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325ix hard start - poor MPG

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    325ix hard start - poor MPG


    Recently have developed a slight problem with my m20.
    Sometimes, hot starts would require a blip or two of the throttle to get it going, but it was extremely inconsistent and not sure what caused it. But yesterday, the car was cold, about 32* and it took about 30 seconds of accelerator fluttering to get it to finally start, and when it did, it took a minute or two to clean up.

    The car runs fine and feels like typical m20 power, but MPG has been steadily declining over the past 2 months or so.

    Recent maintenance:

    Cap and rotor
    Timing belt
    Water pump
    Intake boot
    Valve adjustment

    The issue was present before these maintenance items, so I don't think that's the cause.

    Any idea where I should start? I am thinking a fuel filter and o2 sensor, mainly because I don't know when they were done last.


    You are getting too much fuel in the cold start process. See if your intake boot elbow has raw fuel in it. It could be a leaky injector.


      Thanks for the tip! Will check that tonight. Leaky injectors could definitely be the case, if so, should I replace them, rebuild them, or upgrade to better injectors? I am not looking to push every HP out of this car. MPG is more important to me.


        If it were me, I would send the injectors to a professional shop to be cleaned and balanced. If the injector is bad/leaky, they can tell you and then I would replace it.


          Some other components that can cause hard start are:
          CPS - Good idea to replace on any vintage BMW.
          AFM - cleaning inside the port will do nothing. Good AFM's are ones that have been refurbished. No such thing as a "working or good" 30 year old AFM.
          Vacuum leaks - Very common and well known issue
          TPS sensor - rarely fail but can become out of adjustment. If this is not adjusted, your DME will never know its at idle.
          DME temp sensor - this tells your DME the temp of your coolant, important for starting and fuel control.
          Failing DME - change in temp can make the DME work well or not work well. Copper oxidation can also make a DME fail.
          Failing engine ground cable - consider replacing or checking this cable as its responsible for grounding all engine electronics. Cold moist mornings can cause excessive resistance between the cable and chassis ground.
          Weak starter - I have seen new or rebuilt starters significantly decrease engine start times but least likely with your symptoms.

          Maybe a few other things that others can chime in if I forgot.

          Good luck.
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            Thanks both for the input. I am going to do all new vac lines, rebuilt injectors, fuel filter,a fuel lines and an o2 sensor.

            What else do I need when doing the injectors? Any other gaskets?


              I am not sure when you send them off if they come back with new injector o-rings or not. If not, you will need those. Before you pull the injectors, I suggest you vacuum off or blow compressed air over the injector ports to keep debris from falling into the engine. If its gunky, try and clean it up as much as possible. You will have to remove the valve cover vent hose, if its old, it will most likely crack or break. May want to have a spare on hand.


                Installed rebuilt injectors. Did a few other things like vacuum lines and fuel lines, fuel filter etc.

                Getting good MPG now and great power, but the intermittent warm hard start issue is still there.

                I tested my TPS and it is good but I cannot get stomp codes to show. I even bought a new TPS just to test and still no go. Both TPSs were adjusted correctly and I did test them with a meter.

                Something wrong with my TPS wiring or something else going on? I don't have a check engine light on but I thought it would be a good way to test the TPS?


                  Verify your fuel pressure is constant, even when warm. My fuel pump developed a leak after it warmed up during use, causing pressure to drop and fuel to leak out of the pump. It was easy to find on my early model because the fuel pump is outside the car. Replacing it fixed the problem.

                  it's a Kenny Powers quote on wheels


                    Don't forget to check the aluminum elbow going into the throttle body. It is a common problem with "hot start" issues. This elbow can come loose when the engine gets hot and creates a vacuum leak.


                      Good to know. Thanks both of you. Any ideas what's going on with the TPS?


                        Start at the TPS. SThe crankcase venting tends to dump oil in them and ruin the switch inside. The m20/m30 TPS is actually a 3 way switch, not a potentiometer as in most vehicles. It has a switch inside to tell the ECU to go to the idle table, and if that switch fails then the ECU "thinks" your foot is slightly on the accelerator pedal and is giving the fuel to suit. When the WOT switch is faulty, the car will lack power as the ECU will never jump to the WOT map where the timing is much more advanced (and sprays fuel to suit).

                        If your TPS is working, there's two other places the ECU uses to richen up the mixture for cold starts. The blue coolant temp sensor has a LOT to do with fueling, there's actually a multiplier on the math in the ECU maps for it. Finally, the IAT sensor that's inside the AFM - which has far less to do directly with the ECU fueling, but is still temp related.

                        Also, don't dismiss the IAC. If it's sticky in the near or closed position, the car will run excessively rich, specially at idle, low throttle (as with the TPS).
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                          I've tested the TPS working properly on and off the car. It is adjusted property too. I tried to get the stomp codes to show (it's not throwing a code, just thought it'd be a good way to check the TPS), but it wouldn't work. I even had someone turn the key while I manually move the throttle with my hands to full open, closed and back 5x. Still no stomp codes.

                          Any ideas?


                            Originally posted by Gregs///M View Post
                            Some other components that can cause hard start are:
                            Failing engine ground cable - consider replacing or checking this cable as its responsible for grounding all engine electronics. Cold moist mornings can cause excessive resistance between the cable and chassis ground.

                            Maybe a few other things that others can chime in if I forgot.

                            Good luck.

                            Just had a random thought. On cold mornings the car shows battery light and brake fluid light when first started. If I blip the throttle the lights go away.


                              Check the voltage at the battery with the car running and not. Running should be 14v, not should be at least 12.5v. If it is low while running, the alternator could be bad.

                              Fuel pressure regulator could be bad - test fuel pressure while running and if possible, while driving.

                              Run a smoke test to check for any vacuum leaks.

                              Definitely change the DME temp sensor if you haven't already. They are cheap. Don't get it confused with the sensor for the gauge. They are different, but located next to each other in the head.

                              Use an ohm meter to check for continuity between the TPS plug and the DME plug. You could have a short or broken wire.