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RMillerPhD
RMillerPhD, ASE Certified Technician
Category: Car
Satisfied Customers: 374
Experience:  26 year of import auto repair, Owned and operated a Repair shop and Automotive Machine Shop
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1989 vw cabriolet: cranks..it sits idling or driving down the highway

Resolved Question:

My 1989 vw cabriolet, starts and runs for awhile, then it stalls, when I try to restart it cranks over but does not start, if I let it sit for a couple hours, and then try it starts right up. This can happen either as it sits idling or driving down the highway. The run time can be 10 minutes to 30 minutes or it will run for hours with no stall. I've changed the coil, fuel filter and fuel pump.
Submitted: 6 years ago.
Category: Car
Expert:  RMillerPhD replied 6 years ago.

If the engine is spinning, but does not start then:

The first thing you want to isolate is whether the problem is fuel or spark. Buy a can of Carburetor Spray and spray into the air intake system and see if the car tries to start on this flammable spray. I typically remove a vacuum line and spray directly into the intake manifold using the plastic tubing which comes with the carburetor spray can.

1. If the car does not attempt to start, the problem is with the ignition system.

2. If the car attempts to start, the problem is in the Fuel system.

NOTE: Be careful with the Carburetor Spray you don't want to set the engine on fire. Make sure the fluid gets into the manifold.

If the engine does not attempt to start. Verify that you have no spark. Remove a spark plug wire from a spark plug. Place an insulated Phillips screw drive into the end of the wire and allow the metal shank to be 1/4" or 1 cm from the metal on the engine and see if you have spark, when someone cranks the engine. BE CAREFUL not to shock yourself. You will not be injured but you will not be happy with the jolt

If I were a betting person, I would put my bet on the Crank angle or Cam angle sensors. (Which is a Hall Effect signal Generator on this model) This is a common problem with your description.

Let me know how your testing goes. ONCE you have tested a few of this basic issue let me know and then we can focus on the issue that comes up as a problem (Fuel or Spark?)

We can move the next tests once your have determined which system has failed.

YOU will probably find that an electronic component is failing when it gets hot. The transistor in the device is failing then when it cools down the transistor begins to operate. OR a relay is failing when it gets hot. I would bet on the transistor which is the more probable cause.

Hall effect or the ignition module is what i would inspect if the car does not attempt to start with carb. spray.

Diagram and key

Diagram1

Diagram2

Diagram3

Diagram4

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Customer: replied 6 years ago.
Reply to RMillerPhD's Post: I tried for spark, first few cranks were no, then I got spark and it started. I put theplug wire back on and started it again, it ran. I started it and let it run, turned it off and did it again, 3 times it worked on the 4th try, no start and no spark on rechecking. That is where I stand right know.
Expert:  RMillerPhD replied 6 years ago.
There are 2 possibilities

See diagram 1 HAll Generator.

You need to test for AC voltage on middle wire at Ignition control module terminal 6.

NOTE: I use an oscilloscope to verify and a voltmeter is not the best way to test

If you don't get any voltage (AC setting 1.5 to 2.5) or with the Oscilloscope a 5 volt square wave pattern. The Hall generator is bad.

The odds are that the Hall Generator is bad, but it could be the module.

Test and let me know the results.
RMillerPhD, ASE Certified Technician
Category: Car
Satisfied Customers: 374
Experience: 26 year of import auto repair, Owned and operated a Repair shop and Automotive Machine Shop
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