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87-91 chevy TBI issues, "Code 42"

Discussion in 'Engine & Performance' started by Vbb199, Jun 28, 2020.

  1. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    So today, having some spare time, I decided to check the codes that occasionally comes on my suburban (check engine),

    I get the standard "no iginiton signal" which is supposed to be there

    Then

    (flash) (flash) (flash) pause (flash) (flash) x3
    (flash) (flash) (flash) (flash) pause (flash) (flash) x3


    Code 32 and code 42.
    32= EGR
    42= electronic spark timing
    I suspected the EGR and coolant temp sensor, but I was mistaken.

    Any who.
    I took the cat off, because I haven't actually gotten around to clearing that. Intended on replacing it with a straight 3" pipe to run from the "Y" to the muffler, never got around to it. So I've been Enjoying cruising around town today with no cat, and no muffler. Sounds great. LMAO. Not really. Every Tom, dick and Harry has looks when I gotta accelerate.

    Truck did indeed perform better, but it still seems like a total turd (and when I say total turd, I mean running like a stock tbi 350, not REALLY a turd)

    The truck when I first got it running would boil the tires from a stop if I stood on the gas.


    ANYWAY.
    The issue is, timing isn't advancing upon acceleration, hence "code 42"

    It's just going off the base timing, and a little extra the iginiton module gives it to crank the truck, so whatever dad originally set it at when he dropped the crate Motor in, it's running at that, plus a little bump from the module, and maxing out.
    I have NO REASON to think timing is off, and I will not mess with timing. Goblins didn't get in there and turn the timing down when I was asleep.
    But......
    This code Explains why it feels like someone stole 200 ponies from it.

    Now from what I've understood, the way the system works is, anything below 4-600 rpms, the ignition module advances the timing (ideally for cranking), after that 4-600 the module goes into bypass mode and the EST (electronic spark timing) controls timing, and advances as necessary when you apply throttle.

    Not 100% sure how that works yet, maybe off of the MAP sensor? Something.
    Ironically 3 months ago, I checked codes. And I had a MAP sensor error, now I don't.


    So the motor as is is running and driving off of its base timing.
    It's a fucking turd.

    Has anyone dealt with this code before? Maybe some suggestions where to start?

    I don't want "well this seems obvious, try this"

    I'm looking ideally for someone who's dealt with it before and can share an experience with me.

    While the previous sentence came off really nasty, I don't mean it that way, I just mean im looking for someone else to suggest something else I'm not thinking about :)

    Current strategy is to check for a short at the EST (by the way the EST is plugged in), maybe change the map?, that and replace the module, which I'm told is below the dist cap.

    Anyone else have any suggestions?

    Thanks guys as always.
     
  2. The88

    The88 What's broken this time?

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    I know exactly what that issue is. The burban needs a 502
     
  3. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    GO BIG OR GO HOME!
    632 CUBIC INCHES!

    700r4 will last a whole fucking 2 shifts and then shit the bed lol lol lol
     
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  4. The88

    The88 What's broken this time?

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    The best 2 shifts of its life.
     
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  5. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    Ive been fiddling with that bitch, you can checkout the k5 thread, been doing a little here and there when I have time.

    Hoping sometime mid week I'll give it another run, video it and run it a little harder.
     
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  6. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    It's occurred to me after thinking on it, dad set the base timing at like, 6-8° btdc, which means since it's just mechanically advancing for cranking, at most I'm at like..... 15-20° total timing at WOT.
    Sheesh.
    Explains why it's so terrible
     
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  7. JoeR Jr

    JoeR Jr Full Access Member

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    Code 42 can be a pain in the ass. It can be caused by a few different things.
    the PCM sets the code when the EST wire is grounded when it isn't supposed to be, or if it has power on it when it isn't supposed to. While the test procedure lists a bunch of resistance values, the're not a lot of use for intermittent failures.

    So here's how the circuit works:
    When cranking or running, the ignition module ALWAYS puts out a 5V square wave on the Reference wire going to the PCM. This wire is purple with a white stripe. If you measure it with a lab scope, that's what you will see.
    When cranking or running the PCM ALWAYS TRIES to put a 5V square wave back to the ignition module with the proper timing advance on it. This is the White EST wire.
    The ignition module is capable of listening to the EST wire, or ignoring it. When it ignores it, the wire is mostly grounded. If you were to look at it on a lab scope, you'd still see the square wave, but it's not 5V any more. It's low voltage, don't remember exactly what it is, but it's under 1V.
    The module doesn't decide on it's own when to listen to the EST wire. The PCM tells it what to do. It does this with the bypass line. That wire is brown with a black tracer. It has no voltage on it when you are in base timing mode and 5V on it when you are in bypass mode (listening to the EST).

    When cranking, you will have a 5V square wave on the Ref Wire(ppl/wh) going from the module to the PCM. The bypass wire(brn/bk) will have 0V on it.
    The EST wire (wh) will have a very low voltage square wave on it because the module is grounding it. You will be in base time mode.
    A code 42 can set while cranking if it sees over 1V on the EST. Possible problems here are the EST wire has a good connection at the PCM but not at the module end. The module can't ground it. The PCM sees voltage on the EST line during cranking and a code 42 sets.

    When running you will still have a 5V square wave on the Ref Wire (ppl/wh).
    The PCM puts 5V on the bypass wire(brn/bk) because it wants to take over timing advance duties.
    The EST wire (wh) will now have a 5V square wave on it and the PCM is controlling timing advance.
    A code 42 can set with the engine running if the EST line doesn't have a 5V square wave on it. This could be caused by a short to ground on the EST (wh) line. It could also be caused by an open between the PCM and module on the bypass wire (brn/bk). If that wire goes open, the module will short the bypass wire back to ground.

    Look at the attached pic. Ignore channel 1. It's a crank sensor and doesn't apply here.

    Channel 4 is the Bypass wire.
    Channel 2 is ignition reference. It's the signal to the PCM with base timing on it.
    Channel 3 is EST. It has the timing commands from the PCM, but you can see it's goofy. It should look just the the Ref signal, just leading it a little. you can see a small extra pulse and then it goes dead. The PCM failed on this car. As soon as the PCM realized, it it removed the bypass voltage(channel 4), grounding the EST and putting the car in base timing. That's the way it's supposed to work.

    Back to your truck. Before you go too crazy, make sure the 4 wire distributor harness is as far away from the plug wires as possible. Unfortunately stray secondary voltage can make the PCM think there is voltage on the EST wire and it sets a false 42.
    After that I'd check the 4 wire harness for integrity.
    After that you need a scope to diagnose it.

    Sorry for the long post, but you wanted someone that had dealt with the code before. that's my 2 cents.

    Joe

    est.jpg
     
    Last edited: Jun 29, 2020 at 11:21 PM
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  8. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    Thats a whole lot of info!
    WOW!
    Thank you so much!
    If I cannot do just simple diagnosing of the situation, is there a chance you might could give me a hand? I'm right over in Salisbury NC.

    I can buy your beer, gas, food, whatever you like.
     
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  9. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    I haven't even opened the hood to see if a plug wire is laying on the dist wires yet to be honest. I've been caught up in fixing the ragged out exhaust and plugged up cat.

    Should be done messing with that tomorrow and can move onto diagnosing why timing isn't advancing.
     
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  10. JoeR Jr

    JoeR Jr Full Access Member

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    I don't mind helping you at all. You'd have to come to me though. My days of doing diagnostics in other places are over. I always end up needing something I didn't bring with me. Let me know if you want to stop by.
    Joe
     
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  11. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    Sure thing. Maybe mid week I'll be diagnosing why timing isn't working right.

    Hoping it's just something simple like a bad module, damaged harness, or just the plug wire laying near it
     
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  12. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    Once this is all resolved, you may Wana consider pinning this @HotRodPC

    I Hear and have read code 42 is a pretty frequent occurrence for the TBI crowd with alot of lore, theories and otherwise misinformation all across the web. I pieced together what I have thru research in books, the web, and @bucket help, and then Joe here has REALLY added some vital information to this whole thing.
     
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  13. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    We'll what do you know.

    Cyl# 1's plug wire is laying RIGHT on the module harness @JoeR Jr

    20200701_190906.jpg
     
  14. JoeR Jr

    JoeR Jr Full Access Member

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    Hopefully that clears it up! It would be great of it's that easy, right?
    Joe
     
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  15. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    Joe did an excellent job of explaining the circuit. The only thing I’ll add is that a bitchy (not good but not totally failed) pickup coil can trip this code intermittently. It’ll test within spec, but it’s gone when you actually go to use it. From what I’ve seen online, this isn’t a super common problem, and it sounds like this is a very hard code so I’d bet everything the issue is in the actual wiring, and I’d do every voltage probe and continuity test before saying it’s the ECM unless you have a good spare lying around and want to play with it. My experience, though, is it’s not gonna let you win that easily.
     
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