Ok ! I have had a problem for a while with the vette breaking up and stumbling at 2400rpm . What it has come down to , is spark advance ! With the EST connected , it starts breaking up around 2400 rpm on the highway , unplug the EST and it runs fine !

I have taken it to the chip programmer and everything reads out fine in programming and on the scanner . I have replaced the ESC module as a precaution and the knock sensor . No codes !

With base timing at 6* ,I connect the EST , at an idle , with a timing light , it advances to about 36*.

87 vette , 406 ci auto

If the programming is correct , what is causing it to advance ?

 

With the EST connected, the distributor does not control the timing, the ECM does. With the EST DISCONNECTED, the engine runs off the base timing you set with the timing light and a small amount of timing in the ignition module (under the distributor cap).

What was the engine speed when you got the 36 degree reading?

How ae you able to determine it's running at 36 degrees BTDC; the timing tab doesn't read that high?

36 degrees with the EST connected and the engine idling at a normal RPM is TOO much, something's amiss. Reading should be in the low 20s - like 22 degrees BTDC. So it looks like you have in the area of 14 degrees too much timing.

So, there are a few things that you should check that may cause this:

PROM burned with too much timing in it (but since you said it checked okay, I guess that one's off the table)

Base timing set incorrectly:

1. Timing was set with the EST connected, not disconnected as it should have been,

2. Wrong timing tab on the timing chain cover

3. TDC mark on the harmonic damper incorrectly positioned;
A. May have slipped or
B. The accuracy of the mark location not verified with position of
#1 piston

4. Battery not disconnected to clear the error code that was set when ECM detected the engine running with the EST disconnected

5. Timing being checked with the light connected to the wrong plug wire

6. Malfunctioning timing light

These are just my thoughts. Keep us posted on what you come up with.

Jake

 

Ok ! The engine speed that I got the 36* was 950 rpm , idling ,the timing was set with the EST disconnected @ 6*. The balancer is marked like a timing tape . It has the right timing tab and the balancer hasn't slipped . I have checked to see if the timing mark is at zero while at TDC and it is . I will try another timing light , I have 2. Could it have anything to do with being in open or closed loop ?

 

I can't see how the loop status can effect timing THAT much. I'll hjave to chew on this condition some more to see if I can come up with anything else.

BTW, had the distributor been out lately; cam change, etc.?

Could the timing tape be for a different diameter balancer?? Can you verify 36 degrees with a data logger, like a ScanTool, Diacom, etc.?

Jake

 

Hey ! Jake , the balancer is actually engraved or stamped with all of the degrees on it , when its made . I compared it to another to see if it has slipped but it hasn't . I thought the timing gear may have slipped , but its ok , its rare that a double roller does that . It has something to do electronically , with the spark advance or maybe the computer . I'm just not familiar with it or where to look for problems . I have checked all the wiring and can't find anything . Maybe theres a manual that tells where to look , for this advance problem ? Thanks ! ..........John

 

What if the timing light were connected to the wrong plug wire? Would that make the reading invalid but consistent?

-Chris

 

Thats a good question ! I have changed the plug wires , on the cap ,moving them clockwise from number one ,then counter clockwise . I even tried the same thing with the distributor gear , then changed it 180* to see if that made a difference , it didn't help ! I'm wondering if the computer isn't getting the right signal or is sending the wrong signal ? I'm open for sugggestions !..................Thanks !