Missing under load after tune-up.

[Thomas Moore]

Hi all. I need some help. I have a 1993 GMC Sierra 1500 SLE 5.7 Automatic with316k on the clock. The truck was missing but ran pretty good and reliably. An experienced mechanic friend offered to help me replace my leaking radiator, do a tune up and clean up the grease and oil from leaking valve covers as well as replace the gaskets. One that day we did the following:

• Cleaned the entire top side of engine with simple green and water from garden hose with fairly low pressure.
• Replaced valve cover gaskets.
• Replaced all spark plugs and spark plug wires.
• Replaced radiator, hoses, and thermostat.
• Cleaned up wiring (bare sections, bad routing, etc)
• Replaced cracked vacuum lines.
• Replace PCV valve.

We cranked it up and let it warm up. It sounded great. As I live an hour and fifteen minutes away and it was late, I took off before a test drive. It was missing horribly under load. I drove home discussing it on the phone with my friend. We thought most likely water on/in distributor cap.

I took the cap off and let it dry for a couple days but it didn’t help.

I then replaced the cap and rotor but that did nothing.

I drove back down to my friend and tested the timing. We determined that the timing chain does have slack and I will be replacing it but in my head this shouldn’t be the cause of the problem because it would not explain being so much worse after the tune up.

I used the timing gun to see if the coil was cutting out, testing each wire. Cylinder 1 does cut out, but the rest seem not to. My thought was if the coil is going bad it would affect all cylinders the same.

After some research we decided to replace the electronic spark control module in the distributor but again it did nothing.

I am at a complete loss at this point. Anyone have any suggestions?

Thanks so much,

Tom

 

[Thomas Moore]

Also meant to post the codes I got.
12, 43, 66, 67

 

[Schurkey]

I used the timing gun to see if the coil was cutting out, testing each wire. Cylinder 1 does cut out, but the rest seem not to. My thought was if the coil is going bad it would affect all cylinders the same.

Defective part somewhere in the #1 secondary circuit--spark plug, plug wire, distributor cap.

If it was the rotor, coil wire, or coil, it would affect all cylinders.

Using a timing light like that can show faults, but a spark-tester calibrated for HEI is better because it loads the system with higher voltage, AND verifies that the coil output is sufficient.

After some research we decided to replace the electronic spark control module in the distributor but again it did nothing.

The module would affect all cylinders.

I am at a complete loss at this point. Anyone have any suggestions?

Also meant to post the codes I got.
12, 43, 66, 67

In the future, please post the code definitions along with the code numbers.
12 = No distributor reference pulses. TOTALLY NORMAL when the ignition is "on" but the engine isn't cranking or running.

43 = ESC fault.
www.freeautomechanic.com/gm-code-43-electronic-spark-control.html

GM Code 43 Electronic Spark Control Fault - FreeAutoMechanic

Trouble Code 43 indicates that there may be a malfunction in the Electronic Spark Control (ESC) circuit. ESC is used to sense spark knock (pinging) and

www.freeautomechanic.com

66 & 67 = A/C pressure problem
www.freeautomechanic.com/gm-code-66-air-conditioning-pressure-switch-fault.html

GM Code 66 Air Conditioning Pressure Switch Fault - FreeAutoMechanic

Trouble Code 66 indicates that A/C pressure sensor has determined that the A/C refrigerant system pressure is either too low

www.freeautomechanic.com

GM OBD Trouble Code 67

A/C Pressure sensor circuit error The ECM monitors the A/C system pressure sensor circuit. If the system pressure is to l

repairpal.com

43 seems most-likely related to the driveability problem. Verify that the ESC module (if used--later vehicles had it integrated into the computer) isn't wet at the connector, and the wire harness is in good condition. Then follow the service manual diagnostic procedure. Service manuals can be downloaded for free using links available on this web site.
www.gmt400.com/threads/88-98-service-manuals.43575/

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

 

[Thomas Moore]

Thank you Schurkey for this incredibly detailed reply. I am grateful for your time. I will in the future add the code descriptions as well as the numbers.

I did download the service manual and will continue diagnostics using it.

When I replaced the ESC module the tab on the plug inside the distributor broke off. In AZ all plastic gets brittle fast. The fit was/is tight so I think it will be okay. Should I look for a replacement connector and try to unpin the old connector and replace it?

My friend believes the timing chain is at fault here because when we tested it (I think it was 18deg advanced) then shut it off, restarted and retested, it was different (I think a further 13 deg advanced).

I know it needs replaced but I don't agree that it's the problem because the problem started after we did all that work the first day. It seems most likely to have been caused by something we did.

In your opinion do you think the timing chain could be the whole problem?

 

[Schurkey]

When I replaced the ESC module the tab on the plug inside the distributor broke off.

The ESC module is not inside the distributor. That's the ignition module. On my '88 K1500, the ESC module is on a bracket located on the passenger side of the throttle body. Later trucks have it integrated into the computer.

My friend believes the timing chain is at fault here because when we tested it (I think it was 18deg advanced) then shut it off, restarted and retested, it was different (I think a further 13 deg advanced).

"IT" the cam timing, or "it" the ignition timing? How was "it" tested?

It seems most likely to have been caused by something we did.

Agreed. Find out why #1 is misfiring. Follow the service manual procedure for diagnosing Code 43.

In your opinion do you think the timing chain could be the whole problem?

Seems unlikely.

 

[tayto]

run the truck at night when it's really dark, or do in the garage with all the lights off to see if the spark is going somewhere its not suppose too. have seen folks install new plugs and cracked them while tightening. i would also verify firing order/wires

 

[Thomas Moore]

Schurkey - Apologies for being unclear and thanks for the help. The "it" in this case is ignition timing. I will find and follow code 43 diagnostic procedures in the service manual.

I just read that the esc on my truck might be located on the lower drivers side block, below the exhaust. seems like an odd location for a vital electrical component but I'll check it out this afternoon.

tayto - Thank you for the great suggestion. It's very obvious that auto mechanics is not my strong suit but I love learning all these little tricks to help diagnose. Thanks!

 

[Schurkey]

I just read that the esc on my truck might be located on the lower drivers side block, below the exhaust. seems like an odd location for a vital electrical component but I'll check it out this afternoon.

Where are you reading this?

Seems very unlikely. The ESC module should be just to the passenger side of the throttle body. The knock sensor would be on the passenger side block, below the exhaust--just in front of the starter motor.

 

[Thomas Moore]

Well....I think that suggestion was on something like Fixya, probably not the best reference but I can't find the damned thing so it was worth a try but it's not there. Not anywhere that I can find so I guess maybe it's in the ECM or PCM or Engine Control Computer. I've been looking for the module below as this is what O'Rie

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lly Auto and Rock Auto show as the ESC for this truck.

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This is a pic of my engine compartment, passenger side near throttle body where I think most vehicles have them mounted.

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Does anyone know what the module just to the left of the distributor is?

Today I replaced the knock sensor and plug. The code 43 is gone, the only remaining code is 67. The truck still runs bad (missing) so next step, since I need to replace the timing chain anyway, is replacing that.

 

[GoToGuy]

If you downloaded the factory service manual, everything is in there. If you look, the locations of every single part, the whole truck, is diagramed. Section 8 is all electric, every wire, all loom runs, every circuit, all the connectors, what they look like. . In section 1 if you page through you will see top looking down on the engine compartment showing locations of all engine electronic/ and or sensor locations. Try to avoid abbreviations that are not standard.