Engine Dies When Normal Operating Temp is Reached, but then.......

[88Comjet]

I've got a 88 Jetta that dies on me every time the car reaches normal operating temperature. I'll be driving down the highway at 55mph, and the tach drops to zero. No bucking or coughing like the fuel pump is cutting out. I still have power to the rest of the car (radio, lights, etc.). If the car doesn't kick back in, I have to pull off the road and try to restart it, but it's very hard to start. I have to try repeatedly for a couple of minutes. It eventually fires back up and runs fine for the rest of the commute. Today, I went out to my car about a hour before I left work and started it up and drove it around the parking lot. I parked it, but let it idle for a few minutes. Once the temp gauge got to normal operating temp, the car shut off. It was hard to start as usual, but ran fine when I started it back up. Very strange. I'm going to drive my truck to work in the meantime so I don't run the risk of it deciding not to restart and try to get the car to die in my driveway this weekend to verify that it's a spark issue, but in the meantime does anybody have any leads as to what could be causing this?

[ZWStewart]

Cold start valve malfunction?

or Might be an o2 sensor. When the engine is cold, it uses the coolant temp swtich, then when warmed up it switches to the o2 sensor.

[WaterWheels]

Nothing in the Digafant system should kill the engine when it reaches operating temperature. The O2 sensor is constantly monitored. Once the sensor reaches operational temperature and sends a good signal to the ECU, it starts adjusting the mixture. The sensor reaches operational temperature in about 30 seconds, way before the engine reaches its temperature.

Have you done any work to the car? None of the sensors which run the Digifant system really cause the engine to die as you describe. The blue engine temperature sensor if bad can cause either hot or cold issues, but to just flat out cause it to stop dead I can't see happening. But if there is a dead short in say the tachometer or temperature warning system or the fuel pump relay, the coil could get grounded out and kill the engine. Some of the coil grounding is done inside the Digifant ECU, so it could be a problem itself I guess.

[Seax_Smith]

Use a fused jumper and jump your fuel pump relay. Odds are the problem will go away. Do not the the jumper in place unless you are driving the car. What the jumper does is run the fuel pumps even if the ignition is off.

[Fat Rabbit]

I guess we are presuming that your '88 Jettta is Digifant. Hopefully you found something out this past weekend if you found time to work on your car. When I had a similar Digifant issue it turned out to be the Digifant Control Unit Relay that was dropping out. That relay supplies power to the injectors as well as the control unit itself. PN 357-906-381

IIRC on some early Digifant models the injectors were powered through the fuel pump relay so it also might be wise to check that relay by jumpering as previously suggested. PN 191-906-383

The ignition switch can cause a similar issue (dropping out Circuit 15 to the coil) as can other parts such as the coil, the Hall Sender and the ECU itself.

Look first at the relays and then the ignition switch. You can jumper the relays on the fuse and relay panel. My jumper is a short piece of wire with male spade terminals at each end. The fuel pump relay is jumpered at the two larger perpindicular terminals under it and the Digifant relay is jumpered in a similar fashion across terminals 30 and 87 on the fuse and relay panel under that relay.

The prescription for a CIS injected car is essentially the same -- jumper the FP Relay and see if the issue continues. It it does -- replace the electrical part of the ignition switch.

If you get to the ignition switch replacement PM me for suggestions as to how to do it. FR

[green_slc]

Cold start valve malfunction?

or Might be an o2 sensor. When the engine is cold, it uses the coolant temp swtich, then when warmed up it switches to the o2 sensor.

O2 sensor or wiring to that sensor is my guess. ECU probably see lean so it thatdumps extra fuel and flood the engine. That's why is runs ok when cold. Once up to temp and switching to closed loop mode, it reads the bad O2 signal and die. Had this problem before

[ziddey]

O2 sensor or wiring to that sensor is my guess. ECU probably see lean so it thatdumps extra fuel and flood the engine. That's why is runs ok when cold. Once up to temp and switching to closed loop mode, it reads the bad O2 signal and die. Had this problem before

No cold start valve for Digifant, and that's not relevant once the car is started anyway.

If the tach is dropping out, it's an ignition sided issue, not fuel, so o2 sensor is out.

Follow Fat Rabbit's post on troubleshooting Digifant.

Again, this is assuming the OP has a Digifant 88.

As far as generic advice goes, check all your grounds!

[ZWStewart]

If you simply unplug the harness to the O2 sensor, the car will run, but rich with poor gas milage. Try unplugging it and see if the problem goes away.