Geez. the 91 formula 5.7 went from running like a bat out of hell last night to a giant turd tonight. My wife was driving it today (first mistake) and she said the check engine light came on and the speedometer was not working while driving. That was wierd enough. When I get home, it starts fine but it never idles down from where it is when you start it and if you give it any gas it misses completely. If you try to hold the rpms up at 2 or 3000 it runs high and then low, etc even while holding the accelerator steady. OK I figure Ill use my newly aquired knowledge on checking the coded with a paper clip. NO DICE. The engine light just stays on and never flashes. S##t, im getting really nervous about his time.

OK heres what I did last night, the car has always had kind of a hesitation when you first give it some gas. My mechanic fiend said to check the Throttle position sensor by unplugging it, and putting an ohmeter across two of the terminals, then move the throttle and see if resistance decreases in a normal steady fashion. I did that last night and it looked OK. I had the key off during this operation.

I checked all the fuses tonight, ok. Im thinking that the ECM is damaged somehow, why wouldnt it flash the code? Either it just went out today when wife is driving, or something I did last night caused it to fail.
Man this is a bummer, I was looking forward to a fun weekend of driving in our rare nice weather here. PLEASE HELP.

 

If I have to get a new one, how much dinero? also should I get an aftermarket one, performance chip etc?

 

When you tired the paper clip thing, which holes (for lack of a better word) did you put them in?
The older birds and maros use 6 and 12 (if I remeber correctly) and the newer ones use 5 and 6.

I'm not 100% sure about which numbers, but I do know that it's the last two, on the top row (i.e. all the way to the right, all the way up)

You should at least get a 12 code I would think... eh, well except if the ECM is dead...

Good Luck, and let us know how things work out...

 

Those upper two holes all the way to the right are the ones I used, A and B. I did the same deal last week in the same holes and it worked perfect, flashed all the codes. thats why im afraid its the ECM. $%%^^&%$

 

the check engine light is always on? the paper clip trick doesn't yield a code 12. could be a loose ground? no blown ecm fuse. so it just might be the ecm itself.

 

That pretty much it. The check engine light stays on, the ecm fuse is OK, and not even a code 12 , Im screwed. How much does an ECM cost?

 

Double check to make sure your paper clip is touching the contacts, if you're still not getting a code 12, turn everything off. Disconnect the battery (both pos and neg) for 30 minutes, then reconnect the battery and try the paperclip again. If you've still got nothing let me know, I've got a 90-92 ECM sitting here that I'm not using that I'd sell for a decent price.

 

OK, Ive fiddled with the paper clip about a dozen times so I know its touching. I did disconnect the battery but only the negative cable for a minute or so, im going now to the garage to connect both for 30 minutes, will report back. Do you think this business about my wife saying that the speedo didn't work when she first noticed this this pm is significant? the speedo on a 91 is not througth a mechanical cable right? How is it controlled?/??


thanks

 

By the way, save tha ECM, and what's a decent price? I dont know how much these things are. PM me.
Larry

 

I was thinkin' about $40 + shipping on the ECM...

Disconnecting the ECM should reset it. 30 minutes is overkill but its being 100% sure its cleared.

The speedo is electrical, fed from a sensor in the tranny tailshaft. If the speedo went out it could be a fuse, or a shorted wire somewhere. The VSS feeds to the ECM also, so its possible that the ECM failed. It'd be worth your trouble to check your fuses, just pulled them out and visually check them, its pretty easy to tell if they're blown. Also if you have a 12v test lamp you can save yourself some time, since you wouldn't have to remove the fuses to test.

 

I pulled all the fuses earlier and checked them. They all looked oK, except how do you tell if the big solid metal 30a are good, no visual element?

Its getting wierder. I went out to disconnect the battery and I thought I would try starting. Started perfect, no check engine light, idle and throttle response normal. Then after about a minute, boom, engine light on, crummy idle, poor throttle response. then off again, then stayed on. tried paper clip, it started flashing, then stayed on when the idle got bad again. I turns out this corresponds to one of the fans going on and off. This car has two fans, the passenger side one always turned on with the key ever since I got it. I knew this wasn't normal but I thought maybe someone rigged it that way to keep it cooler, and it didin't affect anything. When the drive side one comes on, this is when the engine light stays on, and it runs like crap, and I cant check the codes. I guess when I came home from work earlier it was still warm from the wife driving it and the second fan was always on, just now it had cooled enough for the second fan not to come on immediately and when it did, it killed the code checking. Whats wierd is both fans come on now when its warm with just the key, no motor. The battery is disonnected and Im goig to do the reset you suggested.

 

Heres the deal. After reconnecting the battery cables, put the paper clip in, turned key on, passenger fan comes on like normally does (i dont know if this is normal for everyone), got code 12 three times, got code 12 fourth time, (indicating no stored codes), then the drivers fan came on and the engine light stayed on. Didn't start it because I know what im going to get. Ive always heard about these weird electrical problems but looks like I finally got one of my own.

Any ideas????

 

Now I probably really f$$cked up. I accidentally started it to put it in the garage with the jumper in place. The check engine light was flashing, should have been a dead giveaway. Took out the jumper and it doens't seem to be any worse. Same as before, runs fine until second fan comes on, then runs like crap and check engine light comes on. It sure is easy to forget that jumper. Is that automatice ECM damage or not?

going to bed bummed with a broken car.
Larry

 

Running the car with the jumper in place isn't a problem, it just puts it in a diagnostic mode. Also the fan should run when the jumper is in place. Sounds like you're problem is relating to the second fan. I havent messed with the fans so I can't give you much to go on, but your ECM is probably fine. I'd start looking at the fan switch (passengers side head)... maybe try unplugging the fan switch or try replacing it.... you could also have a short somewhere that causes everything to flake out, but thats pretty unlikely and would be hard to find. Hopefully someone reads this thread and recognizes the symptoms.

 

Thanks, I feel a little better. I remember reading that starting with the jumper in there "may result in ECM damage", but it doens't appear to have made it worse. Ive got a firebird gm shop manual here and its got a whole section on coolant fan electrical diagnosis. For example, if the coolant fan runs with the engine cool and the ac off and the ingition switch on, it's either a ground or the relay. Ive got a voltage tester but not a test light. I can use it to test the voltage. Funny like I said, the passenger fan has been coming on with the key every since I go the car with no ill effects, but now that the driver side is coming on, its messing up the ECM. Im going to go thru this procedure and make sure the fans are running correctly, then we'll see.

thanks again

 

The Heavy duty coolant fan switch was bad, causing the fan to run all the time. REplaced the switch, then the regular coolant fan began coming on with just the ignition switch in run position and motor off. I disconnected both the AC pressure switch and coolant sensor switch that supposedly turn this fan on, and it still turns on with just the ignition switch on. Both relays check out ok. If I start the car, the driver side fan comes on and the engine light comes on, the car runs like crap. Also I cant check any codes, the engine light just stays on. There arent' any codes anyway. I checked them several times before and there werent any. I tried disconnecting the relay to the driver side fan, and of course it doesnt start with the ignition on, but if you start the car you get the same thing, engine light on, runs like crap.

I hate this. It still seems like the ECM is bad. Auto zone has an eCm for $94. You turn you old one in an put the old chip in the new ECM. Could it be the chip too? They have a hypetech thermomaster chip for $164, is this any good? whats better?

 

Hate to cut down any parts stores, but I wouldn't let them have my ecm. I think napa sells them for about the same price, with a lifetime warrentee. It could also be just the prom chip that has died. Try disconnecting the batt. unplug the ecm, pull the chip. Becareful [ static electricity] will ruin the prom. Make sure you are grounded when working with the ecm. Plug everything back in and check. IF you have a ttop car that leaks, may have caused a bad connection. Also check the ecm fuse[ out by the batt] , could be loose.

 

I already pulled unplugged the ECM and pulled the prom as part of the gm shop manual procedure for what to if the check engine light stays on with the jumper (no code 12). Plugged everything back in, no help. I checked all the fuses in the fuse block but I dont know where the ecm fuse near the battery is. Ill look in the shop manual, but maybe you could tell me in case I don't find it.

Im going to pull the ecm from my 91 z28 305 and put in the firebird chip, and see if that solves it. If it does Im off to look for a NAPa store for a new ecm, if not, ?????

Please let me know where that ecm fuse near the battery is, thanks

 

Larry, look right behind the battery towards the firewall. There should be a black capsule with two wires going to it. Pull the cover off[ maybe kinda hard] , check that fuse. That's the one I pull when I change chips in my car.

 

I found a fuse capsule between the battery and the radiator mounte there on the body. Opened it found a 20amp fuse in good condition. Oh well, thanks for trying.

 

Larry Check to make sure you have uninterupted power to the ecm BEFORE you change it out. Although I had the same problem when I first got my car going. ECM. I just wanted you to try the things I was told to try.

 

I really appreciate anything that you can tell about your experience. Im new at this an am trying to not appear to stupid, but how do you suggest I determine if I have uninteruppted power to the ecm? I have voltage tester, test light, etc

 

If I remember right it is a orange and black wire going to the ecm. Check you book for the wiring diagram. I used a safety pin in the wire and hooked to a volt tester, not a light. You could have a power broken wire.

 

It sounds like the Prom is bad.
Pull the chip out and try and start the car and see if it runs the same as if the chip were in the car. I was driving my car around for a couple of minths with a bad Prom that I burned and thought it was my computer not functioning right.

 

I don't think this has anything to do with the ecm fuse near the battery. If that fuse is blown, the car won't run, if it isn't, it will. Same goes for a broken/loose power wire. Your car runs so those are out of the mix. It sounds to me like you have a loose/worn wire in the ALDL connector thats shorting and causing the ecm to go into diagnostic mode. If it does this while the car is running, it will a)cause the primary fan to come on during the run (the other fan is controlled by the a/c switch or the coolant temp. switch in the head), b)cause the check engine light to go on (when its running, this light will flash on and off for certain periods of time corresponding with the fuel integrator in the ecm prom), and c)bypass the EST and force the timing to base timing at all times (cause the car to run like crap because the ecm won't be advancing the timing). Now it still could be the ecm. A bad transistor here or there can you ruin your day.......

 

ECM draws a negligible amount of current compared to coolant fans. Splices don't usually fail totally at first, instead the resistance slowly rises as the joint becomes more and more corroded. Remember voltage drop formula is v=ir.

There's a splice under the hood with one branch getting power from the battery to the ECM and to the coolant fans. A badly corroded joint will cause a big voltage drop when the coolant fans engage and will starve the poor ecm for power. To check for sure, hook up a voltmeter to the ECM power pins.

Also remember that for electricity to flow, not only do you need a positive power lead, you also need a good ground. In some years, the coolant fans shared a common ground with the electronic spark module and other critical engine components.

 

I borrowed a friends mem-cal chip and it didn't help. Replaced the ECM and the fan quit running with just the key on and the code 12 flashed with the jumper. This indicated the ecm was bad. However, it still didn't start and run right with my old chip. PUt the borrowed chip in and it ran like a bat out of hell in. conclusion, both the ecm and chip were bad. Bad luck I guess. Im going to buy a fastchip standard chip for 91 firebird 350 a/t. Its only $160 and thats only a little more than stock. When I have time Im going to learn how to burn my own.

 

It seems to run fine with the reconditoned ECM and my friends borrowed chip. So im going to buy a new chip to put in, but, Im wondering if I do an electrical test to be sure that I don't have a wiring problem that will cause the new ones to burn out.

Any suggestions?

 

Have you done any electrical wiring modifications lately? If so, then double check those.
Other than that, I am not sure what to check for.

 

I have changed the stock coil to a MSD Blaster GM coil, and put on an MSD cap and rotor. Ive got the MSD 8mm wires but havent put them on yet. Did change the plugs, which involve a lot of movement of the wires. The only other thing is changing the HD coolant fan switch, but I did that after the problem. Im just bening paranoid becuase im afraid Ill put in the recon ECM and new $160 fastchip and theyll burn out again. PLEASE ***, NOOOOO.