1stL98

hey everyone i just got an 88 she misfires with the est plugged in if i unplug the est bypass timing is at 6deg.i did a compltete tune up,it ran like that before,checked fuel psi its 39psi.it has bosch 3,thanks jon!i have a snap on mt2500 scanner there are no codes,it has a new ac delco ecm, im at a loss on this one any ideas wud be outstanding

GREGGPENN

Any chance your spark wires are interfering with the reference signal wire? (It's possible.)

1stL98

i replaced them with delco wires and carfully put them all in the plastic retainers,and made sure none were touching anything.wen u say reference wires are u talking about the wires for the esc module?

GREGGPENN

Yep. That's how the ECM knows where the dizzy's at (position). If they are not close enough to interfer with the EST reference signal, look to make sure you didn't pull-out/damage one of the 4 EST wires/plug.

1stL98

i checked all the wires at the 4 pin plug behind the dist.is there a test procedure to check the pick up coil voltage? i see no corrosion on any of the pins,like i said in my first post if i bypass the est,the idle smooths rite out and runs excellent all thru the rpm range.then wen i plug it in she starts to misfire especially just off idle

coupeguy2001

I had a bad ESC, it loped at idle pretty bad, and when I unplugged it, it ran good.
I bought a new one, and installed it. it ran great after that.

1stL98

i thought about that,so i pulled the one out of my 85(i think they are the same,but not sure)i didnt make a difference.but if they are not the same wudnt work anyway.i read there is an upgrade for the esc module if im gonna buy one i want the best

1stL98

is the 85 est module the same as the 88?

Muffin

I guess not. The Ecklers catalog shows an AC Delco ESC for 84-86. Nothing for 88 and up. There is a upgrade form GM but I have lost the part number.

coupeguy2001

I don't know if they are the same, I bought an 89 harness a while back, and it had all the relays and things on it. I used the esc on my 86, and that is what fixed my loping/surging.
Don't know if it was an 89 unit or not since I didn't actually take it out of the car, I just received the harness in a box.
I would believe the 90-91 units might be different because of the MAP style computer and tuning.

1stL98

i just installed a new ac delco est module and no change,ive seen alot of wierd stuff on vettes over the years but this one has the top spot im at a loss the wierd part is no codes and runs smooth with the module unplugged!! ive checked and even replaced the pick up coil and icm

GREGGPENN

Check to see if you have two wires reversed. Use an ohm tester to confirm which wire is which if they are buried.

By misfire, do you mean backfire or no fire? You could also check for proper connection from your esc ALL the way to your ECM but that's a pain.

What about the ignition module in the dizzy. Anyone know what a bad one would do?

Do you know if the chip has been altered? Was it running correctly when you bought it?

1stL98

it was running that way wen i bought it.so as part of the major tune up which i always do on any vette i buy i replaced plugs wires and clean the throttle body etc.ive used ac delco parts and i have a snap on scanner.i had another set of new wires for my 85 and swapped them no change.compression is 160psi on all cylinders the prom is the factory original wud a prom be screwed up? she misfires at idle and worsens just off idle,there is no backfire or black smoke

JOHN89

When you changed the magnetic pickup did you adjust the gap between it and the reluctor ring? Most require .008'-.012" gap. about the thickness of a match book. Do a yahoo search on this. Plenty of info out there on it.

I had the very same problem. Mine would actually stall after a while. I found the gap at 0.300". I would look there as your symptoms sound sneekily similiar to mine.

Hope this helps,
John

1stL98

air gap?ive never heard of that procedure i didnt know the pick up coil gap was adjustable.do u remember the procedure at all?

JOHN89

rotate the distributor so that the reluctor is pointing right at the magnetic pickup (centered). Measure the gap between the reluctor point and mag pickup. It recommends using a non metallic feeler gauge. I used a piece of card board that was approximately .012". Loosen the two screws that hold down the magnetic pickup. They are not grooved and don't realy look adjustable but they are. The holes are larger than the hold down screws. You have to **** the pickup towards the reluctor pointer with the correct thickness feeler gauge (card board) inserted and tighten down the screws. I have a small style in my car which is a little different but the theory is the same. The important thing is to rule out the gap being too large. Pull off your cap and rotor and take a quick look at it. You will be able to tell ather quickly . Bump the ignition key so the dist moves until one of the pointers points directly at the pickup.

I'll try to post a photo so you get the idea.

Cheers,
John

JOHN89

http://image.highperformancepontiac....tized_12_z.jpg

this style doesn't look adjustable.


http://image.circletrack.com/f/92744...llory_hei+.jpg

this style is adjustable

1stL98

excellent info john but i have a totally stock hei in my 88 i looked at the pick up coil and it doesnt seem adjustable

JOHN89

which one does it look like in the phots I posted?

1stL98

the first image, but it has no vacuum advance