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Thread: Dwell Stuck at 50% am I correct to suspect the O2 sensor?

  1. #1
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    Question Dwell Stuck at 50% am I correct to suspect the O2 sensor?

    My car has a very hard time starting. I have aftermarket cams which make the idle very lopey, I don't know for sure but I'm guessing this means I need more fuel than an OEM cam'd car. I put an o-scope and the dwell meter on the thing today and it's stuck at 50% duty cycle when the car is completely warmed up.

    Anyone know if I can replace the lambda o2 sensor with this cheapo one from AutoZone?

    Thanks,
    -Greg

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    Administrator Ron's Avatar
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    This is the one AutoZone says will fit.

    It basically has no affect on the engine while cold or during starting because the sensor must see 572F (300C) minimum...

    FWIW, steady 50% duty cycle could be:
    No signal from the O2 sensor (bad sensor or wiring open), WSM D:04:15
    WOT switch stuck/shorted internally, wiring shorted to ground. WSM D:04:15

  3. #3
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    tune work

    Quote Originally Posted by Ron View Post
    This is the one AutoZone says will fit.

    It basically has no affect on the engine while cold or during starting because the sensor must see 572F (300C) minimum...

    FWIW, steady 50% duty cycle could be:
    No signal from the O2 sensor (bad sensor or wiring open), WSM D:04:15
    WOT switch stuck/shorted internally, wiring shorted to ground. WSM D:04:15
    Thanks Ron, I went out and bought the O2 sensor I posted before for $16 and just cut the end off and soldered on the end from my old one. I took the car for a spin and got it all warmed up and then set up my o-scope to measure the dwell. I was able to tweak it into the a range of ~34% - ~58% according to the duty cycle calculation -- does that sound about right?

    Here's a video of my tune after warming it up:


    I'm curious about this because it seems like once I get the fuel mixture set properly it seems my car will not start. It's like it doesn't have enough cranking fuel. I'm wondering if it's because of the hot cams... unfortunately this has always been a problem so I'm not sure how to solve it properly. Any ideas?

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    hard starting

    Here's a video of the first two cold start attempts:

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    Under Ron's watchful eye. Glory be to Ron! Michael's Avatar
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    I find that anything other than an analogue meter is hard to set the mixture with but I'm not that smart. Watching that needle swing makes it easier (for me anyway).

    As far as your cold start goes, two or three things;
    Check your battery voltage and charge if needed. Sounds like it's cranking slow and your volt gauge looks pretty low. I found with mine that it started a lot easier when the alternator was functioning properly. Cleaning the ground on the frame below the expansion tank could also help as well as the one on the radiator bracket.

    Also look at the o rings in the primary pressure regulator in the fuel distributer. If they are old just replace them because even the smallest imperfection or damage can cause a hard hot start but if they are really bad it will also affect cold start. Thing is there is no way to really check them without removing them and they get hard and brittle when old. It's a cheap and very easy fix but be sure you know that the plunger is spring loaded when you remove that nut.

    I would not worry too much about mixture setting right now. Once you fix your cold start issue and possible other problems such as a vacuum leak, etc. The mixture is usually the last thing you do.
    Last edited by Michael; 01-05-2020 at 09:32 PM.

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    By hot cams, you mean non-OE cams? Timing could be a factor.

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    Hard cold starting usually means a problem with the cold start valve or other associated parts like the TTS or the wiring. If you have the duty cycle set, leave it alone. If you have after-market cams you may need to adjust the base timing. While checking the timing you should check the mechanical and vacuum advance to make sure they are working properly. The dwell or O2 sensor has no effect on cold starts. If the battery or any connections are not good and the voltage drops low enough the ignition ECU won't work so that means no spark and no start. The main thing that controls the motor during warm-up is the CPR. To test if the cold start system is working, do the plug swap. If the cold motor starts easier then figure out why the CSV is not working right.
    David Teitelbaum

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    Administrator Ron's Avatar
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    Quote Originally Posted by spikeygg View Post
    Thanks Ron, I went out and bought the O2 sensor I posted before for $16 and just cut the end off and soldered on the end from my old one. I took the car for a spin and got it all warmed up and then set up my o-scope to measure the dwell. I was able to tweak it into the a range of ~34% - ~58% according to the duty cycle calculation -- does that sound about right?

    Here's a video of my tune after warming it up:

    I'm curious about this because it seems like once I get the fuel mixture set properly it seems my car will not start. It's like it doesn't have enough cranking fuel. I'm wondering if it's because of the hot cams... unfortunately this has always been a problem so I'm not sure how to solve it properly. Any ideas?
    Yes, that sounds very close to ideal for stock DutyCycle (pulsating, centered at 44.44%, hot).

    Seems to be some confusion... Are/were you having starting problems hot and cold?

    Your temp, volts, and RPM all look low to me in the vid.
    Let the fans cycle a couple of times to ensure everything is warm. Use a reliable meter for volts & RPM. Set CO last, as Michael suggested, ALWAYS. (Again, it's not the cause of starting problems.)

    If all of the basic settings are to spec, I'd suggest checking the Control and Primary pressures (Don't disturb the PPR valve unless pressure is incorrect.)

    At this point, I agree with the WUR/CPR or timing as primary suspects. If cold start problems only, I'd add the CSV as David suggested.

  9. #9
    Senior Member Bitsyncmaster's Avatar
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    Electronic instruments use positive pulse timing for duty cycle numbers and automotive instruments use the negative pulses. So when your scope says 40% your really at 60%. With your wide open throttle switch pressed you are running at 60% solid. So press it and I bet your scope says 40%.

    So your looking to have the scope reading centered around 60%.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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    checking mixture with WOT switch enabled

    Quote Originally Posted by Bitsyncmaster View Post
    Electronic instruments use positive pulse timing for duty cycle numbers and automotive instruments use the negative pulses. So when your scope says 40% your really at 60%. With your wide open throttle switch pressed you are running at 60% solid. So press it and I bet your scope says 40%.

    So your looking to have the scope reading centered around 60%.
    Crap, I was worried about that. I can measure +Duty and/or -Duty on the scope and I chose +Duty because I wasn't sure if it should be high time or low time. Unfortunately, I removed my WOT switch from my car -- it was broken and causing other issues. Can I just ground the WOT pin to enable the WOT mode? Or maybe I need to short the two pins together? That should be an easy way to check.

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