BMW Hard to Start Warm
After two years, no help from the dealer or the Indy shop and countless troubleshooting steps, I have finally solved the bad start issue on my BMW Z4. Hey guys and welcome back to the SimpleCarGuy channel! Today, I will go over all of the steps I took to try and solve this problem over the last 2 years, what I’ve learned in the process and how I accidentally solved it.
To put a lot of this into perspective, I got the car with a blown engine and swapped in a newer 30k-mile engine in my garage. Since I didn’t use the same exactly engine as I took out, I had to swap a lot of parts in the engine replacement process from the old engine. As a result, I had a few issues once everything was finished, but nothing I couldn’t solve. Except for this bad starting issue. When the car was cold, it would start no problem. However, after the engine was fully warmed up, there was a chance that it wouldn’t start or hesitate while starting. It didn’t help that the car has auto stop and start option and would turn the engine off at each red light.
From the information I have given you so far, what do you think the solution to this problem is? Leave your comments below and NO cheating! Now would also be the best time to hit that LIKE button to support the channel!
Since I believed the issue had something to do with the engine swap, I started troubleshooting some of the more obvious items. I rechecked and retightened all of the ground wires as well as the power wires going to and from the alternator and starter, checked all of the fuses and everything else I could think of. None of it made a difference. To be clear, I never had ANY codes on the car related to this issue.
With the wiring not being the problem, I got to thinking that it must be something to do with the ignition system. The most obvious item that came to mind and something I definitely touched during the swap was the crankshaft position sensor, this sensor tells the rotational position of the crank to the ECU and when it goes back, shows similar symptoms to what I had. Luckily, these are very cheap on eBay, so I swap it out but the problem remained. Similarly, the camshaft sensors monitor the speed and position of intake and exhaust camshaft. ECU uses this information for ignition and fuel control, but after swapping these out as well, I still had the same starting problem.
At this point, I’ve posted on forums, groups and consulted with a few friends and of course everyone gave a different solutions and mostly blamed different sensors that are known to fail on a BMWs. Since I like tinkering with cars, I got right to it! I’ve swapped out the air intake temperature sensor, cleaned and later swapped the mass air flow sensor, tested the coolant temperature sensor on the engine block, unplugged the O2 sensors to see if that made any difference and even looked into the oil pressure sensor as well as checked the oil pressure when the engine was fully warmed up. All was normal, still no codes or any other indication of something being wrong with the engine.
Just to check some items off my list, I smoke tested the vacuum system on the engine to look for any leaks, replaced the vacuum solenoid and even cleaned the PCV valve in the process. Of course the results were still the same.
Knowing that the wires are all secure, the ignition system is operating as expected, most of the sensors on the engine have been replaced or tested and there were no vacuum leaks on the car lead me to believe that it has something to do with the fuel delivery. I started with a big one and replaced the HPFP as I had a spare one from my old engine. The HPFP is one of the common issues on this engine that can cause very similar symptoms. However, even after replacing the fuel pump, the fuel pressure regulator AND testing the fuel rail pressure, my starting situation didn’t change.
Things I’ve Learned:
All of the items I have mentioned so far have had no effect, but replacing the injectors did make a change, just not to how the car started. What I learned was that there were 2 different injector types on the N20 engine, the EU5 and the EU6. Although practically the same, at full throttle, I would get a lean code pop up once in a while. After replacing the newer EU6 injectors with the EU5 Injectors from the old engine, those codes never re-appeared. The reason I’m mentioning this is in case you are troubleshooting a similar issue and didn’t think to look at the injector version.
So, I definitely learned a lot in the process but after all of this time, the car would still be hard to start when warm. Since I couldn’t figure it out, I decided to take the car to a local Indy shop that specializes in German cars. They spent a couple of days troubleshooting the car and at the end told me that it ‘might’ be the Valvetronic servo motor that needs to be replaced. Looking online, it looked like a possible solution, but before I took the engine apart, I brought it to a BMW dealer to check out as well. They scanned the car, logged some data, reset the adaptations on the DME and sent me home. The issue was back before I even got home. Not wanting to pay $200/hour for further troubleshooting, I left it alone and drove the car as is for a while.
Knowing everything you know now, have you figured out what the issue was? The solution was purely accidental. If you follow my channel, you may have seen a video where I replaced the starter motor on this car. After I removed the old one and got the new one out of the box, I noticed that they were different in size, color and even OE manufacturer. I was sure I ordered the correct starter as I went by part number that came up for my VIN. The reason these two were different is because the starter that was install on my car was for N20 engines that do NOT have the Auto Start/Stop functionality. Someone in the past, installed a starter that technically fit, but was not made for this version of the car. After installing the correct starter and driving for about a week now, I have had absolutely zero issues! This just shows you that you can try 100 different things, but sometimes the solution is hiding in plain sight. I hope some of the troubleshooting steps I talked about help someone fix their cars that have a similar starting problem. For now, I will continue enjoying my BMW Z4 now that it’s 100% issue free. Thank you so much for watching and I’ll see you in the next one!