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Thread: GM TBI on a 1966 283 Engine - Getting a Code 42

  1. #1
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    GM TBI on a 1966 283 Engine - Code 42 and Other Problems

    After introducing myself on the other thread I'm just going to jump in and tell you my GM TBI issue...

    I have a 1966 Chevrolet El Camino with a 283 engine, 2bbl, powerglide, A/C and P/S. The vehicle is mostly stock including the engine.

    After years of frustration with points & condenser and carburetor issues I thought updating to a 1990's GM TBI was just the ticket. I bought a kit from a company I found via the internet. As you know, there are several kits out there and they all seem very similar to me. The kit I purchased included the wiring, most of the hard parts and some basic instructions and troubleshooting tips. I've spent the better part of 8 weeks trying to get this thing to work. Here are the details:

    I was able to install the TBI system in about a week. Ran the return fuel line, changed the fuel sending unit to allow for a return line, installed an external fuel pump (relatively low, near the bottom of the tank), pulled the old distributor, installed the small HEI distributor, attached the TBI adapter plate, TBI, ran the wires.... Presto, the engine started the first time. Wow was I ever happy. The thing ran great and I was giving myself high-five's for doing such a good job. I took the car for a test drive and marveled with the throttle response, etc. Then the check engine light came on - code 42.

    Under a variety of conditions I get a code 42 indicating something is askew with the timing/spark. I installed a new ignition module in the distributor today. No change, still get code 42 under a variety of situations - cold engine, hot engine, idling, etc. If there is any consistency to the check engine light turning on I guess I'd say it's during relatively low-speed operation. But I've even had it happen at highway speed.

    I don't want to tune and tweak and wring every HP out of the engine like many people on this forum. I just want a vehicle that's reliable enough for my wife to feel comfortable driving it. Unfortunately, that's not the case as this time.

    I have a 1320 Electrontics scan tool that allows me to capture data. I can see the point when code 42 is triggered but I have no idea how to diagnose this issue any further.

    Any ideas?
    Last edited by nvestysly; 08-01-2018 at 02:13 PM.

  2. #2
    Super Moderator dave w's Avatar
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    Perhaps this link will help? It for a similar TBI system, so the troubleshooting is EXACTLY the same for your TBI system.

    http://www.fieros.de/en/v6help/code42.html

    dave w

  3. #3
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    Thanks for the reply. I found similar information in the form of a PDF on the gearhead-efi site. I've excerpted the information regarding code 42 and included it below. Most of the troubleshooting tips point toward wiring/connector issues with the odd situation concluding with a faulty ECM.

    Guess I'll be getting out the multi-meter and spending some time under the hood and under the dash.
    Attached Files Attached Files

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    An update...

    Yesterday when I installed the new ignition module I think I forgot to clear the previous code 42. OOPS!

    Regardless of what I did, or did not do, yesterday... I cleared the codes today. I idled the engine for an extended period of time - in total I ran it for probably 40 minutes. What didn't happen? No check engine light today. I may try again tomorrow to see if that trend continues. However, a few odd things happened and they've happened previously so any insight you can provide would be helpful.

    1) A few minutes after starting the engine - it was not up to operating temperature of 180 yet - the engine died. No check engine light. I tried to restart and it did not start easily. It turns over fine. Everything seems to work - radio, lights, horn. It catches a time or two but is very difficult to start. I sit there hoping I don't run the battery down. I try starting a few more times and ultimately end up putting the accelerator pedal to floor and hope like he!! it will start. This isn't too bad if it happens in the garage like it did today. But if it happens in traffic it's a BIG problem. This same issue has happened when the engine is cold, when it's hot and everything in between.

    2) After 30 minutes of idling the engine the IAC count went way up to around 200 and even higher up to around 250. The TPS was still at approximately 0.50 (after all it's supposed to be idling). But the wacky high IAC count makes the engine rev up to 2000+ RPM. I gently revved the engine even higher and let it come back down but the IAC kept it at 2000+ RPM. I punched the accelerator and quickly released it and it came back down to normal idle with the IAC coming down to less than 20, less than 10 and then settling around 0. What the heck?! This thing has a mind of it's own. More mild versions of this have occurred in the past when I'm driving the vehicle. I hear the hiss/sucking that indicates the IAC is open and the engine doesn't want to slow down. It's usually not an issue but if it clicked up to 200-250 and the took off when I'm idling at a traffic light that would be a BIG problem!

    3) One final crazy thing. All the aforementioned stuff had happened and I continued running the engine. Mostly at an idle. Sometimes putting the car in Drive, sometimes running the A/C all in an attempt to see if the check engine light would come on. So now I'm back to idle and the engine just quits - skips a beat and dies. This time it started right back up. As I said, this thing has a mind of it's own.

    So the million dollar question is... should I replace the ECM? I can buy another ECM at Summit Racing and put my chips into it. Would this be worth the $75 to see if that's the problem? I simply don't know how to diagnose this TBI system.

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    This is some ironic stuff. After spending years fighting points and condenser, you upgrade to a fight with a much more complex system. Usually these swaps do not cause more headaches than they cure.

    Could you provide the part number for the ecm? And is it using a stock GM file or one that was modified to match your engine?

    Thanks,

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    The ECM is 1227747and was remanufactured although I'm not sure which "brand" it is. The PROM is modified by the guy who sells the kits. I don't know exactly how it's modified (which data) but the seller asked questions about engine size, transmission, A/C, thermostat temp, 2bbl, factory timing spec, idle RPM, etc.

    The original chip the seller made for my car was not a good match and I was having additional problems early on. The idle speed was too high in the chip so the computer was trying to compensate for what it thought was low idle. We fixed that with a replacement chip and the car is running much better than it did originally. I didn't put all that into my original post because I didn't want to confuse the issue. However, the check engine light and problems I described in the posts above are persisting.

    I looked at Summit Racing for a replacement ECM. They have a 1227747 replacement from Cardone for $74 and a replacement from AC Delco for $107. Both are remanufactured. I'm leaning toward AC Delco although it goes by a newer part number 88999146. I guess the newer number supercedes 1227747.

    The seller of the TBI kit is willing for me to return the ECM and he will put it in his truck and run it for several days. That should verify whether the ECM is at fault. While I'm appreciative of the seller's willingness to help that whole process could take as long as two weeks - sending, testing, returning.... On the other hand, I can have a replacement ECM from Summit or any number of other parts dealers today.

    A few more details about the system I installed:

    - external electric fuel pump is providing 12 psi to the TBI
    - MAP sensor
    - 180 degree thermostat
    - A/C compensation increases RPM in an attempt to overcome the huge load of the A6 compressor
    - engine coolant temperature sensor is in the block side of the water (not the radiator side)
    - installed new small-cap electronic distributor
    - TPS is set to 0.50 volts but sometimes varies a small amount (0,49 - 0.52) after driving the car. It doesn't seem to always return to 0.50
    - when the engine is idling normally the IAC is anywhere from 0 to 10, sometimes a bit higher
    - the VSS is disabled, as is the knock sensor, any other sensors not mentioned above have been disabled.
    - ECM is mounted in the cab, and it has plenty of air around it and is easy to access (it's not way up under the dash)
    - the two relays are attached to the bottom of the ECM with adhesive. I did not screw the relay tabs to a piece of metal
    Last edited by nvestysly; 08-02-2018 at 07:41 PM. Reason: added MAP sensor to list

  7. #7
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    Lots of rain recently but the weather is clearing today. A test drive with the new ignition module is on the list of things to do.

    This TBI system does not include a knock sensor. The seller said I didn't need one. What is the Knock Counter referring to in the attached screen capture? The picture is from the 1320 Electronics scan tool I'm using.
    Attached Images Attached Images
    Last edited by nvestysly; 08-03-2018 at 08:51 AM. Reason: corrected spelling

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    Matters have gone from bad to worse. I barely made it around the block - the engine died three times! The first time it took quite a long time to fire up - probably 10 seconds - which seems like an eternity when I was blocking an intersection. The second time it restarted quickly and the third time it took a few seconds. Each time the engine died my foot was on the gas pedal although I was going at low speed in our neighborhood. The engine just died - lost power completely. As I said previously, the lights, horn, radio and other electrical items continue to operate.

    I'm going to try to duplicate the situation in my garage. I'm plan to pull/push/tug on every connector and wire I can get my hands on to see if I can make it "die" in a repeatable fashion.

    I purchased an AC Delco ECM but I'm not going to install that yet. Once it's opened I cannot return it for full value so I'll wait to see where my troubleshooting ideas take me.

    Any ideas and constructive comments are welcome.

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    Could be the ICM even though you just replaced it. Electronics usually fail in first few hours of use or after many hours of use. Would be helpful if when it dies, you can check for spark and also for fuel spray from injectors. I lost an ICM recently and injectors would spray fuel but no spark. The coil driver portion of the ICM failed. After engine cooled off, it would start back up.


    Hope this helps
    Brian

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    The ignition system is designed so the ICM operates independently of the ECM until the ECM is ready to take control of spark timing. The ECM signals it is ready by applying 5V on the tan and black "Bypass" wire connected to the module. Some of the vehicles equipped with '7747 ecm's had instructions to set ignition timing by jumpering two pins in a connector under the dash. Others used instructions that said the Bypass wire should be disconnected under the hood. Problems can occur if you use the wrong set of instructions as the ignition timing can be off significantly.

    If the ignition timing is correct it's possible to force the ICM to ignore the ECM. This can be an important step in diagnosing a spark problem. If the ECM is disconnected and the coil stops producing spark it's obvious the ECM is not the culprit. Locate the tan/black Bypass wire to the ecm and disconnect the wire to keep the ECM out of the picture.

    Even though the engine is not equipped with a knock sensor the ECM still uses knock counts. The value in the knock counter will be different each time the engine is started but should not change during the time the engine is running. As long as they do not change while the engine is running the ECM will think there is no knock.

  11. #11
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    Quote Originally Posted by 1project2many View Post
    The ignition system is designed so the ICM operates independently of the ECM until the ECM is ready to take control of spark timing. The ECM signals it is ready by applying 5V on the tan and black "Bypass" wire connected to the module. Some of the vehicles equipped with '7747 ecm's had instructions to set ignition timing by jumpering two pins in a connector under the dash. Others used instructions that said the Bypass wire should be disconnected under the hood. Problems can occur if you use the wrong set of instructions as the ignition timing can be off significantly.

    If the ignition timing is correct it's possible to force the ICM to ignore the ECM. This can be an important step in diagnosing a spark problem. If the ECM is disconnected and the coil stops producing spark it's obvious the ECM is not the culprit. Locate the tan/black Bypass wire to the ecm and disconnect the wire to keep the ECM out of the picture.

    Even though the engine is not equipped with a knock sensor the ECM still uses knock counts. The value in the knock counter will be different each time the engine is started but should not change during the time the engine is running. As long as they do not change while the engine is running the ECM will think there is no knock.
    Okay, I know this sounds simple to many on this forum but let me restate what I think you're saying to see if I understand the process...

    My instructions indicate to set the timing with the bypass wire disconnected. My factory timing spec for this engine is 4 degrees. I disconnect the bypass wire and set the timing at 4. That creates a code in the ECM and the CEL is on. I have to clear that code by disconnecting power to the ECM. I reconnect the bypass wire. Start the engine, runs fine. The timing is now advanced quite a bit at idle - maybe 12, 14, 16 degrees. I don't know for sure but it's certainly advanced and the engine is running fine. That means the ECM is controlling the advance, right?

    So in the past, when the engine stumbled, the CEL came on and I get a code 42. The timing reverts back to 4 degrees - I've measured it in this condition and it's definitely back to 4 degrees. In some ways that's the same as disconnecting the bypass wire. The engine still runs, I guess with no advance, so it's a bit sluggish and I can limp home in that mode. However, when this happens the code 42 doesn't seem to be a "hard" code. As soon as I turn off the ignition and restart the engine everything is fine. The CEL is off, the spark advance is back to where it should be and all is well until the next time the engine stumbles.

    I'm probably going around in circles here but I don't understand what the difference is between 1) physically disconnecting the bypass wire which sets a hard code 42 and the CEL remains on until I clear the codes compared to 2) the random phenomenon that is occurring in which it appears the timing goes back to limp home mode, indicates code 42, but doesn't set a hard code 42.

    I haven't been able to idle (or drive) the car very much since I replaced the ICM. I'm now preoccupied with the fact that the engine stalls every few minutes. I've been looking into that and may have more information tomorrow.

    Next time I operate the engine I'll look at the Knock Counts. I think this value changes while the engine is running but I know for sure there is no knock sensor. I'll report more on this tomorrow.

    Thanks again for the replies. I'm learning as I go. As I said in my introduction thread I don't feel I know enough to be a gearhead, just enough to be a dangerous shade tree mechanic!

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    I would view this situation as a box of unknown problems. The computer tune may or may not be right for the engine. The wiring harness may or may not have a problem. The ignition module may or may not have a problem. The distributor may or may not be working correctly. System voltage may or may not be causing a problem. Or the Gods of Mechanical Things may or may not be entertaining themselves by altering the laws of physics and logic around the car whenever it's in use. The goal, to me, would be to solve for unknowns while watching out for wrenches falling horizontally and bolts of electricity randomly leaping from the cat.

    I'm probably going around in circles here but I don't understand what the difference is between 1) physically disconnecting the bypass wire which sets a hard code 42 and the CEL remains on until I clear the codes compared to 2) the random phenomenon that is occurring in which it appears the timing goes back to limp home mode, indicates code 42, but doesn't set a hard code 42.
    I think you've got the basic operation down. Your observation of the difference is a good one. It is not typical behavior for the ecm. Typically if the ecm sees a problem on the bypass line it will store code 42. However, there are certain conditions where the ecm will determine there is a problem currently but it will not set the code. Those problems can be very difficult to diagnose. They are often caused by voltage fluctuations on the bypass or EST line and technicians from the early days of GM EFI all have a story or two about trying to cure an intermittent code 42.

    You have also reported " I try starting a few more times and ultimately end up putting the accelerator pedal to floor and hope like he!! it will start." Did the engine start while putting the accelerator pedal to the floor and cranking? Or did you need to use a different method? If it started while putting foot to floor the engine may have flooded during cranking or just before stalling, possibly an indication that more tuning is needed. Which may or may not be part of the stalling problem.

    You also said "So in the past, when the engine stumbled, the CEL came on and I get a code 42. The timing reverts back to 4 degrees - I've measured it in this condition and it's definitely back to 4 degrees. " This is an excellent diagnostic step. This is also correct behavior for the ecm. When the ecm senses an incorrect voltage on the bypass line it will flag a problem with the check engine light, send up a code 42, and stop attempting to control ignition timing. Timing should revert to base timing, in your case 4 degrees. This is also a good sign, at least as far as discounting the theory that the gods are playing games with your car.

    Please tell me if this list covers all the symptoms:
    1) Check engine light / "soft" code 42 occurs under various conditions
    2) Engine stalls very frequently after ICM replacement, primarily at low speeds. Restart can be difficult.
    3) ECM will, at times, command increasing IAC counts causing very high engine speed

    Is this list complete? Did I miss anything?

  13. #13
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    Yes, that captures the basic problems I’ve experienced. The past six weeks are a bit of a blurr so I sometimes have difficulty recalling which problem retates to which period of time and how that may or may not relate to what’s currently happening.

    The latest issue is certainly the frequent stalling/dying for no apparent reason and it’s not setting a code.
    Last edited by nvestysly; 08-04-2018 at 09:17 AM. Reason: typo

  14. #14
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    Quote Originally Posted by 1project2many View Post

    Please tell me if this list covers all the symptoms:
    1) Check engine light / "soft" code 42 occurs under various conditions
    2) Engine stalls very frequently after ICM replacement, primarily at low speeds. Restart can be difficult.
    3) ECM will, at times, command increasing IAC counts causing very high engine speed

    Is this list complete? Did I miss anything?
    I'm back at home now with access to a regular keyboard so I can type more easily. The earlier post was from a smart phone.

    So I'll elaborate on item 2) Engine stalls very frequently after ICM replacement, primarily at low speeds. Restart can be difficult.

    I recall now that stalling/dying occurred with the previous ICM but it was much more infrequent. It was at times difficult to start with the previous ICM as well as the replacement ICM. The situation does seem to be worse (increased frequency) with the new ICM but my guess is that's a coincidence. My guess is that whatever is causing the engine to stall/die is some other issue but maybe affected when I replaced the ICM.

    I just ran the engine for 5 or 6 minutes and it died again. Upon trying to restart I was able to check for a spark at one of the plug wires where it attaches to the distributor cap. No spark. I tried restarting 4 or 5 times just a few seconds each time. No spark. Finally, after several more attempts there was spark and the engine started immediately. I did not touch the accelerator pedal. My previous comments related to pushing the pedal to the floor probably have nothing to do with whether the engine was going to start. I think I was simply reaching a level of frustration and the only thing I could think to do was push the pedal to the floor. I'm beginning to wonder (recall?) if similar things hadn't happened back in the carburetor days I just don't have a clear memory of those situations as the car has been in the garage, mostly unused, for nearly a year until the TBI system was installed recently.

    Yesterday, at one point when I was troubleshooting I tugged/pushed/ pulled on every wire and connector I could find. The two connectors at the distributor, the IAC, TPS, coolant, ECM connectors - nothing I did made the engine die. So I'm increasingly believing that it's not caused by a loose wire. At least not a loose wire or connection related to the TBI install.

    Many years ago the car would die but all electrical circuits would die with it. The horn, lights, radio, you name, all dead. I replaced the under hood wire harness and that seemed to fix the problem. Perhaps something like that is happening again. I'll have to do some research on which wires go where and which items are in the ignition "loop."

    This thing is driving me crazy!

    ON EDIT: The knock counter on the 1320 Electronics software is changing while the engine is running. The counter started at zero when I started the car but changes wildly (sort of like the O2 numbers change wildly) from 50 - 200 - 75 - 275 - 40 - 195... with every data record.
    Last edited by nvestysly; 08-04-2018 at 12:18 PM. Reason: added comment

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    I believe focusing on the stalling / hard start might be a good place to enter into this fight. The suggestion to disable the ecm controlled spark advance is a diagnostic step. This completely eliminates ecm control of the timing. If there is no spark with the bypass connector disconnected then the distributor, coil, and wiring must be checked. Alternatively if the engine stalls while the ecm is controlling timing you can disconnect the bypass connector to see if the engine restarts.

    Other tests if there's no spark could include checking resistance of pickup coil and watching the laptop to see if RPM is showing greater than zero while cranking. I've just gone through this today with a truck that only sees daylight a handful of times at best.

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