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Thread: Engine hesitates and eventually stalls

  1. #1
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    Engine hesitates and eventually stalls

    Hi fellow Sixers,

    I was driving the yesterday and while in 4th gear I felt the car losing power as I gave it more gas. It gradually slowed down and eventually stalled out BUT I am able to restart it in neutral and idling although very rough. But once I engaged into 1st gear and starts moving, the car would stall out again and the cycle continues. After a few tries, mostly normal power resumed and I was able to drive home but now with the Check Engine light on.

    I looked around upon parking the car but don't see any leaks, any smoke or anything obvious that jumped out (and yes I do have gas in the tank). The car will start fine with the check engine light on and I need some help in diagnosing the issue. My initial feeling in something fuel related. Maybe dying fuel pump, fuel pressure regulator, etc. I will check the plugs and air filter but they are fairly new. Alternator maybe? But I would think the charging light would come on and not behave like it did.

    Any other ideas I should look out for? Thanks all.

    88 635 5MT

  2. #2
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    Could also be vacuum leak. When you get it started, pull out the dipstick. Does the rpm drop noticeably? Normally it should. If you have vacuum leak it will not. Check that, but I think you are right about fuel issue.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

  3. #3
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    I can’t remember the symptom but back in the day the airflow meter would have breaks in the copper tracks under the black cap and the airflow meter door has an arm inside that moves against the track well over time the tracks get worn thru so it will have a brief continuity break then as it goes passed the break the continuity continues the techs would check it with a volt ohm meter

  4. #4
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    With the engine running I pulled the dipstick and the engine sputters with slight drop in RPM. I rev it up a few times and could smell gas fumes but no leaks. But you could tell that it's not on full power.

    What kind of diagnostic tool are people using today? I remember the PEAK tool back in the day but I'm hearing Schwaben is the new scan tool now with the 20-pin adaptor? Wonder if it's worth the investment.

  5. #5
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    Quote Originally Posted by conditioner View Post
    With the engine running I pulled the dipstick and the engine sputters with slight drop in RPM. I rev it up a few times and could smell gas fumes but no leaks. But you could tell that it's not on full power.

    What kind of diagnostic tool are people using today? I remember the PEAK tool back in the day but I'm hearing Schwaben is the new scan tool now with the 20-pin adaptor? Wonder if it's worth the investment.
    If you suspect fuel related issues, and I second that, get yourself a fuel pressure gauge. The engine can usually idle ok with slight or no pressure but when demanding more fuel under power is where it will cough and die. Also, if your engine is a b34, check the two sensors mounted on the bell housing and their associated plastic connectors up top on the aft intake manifold strut. The two sensors are needed for start and run.
    Disregard the last statement. I see you have and 88 so it will be a B35 with a single sensor up front on the toothed vibe dampner. The fuel issue still stands though.
    Last edited by carsnplanes; 10-27-2020 at 09:22 AM.
    '88 635, '92 325IC

  6. #6
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    Fired up the car this morning and drove around a few blocks with some moderate acceleration, no issues. Check engine light is also off. Hmm...Now i'm really nervous about taking it any where a few miles away. I did hear some clanking noise around the flywheel area when turning over but no hesitation or hard start. I pulled the vacuum hose off the fuel pressure regulator and did not see any fuel so I'm guessing the FPR is good. The fuel filter has not been replaced in a while so I'm thinking doing that and maybe pre-emptively replacing the in-line exterior pump? Any thoughts?

  7. #7
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    Check the fuel pressure with a gauge, if it's not pumping correct
    pressure then look at the FPR and filter first before changing the pump.
    You should confirm it's the pump for sure as it may not solve your problem.
    Taking the vacuum line off changes the pressure. Gas usually doesn't
    leak from there.
    Last edited by 1986series6; 10-27-2020 at 08:45 AM.

  8. #8
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    "fuel issues" doesn't necessarily just mean fuel pressure or pump related. It could mean venting, or water , or clogged pump strainer/sock that releases its crap over night, only to be sucked back up again and clog the sock the next day or day after. It could be any of those things. The engine light means something affected the firing (misfire) of the engine and even a bad batch of gas with water could do that. If you don't know the condition of the external fuel filter, that's a good place to start to get a known baseline condition of it. Also for venting, make sure your gas cap vent is free and clear. Drive with it off for a few miles if you have to just to test. A faulty FPR can be a sticky valve seat not just a torn diaphragm and leaking fuel. At low vacuum settings, it should more or less block off the return to keep fuel in the rail. Other areas are, torn intake boot. I doubt that as you say your car starts right up.
    The B35 is pretty good as there are fewer sensors with the 1.3 Motronic than it predecessor B34 with Cold Start Valve, two bell housing sensors, Thermo time valve and sensor etc.
    Definitely put a gauge on it though to know that when it does work, what is its delivery pressure that you are working with.
    Last edited by carsnplanes; 10-27-2020 at 09:23 AM.
    '88 635, '92 325IC

  9. #9
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    Jody wou;d a "smogged" car have a vented gas cap" Here in AZ when you go through the smog test they use their own gas cap to make sure there is no venting there.
    81 Euro undergoing total nut and bolt restoration
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  10. #10
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    Quote Originally Posted by Bert Poliakoff View Post
    Jody wou;d a "smogged" car have a vented gas cap" Here in AZ when you go through the smog test they use their own gas cap to make sure there is no venting there.
    Bert,
    Venting for gas tanks and caps would be negative pressure venting as in being able to suck/breathe in air but not necessarily expel it. Expelling it would be through the evap system /charcoal canister. The cap should have a one way valve in it and if blocked, will suffocate the tank.
    '88 635, '92 325IC

  11. #11
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    If you want to test your fuel pump there is a very easy test to do. Most people refer to fuel pressure, which is important but what is more important is flow. As carsnplanes said about increasing demand when you put your foot down.When you buy an up rated pump they are referred to in gallons per hour. In Europe litres per minute.
    In a pump as velocity increases pressure decreases (Bernoulli principle). So the pump is rated at a certain flow rate at a given pressure. As the flow of fuel increases the pressure drops. Think of putting your thumb over the end of a hose pipe, the pressure increases in the hose. Remove your thumb and the pressure drops and the flow increases. A nozzle can be used to convert the pressure into velocity to project the water further.

    The specs for the BMW pump are
    Voltage 12volts +/-05volts (car not running)
    Delivery pressure 2.5 -3.5bar (controlled by the fuel pressure regulator)
    Flow rate (to 1986) 2.2 litres a minute minimum
    Flow rate (1986 on) 1.9 litres a minute minimum

    So if you disconnect the fuel supply hose and run the pump it should fill a US gallon container (3.79litres) in two minutes (1986 on) and less than two minutes (pre 1986)
    If your pump can do that then there is not a problem with the pump and pre supply pump if fitted. You can then eliminate it as the problem.

    The next thing is to check the regulated pressure with a gauge. If you do not have a gauge run the pump and check that you can hear/feel the fuel returning to the tank. I tells you that the regulator is controlling the pressure and the return line is not blocked. It does not tell you if it is the correct operating pressure in the fuel rail.

    Finding the problem is a process of elimination the "Sherlock Holmes approach"

    "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?



    I never guess. It is a shocking habit destructive to the logical faculty.




    You see, but you do not observe."



    Arthur Conan Doyle.

  12. #12
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    So...going back to the original subject line; "Engine hesitates then eventually stalls". This is usually not a behavior of a bad or faulty TPS. Take each function of the TPS for a minute to see if each of them are functioning as expected.
    Idle phase: when at idle stop, audible click, TPS internal switch completes a ground to an input to the ECU to tell the ECU to send a proper idle mixture of fuel to the injectors.
    Mid range or off-idle range: no input to ECU, This range between idle and WOT is a pre-programmed average mixture that is right for normal part throttle driving.
    Wide Open Throttle WOT: This completes a different ground input to the ECU to tell the ECU it requires a richer mixture and to adjust timing for a full load demand.

    Now let's look at a defective TPS:
    Defective Idle stop: Improper idle mixture at idle, so the engine could run richer than normal as the ECU doesn't know it's at the idle stop.
    Mid range area: (TPS area between idle and WOT) There should be no change as there is pre-prgrammed mapping of fuel and spark and no input to the ECU, however if ECU is receiving an improper ground of either idle or WOT in this free range area, the part throttle drivability could be affected such as too lean at part throttle or too rich. This could happen if any of those two wire inputs, idle or WOT are chafed to ground somewhere.
    No WOT signal: the ECU will not know a change in fuel and timing is required and you will at worst get sluggish full-load condition or pinging due to a lean mixture with your foot to the floor.

    These faults still should not cause "hesitation or eventual stalling".
    "Eventual stalling" makes me think of a gradual depletion of fuel. You didn't say when this eventual stalling happens. During idle or part throttle or WOT.
    Last edited by carsnplanes; 10-29-2020 at 09:07 AM.
    '88 635, '92 325IC

  13. #13
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    Thanks all for chiming in. So it appears the car is back to normal again after driving it a few times. But I'm still a little nervous. I've had this car for about 15 years and never have had to replace anything fuel related other than the fuel filter and a couple of fuel lines which I did a few years ago. So it's possible the filter is getting dirty, bad gas or one of the fuel delivery parts is getting weak. I plan to keep it for a while and might just pre-emptively replace some parts. I think I'll do the filter and pump at the same time since it's next to each other.

    -carsnplanes : I never considered the TPS as a possible cause but since you addressed it, it's something to think about. But to recap, I'm driving in 4th gear around 45mph, I give it some gas to accelerate and suddenly felt a loss of power and the car begins to slow despite giving it more gas ( never WOT at any time). After a few hundred yards, the engine dies, stops and I turn the car off. I immediately turn it back on and it starts fine but soon as I engage into first gear with some gas, again the engine dies. After 3-4 times like this, the car regains power as "normal" and I was able to drive home without incident. So far so good (fingers crossed).

  14. #14
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    Quote Originally Posted by conditioner View Post
    Thanks all for chiming in. So it appears the car is back to normal again after driving it a few times. But I'm still a little nervous. I've had this car for about 15 years and never have had to replace anything fuel related other than the fuel filter and a couple of fuel lines which I did a few years ago. So it's possible the filter is getting dirty, bad gas or one of the fuel delivery parts is getting weak. I plan to keep it for a while and might just pre-emptively replace some parts. I think I'll do the filter and pump at the same time since it's next to each other.

    -carsnplanes : I never considered the TPS as a possible cause but since you addressed it, it's something to think about. But to recap, I'm driving in 4th gear around 45mph, I give it some gas to accelerate and suddenly felt a loss of power and the car begins to slow despite giving it more gas ( never WOT at any time). After a few hundred yards, the engine dies, stops and I turn the car off. I immediately turn it back on and it starts fine but soon as I engage into first gear with some gas, again the engine dies. After 3-4 times like this, the car regains power as "normal" and I was able to drive home without incident. So far so good (fingers crossed).
    Conditioner, Yes, the TPS was one of the items on somebody's list in the thread and while we all were taking notes, just wanted to basically rule that out out based on the fact that your symptoms didn't really match the failure of that component. I agree it feels more like a fuel delivery issue. I have had water that caused more of a stumbling rather than a complete and eventual shutdown. It still may be bad fuel or a clogged filter or pump filter sock.
    '88 635, '92 325IC

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