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Thread: Lean surge 7.4L with TBI

  1. #16
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    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...
    Hmm... Neukam is spot on. The TBI FPR is a typical diaphragm plus spring type regulator. Fuel is exposed to one side of the diaphragm, the other side, with the spring, is exposed to atmospheric pressure through a hole in the regulator hat. Because of this port, changes in atmospheric pressure will change the fuel pressure. The physical placement of injectors and regulator atop the throttle body and inside the air cleaner ensure the pressure at the bottom of the injector and pressure at the atmosphere port of the regulator remain the same relatively speaking. This is well known science.

    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.
    Understand this is not the OEM method, it's not an "improved" method, and it's not necessary for a stock or mildly modified vehicle. This is a technique designed to allow one to obtain maximum fuel delivery under load while reducing fuel delivery at low load. It reduces the total range of VE values across the fueling tables and can reduce the work required to obtain good driveability. This technique is generally used for higher HP vehicles and its a workaround for the limits of the OE ecm and TBI injectors.

    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...
    I haven't talked with Neukam much but I have to wonder if the poster spent time at a GM facility or a GM dealership in the '90s. We are hearing of things that could be considered arcane knowledge today. The one wire O2 is really a two wire O2 without the second wire. The "wire" for the signal return is found in the body of the sensor. The sensor is attached to the exhaust pipes which provide a great path for ground. And the exhaust pipes are connected to the block. But the exhaust pipes and engine are hung in rubber which provides a poor signal path to the body. Engineers connected the signal return from the ecm to the rear of the engine block or cylinder head, close to the exhaust pipe without connecting directly to the pipe. Neukam is suggesting there's a possibility that ground is bad which would cause a false lean condition. Neukam is further suggesting the rare but real possibility that the ground for the heater has been connected to the same point as the ecm O2 return wire, and that the a bad connection to the block combined with heater power unable to find ground could add to the issue.

  2. #17
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    1project2many has pretty well summed up what I was about to say.

    Quote Originally Posted by 1project2many View Post
    Hmm... Neukam is spot on. The TBI FPR is a typical diaphragm plus spring type regulator. Fuel is exposed to one side of the diaphragm, the other side, with the spring, is exposed to atmospheric pressure through a hole in the regulator hat. Because of this port, changes in atmospheric pressure will change the fuel pressure. The physical placement of injectors and regulator atop the throttle body and inside the air cleaner ensure the pressure at the bottom of the injector and pressure at the atmosphere port of the regulator remain the same relatively speaking. This is well known science.
    I just took a picture of the port on the regulator that is used for reference. I will edit this post and upload it from my phone. Many people don't understand this function of the regulator because it is not as obvious as a typical multiport EFI system with a vacuum line attached to the regulator, but the function is the same.

    Understand this is not the OEM method, it's not an "improved" method, and it's not necessary for a stock or mildly modified vehicle. This is a technique designed to allow one to obtain maximum fuel delivery under load while reducing fuel delivery at low load. It reduces the total range of VE values across the fueling tables and can reduce the work required to obtain good driveability. This technique is generally used for higher HP vehicles and its a workaround for the limits of the OE ecm and TBI injectors.
    The large peak and hold TBI injectors do not meter fuel well at very low pulse width. As injector size is increased, or fuel pressure is increased, the injectors cannot accurately meter fuel at the reduced pulse width required at low load with the higher flowing injectors. Vacuum referencing the regulator and retuning the VE tables can work well to solve the problem, and in some cases may be the only way to solve the problem, but is technically not the "proper" way to do things. It is merely a hack to a compensate for the limitations of the factory TBI system.

    I haven't talked with Neukam much but I have to wonder if the poster spent time at a GM facility or a GM dealership in the '90s. We are hearing of things that could be considered arcane knowledge today.
    I've never worked for GM. I started working on cars in 2001 at an independent shop that, do to it's location, catered to many customers with lesser financial means. Most of the cars I worked on were from the mid eighties to mid nineties and many of them were "problem cars". After working there for 2 years, I went to NASCAR Technical Institute in Mooresville, NC. I had a partial scholarship. I helped some of the instructors teach some of the other students that were having problems understanding, and I learned a lot from some of them who were extremely knowledgeable. I believe several of them had worked for GM, and one of them may have done some programming on these systems. After graduation, I worked at an independent dealer and repair shop that specialized in foreign cars. There I worked on everything from Geo to Lambo, but mostly on older German cars. After that I worked at another independent repair shop where I again worked on whatever came through the door. I don't claim to be the most knowledgeable person here, but I did gain a lot of knowledge and experience over about 10 years, before changing careers. I now work with train control signals for the local transit company.

    The one wire O2 is really a two wire O2 without the second wire. The "wire" for the signal return is found in the body of the sensor. The sensor is attached to the exhaust pipes which provide a great path for ground. And the exhaust pipes are connected to the block. But the exhaust pipes and engine are hung in rubber which provides a poor signal path to the body. Engineers connected the signal return from the ecm to the rear of the engine block or cylinder head, close to the exhaust pipe without connecting directly to the pipe.
    I just want to emphasize that this signal return is (and must be) dedicated solely to the O2 sensor. Assuming the ECM is 1227747, it should be connected to ECM pin D6.

    Neukam is suggesting there's a possibility that ground is bad which would cause a false lean condition. Neukam is further suggesting the rare but real possibility that the ground for the heater has been connected to the same point as the ecm O2 return wire, and that the a bad connection to the block combined with heater power unable to find ground could add to the issue.
    I'm also suggesting the rare but real possibility that the ground for the sensor has been left unconnected, or broken off after performing other work on the truck. You say the truck is running rich, and O2 voltage is reading low, and BLM is high. It would make sense that there's a problem with the O2 sensor circuit. It is possible, but I don't know how likely, that the ECM is raising the idle in an attempt to heat the O2 sensor and force a response. I don't know enough about $42 control strategy to know if that is the case.

    Edit: in the last paragraph I wrote that it was possible the ground for the heater has been left unconnected. I meant to write sensor. I had a brain fart. Also uploaded a photo of the stock regulator showing the opening. Both spring pressure and air pressure act upon the diaphragm to regulate the fuel pressure differential across the injector.
    Attached Images Attached Images
    Last edited by Neukam; 10-21-2020 at 06:33 PM.

  3. #18
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    Neukam - still reading through and digesting responses. But I will say one more time that a single wire O2 sensor does NOT have a dedicated ground that goes back to the ECU. It has 1 single signal wire...no ground wire...thus the term "single wire". There is no O2 ground wire in my harness.

    O2 sensor on this is in the rear of the driver's side exhaust manifold. So it is grounded to the engine. The heater circuit is grounded to the body inside the cab.

    I pulled all of the ECU related grounds yesterday and cleaned and reinstalled.

    All my testing right now is with the air cleaner off, so the remote FPR is getting exactly the same atmospheric reference as the stock FPR would have under these conditions. I backed the fuel pressure down to 9psi (the factory lower spec) and it is still extremely rich at idle (like eyes watering, force me out of the garage rich), and it made the surge worse. I believe I have eliminated the possibility of a vac leak under the intake - I was getting about 6 inches of vac in the crankcase and when I disconnected the PCV and plugged the hole in the valve cover I had no vacuum in the crankcase.

    At this point I'm ready to pull the harness apart and look for wiring issues. First I'm going to reinstall the factory FPR and see if the issue resolves itself.
    Last edited by Dieselsj; 10-21-2020 at 08:44 PM.

  4. #19
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    Quote Originally Posted by Dieselsj View Post
    Neukam - still reading through and digesting responses. But I will say one more time that a single wire O2 sensor does NOT have a dedicated ground that goes back to the ECU. It has 1 single signal wire...no ground wire...thus the term "single wire". There is no O2 ground wire in my harness.
    I will say one more time that the single wire O2 sensor DOES have a dedicated ground that goes back to the ECU, but it does not go directly to the sensor

    O2 sensor on this is in the rear of the driver's side exhaust manifold. So it is grounded to the engine.
    O2 sensor is grounded through the exhaust pipe, which is grounded through the exhaust manifolds, which are grounded through the engine, which should be referenced through the tan wire bolted to the back of the engine to pin D6. If that wire isn't there, then that's a problem.

    The heater circuit is grounded to the body inside the cab.
    This should be fine.

    I pulled all of the ECU related grounds yesterday and cleaned and reinstalled.
    Did you specifically check that tan wire? I'd measure resistance from the body of the sensor to pin D6 on the ECM. I'd check the signal wire from the sensor to the ECM pin D7 as well.

    All my testing right now is with the air cleaner off, so the remote FPR is getting exactly the same atmospheric reference as the stock FPR would have under these conditions. I backed the fuel pressure down to 9psi (the factory lower spec) and it is still extremely rich at idle (like eyes watering, force me out of the garage rich), and it made the surge worse.
    I don't think the FPR is causing your problems, I just wanted you to have proper information about how the system is supposed to work. If you had told me that you had the FPR connected to manifold vacuum, I would have advised you that that was at least part of the problem.

  5. #20
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    Forget my 3 wire O2. Let's say I went back to my factory stock single wire O2. There is a single purple wire on the sensor. Nothing else. Sensor is mounted in the driver's side exhaust manifold, not in the exhaust pipe. Where would I find this dedicated ground wire that you keep mentioning?

    Hmmm...re-reading your response...I think I see our miscommunication. The ground wire isn't actually on the sensor itself, you are saying there is a dedicated ground from the ECU to the engine block that is for the O2 signal? If that is the case, then I'll go crawl around under the truck and see what is there. As far as I remember the only ground on the back of the head is the engine-to-body grounding strap. It is totally possible that I messed something up when I did the TH400>NV4500 swap. I will also check for continuity between that pin on the ECU and the engine.

    Good stuff. Keep it coming. Sorry if I'm seeming frustrated. I'm seeing why people say these systems suck and they go back to carbs. No...I'm not going back to carbs.

  6. #21
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    I'm not sure on the 7.4L but, on the 5.7L the grounds to the ECM are on the T-Stat housing.

  7. #22
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    Here's some diagrams from my 88, they s/b close to yours. Grounds 107 Black/White and 108 Tan are the ones.
    88 C1500 Sensors.JPG
    88 C1500 Grounds.JPG

  8. #23
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    Upon further investigation, I found this which shows the Tan wire from the ECM for the O2 sensor.
    88 C1500 O2 ground From ECM.JPG

  9. #24
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    Quote Originally Posted by PlayingWithTBI View Post
    I'm not sure on the 7.4L but, on the 5.7L the grounds to the ECM are on the T-Stat housing.
    Appears to be where mine are also. I traced the harness from where it comes through the firewall and I have all grounding locations accounted for.

    Next I'll be checking continuity and resistance on them.

  10. #25
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    Quote Originally Posted by PlayingWithTBI View Post
    Upon further investigation, I found this which shows the Tan wire from the ECM for the O2 sensor.
    88 C1500 O2 ground From ECM.JPG
    I have G108. Mine has 2 wires also. What I didn't see was G107 - appears to be a single wire ground. I need to find that one.

  11. #26
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    Quote Originally Posted by Dieselsj View Post
    Hmmm...re-reading your response...I think I see our miscommunication. The ground wire isn't actually on the sensor itself, you are saying there is a dedicated ground from the ECU to the engine block that is for the O2 signal?
    That is exactly what I'm saying.

    If that is the case, then I'll go crawl around under the truck and see what is there. As far as I remember the only ground on the back of the head is the engine-to-body grounding strap. It is totally possible that I messed something up when I did the TH400>NV4500 swap. I will also check for continuity between that pin on the ECU and the engine.
    It will be a small gauge tan wire, maybe 18awg somewhere on the back of the engine. It should be mounted by itself, not doubled up with any other ground wires.

    Good stuff. Keep it coming. Sorry if I'm seeming frustrated. I'm seeing why people say these systems suck and they go back to carbs. No...I'm not going back to carbs.
    It's understandable to be frustrated. When these systems are right, they give good drivability, and are generally very reliable. GM could have put a "better" system on the trucks, but the TBI was relatively simple, reliable, and cost effective enough that GM decided it was good enough to get the job done. I really can't disagree.


    Quote Originally Posted by PlayingWithTBI View Post
    I'm not sure on the 7.4L but, on the 5.7L the grounds to the ECM are on the T-Stat housing.
    The ECM does have grounds on thermostat housing, but the O2 ground is definitely on the back of the engine on a SBC. I'm pretty sure the O2 ground is in the same area on the BBC.

  12. #27
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    O2. Use a air powered Die Grinder with a wire brush. Remove the O2. Run the wire brush over the Ex manifold until it shines bare metal. Wire brush off the O2 threads. Anti-seize the O2 threads. screw it in a few times and recoat. Tighten.
    Measure the O2 Ground at the ECM Pin Back probe. DVOM Ground needs to be connected to Battery Negative post. Start/Run engine hot. Measure O2 Grd. Running and hit the throttle to see max volts. More than 0.1/0.2 is a poor ground. (Measuring with DVOM on OHMs Scale is Deceiving. Had some measure 0.1 ohm this way. DVOM Voltage measured 0.4 volts on O2 Ground when throttle hit. Car ran just as a plugged Cat converter. Dealer spent $4,500 in parts for poor ground at head.)

    BBC. China MAP Replace with Delco/GM. in BBC and others under load, Fuel will WICK up the Vac tube. Contaminate the sub-strait of the strain gauge. This shifts the MAP output voltage and the engine runs very lean/LEAN mixture. (No Codes set either back in those days) So lean the EGTs will be over 1600 Deg. Ex Valve grow longer or tulip, then engine destruction. Remove MAP. Place white cloth/Kleenex on palm, Tap vac inlet of MAP. Any liquid of any kind is shifted lean.)

    Insure MAP is manifold Vac. NO LOW POINTS IN MAP LINE. Install MAP at the top of the firewall run vac line, no sags, to Manifold vac.

    BBC If you live in the frozen tundra keep the thermac active in temps starting at 48 deg F and lower Esp on humid days. Stops TB Icing. If not in cold, run the thermac Vac line plugged so thermac is INOP 100% & remove hot sir tube. In MI I never had any iced TB with any carb/TBI BBC. Run air cleaner housing intake tube to Rad support as Suburbans. Helps when under hood temps are Very hot during loads. Use Small Block Large Truck air cleaner TOP. Use Large A/C Filter. (#753? vs #248) Double air flow. BBC needs cold air. In some cases VE may need to be touched up. (Injector pulse width at low %. With higher fuel pressure it is difficult for the Injector to overcome the pres and begins to be unstable. Physics)

    In some yrs Idle BLM was meant to be at 140. Anticipates Full Carbon Canister. No sag with Canister purge at 100%.

    Back to the Farm.

  13. #28
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    Quote Originally Posted by Neukam View Post
    That is exactly what I'm saying.



    It will be a small gauge tan wire, maybe 18awg somewhere on the back of the engine. It should be mounted by itself, not doubled up with any other ground wires.



    It's understandable to be frustrated. When these systems are right, they give good drivability, and are generally very reliable. GM could have put a "better" system on the trucks, but the TBI was relatively simple, reliable, and cost effective enough that GM decided it was good enough to get the job done. I really can't disagree.




    The ECM does have grounds on thermostat housing, but the O2 ground is definitely on the back of the engine on a SBC. I'm pretty sure the O2 ground is in the same area on the BBC.
    This is big block. From everything I am seeing, the O2 ground pin is joined to the ECU ground at the G108 grounding location, which is the tstat housing. I traced the entire harness and there isn't a separate ground wire that I missed.

  14. #29
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    Quote Originally Posted by Dieselsj View Post
    This is big block. From everything I am seeing, the O2 ground pin is joined to the ECU ground at the G108 grounding location, which is the tstat housing. I traced the entire harness and there isn't a separate ground wire that I missed.
    I honestly don't remember any small ground wires going to the back of the engine, only the ground strap that goes from the head to the fire wall. As @Dieselsj said I found both the black/white and tan wires double crimped to the eye on the T-Stat housing. Maybe GM changed its location later on?

    88 C1500 T-Stat grounds.jpg

  15. #30
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    Quote Originally Posted by Dieselsj View Post
    This is big block.
    I'm aware. I don't have access to a big block TBI to look at for reference, and finding reliable info about details like this is difficult. If I had access to one, I would find and verify the o2 ground a send you a photo.

    From everything I am seeing, the O2 ground pin is joined to the ECU ground at the G108 grounding location, which is the tstat housing. I traced the entire harness and there isn't a separate ground wire that I missed.
    Are you seeing this on your truck? Did you trace the wire from D6? I've had inaccurate or incomplete diagrams lead me astray more than a few times, especially on vehicles from that era. I'm not saying that it's wrong, I'm saying verify and be certain. If it is tied to other grounds, I would separate it and give it it's own dedicated mounting point.

    Are you absolutely sure that the sensor is working properly. Check it's output with a meter and if the voltage is low, spray some propane from a cheap torch, or some carb cleaner into the throttle body to make absolutely sure that its running rich. If you're seeing voltage close to 1v at the sensor while adding extra fuel, reconnect it and verify that you get the same reading at the ECM between pins D6 and D7. If it isn't, you'll need to find out why. If you find that the sensor is bad, consider upgrading to a 4 wire sensor, with the sensor ground connected only to D6.

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