Any Lean Burn experts out there?

M Body General Discussion

  1. Nosrus77

    Nosrus77 Member

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    I know, I know - just replace it. I probably will long-term. However, I want to get this 39,000 original mile Dippy on the road like, NOW.

    It'll run beautifully for 10-15 minutes. Starts like a dream. Once she warms up, it'll start to run rough and then stall. Shut it off and fire up again, and its the same thing. Runs great for 10-15 minutes.

    This thing is sooo clean, and the air cleaner has a tag on the computer that says reconditioned with a 2015 date.

    Where should I look first? I'm convinced it's a sensor or switch.
     
  2. 89.Fifth

    89.Fifth Well-Known Member

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    Could be the EGR timer and not the lean burn. Try disconnecting the egr vacuum hose and plugging it. See what happens.
     
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  3. jasperjacko

    jasperjacko Well-Known Member

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    likely the run pickup in the distributor.
     
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  4. Darth-Car

    Darth-Car Well-Known Member

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    Don't forget to check for vacuum leaks. Those little rubber lines crack easy with age, and one leaky line can cause all kinds of fits for the computer.
     
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  5. bbeep79

    bbeep79 Member

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    Contact BudW who is on this site, he is sharp on the Lean Burn set up. I have owned my 1979 Road Runner 318 - 4bbl with Lean Burn for forty years, and it has never been the source of any problems that I have had with poor running. I had a problem with it starting hard the past couple of years, thought it was carb related. Rebuilt the Thermoquad, no changes. I then was starting to suspect the Lean Burn. I described the problem to BudW, and he advised to change the thermostatic choke. it still had some 'spring' to it, but nothing like a new one. Problem solved. I know your problem is different, but Bud pointed me in the right direction. He says too many times the Lean Burn set up is blamed by people who don't understand how it operates. I also know plenty of people who hate Thermoquads, and say get rid of them, but if they are rebuilt and tuned correctly, they are a fine item, in my opinion.
    Contact BudW, he's the Man !
     
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  6. AJ/FormS

    AJ/FormS Well-Known Member

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    Open the gas cap.
    I bet your tank venting system has failed
     
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  7. BudW

    BudW Moderator Staff Member

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    Most people like to Blame the computer (Lean Burn, or whichever name is printed on the box on air cleaner) but it is rarely the cause of most problems. A person will need to rule out all conventional ignition and/or fuel problems first, then start to tackle the computer.

    So many places to start here, but I would agree, the carburetor not getting fuel and distributer pickup coil are way up there on the top 10 things to look for. If a person has a timing light, hook it up (doesn’t matter where light is pointed if light is visible) and run engine. Having a helper, helps a lot.

    Temporarily attaching a clear (glass or plastic) fuel filter is also a good idea. I don’t prefer using a glass or plastic fuel filter for daily use – for testing purposes, they work great.

    Try these two things first and report back, here.
    BudW
     
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  8. Nosrus77

    Nosrus77 Member

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    Will do. IT looks like someone has messed with it before it was parked. New fuel filter - and the kickdown rod was disconnected. I went ahead and ordered new EGR valve and choke thermostat (never hurts to add new) and will mess with it more this weekend. It's rather cold here at the moment (lows in the teens and 20's the last couple of days).

    I appreciate all the responses, guys. This has been a great forum thus far.
     
  9. BudW

    BudW Moderator Staff Member

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    I’ve been sick the last few days and not been online.

    If someone has messed with the system – then there is no telling what it will need to get back working.


    First, getting the car running you are ok with kickdown linkage disconnected. Driving the car – you need to have that connected and working – or you will burn out the transmission in no time.


    With air cleaner wing nut removed (but everything else still connected – of possible), take several pictures of the intake/carburetor area and post here. That might be helpful – if something is missing.

    It takes fuel, fire and compression, in proper amounts and in proper timing – or things wont work, so the three areas to investigate is fuel system, ignition system and engine (compression (ie burnt valves, etc.), camshaft timing (ie: timing chain jumped, etc.).

    I doubt you would have a timing chain problem with that few miles – but it can happen, especially with those plastic timing gears. Things like burnt valves, bent pushrods, broken rocker arms happen all the time and 1 cylinder down can keep engine from starting.

    The carburetor will be computer controlled in ’84 (I think you have an ’84) – but computer only controls a small part of it. Not enough to get car started or for driving short distances. If car won’t start, look down the carburetor bore (with a flashlight) and push throttle back. If you see the fuel jets squirting (accelerator pump system works and there is fuel in the carburetor) then and don’t see fuel drops inside at other times (which means the float sunk and flooding the engine), then I would start looking elsewhere (for now).

    The ignition system is harder to diagnosis on that system (on any older computer-controlled system) – especially on a no start condition.

    My recommendation (for do-it-yourselfers) for order of diagnosis on no start/runs really poor, is:
    Non-computer controlled:
    - Fuel system
    - Ignition system
    - Compression check

    For computer-controlled systems:
    - Fuel system
    - Compression check
    - Ignition system


    For fuel system, first disconnect the fuel line from fuel pump (either at fuel filter (under alternator) or disconnect line at carburetor) and check for fuel volume. Fuel will be under low pressure but will spurt out in cycles (once for every 2 engine revolutions). No fuel or not much - you will need to look into metal line damage (or rust) and check all rubber hoses (front to rear).

    Compression test is straight forward, if you have access to compression tester – which sometimes can be rented from an auto parts store. You should have no more than 20% difference from highest to lowest reading. Either all spark plugs should be out or all but 1 plug should be in or you will not get good results (the former is better than the later method, IMO).

    To check timing chain timing is more difficult. Checking for timing chain wear is not hard at all.

    Remove distributor cap and set aside. Having a helper is worth the effort. Get a 1¼” (32 mm) socket and a long ratchet/breaker bar. I have one that is 2-foot long which works great. Try to always turn the crankshaft in one direction if possible (clockwise when looking from front of car), mainly because if engine is hard to turn – it can loosen the crankshaft bolt – but if engine turns over easy then you don’t have anything to worry about.
    Turn ratchet/breaker bar and watch for disturber rotor for movement (which should be instant). Then reverse direction of crankshaft and make note of how long distributor rotor turns in opposite direction. It should be instant or close to instant. Loose timing chains can be a problem with drivability – but not with hard start (unless timing chain jumped a tooth).

    Ignition with a computer is rather difficult to diagnosis, especially sense FMJ computers have no method for us to communicate with the computer. Getting a spark tester (plugs in between spark plug and wire) or a timing light is about the easiest way to check – but even that is not reliable (unless you get no sparks) when a computer is involved.

    Normal ignition system – there is only 4 areas of failure:
    - Ballast resistor
    Newer Ballast Resistor.png or
    Older Ballast Resistor.jpg
    - ICU (Ignition Control Unit)
    Ignition Module.jpg
    - Pickup coil
    Pickup C1.jpg
    - Wiring harness open/short circuit

    Computer-control system
    - Either (or both) pickup coils
    - Computer
    - Wiring harness (with miles of possibilities)

    Generally, these will cause drivability concerns, and not no start conditions but spark plugs, plug wires, distributor cap and rotor are also on that list. I generally change these items every 30k miles on my FMJ vehicles.

    BudW
     
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  10. Nosrus77

    Nosrus77 Member

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    Appreciate the response Bud. I've replaced the Choke Thermostat, and tonight will do the choke pull off (which is very bad, by the looks of things) and the EGR valve (just for good measure). The distributor pickup should be done by the weekend, I hope. 55-60 hour works weeks don't help. The carb has a 2005 reconditioned tag on it, and the computer has a 2015 reconditioned date. I'll post pics later on, but it looks complete. It's sat for a couple of years, so right now I'm going to try to do some of the above that you mentioned. I feel more and more like it is a maintenance issue, not an overall system failure.

    As I look this car over, I'm blown away by the originality of it. Almost all hoses have the factory markings, as do most parts, to be honest. Spare tire hasn't seen the ground, floor under the trunk carpet is insanely clean and solid, and all the parts that I mentioned a moment ago have the pentastar stamped on them.

    I appreciate the help and guidance. Loving this group so far. . .
     
  11. Nosrus77

    Nosrus77 Member

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    Would any of you mind posting pics of your engine compartments? Specifically the vacuum hose routing and the throttle linkage. I've discovered a few open ports (vacuum solenoid for EGR) and at least one missing spring. Anything could be of assistance.

    I replaced choke thermostat, EGR, and Choke Pull off - very little effect. I unplugged the computer input to the carb, and it ran better. Just trying one thing at a time until she smooths out. Worst case, I will do the lean burn delete, but would prefer to get it working correctly.
     
  12. 89.Fifth

    89.Fifth Well-Known Member

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    Mine's not original anymore so pics of it won't help. Wish you were closer, I'd come by and help you with it.

    Any chance you can do a Skype video call? I could help you there.