Phantom Coolant Sensor Readings.. 91 Golf Digifant II

I have a 91 Golf 1.8 liter. While investigating my rough idle problem I checked the coolant temperature sensor. Something's amiss. The sensor is in the block located directly below the second spark plug from the left. It has two wires and a shield. The jacket is brittle and missing in many places as it had been laying up against the block. The electrical shield is in tact and seems no worse for wear. I measured it various time and it always reads zero ohms. I checked for continuity between the shield and the pin so I know I was reading the correct pins. I had alligator clips and wiggled them around to eat through any corrosion that may have been on the pins. I was using a digital Fluke multimeter. The sensor is therefore bad I say. But wait! The temperature gauge on the dash is working. So I slip a 1,000 ohm resistor in place of the sensor to see if the idle or gauge is effected. Nothing. The engine was pretty much warm during this whole operation. What gives?

Do I not have the coolant temperature sensor? There is another sensor in the head on the right side of the engine just above the output to the heater core. Could that be the coolant temperature and I am looking at the block temperature?

Reply to
mrat
Loading thread data ...

That's the knock sensor. Both coolant sensors are in the water outlets. You have two temp sensors, one for the guage and another for the computer. It's most likely bad and causing your rough idel.

JoBo

Reply to
Jo Bo

[edit] What I ment was the coolant sensor for the computer is bad.
Reply to
Jo Bo

different sensors! ;-) Both sensors, one for the FI and one for the gauge are different, should be on the coolant pipe on the cylinder head (upper radiator hose connects to this pipe).

Check out the sensors and wiring from the ECM plug. It will help you find any wiring/sensor problems with the FI system. ;-) Bentley usually gives good directions on doing this.

later, dave

Reply to
dave AKA vwdoc1

Blue sensor = to the ECU

Black sensor = to something else (the gauge maybe?).

Coolant sensor usually doesn't cause a rough idle...a bad one usually causes the ECU to think the engine is cold and the idle will usually increase (smooth idle, but too fast).

I'd instead remove and clean out the ISV with spray carb/FI cleaner. The ISV is the little metal can atop the valve cover with an electrical connection and a couple of hose connections. Remove that, spray every orifice well with cleaner, flush it out good, reinstall.

If you do want to replace your coolant sensor as a preventive measure to rule it out, they're very inexpensive...about $12 at the dealer. Get a new o-ring for it too. Replace it when the engine is stone cold so you don't burn yourself and so you don't lose too much coolant in the process. You just undo the wiring connector, undo the retaining clip, pull it out, remove o-ring, install new o-ring and sensor, install retaining clip, connect wiring, done.

Reply to
Matt B.

Thanks for being so gracious about our confusion on the sensors. We got up this morning to check things out with the engine stone cold and noticed the two sensors on the radiator hose. We must have had tunnel vision yesterday. We measured the resistance but no time to check against Bentley for the correct values. Indeed one is for the dash gage and the other does increase idle speed which gives the sense of smoother idling. As to the rough idle, we cleaned the throttle body intake side of the butterflys. That helped a lot but not completely. We also checked the ISV per Bentley. It does work to some degree, I just don't know if it is enough. We will probably take your advice and clean it. We may even pop for removing the throttle body and doing a through clean job. Who knows, maybe we'll find a busted gasket or something. We had done a search on rough idle and are just going down the list of things to check.

Reply to
Paul Johnson

Duh, you're in trouble big time. That's the knock sensor and not the coolant gauge sensor.

Reply to
Woodchuck

Got an older VW - same problem.

The two conductor temp sensor which feeds its signal to the Digifant unit most assuredly _can_ cause rough idle, but, early on, it's far more likely to cause hard starting. When it's _completely_ bad, (it doesn't fail all at once, you see), it is an open circuit. If you disconnect it, it may start better, but run funny till you get above about 3100 RPM, when the computer isn't even part of the whole timing/fuel mix/air volume equation.

What did I do, before I got the part? Connected it to start the car, and disconnected it after it was a little warm. As I recall, anyway... :^p

Another thing about rough running, the system is very sensitive to vacuum leaks. Our little friend had a little rubber elbow joint, between the idle air stabilizer and the intake manifold, that went bad. It was a bitch to track down, but once we replaced it, it ran great.

250,000 miles and all

My apologies if I've given old news. I'm just getting re-used to using newsgroups.

jdg

Paul Johns> Thanks for being so gracious about our confusion on the sensors. We

Reply to
Jeffrey D. Glock

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.