coil on the car is it drawing too much voltage from the charging system to operate the rest of the cars needs? THANKS GUYS Pipe

Lichen

I don't know about the dimming lights, but your O2 sensors may be so far back now, that heated O2 sensors may be called for. The car may be dropping out of closed loop at stop lights.

pipe

If it is electrical do I need a 140 amp alternator to keep up? The reason I changed battery's is because the old one was in the car for over 4 yrs. and was shot. The reason I changed the Alternator was because I didn't have the brace on the back side after installing the Headers to stabilize it and when I ran it hard the torque cracked the case. I'm going to check the grounding points and the terminal behind the Battery where all the wires are attached and see if it gets any better. THANKS

rick lambert

You may want to check the old pages-I posted the same question a couple weeks ago and got alot of responses.

Mike_88Z51

In that case I agree with the possibility that it "might" be O2 sensor related. For the first few minutes there are timers in the car that keep the ECM from using O2 sensor feedback and going into closed loop. The ECM uses preprogrammed tables to make mixture calculation decisions instead of O2 feedback. It sounds like the problem "might" be poor info from the O2 sensor to the ECM once the ECM goes into closed loop. If the sensor is not completely bad, but is going out of spec it can give the ECM poor readings that will lead to a bad mix. This can be more obvious at idle. On the other hand, it could be due to a poor minimum idle air setting or IAC problem. The factory manual has detailed directions for setting the minimum idle speed air adjustment. There have been a couple of threads recently where someone posted the steps. Try a search for the posted instructions.


I would not think that the coil would draw too much current for a properly functioning alternator unless it was defective. If the engine slows enough to nearly stop turning, then the alternator will not be able to produce enough current and you could see your symptoms, but usually voltage dips when the blinker activates are due to poor alternator output or bad grounds. I think your idea to check all grounds is an excellent one. Even though they are new, I'd still consider an electrical test of the battery and alternator under load.

I agree with Lichen that it is quite possible the O2 sensors could be so far back due to the headers that the car might be droppng out of closed loop at stop lights, but even if it is happening, I doubt that this is the root cause. From what you say, the car starts out fine the first few minutes after startup and then begins to have a problem once it has warmed up. As I have already noted, the first few minutes after startup there are timers in the car that keep the ECM from using O2 sensor feedback and going into closed loop, and that is the one time it runs fine at idle. It appears to me that you have a problem when you go into closed loop, not due to dropping out to open loop operation.

pipe

Guys thanks so much for all your tips on trying to straightening out my car. First i'm gonna clean all the grounds on the car and clean up the terminal ends to the wires and hit em with a little dielectric grease when I reinstall em. Then i'm gonna weld in a O2 bung closer to the motor in one of the pipes of the headers then see what happens. THANKS AGAIN

pcolt94

First we have to figure out if you have one or two problems. If the engine is surging, it can effect the alternator output especially if you are getting the RPMs down to a stall condition.

One place to start is put a volt meter across the battery and measure the darn voltage. Have someone keep a steady RPM at 800 to 1000 at first. It should remain about constant at 14 to 14.5 volts. Then check at all RPMs 500 to 2000 or so. Turn lights on and see if numbers are the same. It should not change much. If you are 14 or better with lights on at 1000 RPM your probably OK electrically, if not further checking is required.

As for surging this could be another problem. Check for vacuum leeks, hoses, ERG valve, clean throttle body, IAC motor and a host of other items that could cause the idle problems. I’m sure there are many more suggestions out there. Charge on

onedef92

I'd start with a Scan Tool. You'll get to see real-time data AND select individual sensors. Really helpful in tracking down electrical bugaboos.

pipe

Let me run this by you guy's I've added the headers (no cats) and flowmaster mufflers and a few more bolt on's but never had the Chip changed with these changes. Could this be a problem? THANKS FOR ALL THE REPLIES Pipe

bogus

silly question... do you have O2 sensors mounted?