Backfired while starting, now I have a miss!
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Northstar Engines and System Technical Discussion Discussion, Backfired while starting, now I have a miss! in Cadillac Engine Technical Discussion; While starting my 94 STS it backfired the other day and now I have a low RPM miss. WHen it's ...
  1. #1
    therodman is offline Cadillac Owners Fanatic
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    Backfired while starting, now I have a miss!

    While starting my 94 STS it backfired the other day and now I have a low RPM miss. WHen it's cold it sounds like 1 cylinder isn't firing at all until it warms up a bit, then it seems to be hitting on all 8 cyinders. However, while idling, it'll miss every few seconds, maybe 5 to 10 seconds and the RPM's will jump up from about 600 to 800 and then idle back down until it misses again. When I start to drive the car, it'll miss just a little bit at low RPM's but once moving with the RPM's up a bit, the miss goes away.

    I pulled all the plugs and visually, they all look good, the wires also look okay. I know that you can't tell the true condition of a component by the looks but I guess what I'm saying is that there is nothing obvious from a visual inspection!

    The car has never missed before and it started right after the backfire incident so I have to assume the backfire damaged something. I pulled the beauty cover and once again, everything looked just fine. By the way, is the map sensor under the beauty cover near the throttle bore end. If so, is it the little black box sitting on top of the fuel rail. What is that little thing next to the little black box which is pointed downwards and has a set of wires leading into it, on the bottom end it looks like a little loop of brass or copper hanging downward.

    Anyhow, my take is that the blast damaged something and my thinking is that it's a spark plug or a sensor if there is one inside the intake where the blast occurred! I don't see how the blast could've damaged something outside of the intake, for example, a plug wire!

    Anyone with any suggestions or answers would be greatly appreciated!!

    Thanks in advance.....

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  3. #2
    fleishman is offline Cadillac Owners Fanatic
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    Re: Backfired while starting, now I have a miss!

    Unfortunately I do not have any advice for you, but I do have a 94 SLS parts car if you end up needing to replace anything!

    Good luck!

  4. #3
    jackc is offline Cadillac Owners Member
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    Re: Backfired while starting, now I have a miss!

    Quote Originally Posted by therodman View Post
    is the map sensor under the beauty cover near the throttle bore end. If so, is it the little black box sitting on top of the fuel rail.

    Yes - that's the MAP.



    What is that little thing next to the little black box which is pointed downwards and has a set of wires leading into it, on the bottom end it looks like a little loop of brass or copper hanging downward.


    That's probably the fuel pressure regulator - but wires don't go to it. I think what you're seeing are the injector wires


    Anyhow, my take is that the blast damaged something and my thinking is that it's a spark plug or a sensor if there is one inside the intake where the blast occurred! I don't see how the blast could've damaged something outside of the intake, for example, a plug wire!


    You may have fractured a plug or blown crud into an injector.

    Use the power balance test to try and find the cylinder that's giving you problems.

    Anyone with any suggestions or answers would be greatly appreciated!!

    Thanks in advance.....


    3333

  5. #4
    therodman is offline Cadillac Owners Fanatic
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    Talking Re: Backfired while starting, now I have a miss!

    I found the problem to be a spark plug, I decided to start there and changed 1 at a time starting on the front bank of course. I started the engine after changing the #2 plug, still missed. I started the engine after changing the #4 plug and she purrrrrred like a kitten. There has to be crack or something similar which was causing that plug to short out or not fire. At least it was a simple problem and I'm back on the rode again!

    Thanks Jackc for your reply, I will say to the following,

    What is that little thing next to the little black box which is pointed downwards and has a set of wires leading into it, on the bottom end it looks like a little loop of brass or copper hanging downward.
    That's probably the fuel pressure regulator - but wires don't go to it. I think what you're seeing are the injector wires

    that it's not the fuel regulator that I was looking and it wasn't injector wires either. It's right alondside the MAP sensor, the plug goes infrom the top and plugs in straight down, It's a small 2 wire plug and the thing it plugs into looks like a little loop kind of like a figure 8 of brass or copper, maybe about 1/2" long and 1/4" wide. I'm thinking maybe this little unit was surrounded by plastic and I'm looking at what is inside of whatever this is. The engine runs fine so I'm not worried about it but am still curios what it is.

    Of course I posted a reply with the solution to my problem so that others may benefit if they have a similar problem. I must've read 15-20 posts looking for a solution to my problem before I posted. Most of those posts became dead ends where the poster never replied with the answer or solution to their problem. One can assume if the last thing they tried was a spark plug for example and you never see another post from then afterwards, I guess you can assume that was the solution.

    It would be nice if everyone was curtious enough to get back on the computer and post the solution so that others may benefit and to also acknowledge those who took the tme to offer advice!

  6. #5
    jackc is offline Cadillac Owners Member
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    Re: Backfired while starting, now I have a miss!

    Quote Originally Posted by therodman View Post
    It's a small 2 wire plug and the thing it plugs into looks like a little loop kind of like a figure 8 of brass or copper, maybe about 1/2" long and 1/4" wide.
    Its been a few months since I was in mine, so I wasn't recalling that sensor.

    Checking the alldata diagram, it looks like thats the Intake Air Temp sensor.

  7. #6
    therodman is offline Cadillac Owners Fanatic
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    Re: Backfired while starting, now I have a miss!

    Thats what it looks like now that you mentioned a temp. sensor. I appreciate you looking for me, I was wondering what that could've been.

    I've fixed the missing problem but that leaves me with my original problem and that is hard starting once it's ben warmed up and sitting for an hour or two. When I first start the car cold, it starts right up, in fact when I turn the key on, I can hear the fuel pump pressurize the line at which time I turn the key to the start position and it starts right up. Once it's warm, it seems like the line doesn't pressurize up, I can hear the fuel pump just whirrring away.

    Once again, any suggestions would be welcomed!

    Thanks again jackc.....

  8. #7
    Zorb750 is offline Cadillac Owners Connoisseur
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    Re: Backfired while starting, now I have a miss!

    Quote Originally Posted by jackc View Post
    Its been a few months since I was in mine, so I wasn't recalling that sensor.

    Checking the alldata diagram, it looks like thats the Intake Air Temp sensor.
    Manifold Air Temperature. IAT is in the airbox.

  9. #8
    jackc is offline Cadillac Owners Member
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    Re: Backfired while starting, now I have a miss!

    Quote Originally Posted by therodman View Post
    Once it's warm, it seems like the line doesn't pressurize up, I can hear the fuel pump just whirrring away.
    First suspect would be the fuel pressure regulator. Could also be a leaking injector.

  10. #9
    jackc is offline Cadillac Owners Member
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    Re: Backfired while starting, now I have a miss!

    Quote Originally Posted by Zorb750 View Post
    Manifold Air Temperature. IAT is in the airbox.
    I'm looking at my 93 FSM now and it too is calling the sensor next to the MAP
    the IAT. The only thing in a 93 airbox is the filter. 94 should be the same. You may be thinking of a later model.

  11. #10
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    Re: Backfired while starting, now I have a miss!

    I've fixed the missing problem but that leaves me with my original problem and that is hard starting once it's ben warmed up and sitting for an hour or two.
    Classic FPR symptom.

  12. #11
    therodman is offline Cadillac Owners Fanatic
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    Re: Backfired while starting, now I have a miss!

    I believe I checked the FPR once before but may have received bad info on how to check it. I was told to take off the cover, turn the key on and then back off, if it was bad, fuel would come out the inlet where the vaccum line would be connected if there was a vacuum line. I guess I assumed since I didn't see any fuel, the FPR was good when maybe in fact it is bad!

    Do you suggest I change the FPR and go from there, I kind of remember the price being about $50 so even if it doesn't correct the problem it's not that big of a deal?

    Thanks Jackc and Ranger for your input!

  13. #12
    Ranger's Avatar
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    Re: Backfired while starting, now I have a miss!

    You checked it correctly. You can also do it with the engine at idle. If no fuel leaks from the nipple, it is good.

  14. #13
    therodman is offline Cadillac Owners Fanatic
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    Re: Backfired while starting, now I have a miss!

    I now have a new fuel pressure regulator and will be changing it soon, in the morning probably. I still have my doubts that the problem is the regulator so any suggestions to the contrary would be appreciated. I just don't see how a regulator could work properly when the engine is cold from sitting over night but when it's warmed up, sitting about 15-30 minutes up to a couple of hours, the enigine won't start and acts like it's flooded with the regulator being the culprit. To my knowledge, the regulator is nothing more than a diagphram with a spring. How could the rngine being warm cause it to fail?

    I asked a local mechanic what he thought and he said an engine temperature sensor may be faulty. He stated that it wasn't the same sensor that goes to the "gauge - readout" but rather something about the incorrect info getting to the computer. He said that is how the modern day cars with injectors are set up to start. The computer uses the engine temp info to either richen or lean out the mixture to start the car, a modern day choke system.

    Ranger mentioned to check the FRPR regulator at idle which is probably what I'll do. Remove the cover, start the engine and let it idle. I'll warm it up just a bit prior to removing the cover so that it doesn't want to idle a little higher on it's own. If no fule is leaking out of the nipple on the FPR, then I should assume that the FPR is good, is that correct?

    Let's assume that the FPR is good, where do I go next, what would be the very next thing to check?

    On another note, going back to the FPR, even if it did leak, let's say just a very tiny amount of fuel. How would that flood the engine because that fuel cannot make it's way into the injector tubes but rather lay at the bottom of the valley area. I'm having a hard time visualizing how excess fuel could get into an injector tube without it being the injector itself. Obviously I'm puzzled and that's why I'm here asking these questions. I trust everyones judegement who have responded but I like to know why, even if a certain part corrects a problem and that problem is resolved, I still want to know why or how that corrected the problem. I know, a lot of people take their car to a garage and if they drive away with everything working they don't care or want to know why, they just pay and go! I'm just the curious type I guess....

  15. #14
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    Re: Backfired while starting, now I have a miss!

    You are correct. The FPR is a diaphragm with fuel on on side and vacuum on the other. If the diaphragm leaks, fuel is sucked into the manifold and the engine runs rich. When you shut down, the 45 psi fuel pressure bleeds down via the leaky diaphragm and the raw fuel sits in the vacuum hose or in the manifold. On a hot start, the engine is very rich with extra unmetered fuel. It's much like starting a carbureted car hot with the chock on (rich).

  16. #15
    therodman is offline Cadillac Owners Fanatic
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    Talking Re: Backfired while starting, now I have a miss!

    Ranger and everyone who replied or helped in any way, a big thank you! I changed the FPR this morning , what an easy job, could not have taken more than 20 minutes. I never could make my old FPR leak out of the vacuum nipple but there was some gas residue at the base of the nipple. On my car the FPR sits with the vacuum nipple upright so if it leaked from there, it would leave some petro at the base of the nipple because it has a little bit of a bowl shape at the base. Didn't really make any difference what I thought or could find, I really had no choice being a backyard mechanic with no diagnostic equipment, I had to start somewhere.

    I can't believe I didn't do this a long time ago, it was so easy! Anyhow, I believe it has corrected the problem. So far my car has started the way it should with the most time sitting after warm up so far about 30 minutes. After 30 minutes sitting it started right up where prior to changing the FPR, I'm pretty sure after 30 minutes I would have to hold the accelerator to the floor and then crnak it a couple of times, maybe 3-5 seconds each crank prior to it starting, now 2 revolutions at the most and it's running, that's without touching the accelerator.

    If it starts like that after sitting an hour, then I know it's fixed. The fuel pump actually sounds a bit different when it pressures up, not quite as high a pitch, like it's not having to turn as much or fast to pressurize the line.

    I'll report back after a day or two to let you all know the final verdict. The backfire I'm pretty sure was a bad spark plug and maybe, just maybe, the FPR contributed to the backfire problem but I doubt it. Since I changed that 1 plug, no more backfires but I've also held the accelerator down to the floor everytime I've started it warm for the past month or two!

    Once again, thank you for the help!!

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