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Thread: M30 Wizards Please help :-)

  1. #1
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    M30 Wizards Please help :-)

    1984 733I

    Issue.

    1. Car runs amazing as it warms and even for quite some time at Normal operating temperature. Occasional stumbles do happen like a miss but generally throttle is smooth with good power.

    2. After some time at normal operating temp whether at a steady 75 on the highway, or around town the car just loses power and does not want to rev.

    3 I turn the car off and back on again immediately and normal operation returns for a short period of time before it does it again.


    Car has 1 fuel pump conversion, Removed the fuel strainer, fuel Filter is new and facing the proper direction.
    AFM has been cleaned and re-tracked.
    All vacuum hoses in the engine bay have been replaced.
    Coil has been replaced, new distributer and wires.

    When the car gets to normal operating temp eventually it just poops out.

    Not sure where to go with this one. it seems like fuel, but that should be all the time then not just at operating temp and that can't explain the return to normal after restarting the car.

    Any help on where to start here would be awesome!

    Thanks

  2. #2
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    00 MRoadster '83 745i,
    Put a fuel pressure gauge on it. May not be the issue, but you need to eliminate it.

  3. #3
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    What PSI should the 733 be?

  4. #4
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    Ran a fuel pressure test today. Engine off tester shows 33 or so psi. Engine idling pressure shows 30. Remove the vacuum hose off the regulator and it goes up to 33 or so. Throttle bums pressure down a touch momentarily and then it returns. It seems to have constant pressure and the regulator seems to work as its supposed to.

    Every so often some sort of mode switches to some sort of idle mode and then the car just runs like its stumbling. The video below shows what I mean at the end.

    https://drive.google.com/file/d/1Rm5...ew?usp=sharing

    It would seem that it's not a fuel system issue, although I did not drive the car and put it under the same conditions as I experienced the problem yet. Still its a new fuel pump so I'm inclined to believe fuel delivery is not causing my problem.

    What should I test next that could be causing the issue and thanks for the help!

  5. #5
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    1997 m3 4dr - e28.....
    cant say i ve had this issue with a m30 but with an m20 in an e28 i had a very similar issue with the coolant temp sensor intermittently freaking out. didnt download your video but your description sounds familiar. took me ages to figure out

  6. #6
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    I just replaced that today so i'll have to take it out for an extended drive tomorrow and see what happens.

  7. #7
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    Took the car out for an extended ride today. Hooked up the Fuel pressure gauge and put it so i could see it through the windshield. We are losing fuel pressure over time as the car runs. dropped as low as 15 psi at one point. Turn the car off for a second and then back on and it returns to normal for a short time. Let the car sit for 15 minutes and it will be fine for a bit longer.

    My running theory on this. The car actually did this when I bought it and it was running before I did the full engine rebuild and replaced LOTS of parts. So if I eliminate the stuff I replaced and figure it was something on the car we have the fuel pressure regulator which seems to work correctly after testing. Possible vacuum issue effecting the fuel pressure reg as the car heats up. A clog in one of the lines but that would not likely be effected by heat or use. and the tank.

    Here is my running theory and please correct me or point me in the proper direction is you think I'm off base here. I think there is rust and crap in the fuel tank. over time the pump sucks it up to the filter and it effects how much fuel is feeding into the pump lowering the fuel pressure. I turn off the pump for a moment and some of it drops back down to the bottom of the tank. when i turn it back on the cycle start over again.

    My evaporative tank lines all leak so i have to do that job anyway to replace those, so all I can do is drop the tank and have a look.

    I would love to hear your thoughts on this. and especially anything else you might this is the cause now that we know its fuel pressure.

    Thanks!

  8. #8
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    00 MRoadster '83 745i,
    Very UNLIKELY to be the fpr, as it normally causes too much fuel pressure when malfunctioning.

    You may be on the right track. You will know as soon as you pull out the pump and sending unit assembly.

  9. #9
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    Fuel Pressure issue resolved!

    Unfortunately it wasn't a dirty tank. Pulled the tank and it looked pretty clean actually. Rusted out evap lines needed tending anyway so I fixed that and replaced all the evap hoses.

    Turns out it was the fuel pump....Sort of.

    So I did the 2 fuel pump conversion to one. But I used the old wiring and just hooked it up. Turns out old wires and powerful pumps don't necessarily get along. What was happening is the old wires would heat up and act as resistors causing the pump to get less juice than it needed. I rewired thee Fuel pump, and drove the crap out of it for an hour and never lost pressure, and it was a HOT one today like 98 so if it was heat related it would have happened today.

    The problem is the car still stumbles all over the place. It just doesn't rev freely. I feel like if I pull some throttle and re-apply it helps. sort of. There is still something not right.

    What could be causing this issue? I can't imagine its vacuum related because all the hoses are good and the fuel pressure regulator is getting the proper vacuum.

    New distributer, new plugs and wires, new reference and speed sensor.

    I've cleaned the AFM multiple times and re-tracked it.

    All I got is the motronic brain and the throttle position sensor as possible causes. Im guessing there are a lot more things that could cause it though so Please let me know what you think it could be.

    Thanks!

  10. #10
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    Tested out the TPS today. It reads properly. so that leaves the AFM or the Motronic brain unless there is something I am missing in the equation here. Ts there something else I should be testing?

    Thanks

  11. #11
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    00 MRoadster '83 745i,
    What is the number on the ecm?

    What is the number on the AFM?

  12. #12
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    What is the history on the car as far as how long you have had it and how often is it driven. Based on your descriptions my first thought would be distributor or spark related. Cars that aren't driven often can have the issues you describe where they drive fine until they get hot then start to misfire. It can also get worse to the point that car won't start or it will backfire. Sounds like you already replaced that and checked everything else so unfortunately I have nothing else to add.


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  13. #13
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    Decided to go parts cannon on the car at this point.

    @lynnbilodeau I dont have much history on the car. I just finished a complete rebuild of the engine and got it all back together so most everything is new and in just trying to get it running proper.

    I replaced the Motronic brain and the Maf with rebuilt units and the car now goes great. The only remaining issue is a crap idle. It try's to do the 800 rpm idle but its just rough like some cylinders aren't firing. I blip the throttle and it settles in at 1100 or so rpm and its fine.

    I would love to solve the idle issue if anyone has any ideas on it.

    Now the only non functional system on the car is the cruise control. I'll tackle that at some point. Thanks for all the input and please let me know if you have an idea of what causes the car to miss when it trys to idle.

    Thanks!

  14. #14
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    Took the car out for a long trip today. Ended up with the exact same issue it just took an hour to show up instead of 30 minutes. I push the peddle to the floor and nothing. its like someone disconnected the throttle cable. I turn the car off and back on and it returns to normal for a short period of time.

    I really need help with this one. I dont know what else to test.

    New AFM
    New Motronic Brain
    Fuel pump Rewired. (One pump conversion)

    The issue seems exactly the same as it did before where the car would lose fuel pressure, but the pump is now rewired. The fuel tank is clean, the fuel filter is new.
    Rotor new, cap new, plugs new, wires new. I know new parts can be bad for sure but something has to make sense to exhibit the issue when at running temp, and it ran fine today at running temp for an hour before exhibiting the issue. I am actually thinking its the fuel pump overheating or something at this point, but I don't know where to start.

    Thanks for the help in advance.

  15. #15
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    00 MRoadster '83 745i,
    If it is acting the same way, you need to look for the same thing: falling fuel pressure.

    Although I said it was highly unlikely the FPR, it is still possible.

    Hook up a fuel pressure gauge. When it starts to drop, get out and crimp the return line with a pair of pliers. If you are worried about damaging the hose, put a rag or something it the jaws to keep from pinching. They make pliers just for such applications, but I wouldn't buy a pair just for this test.

    If the fuel pressure immediately rises, you found your problem. If it doesn't, then suspect a collapsing fuel hose between the tank and the pump. I think you already ruled out any trash on the bottom of the tank plugging up the screen on the pickup, right?

  16. #16
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    Thanks for the reply. I did fully clean the tank a week or so ago.

    I hooked up the fuel pressure tester again and sure enough its losing fuel pressure again. I have a theory so I tested the charging system. My thought was that maybe the battery is running down over time and not giving the pump the power it needs to run properly since I've already rewired it.

    A few tests later and it does seem that with lights on, the AC, and the stereo the alternator was not providing enough juice to keep a full charge, but it also wasn't dropping below 13 volts. I don't use the car at night so that situation with lights on was extremely unlikely but I replaced the alternator and it seems to provide just enough juice at this point. I may need to replace some of the old wiring directly off the alternator and the battery and that may give it a bit of a boost, but ill probably do an alternator update at some point.

    I did replace the fuel pressure regulator and it didn't fix the issue. :-(

    I have a new Fuel pump coming tomorrow and I will replace the fuel filter again at the same time.

    The collapsing hose thing it kinda interesting. Most of the rubber hosing has been replaced so It's pretty unlikely, also it only has one pump. I did the duel pump delete, so it only has a high pressure pump in the tank. After tomorrow I am really running out of places to look.

    Hopefully the new pump fixes the issue and it was just defective.

    Thanks again for the help.

  17. #17
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    Installed the new fuel pump. Something interesting happened. I suppose this was a question I should have asked before but never thought about it.

    The fuel pressure gauge maintained pressure after the car was turned off. In fact most of the pressure for a really long time. I am new to this so I didn't know that was a thing.

    I am assuming the rail is supposed to hold pressure after the car is off. It makes sense that when it does, the pump doesn't have to work nearly as hard to maintain the pressure.

    I didn't have time to take the car out and see if it solves the issue because its developed a massive exhaust leak I need to take care of first, but hopefully tomorrow.

  18. #18
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    There should be a pressure kept after switching engine off to prevent fuel from boiling while engine is still hot. Second, as you stated, pump does not have to build a pressure from zero each time engine is cranked.

    I don't know e23 but for e32 there is a relay for fuel pump only.
    Long shot but my mirrors relay was providing 12v but not enough amperage. Just a thought

  19. #19
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    Thanks for the reply @zapass. I kinda figured the system was meant to continue to hold pressure after the car was off. I just never thought about it until I saw it actually happen with the new pump.

    The Fuel pump has a dedicated relay and has actually been completely rewired. :-)

    Car is at the muffler shop but I should get it back tomorrow to really test it out and see if the issue is resolved. With the exhaust leak it was idling really rough.

  20. #20
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    Yes, on most bimmers, the system holds pressure after shut off. I pulled a fuel line off a 535i that had been sitting in the back lot at the shop for two years. Still had pressure.

    As for the rest:

    I hooked up the fuel pressure tester again and sure enough its losing fuel pressure again. I have a theory so I tested the charging system. My thought was that maybe the battery is running down over time and not giving the pump the power it needs to run properly since I've already rewired it.

    If you aren't dropping below 13 volts, then the alternator is not the problem for your drop in fuel pressure. IMHO, even at 11 volts, it would pump plenty. A drop to 11 could cause other issues with sensors that get reference signals. But I would bet money that the pump would still give you at least 50 psi no problem.

    I did replace the fuel pressure regulator and it didn't fix the issue. :-(

    Did you first pinch off the return line, like I suggested? If you had, you most likely would have seen that it didn't matter.

    I have a new Fuel pump coming tomorrow and I will replace the fuel filter again at the same time.

    Hopefully the new pump fixes the issue and it was just defective.

    I am not saying a new pump won't fix it; but I have never seen a pump that "lost pressure" all on its own. It is normally something else. Look for a kinked or partially pinched hose between the tank and the fuel pump. Hard to diagnose on line, but I REALLY suspect something before the pump. Also, I don't remember how long it runs before it loses pressure.

  21. #21
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    @lynnbulodeau

    Thanks for the reply.

    The system never held pressure until I replaced the fuel pump so that had to be part of the problem. After replacing the fuel pump I flogged the car for a couple hours and was convinced the problem was resolved.

    On the way home it lost pressure again but this time it was different. It lost pressure and just wouldn't start.

    To me this was the greatest result I could have gotten. I figured it has to be a line blockage somewhere.

    Towed the car home and set about replacing every soft line and using pressurized air to blow out the hard lines. I really think I blew out a lot of old crap in the line. or it least it seemed so.

    Hooked everything back up and sure enough the pump is only giving me 20PSI on the fuel pressure guage.

    Pulled the in tank pump and the rubber hose was blown. Replaced the hose, put the pump back in and for the first time got a touch over 40 PSI with the pump on. Before this point I never got over 30. turned off the pump and the system holds pressure. Starts right up and runs better than ever, at least in the garage.

    I have thought this issue was fixed so many times now I'm not even gonna say it. lol I'll take it out and run it hard tomorrow and see if the issue re-appears, but fingers crossed.

  22. #22
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    Hopefully new pump and lines takes care of it.

  23. #23
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    On e32 forum there is a thread about submersible fuel hoses, maybe you will find it useful:
    https://www.bimmerforums.com/forum/s...-BMW-fuel-hose

  24. #24
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    Very Useful! Thanks I'm hoping to have this sorted tomorrow.

  25. #25
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    I think its FINALY sorted.

    I had to take the time to modify the pump housing. This time I brazed on a smaller nipple that matched the size of the one coming off the pump.

    I never did anything like this before, but it wasnt that hard. A torch, lots of flux and silver solder did the trick.

    It took a few times to get it totally sealed. I would braise it and then vacuum test it. if it didn't hold vacuum I would coat the area with flux and apply vacuum again. That would suck the the flux into the cracks and made it easier for the solder to flow in there and seal.

    once it held vacuum it was mission accomplished.

    I used an in tank rated hose and put everything back together.

    fuel pressure tester shows good pressure and holds it for a really long time after the car is off.

    75 mile drive on a really hot day with no issues what so ever. Felt pretty dam good.

    Other than a crap Idle its ready to roll.

    Have a smoke tester ready to test the intake and exhaust to try to see if there is a leak causing the rough idle, but that is for another thread.

    Thanks for all the feedback and help on this. I couldn't have figured it out without this forum.

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