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Intermittent EST / Code 42 — 1989 K5 Blazer 5.7L TBI, 700R4

clutch2k3

clutch2k3

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Been chasing an intermittent Code 42 for a few months now and running out of ideas.

What's been replaced:
  • Spark plug wires ((ACDelco)
  • Coil (GM Genuine)
  • ICM (GM Genuine)
  • Distributor assembly (GM Genuine)
  • Both coil connectors (black and grey)
  • 2-pin and 4-pin ICM connectors

Other relevant work:

  • ECM/ICM signal wires (EST, Bypass, Reference, Ground) rerouted away from plug wires
  • EGR and EVAP deleted
  • Long tube headers, custom exhaust, 4.10 gears, 2" lift

The fault is completely random — no conditions I can identify to reproduce it. The Drivability shop manual for intermittent Code 42 basically just says route plug wires away from ICM wires and check for broken wires, which I've already done.

Any ideas appreciated.
 
Is there any noticeable change to the way it run? Fuel economy hasn't gone to heck? Plugs look ok (I'm assuming the manual thinks its ignition related)?

Both of the intermittent codes I've had were poor connections at the ECM. I do a lot of connecting/disconnecting of the ECM so that could be my specific issue, but with the age and vibration at this stage, even left untouched they can be an issue I'm sure.

Slightly different setup (TPI running electric fans), but my fan(s) stopped working randomly. Did everything I could to test, narrowed it down to the fan relay control wire from the ECM. Almost to the point of digging into the wire loom, but decided I needed to try and get that pin/terminal connection at the ECM more positive because I just didn't see the wire being the problem, and sure enough, that terminal was the problem. False result from the continuity test because the terminals don't lend themselves to making solid contact and I don't stab the wiring jacket.

When I first converted to injection I also got an intermittent MAF code and the terminal lock on one of those wires had failed so was sometimes losing contact with the ECM pin.
 
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Is there any noticeable change to the way it run? - yeah, its never run better! except when it looses timing ...

Fuel economy hasn't gone to heck? - nope

Plugs look ok - they look fine to me. I'm not a seasoned plug reader though. I've pulled them quite a bit since summer 2025 and they always look ok to me.

I'm assuming the manual thinks its ignition related? - well, timing drops out and it throw a code 42 w/SES light so, if somehting else is causing that, thats beyond my diagnostic capabilities.

Both of the intermittent codes I've had were poor connections at the ECM. I do a lot of connecting/disconnecting of the ECM so that could be my specific issue, but with the age and vibration at this stage, even left untouched they can be an issue I'm sure. - I just took out the ECM, reseated both of those chip thingies and shot some DeoxIT in both sides of the connectors. all the male & female pins looked OK to me.

Slightly different setup (TPI running electric fans), but my fan(s) stopped working randomly. Did everything I could to test, narrowed it down to the fan relay control wire from the ECM. Almost to the point of digging into the wire loom, but decided I needed to try and get that pin/terminal connection at the ECM more positive because I just didn't see the wire being the problem, and sure enough, that terminal was the problem.

When I first converted to injection I also got an intermittent MAF code and the terminal lock on one of those wires had failed so was sometimes losing contact with the ECM pin. - so you're thinking I should double check the the 4x ECM/ICM terminals at the ECM, right?

False result from the continuity test because the terminals don't lend themselves to making solid contact and I don't stab the wiring jacket. - oh man, I went through this exact same thing with the black coil connector, so painful when the multimeter lies ...
 
It wouldn't hurt to try and tighten up the terminals IMO. Really no practical way I can think of to test that the connection is good. At least when they are corroded it's a visible indicator. With a loose connection, and terminals that small, it's near impossible to see where the problem lies. I just added a bit of spring pressure to the terminal on the connector side by adding a very slight bend of the terminal wall and that did it.

You are seeing timing drop off while it's running, implying you are datalogging or using something to watch timing while it's running?

Intermittents are tough. You've tried to wiggle the wires/connections while it's running to see if you can trigger it?
 
Yes, I am datalogging and I see code 42 enter into the logs. the only noteworthy thing I noticed that happened just before this last code 42 was that BLM was enabled. I'm not sure what to make of that. AFAIK the ECM immediately takes command of timing after cranking (>400 RPM), even in open loop.

At this point though I don't need the data logs to tell me I lost spark advance, I can tell by throttle response and the idle RPM. and the SES :-)

based on my research, stock TBI ECMs do not log spark timing, with or without tunerpro RT. On modded TBI ECMs timing is definitly logged (the ECMs modded for Jeeps, i forget the name of the company, begins with an "H"?).

Yeah, I tried wiggling wires but I think its time for me to level up my wiggling skills. I don't think I'm being violent enough.
 
wow, someone posted in that thread that they replaced their TBI with a carb because of longstanding code 42 issues. yikes.

I spent a few hours last night rerouting the plug wires and tidying up the wires so nothing is touching nothing. hopefully I'll test drive it today. but it can take multiple drives over multiple days for it to trigger.
 
Just completed the first test drive around town since the extreme plug wire reroute — lots of stop and go, long red lights. No Code 42. Fingers remain crossed.

I've also since come across this post. Doesn't help me but might help others.
 
I have a new respect for shooting 30,000 volts down a wire that's routed in the open. Coil packs make so much sense to me now.

I always assumed the insulation on plug wires contained the voltage, now I realize they don't contain that large electric field around each of them. I've gotten a decent jolt grabbing a wire while leaning on the fender. I didn't think that was possible through the 8mm insulation until it happened.
 
I generally just try to avoid spark period for that reason lol. Doesn't hurt, but I don't like the surprise.

I've not messed with plug wires a ton, but unless the jacket is failing, they SHOULD be something you can touch without getting shocked, no?

Genuinely curious, I know that plug wires are like everything else nowadays, just because it looks like quality, or is sold as being quality, doesn't mean it is.
 
I pinch them all the time, you can fell the spark run through them. I can tell if a cylinder is not hitting by pinching the spark plug wire between index and thumb.
 
Wes Harden said:
I pinch them all the time, you can fell the spark run through them. I can tell if a cylinder is not hitting by pinching the spark plug wire between index and thumb.
Click to expand...
:lol thats like the most car-mechanic-guy thing ever said lol
 
I just double checked my understanding and I'm confident this is true based on theory and my experience:

When holding an intact plug wire on a running engine, electrons won't directly flow through your body due to the insulating jacket. However, your body capacitively couples to the high-voltage field (20k-40kV) inside the wire. If another part of your body is grounded, induced charge has a discharge path through you, and you'll feel a shock with each firing event. If you're floating, the effect is more limited — your body charges and discharges with the field but has nowhere to dump.
 
clutch2k3 said:
I just double checked my understanding and I'm confident this is true based on theory and my experience:

When holding an intact plug wire on a running engine, electrons won't directly flow through your body due to the insulating jacket. However, your body capacitively couples to the high-voltage field (20k-40kV) inside the wire. If another part of your body is grounded, induced charge has a discharge path through you, and you'll feel a shock with each firing event. If you're floating, the effect is more limited — your body charges and discharges with the field but has nowhere to dump.
Click to expand...
True when the wire is in good shape.
 
Its been about 10 rides since I re-rerouted the plug wires and no code 42.

If this actually ends up being the problem, that will make 2 separate issues I've had with plug wire routing since last summer. The first one (misfiring without code 42) took me ~6 months to figure out, the second one (code 42 without misfiring) took me ~2 months to figure out. Lesson learned is that plug wires can't be near each other and can't be near wires carrying sensitive signals. In my defense, during the second plug wire issue: the black coil connector was bad, the POS China EGR valve was leaking, the EVAP starting leaking fuel vapor into the intake, the brake booster started leaking air into the intake, the target idle RPM in the PROM is set 100 RPM low, and the governor in the 700R4 shifts into 4th way too soon (the ECM ends up locking the convertor under 35 MPH while in 4th).

Thanks for all the help! (fingers still crossed)
 
@clutch2k3 I just want to thank you for the content you post about on this forum. I have an identical truck (1989 / 350 / 700R4) and reading through your threads is helping me either confirm my suspicions on mine or give me new ideas to check out. It seems like we have gone through a lot of the same struggles.

I might start tagging you on my build thread when I run into miscellaneous crap just to see if you've dealt with it before.
 
shima said:
@clutch2k3 I just want to thank you for the content you post about on this forum. I have an identical truck (1989 / 350 / 700R4) and reading through your threads is helping me either confirm my suspicions on mine or give me new ideas to check out. It seems like we have gone through a lot of the same struggles.

I might start tagging you on my build thread when I run into miscellaneous crap just to see if you've dealt with it before.
Click to expand...
Great feedback! Good to hear my ride on the struggle bus might be helping someone else with squarebody love!

Full disclosure: @Wes Harden is like 99.999% responsible for getting my truck running right. There is no way in the world I would have been able to get that AC system working without his help.
 
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