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EZ-EFI 2.0;EZ-LS; LM-7 (5.3L) Backfiring through TB

Discussion in 'FAST Support Forum' started by Bethel McMillan, Jun 17, 2018.

  1. Bethel McMillan

    Bethel McMillan New Member

    Joined:
    Jul 8, 2014
    Location:
    Aurora, CO
    History: Have a 70 Chevelle with the 5.3 LM7 Iron block. Got the above system and installed with a single-plane from Holley. Sat for a couple years due to life getting in the way. Recently, I was able to get back on the project. Drove it around the block to shake it out.

    Occasionally, while starting and let it warm up, shutting it down, restarting, it would "pop" back through the TB. Scares the wife and dog. But it would start on the next try.

    Until today. I took her out on her longest run yet, about 10 miles, nothing fancy. About 7 miles in, I was at a light, and begin to accelerate when it backfired, loud and it died. for the remainder of the trip it would stop running at lights and when I tried accelarting. At one point, thankfully, near my garage, at cruise, it backfired and quit.

    Back in the garage, I looked into the throttle-body and found the expected soot in the TB.

    It never overheated, no leaks, fuel pump was at the correct pressure (43). If it was a Gen 350, I would have checked the wire order, but it's not, and I did verify the coil wiring.

    My other though is valves not closing. I didn't have time yet to check the plugs, previously discussed wife was waiting on me. I will fire it up tomorrow and after it's warmed up, check the plugs.

    Anybody got other suggestions or had similar experience?
     
  2. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Check the MAP sensor. It should read close to 100 on the handheld with the key on, engine off. With the engine running, you should see MAP numbers below 65. Also check vacuum hoses and the IAC/TPS calibration. Check all settingss in the Advanced Menu.

    Be sure the rotor is good and adjustments haven't changed, and the cap is clean inside with a good button. Be sure none of the plug wires are leaking. If you don't find anything, you'll need to check the wiring of the CD box and the FAST computer. Corrosion could be causing a problem.
     
  3. Bethel McMillan

    Bethel McMillan New Member

    Joined:
    Jul 8, 2014
    Location:
    Aurora, CO
    Well, it's an LS engine, no rotor.

    But I did find the MAP sensor reading 81 when off. If I'm not mistaken, should I be able to replace the MAP sensor myself? Is there other steps I can take to correct the reading?

    On cold-start, it's all fine. But after it warms up, I start having issues, especially backfire through manifold.

    I am use EZ-LS controller. the advance timing is set to 20 degrees, but at the 'Timing Verification' screen, it is 50. I am beginning to suspect this screen does not apply to EZ-LS controller. I did adjust it to +20, but after I start the engine, it is back at +50.

    This is the only problem keeping me from enjoying the fruits of my labor.
     
  4. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Yes, LS - I forgot. Maybe I should have thought LSMFT.:) Yes, you have a MAP problem - and it may be the entire source of your problem. You can replace the MAP with a standard GM one bar sensor. If you use the FAST wiring adapter and the old GM style one bar sensor, you leave the old TBI sensor in place as a plug.

    You can just replace the TBI mounted GM/FAST one bar MAP using a Allen key for the one MAP mounting screw. In either case, this can be done without removing the end cover. It only takes a few minutes. I replaced one with the old GM style wiring adapter and old style GM MAP, and later replaced it with a new GM TBI style MAP purchased from eBay for $7. The $7 eBay replacement has never been problem. The original TBI FAST MAP was killed due to fuel reversion before I switched from a dual plane intake to a single plane.

    If you are having problems with backfiring due to other issues, it may be best to use the FAST wiring adapter with an the old style GM MAP connected using a vacuum line connection. This will eliminate replacements of the MAP.
     
  5. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    When you install a new MAP, be sure to do a new Wizard reset. Then, using the Advanced menu: Set A/F idle 13.8, cruise A/F 14.1. WOT 12.6. Set timing idle advance to 20 degrees and all-in to 32 degrees @ 4000 rpm. Set vacuum advance 10 degrees @ 45% load. Be sure to reset the IAC and TPS after this as needed. This should get you started off good for later fine tuning. Plugs should be the stock heat range using the factory set gap and copper core - nothing extravagant. Also, use no anti-seize and be sure to use the correct torque. Plated plugs need no anti-seize. Changing the gap and anti-seize use are two of the major causes of problems.
     
  6. Bethel McMillan

    Bethel McMillan New Member

    Joined:
    Jul 8, 2014
    Location:
    Aurora, CO
    Well, <redacted>.

    So I replaced the MAP sensor and verified no leaks, vacuum or otherwise. The MAP sensor show 84. Hmm, maybe it's my attitude, I mean, altitude. I'm in Denver, so I'm thinking this is a normal reading.

    However, I did redo the setup, using the above suggested settings. No fire. I have to add throttle, to watch it backfire through the TB while cranking, so there is spark somewhere.

    This weekend, I'm removing all the electronics and checking, again, the connections. Whilst the electronics are out, I'm pulling the heads and checking for issues with valves, push rods, etc.

    I'm hoping it's something simple...
     
  7. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Yes, Denver altitude will lower the MAP reading - but I believe the reading should still be in the lower 90's there. Be sure to check the coil is good and in phase, along with the ignition box. Also check the pins in the harnesses. If this doesn't help, you may need to RMA the ECU and handheld. FAST Tech should be able to troubleshoot the EZ-LS controller also.
     
  8. Denis

    Denis Member

    Joined:
    Dec 2, 2009
    Location:
    Mendocino County, Northern Calif
    Just did an internet search for atmospheric pressure in Denver and here is what I found: The average atmospheric pressure in Denver (elevation 1610 m) is 83:4 kPa. Another website said it was 84kPa.

    Sounds like the MAP sensor is reading correctly.
     
  9. Bethel McMillan

    Bethel McMillan New Member

    Joined:
    Jul 8, 2014
    Location:
    Aurora, CO
    Thank you, Denis.

    I'm trying to find time to go through the wiring and verify all the connections. I will keep it everyone update to date on my progress. I just wish Life would let me have some free time...
    ==Mac
     
  10. Bethel McMillan

    Bethel McMillan New Member

    Joined:
    Jul 8, 2014
    Location:
    Aurora, CO
    Tried it all, now to pull the heads, first step, remove the spark plugs:

    IMG_20180807_123008.jpg

    Can anyone see what's wrong with this picture?

    Lucky for me, I am on vacation.

    I'll post updates.
     
    Last edited: Aug 7, 2018
  11. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Aside from the broken parts mechanical damage in the one cylinder: Number one, I would never run those plugs. A standard gap/electrode plug runs just as well - often better. Two, I don't know what heads you have, but flat gasket seat plugs are preferable to angle seat plugs when compatible.

    The damaged plug shows the source of your "pop" and fueling problem. Good luck with the repairs.
     

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