Story by Jack Heideman
At some point, everyone encounters a car that doesn’t run. Figuring out the source of the problem can be frustrating.
While it’s fairly common for people to just throw new parts at their inoperable cars, we’ve learned that taking a methodical approach is not only less aggravating, but more successful than guessing. When we bought an E30-chassis BMW 3 Series that wasn’t running, we put our methods to the test.
An engine needs fuel and spark to run, so we tested those first. To check for fuel, we disconnected a fuel line, routed it to a catch can, and briefly turned on the car.
Since fuel flowed into our catch can, we moved on to test for a spark. We disconnected the coil wire from the distributor and held it about a quarter of an inch away from a ground. When we cranked the engine over, there was no spark. Our method of diagnosis was starting to pay off.
At this point we brought out the Bentley manual to see what tests it recommended to determine what was causing the lack of spark. It had us check for voltage at the coil, visually inspect the distributor cap and rotor, and measure the crank position sensors with an ohmmeter for the proper 960 ohms of resistance. Our car passed these tests.
The next test was to spin the engine over and see how much voltage each of the crank position sensors produced. One sensor produced a small AC voltage and the other produced none. Our methods had quickly helped us find the problem. That was the good news.
The bad news came next, when we pulled the failed crank position sensor. We saw that the top dead center peg on the flywheel had broken off. Our hope for an easy fix quickly dissolved.
To get to this peg, we had to pull out the transmission. Once the transmission was out, we saw that a piece of the throwout bearing had broken off. Its shrapnel had taken off the peg on the flywheel.
Instead of buying an expensive replacement flywheel, we sought advice from GRM friend and Redline Performance owner Rennie Bryant, who told us the dimensions of the peg. We then called on a machinist friend, Bill Wiswedel, to create a new one for us.
With the new peg installed in the old flywheel, we put the car back together using a clutch kit from BimmerWorld and some bushings and hardware sourced from Pelican Parts.
The car started right up, proving our diagnosis correct. However, during our first drive we realized there was another problem: The car would barely run above 3000 rpm. Now we had a drivability mystery to solve.
Idle and drivability problems are often caused by vacuum leaks, but spraying carb cleaner on the intake tract and vacuum lines confirmed that a leak wasn’t our culprit. Next we checked if the engine had consistent spark. We connected our timing light to each plug wire, and bingo: At 3000 rpm, we were losing spark at cylinders one and six.
Now we knew the issue was related to spark, but we needed to find out why. We checked the plug wires with an ohmmeter–no issues. We visually inspected the distributor cap and rotor–nothing wrong there, either. Since we had a used cap and rotor on hand, we tried them anyway. We were still dropping spark above 3000 rpm.
We were feeling stumped, but then it hit us: What if a crank position sensor was damaged by the throwout bearing shrapnel? This was a $60 hunch, but luckily a new sensor from Pelican Parts fixed the problem. Although we took a bit of a guess at the end, our methodical approach meant it was an educated one.
Step By Step
1. Our latest BMW find wouldn’t run. We first checked for fuel by disconnecting the fuel line and watching for flow into our catch can. We were careful to keep our distance, as fuel flows at pretty high pressure. When working with fuel, remember to be safe: Have a fire bottle at the ready, ensure that the area is spark-free and well ventilated, and inform all involved of the potential hazards.
2. Since the engine was getting fuel, we tested for spark by holding the coil wire about a quarter-inch away from a ground. Nothing.
3. We read our Bentley manual to find out how to diagnose our no-spark issue.
4. The next test focused on our car’s two crank position sensors. Step one was to check that their impedance was about 960 ohms with the engine off. Step two was to test for voltage when cranking the engine: One showed 3 volts and the other 0. The good news was that we now knew the source of the problem. The bad news was that we had a lot of work ahead of us.
5. In order to access the crank position sensor on the flywheel, we had to first pull the transmission. Fortunately, we were able to repair the flywheel with a new peg so our sensor could pick up a signal. As we’d later learn, this is somewhat common failure on these cars.
6. Even though our car was now running, it had drivability issues. Now it was time to check for consistent spark on all six cylinders. We had an assistant slowly rev the engine while we monitored spark on each plug lead using a timing light. Once the engine hit 3000 rpm, the timing light revealed that the spark was not consistent on cylinders one and six.
7. We checked the resistance of each plug wire and found no issues.
8. Finally, we uncovered the source of the drivability issue: a bad crank position sensor. By being methodical, we solved our problem without replacing any good parts.
Comments
The last one I had it was out of gas , but the gauge said 1/4 !
Crawl under the car and tap the tank if you can get to it 🙂
FPZguy
New Reader
2/13/19 3:48 p.m.
After paying $770 for a shop to replace my Volvo XC70 fuel pump, the car stopped dead when I pulled off the Interstate at a gas station. After a while it cooled off and I got it started again. The problem was really the fuel pressure sensor! $57 and a 10mm wrench was the fix. If you have a high mileage car just replace the three key sensors – fuel, cam shaft and crank shaft, about $120-$200 total and play it safe.
Hmm. I thought you were supposed to ask on the internet in a way that’s worded to insult the entire mechanic profession, ignore the advice on how to actually troubleshoot the problem and replace every part that’s mentioned as having once caused random other cars to not start.
Here are some that you missed:
- The car has a security system with a magnet that must be applied to the A-pillar to close a hidden switch.
- The shift-park interlock switch broke, disabling the starter circuit.
- The gas from the spare can you added was full of dead spiders and pine needles and has clogged the fuel lines.
- The PO was not an electrical genius, you must have the radio on to start the car, but the headlights must be off.
- Yes, that key fits in the ignition and allows it to turn, but it is for your other Impala and won’t start this one.
t321sg
New Reader
4/17/21 4:15 p.m.
Just because you have fuel coming out the line, doesn’t mean its getting fuel at the right pressure. For fuel injected cars, you should put a pressure gauge on the fuel rail.
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