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Thread: Mystery Problem

  1. #11

    Default

    hook it up to an oscilloscope, so you can see the firing pattern, i suspect it may have something to do with loss of energy/power, see if by any chance the backfire gets worse when the lights are on, also diconect the battery for about a minute so the adaptations reset on the ECU, of course that is done with all off.
    if the back fire continues look into the alternator not having enough output to maintain charge, make sure terminals are secure and clean. also there is a body to engine ground at the starter that normally gets forgotten on engine swaps.

  2. #12

    Angry Backfire through intake under load

    Well,

    The battery terminals are clean, the grounds checked and all are fine. The alternator is working fine even under load. The compression was checked again and it is fine. The number 3 spark plug is very black worse that the rest. We replaced that plug. The plugs all look like the engine is running rich. We switched out the distributor again, retimed it and nothing changed. We ran water over the valley and around the top of the engine looking for a vacuum leak. We checked the plug wires and all are firing correctly and the coil wire is fine. All the wire and plugs are new. We checked the fuel to see if it was aerated and it did not have any bubbles in it.

    So what can it be?

  3. #13
    Join Date
    Sep 2007
    Location
    Philadelphia, PA
    Posts
    122

    Default Crossed wires.

    I read that you checked the wires... However this is indicative of a firing order mishap.
    1990 RRC, 3.9L, AUTO "Willow Pond" (Sold)
    1996 D1, 4.0L, AUTO "Lucas"
    1999 RR, 4.0L, AUTO "Belle" (Sold)

  4. #14
    Join Date
    Feb 2007
    Location
    From Vermont, right now I am still traveling from west to east and back every year
    Posts
    37

    Default

    Does it back fire when you rev it up under the hood by hand or just when you are driving!

  5. #15
    Join Date
    Feb 2007
    Location
    From Vermont, right now I am still traveling from west to east and back every year
    Posts
    37

    Default

    I have had experience with fuel injection systems that have the o2 sensors fail and give the computer wrong inputs obout fuel mixture. When they where doing this the fuel wixture became rich which inturn fouled the plugs. Have you unpluged the o2 sensors to see if it makes a difference! just an idea!

  6. #16

    Default

    Quote Originally Posted by Dan
    Does it back fire when you rev it up under the hood by hand or just when you are driving!
    I thought it just did it under load, but it will do it when you rev it up by hand under the hood.

  7. #17

    Default

    Quote Originally Posted by Dan
    I have had experience with fuel injection systems that have the o2 sensors fail and give the computer wrong inputs obout fuel mixture. When they where doing this the fuel wixture became rich which inturn fouled the plugs. Have you unpluged the o2 sensors to see if it makes a difference! just an idea!
    We did try disconnecting the o2 sensors with no change.
    We also checked everything for vacume leaks.
    I think that it backfires less when the engine is cold.
    I found that if i get it to kick down into passing gear, the backfiring was not there. So at low rpm light load everyting is fine. About 2200 rpm under load, the problem starts. At high rpm under load no problem.

  8. #18

    Default What's next....

    We replaced all the spark plugs, taking out the ones that have all of 750 miles on them. We also checked the wires for proper resistance. We changed the temperature sensor for the ecu. No Change. We tried another known good coil. We are running it on a known good ecu. We changed out the distributor cap, with two new ones to make sure we did not have carbon tracking.

    I have found that if I push it hard and it kicks down into passing gear the intake backfire is gone until it shifts back up.

  9. #19

    Default

    I've had similar probs with my 91 rrc caused by corroded wiring around fuel injectors and temp sensors. Strangely it caused weird thumping noises and loss of power but rarely trip a code.

  10. #20

    Default Problem Solved

    We found a flat cam. The lobe for number 3 exhaust was almost completely gone. The cam was from Rover's North and was Land Rover Cam. We replaced it with a crane cam that ground for a bit more performance. It runs great now.

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