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Thread: Lean surge 7.4L with TBI

  1. #1
    Fuel Injected!
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    Lean surge 7.4L with TBI

    This is on my 1987 Chevy R30.

    Background -
    Bought it a few years ago. TB was worn out, got a new base with bushed shafts, new TPS, IAC, MAP. Also found the wire to the O2 was severed and installed a 3 wire (heated) in place of the stock 1 wire.

    Truck ran well, but always seemed low on power and I'd get an occasional CEL for lean. Upgraded both fuel pumps (2 tanks) to the EP361 (Delco brand) pumps. No more CEL, but still seemed to not pull like it should.

    Did some basic data logging with Winaldl and saw consistent BLM values in the 140-150 range. Everything else looked good and it drove well so I left it alone. I did replace the distributor with a factory GM unit as the stock one was getting noisy and I found a lot of play in the shaft.

    Pulled the factory TH400 and swapped in a NV4500 (overdrive...yay)

    Recently I had the exhaust redone - 2.5" from the manifolds (no headers) to a single 3" muffler. BLM values after that were consistent 150s.

    Installed a Performer TBI intake. Also installed a Aeromotive FPR and removed the FPR on the TBI. This was done in preparation for future mods.

    The issue now is that I have a horrible surge at 1800-2200 RPM. If I roll into the throttle it starts surging horribly to where it isn't even driveable. I've been battling this for a few weeks now.

    So far...
    Checked TPS operation - nice smooth output, nothing funky where the surge is happening.
    MAP output also looks fine.
    Reset IAC.
    Set idle speed so that the IAC counts are right at 20 (see below...).
    Have fuel pressure set right at 12 (have tried as low as 7 and as high as 20).
    Data logging shows that BLM and INT are both into the 150 range. O2 sensor voltage is down consistently under 0.05v.
    Timing set at 4 degrees with wire unplugged (have gone from 0 to 10 advanced while troubleshooting with no change)

    Has the intake/exhaust mods just taken the ECU beyond what it can handle?

    What am I missing here? Almost seems that O2 sees lean, then it dumps in a ton of fuel, sees that its rich and pulls a ton of fuel. Exhaust smells extremely rich while this is happening. Watching the spray from the injectors I can see the increase/decrease in volume while this is happening.

    Something else that I have had for a long time is a high idle. This came in before I started doing the modifications. I do not want to focus on this yet...I need to work out the surge. Just wanted to mention it idle in case they were related. It always wants to idle at about 1000rpm when warmed up. I can set idle to get about 20-25 counts on the IAC, but then the IAC counts creep up and I keep getting a high idle.

    I don't have ability to burn chips. I am looking at the EBL setup from Dynamic. I just received a new 02 sensor (Delco) and was going to give that a shot, then the EBL setup is next (EBL was in my long term plans anyway) so that I can get better data logging to try to trace the issue.

    What am I missing?

  2. #2
    Fuel Injected!
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    I doubt the intake and exhaust are causing enough of a change to cause your issues. Do you have a vacuum line connected to the fuel pressure regulator? If you do, where is it run to? It should be connected so that it references the pressure inside the air cleaner, not manifold vacuum. Do you have the means to monitor the fuel pressure under load?

    I have seen injectors clog or fail. I have also seen injector wiring short to ground, causing an injector to fully open. Do the injector spray patterns look the same, side to side?

  3. #3
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    Regarding the high idle, is your thermostat working properly? Is the ECM reading the ECT properly?

  4. #4
    Fuel Injected!
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    Some things you should try first. Unplug 02 sensor and make some test run. If it is real lean condition, it will stumble really bad. If it is a false lean condition, it will run much better.

    You need to address that high idle speed as top priority. Things to check

    IAC valve
    Thermostat
    Coolant sensor
    Vacuum leak at intake. Plug all vacuum lines to be sure and see how the idle goes. You will need to revert to stock setting the throttle body for the test

  5. #5
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    Quote Originally Posted by Neukam View Post
    I doubt the intake and exhaust are causing enough of a change to cause your issues. Do you have a vacuum line connected to the fuel pressure regulator? If you do, where is it run to? It should be connected so that it references the pressure inside the air cleaner, not manifold vacuum. Do you have the means to monitor the fuel pressure under load?

    I have seen injectors clog or fail. I have also seen injector wiring short to ground, causing an injector to fully open. Do the injector spray patterns look the same, side to side?
    Regulator is not vacuum referenced. Not to get off topic, but I believe the correct location for the vac reference would be manifold vacuum as that would lower fuel pressure under light loads (high vac) and increase fuel pressure under heavy loads (low vac).

    Injector spray patterns are nice even cones, both sides look the same.

  6. #6
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    Quote Originally Posted by Neukam View Post
    Regarding the high idle, is your thermostat working properly? Is the ECM reading the ECT properly?
    Thermostat is good. Per the data logs it is running right at 190 degrees. Sender was changed as part of diagnosing the high idle. I'm assuming that if the winaldl dashboard is showing 190 then all is good with the sender.
    Last edited by Dieselsj; 10-20-2020 at 11:46 PM.

  7. #7
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    Quote Originally Posted by kur4o View Post
    Some things you should try first. Unplug 02 sensor and make some test run. If it is real lean condition, it will stumble really bad. If it is a false lean condition, it will run much better.

    You need to address that high idle speed as top priority. Things to check

    IAC valve
    Thermostat
    Coolant sensor
    Vacuum leak at intake. Plug all vacuum lines to be sure and see how the idle goes. You will need to revert to stock setting the throttle body for the test
    IAC was replaced with a known good unit when I was diagnosing the high idle previously.
    Thermostat and temp sender are good (per data logging)
    High idle was present with the factory intake, and I have since installed the Performer TBI intake. I've sprayed around the intake gaskets and base of TBI with engine running and no change in idle. Only vac ports currently in use on the throttle body are to the charcoal canister, MAP and PCV. One port on the intake is for the HVAC. I have plugged all of those in the past while looking for a vac leak and still had the high idle.

    What do you mean by revert to stock setting on the throttle body?

    I'll run it with the O2 unplugged and see what it does.
    Last edited by Dieselsj; 10-20-2020 at 11:47 PM.

  8. #8
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    What do you mean by revert to stock setting on the throttle body?
    fully close the blades and let iac motor controll the idle speed. Higher rpms means more air is entering the engine than commanded.

    Since you can datalog check the iac counts and see if the commanded rpm match the actual, also check map readings. PCV line is known to draw fresh air.
    You must have missed something.

    High idle is some serious malfunction that needs all of your attention. Pcv

  9. #9
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    Quote Originally Posted by kur4o View Post
    fully close the blades and let iac motor controll the idle speed. Higher rpms means more air is entering the engine than commanded.

    Since you can datalog check the iac counts and see if the commanded rpm match the actual, also check map readings. PCV line is known to draw fresh air.
    You must have missed something.

    High idle is some serious malfunction that needs all of your attention. Pcv
    I know I missed something...that is why I'm here.

    I'm not sure the logs give me commanded vs actual. I do know that the IAC counts will increase, so I don't think it is vacuum related. It is like something is commanding the higher idle. I'll check tonight and see if my logs have the commanded vs actual values.

    Thanks for the inputs so far.

    Before the intake swap/FPR, the high idle didn't really bother me and the truck drove fine. With the lean surge it isn't driveable at all right now which is why I wanted to address that issue.
    Last edited by Dieselsj; 10-21-2020 at 12:01 AM.

  10. #10
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    Unplugged O2 and no change in the way it was running. Same as before...surge at 1800rpm. Didn't matter if engine was cold or fully warmed up. Smelled extremely rich.

  11. #11
    Electronic Ignition!
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    Hello some thoughts here on your headache. The BLM and INT being way high obviously the ECM trying to correct for extreme lean. You could have a vacuum leak into crankcase area, #1 culprit would be intake gasket or crack somewhere. To test idea with engine at idle put your thumb over the breather tube and you should feel a light suction/draw, if no draw pcv or clog could be culprit or excessive "blow by" or "worn out valve guides". If it is real strong pull like your hand on a vacuum cleaner definitely intake into crankcase leak. Another thought is partially open EGR valve or wrong vacuum source opening it. You might even have the wrong injectors in it or clogged fuel filter and/or injectors. Ops almost forgot while at idle watching BLM and/or INT squirt carb cleaner/gasoline/propane into the throttle body , a 5 second shot should make a response if the BLM/INT drop down then it may be injectors if no vacuum leak is found. Hope this helps

  12. #12
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    Been messing with it today. It is extremely rich at idle. Still can't get though 1800rpm without a major surging.

    Crankcase has 6-8 inches of vacuum.

    Runs exactly the same if O2 is plugged in or not.

  13. #13
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    Rebuilt injector pod. Verified injectors are correct. It is running super rich at idle and still bad surge at 1800.

  14. #14
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    Quote Originally Posted by Dieselsj View Post
    Regulator is not vacuum referenced. Not to get off topic, but I believe the correct location for the vac reference would be manifold vacuum as that would lower fuel pressure under light loads (high vac) and increase fuel pressure under heavy loads (low vac)
    Not being vacuum referenced shouldn't cause any problem under light loads or without a restriction in the intake. The job of the fuel pressure regulator in any fuel injection system is to maintain a constant differential between the pressure inside the fuel injector (fuel line, fuel rail, etc) and the pressure outside the fuel injector (throttle body, intake manifold, etc). Because the fuel injector nozzles are not subjected to manifold vacuum, the fuel pressure regulator should not be referenced to manifold vacuum. The factory fuel pressure regulator was located on the throttle body, within the air cleaner, and near the fuel injectors so that it can reference the pressure near the injectors and maintain that constant pressure differential. The aftermarket pressure regulator should be similarly referenced. By referencing the area near the injectors, the system will be able to compensate for any intake/air cleaner restriction, dirty air filter, atmospheric pressure changes, etc.

    Also found the wire to the O2 was severed and installed a 3 wire (heated) in place of the stock 1 wire.
    A 3 wire sensor still references to the exhaust pipe that it is installed to. These systems have a dedicated ground for the O2 sensor. It should be a tan wire. if I remember correctly, its usually attached to the back of the right cylinder head. If that ground wire is compromised, the ECM will not read the O2 sensor properly, will assume the engine is running lean, and will force the engine richer. Make sure that the O2 sensor heater (or anything else) is not grounded through this tan wire and that it is properly attached and making a good electrical connection.

    I saw a guy install a fuel filter backwards once. The truck ran like dookie for about 2 weeks, before it stopped running altogether. Just something else to check.

  15. #15
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    Quote Originally Posted by Neukam View Post
    Not being vacuum referenced shouldn't cause any problem under light loads or without a restriction in the intake. The job of the fuel pressure regulator in any fuel injection system is to maintain a constant differential between the pressure inside the fuel injector (fuel line, fuel rail, etc) and the pressure outside the fuel injector (throttle body, intake manifold, etc). Because the fuel injector nozzles are not subjected to manifold vacuum, the fuel pressure regulator should not be referenced to manifold vacuum. The factory fuel pressure regulator was located on the throttle body, within the air cleaner, and near the fuel injectors so that it can reference the pressure near the injectors and maintain that constant pressure differential. The aftermarket pressure regulator should be similarly referenced. By referencing the area near the injectors, the system will be able to compensate for any intake/air cleaner restriction, dirty air filter, atmospheric pressure changes, etc.



    A 3 wire sensor still references to the exhaust pipe that it is installed to. These systems have a dedicated ground for the O2 sensor. It should be a tan wire. if I remember correctly, its usually attached to the back of the right cylinder head. If that ground wire is compromised, the ECM will not read the O2 sensor properly, will assume the engine is running lean, and will force the engine richer. Make sure that the O2 sensor heater (or anything else) is not grounded through this tan wire and that it is properly attached and making a good electrical connection.

    I saw a guy install a fuel filter backwards once. The truck ran like dookie for about 2 weeks, before it stopped running altogether. Just something else to check.
    While I appreciate the help, I don't totally agree with much of what you are saying here.

    First, there is absolutely ZERO reference for fuel pressure on the stock TBI unit. You don't get a reference simply by placing the regulator "close" to the pod. Factory TBI uses a fixed pressure...there are no pressure changes based on anything in the factory setup. Additionally, as long as the air filter is clean and adequately sized, there should NEVER be any type of pressure differential inside of the filter assembly. I would love to see any supporting articles or data that supports your statement that the FPR should be referenced off of the air filter assembly. I can show you lots of articles that support the FPR using manifold vacuum as the reference. Even the EBL tech articles discuss doing it this way.

    As for the 3 wire O2...the factory O2 on this vehicle is a single wire. A 3 wire simply adds a heating circuit, so the other 2 wires go to a power and ground. A 4 wire sensor will have the dedicated ground for the signal...3 wire does not have a signal reference ground...the ground on the 3 wire ONLY is for the heating circuit. If you remove the ground on the 3 wire it simply disables the heating element and has zero effect on the signal and it works just like a single wire.

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