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Thread: Lean surge 7.4L with TBI

  1. #46
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    tbi injectors do go bad. it took me a few used sets to find a good set back when i was younger and had no money. look at pattern with a timing light and watch the INT/BLM. have you verfied basics here? spark plugs/distributor/rotor/wires? what about fuel pressure? what pump are you running?

  2. #47
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    Quote Originally Posted by tayto View Post
    tbi injectors do go bad. it took me a few used sets to find a good set back when i was younger and had no money. look at pattern with a timing light and watch the INT/BLM. have you verfied basics here? spark plugs/distributor/rotor/wires? what about fuel pressure? what pump are you running?
    Yes I have verified plugs, dist, cap, rotor, wires, ign module, map, tps... Fuel pressure steady at 13psi right now. EP361 fuel pumps, both new (2 tanks...same issue both tanks).

    Pattern on the injectors actually looks fine. I don't see an increase in output though unless I snap the throttle above that lean spot. As I slowly open the throttle when I hit the magic RPM, the INT/BLM go to their max. If I hit the throttle hard and get it past the lean spot then everything comes back to normal on the BLM/INT and O2 voltages

  3. #48
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    It.

    Was.

    The.

    Injectors.

    Broke down and ordered a new pair of injectors last week and got them installed yesterday. First, idle was smooth and the overly rich idle issue was gone. And nice smooth operation as I open the throttle. No more surge. Drove the truck around and it feels like a big block...I always thought it was underpowered but now it feels good. This was with the stock AZFW tune. Tune has a dip in the VE and if I cruise it right at that RPM I can actually trigger a code 44, so I'll be working that out. But for now I have a good driveable vehicle again.

    Also this is the first manual transmission tune I have used since the TH400 > NV4500 swap and I must say the return to idle sucks on this tune...IAC filters are high so it closes very slowly...like foot off the accelerator and truck keeps moving. The automatic tune didn't do that. That will be something else to play with.

    So I guess the moral of the story is that injectors can fail in a way that causes extreme lean conditions at low RPM/high vacuum and rich at idle.

  4. #49
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    So here we are 4 months later and I just went through the same thing. Got replacement injectors under warranty and once again it is running good.

    This one started as getting a lean code during light throttle cruise. Fought that for about 2 months with different tunes and settings and could not get it to go away. Then within about a week span I started getting a hesitation then a bad stumble and then that same lean surge again.

    Once again, new injectors solved the issue.

  5. #50
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    There are only a few things that could cause repeated injector failure. Excess current through the injector could be an issue. If the ecm isn't limiting current when the injector is opened the injector coils can overheat. Or if you have a short to ground on the injector driver wires you could have momentary full current events. That's probably highly unlikely but still possible.

    Fuel issues or fuel contamination can cause problems.

    Does that throttle body have screens around the injectors? Are you using a fuel filter designed for an EFI system? Are you using fuel with greater than 10% alcohol?

    Insufficient fuel circulation might allow the coils to overheat. I would think you'd have driveability issues before the injectors failed but if the pump is not able to produce significantly more pressure than the regulator calls for the truck could run correctly without returning fuel to the tank. I suppose one way to test this might be to go for a good drive then put your hand on the tank to check temperature.

    Have you had any of the bad injectors tested? Understanding how they failed could be helpful.

  6. #51
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    Quote Originally Posted by 1project2many View Post
    There are only a few things that could cause repeated injector failure. Excess current through the injector could be an issue. If the ecm isn't limiting current when the injector is opened the injector coils can overheat. Or if you have a short to ground on the injector driver wires you could have momentary full current events. That's probably highly unlikely but still possible.

    Fuel issues or fuel contamination can cause problems.

    Does that throttle body have screens around the injectors? Are you using a fuel filter designed for an EFI system? Are you using fuel with greater than 10% alcohol?

    Insufficient fuel circulation might allow the coils to overheat. I would think you'd have driveability issues before the injectors failed but if the pump is not able to produce significantly more pressure than the regulator calls for the truck could run correctly without returning fuel to the tank. I suppose one way to test this might be to go for a good drive then put your hand on the tank to check temperature.

    Have you had any of the bad injectors tested? Understanding how they failed could be helpful.
    Thanks.

    Factory screens are on the injectors and under a magnifying glass I don't see any contamination. This was a factory EFI vehicle so I'm using factory EFI filter. And just using regular 87 octane pump gas, so it shouldn't be more than 10%. I have later model high pressure pumps installed and I'm getting good flow back to the tanks. I know the regulator is staying cool.

    I still have the set of injectors that I pulled out back in December. I'll get those tested and see what they say.

  7. #52
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    If they tell you the injectors are fine...?

  8. #53
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    Quote Originally Posted by 1project2many View Post
    If they tell you the injectors are fine...?
    Then I pull it all and the truck gets a Cummins.

  9. #54
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    I understand that. Hard to believe but I'm getting that kind of trouble from a single cylinder engine right now.

    Hopefully you can get this nailed down.

  10. #55
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    Quote Originally Posted by 1project2many View Post
    I understand that. Hard to believe but I'm getting that kind of trouble from a single cylinder engine right now.
    A single cylinder TBI? I'm curious, what is it?

  11. #56
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    Wow getting the light cruise lean issue again. ECU case where the injector drivers are is showing 150+ degrees according to my infrared thermometer after about a 10 minute drive. After about 20 min I'm getting a slight surge and misfire at idle. Both the factory ECU with factory chip/tune and the modified for the programmable chip are having the same issue. I haven't put the temp gun on the injectors themselves yet. I'm thinking I need to split out the entire harness and check everything, starting with how tight the connections are at the ECU and on the injectors. Seeming like a bad feed or ground connection on the injectors which is causing a lot of extra current draw...at least that would explain the heat in the driver section of the board.

  12. #57
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    A single cylinder TBI? I'm curious, what is it?
    Whoops... no. I'm just saying I'm frustrated enough to rip out an engine and change to something completely different.


    ECU case where the injector drivers are is showing 150+ degrees according to my infrared thermometer after about a 10 minute drive.
    You're doing great diagnostic work. I'm not sure that temp is outrageous. I've used four TBI injectors across two drivers before and the ecm definitely got that warm. You could check to see if the ecm is having trouble when hot by bagging up ice cubes then applying them to the ecm when it gets warm. Is there a cool down period where the injectors work ok then don't? Can you get a scope to watch the pattern as the injectors are turned on? It should look similar to the one below. The link gives some reading about peak and hold operation.

    https://autoditex.com/page/peak-hold-injector-53-1.html


  13. #58
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    I don't remember the ECU getting that hot in the past. I never put the temp gun on it, but I did put my hand on it a few times and it definitely got warm but not hot like this.

    Unfortunately I don't have a scope.

    When it gets hot it does start the misfire/stumble/surge at idle. It doesn't do it when it is cool. And it starts acting up a lot sooner on hotter days vs cooler days. Good suggestion to test it though...I have some cold spray that I can use and just hit the heat sink on the drivers and see what happens. I put in old known good ignition coil and ignition module to rule out those items having a heat related failure.

    But it seems like the off idle stuff - the lean cruise and surge - after the issues start, they never go away even after sitting overnight. Almost like the injector is getting so hot that something is affected inside the injector and the damage is permanent. This is why I want to get my thermometer on the injectors themselves to see what is happening there.

  14. #59
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    If the injector coils are being subject to excess current flow they could be getting damaged. Once they're bad they don't recover. Trying another ecm is an option but you're still left holding injectors that don't work correctly.

    Could you remind me which ecm you are using? Some of them require external wiring so the injector driver will operate in peak and hold mode.

  15. #60
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    Quote Originally Posted by 1project2many View Post
    If the injector coils are being subject to excess current flow they could be getting damaged. Once they're bad they don't recover. Trying another ecm is an option but you're still left holding injectors that don't work correctly.

    Could you remind me which ecm you are using? Some of them require external wiring so the injector driver will operate in peak and hold mode.
    7747. I have one that is totally factory stock with a factory chip and one that has been modified to take the 27SF512 chip (for my custom tuning). Both ECMs are reacting similarly.

    Actually remembered something last night...several years ago the truck burned up an ECM. Driving down the freeway I could smell electrical burning and the truck started running horribly. The board had a discolored spot in it where it got hot. What made me remember was running into the original owner of the truck (lives in my neighborhood) and he asked me if I ever figured out the intermittent check engine light issue. That made me remember that it would occasionally throw a lean code when I first got it but it went away for a long time after I cleaned up some things under the hood. Giving me a whole new perspective on this.

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