JOHN89

OK let me ponder on it a bit. I'll get back to you soon.

1stL98

thank u for ur help john,im not given up on her i mean its a corvette

JOHN89

1. Engine idles rough but idles smooth with est disconnected. With the EST disconnected the distributor is running off the module and not the ecm with 6* base timing or whatever you manually set it to. The ECM is not compensating because of input from the sensors. The input sensors are MAF, engine coolant, knock sensor and O2 sensor.

2. You said the injectors are new but Im thinking they arent the problem anyway. Actually I'd probably eliminate the fuel system considering it idles smooth with the EST connector disconnected.

Here are some questions:
How soon after initial start do you experience the rough idle? Does it hunt or surge? Stall? What are the spark plugs gapped at?
Is the coil new? firing order correct? Does it idle smooth with the MAF unpugged? Did you use the white heat sink grease when you replaced the module? What do the sparK plug wires measure out resistance wise? You can use a timing light to see which cylinder is missing. I believe its ignition but you need to do some more troubleshooting.

How ol is the gasoline in the tank? When was the filter changed last?
Just some food for thought.
Keep it going.

1stL98

the miss happens after it warms up a few minutes,or when going into closed loop.the knock sensor is new becauze it was broken,it does go into limp mode with the maf dissconnected,the scanner shows the grams per sec of air flow and are not erratic.itried the timming lite trick to find the cylinder i believe its num4.fuel is fresh and filter is new.she doesnt stall just missfires when i throttle up slightly its really bad.plugs are ac delco rapid fires and are gapped at .045

JOHN89

Ok so closed loop then...Pull out number 4 plug and check the gap. Actually I'd do all of them. Measure the resistance and compare to the other plugs. Measure the resistance of the wires as well. You could swap injector connectors with #2 and see if the miss moves to # 2.

Also check the coil wire crimps wheree the plugs gonnect at the disctributor. I had a bad one one time on an accel super coil. It was the HEI on top of cap.

Yes dont give up. Its a vette!

1stL98

i did that john,didnt seem to move i checked compression all cylinders are at 160psi.i even changed plugs with new ones,no change cap and rotor are new so is the coil

coupeguy2001

Ya know.........the knock sensor between a map engine and a maf engine are different because they increased the compression in 90. If you accidentally got the wrong one and installed it................
Also, if you plug in the EST, are we talking about the ESC? over by the heater? If so, try and plug that in, but disconnect the wire that you disconnect to set the timing with the ESC connected and see if things change.It would seem to me that the ESC is modifying the spark to an overly advanced condition and the knock sensor is telling the computer to pull it back.

1stL98

its wen i unplug the est bypass it smooths out,i changed the knock sensor because it was broken off(plastic part) it ran the same before and after,its the rite sensor verified by the gm part number.i really appreciate the ideas guys.it ran the same before all this work

Square

I had a rough idle in my 86' hat would smooth out when the EST wire was disconnected. Turned out that the harmonic balancer had begun to slip, and what was 6 degrees BTDC indicated, was actually 14 degrees. A new balancer let me set the timing correctly and solved the problem.

1stL98

i didnt think about that!i removed the balancer does any one have a pic of a known good one so i can visually reference the timing mark

GREGGPENN

I don't have the stock balancer, but it looks like the keyway points to TDC, in case that helps.

1stL98

outstandingthanks guys.ive looked at several pics online,and i do believe that it slipped because the timimng mark is not in the same place,but its only 75 dollars for a new one from g.m. and it will be here tommorow.so stayed tuned folks i will post more info tommorow!!!

JOHN89

Yes that would certainly do it! A dial back timing light could verify that. You could also put it on tdc by the harmonic balancer and see where the distributor rotor is pointing with respect to number one on the cap. You could also look at the rockers on number one and verify both are closed. However it is cheap enough to eliminate it from the equation by just replacing it. Good investment actually!

I hope this is the cure for your problem!

Good luck,
John

1stL98

well i recieved the new balancer from gm today and the mark was off by about an inch!! she did slip i installed the balancer reset base timing at 6deg plugged the est in and she runs smooth as silk.this was a new one for me guys.THANKS TO SQUARE FOR THE TIP this forum rules and may the c4 claim its place in the world of excellent cars!!

Square

Your certainly welcome!

The forum helped me when my balancer slipped, so it is the forum that deserves the credit.

I am happy to be of assistance, and happy to hear your idle is smooth!

69427

Guys, there's an awful lot of back and forth talk of ESC and EST here. They are not the same thing.

OP: Which one are you unplugging?

1stL98

its the electronic spark control bypass.

69427

I did electronic spark control calibration for GM for several years. I never heard that term.

I believe you mean electronic spark timing (EST) bypass. The ESC and EST circuits are two distinctly different items. GM could have done a better job naming these circuits to avoid confusion, but they never asked me.