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Thread: Atomic EFI TBI cast fuel rail leak.

  1. #1

    Default Atomic EFI TBI cast fuel rail leak.

    Hi everyone. Out of the blue, our 1977 CJ7 with a SBC 350 just started running badly. At first, I thought it was ignition related. By luck, my wife noticed that one of the four cast fuel rails was really shinny vs the other three. Then we noticed how fuel was dripping out between the cast and aluminum parts. I uploaded a video of the issue on Youtube:

    Is this serviceable, or does the TBI need to be replaced? Thanks.

  2. #2


    Looking at the scan data, the MAP sensor reading indicates low intake manifold vac. For example a stock cam should have a MAP of about 10 inHg or 20 inHg on a vacuum gauge. Possible causes include vacuum leaks, cam timing or ignition timing, EGR stuck open, blocked exhaust. Low vacuum will increase the injector pulse width and cause a rich AFR. Check with a vacuum gauge. To test the MAP sensor, with KOEO it should indicate your barometric pressure. I would sort this problem first.

  3. #3


    This engine is out of a 1970s Blazer, so no EGR. It does have a mild cam, but no idea what the specs are (installed by previous owner at least a decade ago and he can't recall the specs). I recall the vacuum range being closer to 15 inHg when it was running normal a few weeks ago. Does it make a difference if the unit is set to ported with this reading vs un-ported?

    Is the way the fuel is drizzling out between the booster and casting normal? Thanks.
    Last edited by b17gsr; 05-22-2021 at 12:29 PM.

  4. #4


    With KOEO, got 29.5 inHg. I unplugged another fuel injector and the AFR was in the low 13s vs target of 14.

  5. #5


    Found a fitting so I could measure vacuum. My vacuum gauge reads about 7 inHg. Found a few loose bolts here & there, but did not help. Will have to dig through my spare parts to see which gaskets I have and then remove the throttle body to check the condition of the unit and the gasket below it. Some other tid bits in this video:

  6. #6


    I have the MSD Atomic installed on my own car. After inspecting the throttle body l noticed the annular rings are sealed with an o-ring, so you must have a perished o-ring causing the leak. You may be able to carefully replace it yourself. The low intake manifold vacuum may be caused by a faulty gasket or even the brake booster. Clamp the hose and recheck, rev the engine to 2500 RPM and recheck vacuum. If lower again the exhaust is blocked. During a snap throttle, the needle should move quickly to zero and back to idle vacuum. If the needle moves slow, that's another sign the exhaust is blocked.

  7. #7


    Good to know about the annular rings in yours. I did some more troubleshooting this morning. With KOEO, it takes 20 seconds for the A/F ratio to change from warming to a number. Is that typical, or a sign of a tired WBO2 sensor? With the engine around 2500 RPM, it pulls more vacuum. The MAP reads around 12 inHg and the vacuum tester around 16 inHg. So the MAP vs tester are darn close. If I blip the throttle quickly, vacuum goes to zero and goes a bit better than idle vacuum until it settles.

  8. #8


    And I can’t get the IAC to drop below 50 at idle, even after readjusting the throttle blades.

    Tried blocking the vacuum ports, no change (and the rig has manual brakes).
    Last edited by b17gsr; 05-23-2021 at 02:01 PM.

  9. #9


    Decided to order a new WBO2 sensor. Should receive it before next weekend. I'll report back after.

  10. #10


    The WBO2 warmup time is normal and no DTC was logged. The MAP reading and vacuum gauge won't read the same, the handheld is displaying MAP as absolute pressure, the vacuum gauge is gauge pressure. So to work out the intake manifold vacuum from the handheld. With KOEO, MAP is 29.7 inHg (BARO) - MAP at idle 22 inHg = 7.7 inHg intake manifold vacuum. Which you confirmed with the vacuum gauge. Need to find the source of the low vacuum.

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