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1990 350 tbi

I love my Chevy Chevette!
Posts: 1
Joined: 12/12
Posted: 12/01/12 11:00 AM

i just put in a brand new motor an updated the cam to a 270. now it idles up and down. get a code 42 for timing all in all i dont know what to do? please help  

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My first time was in a Chevy
Posts: 218
Joined: 04/12
Posted: 12/01/12 11:58 AM

i will have more later.. but start with this...


Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system.

During cranking, the timing is controlled by the ignition module while the ECM monitors the engine speed. When the engine speed exceeds 400 RPM, the ECM sends a BYPASS signal to the ignition module which switches the timing to ECM control. The ECM calculates what the timing should be then "tells" the ignition module via the EST circuit.

An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be re-started but it will run on ignition module timing.

The conditions for setting this code are:

System in BYPASS mode but the ignition module is still controlling timing

- or -
Engine speed > 600 RPM with no EST pulses (ECM controlled timing) going to the ignition module for 200 msec.

Typical causes for this code include:

1) BYPASS line is open or grounded
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM

#5 is the spark plug wires are TOO CLOSE to the EST module.. or the 4 conductor wiring to the distributer is too close to the spark plug wires... so the corona from the high voltage coming off is creating random signals...

please.. don't start throwing part at this yet.....  i have a bunch more tests and info...

one thing i would like you to do..    pull the distributer cap off...    grab the rotor.. wiggle it around.. is the distributer shaft loose in the housing.. side to side... not up and down.. yes or no...

now... pull the rotor off....   take a bright flashlight and aim it down at the top of the distributer shaft... just below where you pulled the rotor off is the reluctor.. its a stamped steel plate thats riveted on.. under that and visible next to the shaft is a magnet.. look with the flash light and see if the magnet shows any cracks... if it does.. then you are going to need a reman distributer.. one of the popular brands uses a completely different reluctor and pick up coil.. far superior to the OEM design..  

while you have the cap and rotor off..   bring the timing mark up to TDC...  i think that year has TDC as a base timing...
does not matter which way the rotor is pointing for this check...

look at the alignment of the reluctor fingers and the pick up coil teeth.. are they perfectly aligned???  if the timing is correct.. they are aligned...

the idles up and down is another issue...

can you while up on top... grab the throttle lever on the side of the throttle body.. open it slightly.. wiggle it.. does the shaft move around inside the throttle body.. (o )  ( o)   or is it night and tight when slightly open  (o)    when closed the throttle blades hold it off center.. so you have to open it for this test...
if you find play.. you might need to have the throttle body rebushed... a few shops do it... if you have some creative abilities i can give you instructions on how to rebush it..  

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My first time was in a Chevy
Posts: 218
Joined: 04/12
Posted: 12/01/12 12:00 PM

for this second.. also unplug the idle air control valve and see if the surging stops...   this is just a test..

can you measure the manifold vacuum with a gauge and post the results at idle...  

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