Question about 1991 GMC K2500

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Barely runs , timing retarded , only runs when est bypassed

1991 chevy 1500 pickup , 5.7 , barely runs . will run only if electric spark timing is bypassed ,like when you are adjusting the timeing ..I have put in a new module , new ecm , ne wspark control module.. no codes in computer , nothing helped so far

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  • jrivest May 25, 2009

    timing was set to zero , hook bypass back up , clear codes . then when you try to restart timing is so retarded it will hardly run and then die.

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  • Master
  • 4,669 Answers

You have set the timing to zero right? Then after you hook the bypass back up what does the timing go to?

Posted on May 24, 2009

  • Clint Smock
    Clint Smock May 26, 2009

    You said it will run with the bypass line unhooked. How well does it run? Run like normal or when you drive it this way it still runs bad? No power, Slow acell, and running rough?

  • Larry Pearce Mar 17, 2015

    have same problem anyone figure this out

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I have a 1991 chevy 1500 pickup that I use to plow my driveway. I replaced the fuel filter because the truck wasn't starting. Truck starts right up now but only stays on for about 5 minutes then


for shore i am a mechanic i know whats wrong you need to drop your gas tank i know its not your pump ok but there is a big leak insided you tank very simple to fix you need a 6 inch piece of fuel line and two clamp take out the fuel pump and replace the piece of fuel line thats on yur fuel pump assembly ok im glad i could help

Nov 12, 2012 | 1991 Chevrolet C1500

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4 flahes, then 2 trouble code on a 1995 chevy suburban


it is a 42, 96 on up use a OBD-II system to read codes. older than 96 use an OBD-I system to read codes. 42 means - electronic spark timing or bypass fault. this means that 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.

May 22, 2011 | Chevrolet Suburban 1500 Cars & Trucks

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1991 hyundai scoupe; engine cranks but does not start, check engine light came on then off about three times before this occurred. Car has fuel, checked injectors, fuel pump, plugs, rotar. think it might...


You stated the 'CHECK ENGINE' light came 'ON'!... what/which DTC'S (Diagnostic Trouble Codes) OBDI were retrieved? In 1991, OBDI (1) was used. You could retrieve codes by either turning the ignition switch on/off two or three time within 3 seconds and the light would flash for each number (this way you could determine which system was affected). The POSSIBLE CAUSES are many:

Crank Position Sensor
Ignition Coils/Wires
Water/contaminated fuel
Electrical (Relay(s), fuse(s), Poor ground connections, failed ECU (Engine Control Unit), etc.

Are you getting a SPARK to the plugs?... if so, check fuel pump pressure
If you're not getting a spark to the plugs or not getting a fuel injector PULSE, then the crank position sensor MIGHT BE the problem.... NOT SURE!

Too many POSSIBLE or PROBABLE causes.

In order for an engine to RUN, it needs these things:

1. Air flow (intake AND exhaust)
2. Fuel (proper fuel to air mixture)
3. Compression (140 to 2?? psi)
4. Proper Timing (not too advanced/retarded)
5. Ignition (sufficient and at the correct/right time)

Now, after listing these 5 things, without CHECKING each one, your guess is a good as mine.

Mar 22, 2011 | Hyundai Motor 1991 Scoupe

1 Answer

91 firebird 3.1 v6, if timing is set at 0 degrees instead of 10 degrees btdc would it cause strong exhaust and carbon on plugs


Here is the procedure for setting the timing:
1991-92 Vehicles

See Figure 2
When adjusting the timing, refer to the instructions on the emission control sticker inside the engine compartment. If the instructions on the label disagree with the procedure listed below, follow the instructions on the label.
  1. Locate the timing marks on the crankshaft pulley and the front of the engine.
  2. Clean off the marks and coat them with white paint or chalk, if necessary, so that they may be seen.
  3. Run the engine until it reaches normal operating temperature, then shut off the engine.
  4. With the ignition OFF , connect a tachometer to the distributor, and the pickup lead of an inductive timing light to the No. 1 spark plug wire. Connect the timing light power leads according to the manufacturer's instructions.



WARNING Never pierce a secondary ignition wire in order to connect a timing light. A pierced wire may lead to engine misfiring and driveability problems.

  1. Start the engine. With the engine running at normal operating temperature and all accessories OFF, disconnect the EST bypass connector.

An ECM code will be set when the EST bypass connector is disconnected.
  1. Aim the timing light at the timing mark. The line on the balancer or pulley will line up with the timing mark. If a change in timing is necessary, loosen the hold-down clamp bolt at the base of the distributor. While observing the timing mark, rotate the distributor slightly until the line indicates the correct timing. Use the timing specification on the emission control label in the engine compartment. If this label is missing or damaged, refer to the Tune-Up Specifications chart in this Section.
  2. Tighten the distributor hold-down bolt and recheck the ignition timing to make sure the distributor was not moved when the bolt was tightened.
  3. Turn the engine OFF and remove the timing light and tachometer. Reconnect the number 1 spark plug wire, if disconnected.
  4. Connect the EST bypass connector, then check for and clear the ECM code by disconnecting the negative battery cable for at least 30 seconds.

To prevent ECM damage, the key must be in the OFF position when disconnecting or reconnecting battery cables.

0900c1528008e3bd.jpg

Fig. Fig. 2: EST bypass connector

Feb 01, 2011 | 1991 Pontiac Firebird

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Engine stalls driving down the road or on take off. Had code 42. electronic spark tining. E S T circiut fault. Cleared codes and installed a fuel pressure gauge. On test drive the motor, (3.1 EFI) still...


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 and the ECM grounds the EST line. It expects to see no activity on this line at this time. When the ECM enters EST mode, it applies +5 volts to the BYPASS line and expects to see voltage variations on the EST line.
Reasons for the ECM to set Code 42:
1. System in BYPASS mode (i.e. EST line supposedly grounded) but activity sensed on EST line
2. System in EST mode (i.e. BYPASS line driven with +5 volts) but no activity seen on the EST line

Possible causes include:
1. BYPASS line is open or grounded
2. EST line is open or grounded
3. Poor connections between ignition module and ECM
4. Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise
5. Faulty or incorrect ignition module
6. Faulty ECM
7. Faulty Pick Up Coil
8. Faulty Ignition Control Module
9. Faulty Crank Sensor or the Crank Sensor magnet probe could be cracked

Jan 15, 2011 | 1991 Chevrolet Lumina

1 Answer

I have a 1992 gmc sonoma 2 wd 4.3L throtle body i am getting the service engine soon light and I checked the codes and I get number 42- Electronic spark timing bypass circuit or EST circuit is grounded...


CODE 42 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


Thank you for using fixya and keep us posted

Oct 10, 2010 | 1992 GMC Sonoma

1 Answer

What is code 42 for a pontiac 6000 and what do i do to fix it


. This is a common code that is often difficult to diagnose and scan tools can't help. The following understanding of how the electronic spark timing (EST) system works - what is taking place and why - is a big step toward a quicker and easier diagnosis of this circuit. When the engine is turning over, but below the run threshold of 400 RPM, the ECM holds the module's bypass voltage too low (0 volts) to energize its solid-state switch (ignition module). Another way of looking at the solid-state switch is to think of it as a relay. When voltage is applied, the relay is energized. With no voltage applied, the relay is de-energized. The pick-up coil/crank sensor pulses are amplified and shaped by the ignition module. These pulses are used to complete the circuit creating a magnetic field in the ignition coils primary winding that, when collapsing, will induce a high voltage in the secondary winding. Therefore, for every crank sensor/pick-up pulse, the coil is triggered. This is known as the bypass mode or module mode of operation. In this mode, the engine is running on the timing advance that is built into the ignition module. With the voltage low on the bypass wire, the EST is pulled to ground through a resistor in the ignition module. This keeps the voltage on the EST wire at around 300 mV. When the ECM sees the RPM over the run threshold, it will then apply 5 volts to the bypass wire that will activate the solid-state switch (relay). This in turn will switch the EST from ground to the base of the transistor that controls the primary coil. In this mode, the primary coil winding is being triggered by the altered signal sent out from the ECM. The ECM will alter the signal to the ignition module and control the timing based on the inputs from various sensors. This is referred to as the EST mode. This is how the system is designed to work! Now let's look at some of the things that can cause problems and set a Code 42. Under 400 RPM, no voltage on the bypass wire, the ECM expects to see low voltage (300 mV) on the EST line during this condition. If it sees 0 volts, indicating an open in the EST circuit - or higher than 500 mV - it sets a Code 42 and stays in the bypass mode. If the bypass line is open, or grounded, the ignition module will not switch to the EST mode. The ignition module needs the bypass voltage to activate the solid-state switch so the EST voltage will be low over 400 RPM and a Code 42 will set. If the EST line is grounded, the ignition module will switch to the EST - but because the line is grounded, there will be no EST signal. A Code 42 will set. To check the ignition module to see if the solid-state switch is capable of switching when the 5 volts are applied, the engine should not be running and the ECM connectors should be disconnected. An ohm meter and test light can be used. With the ohm meter on the EST circuit, it should read less that 500 ohms (in some cases, a lot less). Using a test light to battery voltage, probe the bypass wire. With this voltage applied to the bypass wire, the solid-state switch inside the ignition module should switch and the ohm meter on the EST circuit should go over 5,000 ohms. There is also another way to check this with the engine running: by removing the bypass and the EST wire from the module or ECM, run a jumper from the reference wire to the EST circuit. The reference signal is the signal that is used in the bypass mode to trigger the primary coil voltage. Apply 5 volts from one of the 5-volt reference circuits or a test light to battery voltage. This voltage on the bypass wire will activate the solid-state switch, in turn switching the module. On some models, the 5 volts or test light will have to be applied before starting or the motor will stall when the voltage is applied to the bypass. If the module is switching OK, the car will continue to run on the reference signal. If it stalls, the ignition module is not switching properly. There are some other checks that can be made when working with an intermittent Code 42. When using a digital volt ohm meter (DVOM), there should be 5 volts on the bypass wire. On the EST wire, with the engine running, there will be close to 2.3 volts. The EST is a 5 volt on/off digital signal, so with a volt meter you will see the average of the high and the low. With the EST signal being a digital on/off signal, if you have a meter that will read in Hz, you can also pick up a Hz reading on the EST wire. Normally, you will see about 28 Hz with the engine idling and will increase with RPM. If the EST circuit would become momentarily grounded, or open, the engine will cut out. The voltage reading will jump down to as low as 1.3 volts and the Hz reading will jump from as low as 17 to 128 Hz. If there is a problem in the EST circuit, the voltage on the bypass wire will not change. If the bypass wire is momentarily grounded or open, the reading on the EST wire will be the same as if it was grounding or going open, but unlike a problem on the EST circuit, the bypass voltage will be pulled low if it becomes grounded or goes open. If this happens fast enough, the engine will cut out. The voltages will vary, but the SES light will not come on and no codes will be set. With the EST circuit open, engine running, the voltage will stay low on the bypass wire and the ECM will not put out the 5 volts. With the EST shorted to ground, the voltage on the bypass wire will be high (5 volts) for a very short time (three seconds or less) then the ECM will remove the 5 volts. Using a min/max on the DVOM is the best way to see this voltage. With the bypass open, the EST circuit OK, the ECM will put out the 5 volts. By using a DVOM on both the EST and bypass wire, you can tap on the ECM, wiggle and tug on the wiring to see if the voltage or Hz reading will change. If it does, this is an indication that you have located the problem area.

Mar 17, 2010 | 1987 Pontiac 6000

1 Answer

Installed a new 5.7 in my 1998 chevy truck and it runs weak


If you have a coil pack instead of a distributor this is correct. If this is that case check your plug wires for possible bad wires and replace your plugs.

Oct 09, 2009 | 1999 Chevrolet Silverado 1500

2 Answers

HOW DO FIX A CODE #42 ON 1991 CHEVY TRUCK


Check to see if the brown and black strip wire is dissconnected by the fuel pump relay if not try dissconnecting the battery for a minute and see if the light comes back on.

Feb 19, 2009 | 1999 Chevrolet Silverado 1500

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