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Thread: Engine running rough cannot set mixture

  1. #11
    Member Dhennick99's Avatar
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    The o2 sensor was recently replaced in an effort to get a handle on the hunting.

  2. #12
    Delorean Guru
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    Tripping the WOT switch will cause the ECU to go to a higher duty cycle, the frequency valve should be buzzing anyway, it will just make it buzz a bit more. Make sure the O2 sensor is connected, the wire didn't get knocked loose. There are tests you can do to the ECU to force it into different duty cycles by grounding the O2 sensor wire or applying a voltage to it. That tests the ECU. You would still have to test the wiring from the ECU to the frequency valve. If you have power to the ECU and the frequency valve isn't buzzing, chances are you have a bad connection between the ECU and the frequency valve. Try plugging and unplugging the connector on the frequency valve several times.
    David Teitelbaum

  3. #13
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    Frequency valve should buzz regardless of O2 sensor.

    If O2 sensor isn't connected, the Lambda ECU can't go into closed loop and will hold the frequency valve at a set duty cycle. You should still hear the frequency valve buzzing, and the dwell reading will be steady and high.

    Test the coil of the frequency valve. With the connector unplugged, you should see a reading between 2-3 ohms between the two pins. Anything above or below indicates bad valve.

    If your valve passes the test, verify the connections are good. Then, with the car on, backprobe the frequency valve, test and verify
    -Battery voltage on Red/Purple wire
    -Pulses on Red/Brown Wire

    you may need a logic probe to see the pulses. DVOM or test light may not work. Harbor Freight has one for $10


    If you're missing battery voltage, issue is with the Lambda relay, or the wiring to/from Lambda Relay
    If you're missing pulses, or pulses are weak, issue is with the Lambda ECU

    If you need to check the ECU, remove the shell on the ECU connector and verify with car running:
    -Battery Voltage at PIN 8, Grn/Yellow. No Voltage? Lambda Relay or wiring
    -Ground at PINS 5 and 16, Black. No Ground? wiring
    -Pulses at PIN 15, Rd/Brown. No Pulse, or weak pulses? Bad ECU
    -You can also test feedback from 02 sensor at PIN 2. Should be between 0-1VDC between PIN 2 and Ground with the engine at operating temp

    Good Luck!
    Last edited by Ethan Rode; 01-14-2021 at 01:20 AM.

  4. #14
    Member Dhennick99's Avatar
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    So I got the frequency valve to buzz and it's working. I was able to set the fuel mixture using a dwell. I have it right in the sweet spot, but the idle is still super high in neutral and park (I have an automatic). I followed a step by step procedure for setting the throttle and curb adjustment screws, but it's proving to be troublesome.

    I'm able to get it set nicely at rest but the bracket that the adjustment screws are mounted into moves slightly when you hit the gas and it throws the idle off. If that bracket would stay put, I think I might be out of the woods and have this issue resolved.

  5. #15
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    Quote Originally Posted by Dhennick99 View Post
    So I got the frequency valve to buzz and it's working. I was able to set the fuel mixture using a dwell. I have it right in the sweet spot, but the idle is still super high in neutral and park (I have an automatic). I followed a step by step procedure for setting the throttle and curb adjustment screws, but it's proving to be troublesome.

    I'm able to get it set nicely at rest but the bracket that the adjustment screws are mounted into moves slightly when you hit the gas and it throws the idle off. If that bracket would stay put, I think I might be out of the woods and have this issue resolved.
    That's great. What did the issue turn out to be?

    So, to set mixture properly, the idle issue needs to be solved. Service manual tells us that idle needs to be raised to 950ish to check mixture using a dwell meter. You can back the idle down to spec after mixture is set. The accuracy of the mixture setting is contingent upon everything else being right, and having a good O2 sensor, if we're using a dwell meter.

    if you're using Dave McKeen's idle ecu, you won't set idle speed with the screws anymore, FYI. You will need to get the throttle linkage issue sorted out, however. Sounds like you might need to pull the throttle body, and check that the ball stud is tight on the crank. There's a spring retaining plate behind the crank that gets wallowed out on the throttle shaft, too. Make sure all these parts are tight, a ball peen hammer will help. You can't have any slop in that linkage if you want a predictable idle. You may also want to verify that your idle speed thermistor is good.
    Last edited by Ethan Rode; 01-14-2021 at 02:54 AM.

  6. #16
    Delorean Guru
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    As Ethan said, you want to eliminate any play in the throttle linkage. That includes checking the ball ends of the quadrant link. You might have to disassemble, clean and lubricate the throttle spool and lube the throttle cable as per the recall. I am also curious what you did to get the frequency valve to buzz. Get that bracket tight too. Your idle motor may need to be cleaned. Make sure the air bleed screws are lightly seated.
    David Teitelbaum

  7. #17
    Member Dhennick99's Avatar
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    I think I may have gotten a handle on it. There was a vacuum leak, one of the lines connected to the T right in front of the idle motor was disconnected. The car was running very lean, once I got that sorted I was able to get a dwell reading. It was trial and error with the throttle and curb idle screws. Once I found the sweet spot, I locked down the nuts and set the mixture. Went on a short drive and the car seemed to be performing as normal. I also went back to the stock ecu.

    20210114_105832.jpg

  8. #18
    Delorean Guru
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    You don't just twiddle those screws. There is a procedure. If you don't have them right you can damage the throttle shaft, the stop screw is supposed to prevent you from overforcing the throttle shaft once the butterfly plates are closed. Then you set the idle switch so it just trips and not depress it any further or you damage the microswitch. There is a nice write up of the procedure somewhere.
    David Teitelbaum

  9. #19
    Member cis6409's Avatar
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    Glad to see your getting the issue sorted.

    What fixed the frequency valve? It would help other owners when similar issues come up.. Was it a wiring issue or was the valve itself bad?

    Shane
    only from the past can we choose the correct path for the future...

  10. #20
    Member Dhennick99's Avatar
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    Quote Originally Posted by David T View Post
    There is a nice write up of the procedure somewhere.
    If you find it, let me know.

    As far as the frequency valve is concerned, once I reattached the vacuum line and richened the fuel mixture I started to get a reading from the dwell.

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