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BMW Hard Starting When Warm – Engine Temperature Sensor

March 24, 2010

This answer is applicable for many BMWs in addition to the one listed below.
I have a 1998 528i touring with 115,000 miles that starts fine when cold, but starts hard (cranks about 2-3 seconds) when hot. If I start it in the morning it starts immediately. When at operating temp (after driving to work), if I turn it off and then try to restart, it will start right up. However, if I let it sit 30 minutes to a couple of hours, it takes some cranking to fire up (will sometimes kick when cranking but takes much longer than is normal). Problably been this way for 3 or 4 months. When it does start, there a fuel smell in the exhaust like it’s running rich …. only on start up. When it’s running, it runs fine; smooth, good idle, good power. No lights lit on dash (Check engine, etc). Spark plugs and all coils (from you guys) were replaced about six months ago. Air filter is a K&N drop-in filter and is clean. Everything else is stock. Fuel filter is about a year old. O2 sensors were replaced and Mass Air Flow sensor was cleaned three weeks ago. I replaced a couple of cracked air hoses around the intake and idle control valve. I’m all out of ideas. Any help would be greatly appreciated.

The symptoms that you’ve described would be consistent with a faulty engine temperature sensor. All BMW models with electronic fuel injection (L-Jetronic, Motronic and all late model applications) use a temperature sensor that monitors the engine coolant temperature and reports back to the ECU (Engine Control unit). In this case, the net effect is that the sensor is “cooling down” too quickly and causing the engine management system to richen the fuel mixture (because it thinks the engine is colder than it is) which floods the engine. The short shut down periods are OK, as the sensor has not started to detect a slight drop in engine temperature yet. However the mid-length shut down periods allow the sensor to start to detect that the engine temp is decreasing and the faulty sensor changes its resistance too quickly …. showing a signal to the ECU that is incorrect. Finally, the longer periods, allow the engine to actually cool down to the point where it can handle the sensor’s resistance change (and corresponding ECU mapping),

Of course, the above points are made without any actual diagnostics, but this would be a common issue that seems to line up with your symptoms.

The Bentley repair manual covers the engine temperature sensor replacement. Basically, you just remove the harness plug , unscrew the sensor and screw in the new one. Note that the sensor is installed into an area where it is exposed to the engine coolant, so you will lose a small amount of coolant during the interchange. Most sensors are relatively easy to access. Again, the Bentley repair manual covers the location. On your 98 528i, the sensor is mounted in the cylinder head, on the intake side, below the intake manifold runners for cylinders #1 & #2. Some other models have the sensor mounted in the thermostat housing or (on V8s) in the water crossover pipe at the rear of the cylinder heads. Additionally, there are some other less common applications.

Click below for the engine temp sensor for your 98 528i:

Click below for engine temperature sensors for other models (Please ID your model and search for “Engine Temperature Sensor”)

Click below for Bentley repair manuals:


Bavarian Autosport

  1. Paul permalink

    I’m having the same issue with my 2001 3.0 X5. I changed the the temperature sensor in the lower radiator pipe and the problem still exists. Spark plugs, air filter, and fuel filter have all been replaced as well. Battery was not replaced but checked out to be fine. Do you have any other suggestions? Any help will be greatly appreciated. THANKS!

    • The temperature sensor that is installed in the lower radiator hose, on M54 6-cylinder applications (note that the post above is for an earlier M52 engine, which does not have the sensor in the lower radiator hose), does give input to the Engine management system. However, this not the main engine temperature input. The main engine temperature sensor is mounted in the cylinder head. It is located under the intake manifold, toward the rear of the head, so access is rather limited. See the photo below (intake manifold is removed, for a different repair).

      BMW M54 Engine Temperature Sensor

      While the temperature sensor certainly could be at fault, I would suggest that you do some other diagnosing before replacing the sensor, as follows:

      * Test for intake system vacuum leaks. Have the intake system tested using a smoke injection machine. This injects a light smoke, under pressure, into the intake system. Any leaks will be readily visible via the smoke that is leaking out.

      * When the spark plugs were replaced, were the connector boots also replaced? The connector boots are the rubber connectors that go between the coils and the spark plugs. Is there any oil in the spark plug wells? The plugs and boots should be replaced at about 60,000 miles. If there is oil in the spark plug wells, the valve cover gasket must be replaced.

      * Check for engine management fault codes. Even if you have not noticed the SERVICE ENGINE SOON or CHECK ENGINE light coming on, check to see if there are any engine management fault codes. We do offer a tool that you can use to check the codes and reset them.

      Let’s start by considering the above points. If no faults are found in these areas, you may wish to replace the temperature sensor and there are also a couple other, more intricate, issues that could be involved.

  2. Paul F. permalink

    I’ve been having this EXACT issue for the past year (2005 325i). Over the winter it didn’t happen but when the weather got warmer it came back. Basically if I start it in the morning (55-65 degrees F) it doesn’t seem to do this, and when I get to work (25 miles) and turn it off, wait 5 minutes it doesn’t happen. But if I go back out after an hour or so (80 degrees F) it happens. I’ve cleaned the MAF and even brought the MAF into work to keep it cool and still have this issue. I don’t have fault code reader nor smoke test. I’ll check the lower temp sensor for any damage tonight.

  3. Mike C permalink

    I have the same issue on my 2002 325xi. Did anyone on this board solve the problem?

    • There can be various source faults for this symptom. One instance may be different from another. Have you checked the points listed above?

      And, of course, anyone who has had experience with this feel free to share your findings with others.

  4. Joe Jr. permalink

    I had the same problem. It turn out to be the FUEL PUMP RELAY. Before you spend any more money or time on this problem – Replace the FUEL PUMP RELAY.

    • Thank you for sharing your experience. A relay with worn/pitted contacts certainly could offer up intermittent operation. If you were to check for pressure in the fuel lines, there should be pressurized fuel that sprays from the loosened connection. Loosen a hose clamp on one of the pressure-side fuel hose connections … such as at the fuel filter (use proper eye protection and safety precautions). If there is no pressure (you may still get fuel leakage, but not a release of pressure), then the pump has not been running. In this case, the relay could be at fault … as well as the pump and other potential faults.

      We do keep the various fuel pump relays in-stock. Just call our agents at 800-535-2002.

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