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Tue May 30, 2006 10:58 am

I own a 1985 VW cabro. Second owner of the car with 124,000 miles. well maintained and running well up to Saturday. The idle is having problems once the car is warm. I noticed the idle would drop around 45 to 5000 rpm's coming out of gear, almost stall, and the idle back to 97 to 10k. That was on saturday. Drove it into work today and it was doing it again, but instead of it correct itself, the idle would pulse up and down. first low RPM and the back up and start over. Any ideas? Dean

Tue May 30, 2006 3:18 pm

Okay, 85 Cabrio - that should still have the old basic CIS system on it.

The idle is affected on this car by the oxygen sensor/frequency valve. Also the warm up regulator on the block. Any vacuum leaks, like the O-rings on the injectors, can throw off the idle too. And gunk in the fuel distributor. There are one or two idle boost valves on there as well, which help to level it out, one of which kicks on when the A/C is running, the other whenever the idle drops too low. If something else is messed up, that 2nd idle boost valve can kick on and off and make the surging actually worse than it would otherwise be.

Oh, I believe you have a fuel transfer pump in the gas tank on this car. Which is accessed by taking the seat cushion out under the back seat and removing a metal plate - it's mounted to the tank in conjunction with the fuel gauge sending unit. When these go out, it usually happens in hot weather. When it dies, the main pump under the car has to work a lot harder. This can result in various running problems, usually hesitation going up hills or around turns, poor starting, and sometimes bad idle. The main pump also makes a louder buzzing sound when the transfer pump goes out, since it has to work harder. Sometimes the car runs OK without the transfer pump going, sometimes it won't run at all. I'd verify that the transfer pump is delivering fuel (it puts out low pressure at high volume, whereas the main pump delivers high pressure at low volume. The only test for the transfer pump is that it delivers 13 ounces in 10 seconds of operation. )

Troubleshooting the rest of the FI system requires a proper fuel pressure tester, an electrical meter and the Bentley manual.

Oh hopefully you meant the car wants to idle at 500 RPM and not 5000! That would be a bit high. :wink:

-Andy

Wed May 31, 2006 7:37 am

Andy, thanks for all the great info. I had read about spraying some brake cleaner around the air lines to check for air leaks. Tried last night, but no luck. So much for it being simple. While playing the spray game, I did hear a loud click when the idle would drop very low. After reading your thread this morning, I believe it to be one of the two idle boost valves you wrote of. There is a white one and a black one. It was coming from the black unit. Could that be the trouble or as you wrote, something else is messed up and the boost is making matters worse? I will call today to get prices on the oxyg sens/freq valve. Our local temp has been getting hotter. Makes me wonder about the trans pump. My wife said she had heard a buzzing sound before, but I never heard it. doesnt seem to happen all the time.
The car runs great other wise. It will climb through the gears without hesitation, fast or slow. (O-ya, almost stalls @ 450 rpm and idles well @ 900 rpm :shock:) The old german girl has been a great car. I'll have to figure out whats wrong this weekend. Had to pull out my 70' Westy this morning to go to work. much easier to work on...Dean

Wed May 31, 2006 11:07 am

TwoVW wrote: had read about spraying some brake cleaner around the air lines to check for air leaks.

Brake cleaner might not be the best thing, since many formulations of it are non-flammable. For such a test you want a highly flammable spray, such as ether starting fluid or carb cleaner to indicate an intake leak. Just beware of fire danger.

Yeah, the click is that idle boost valve.

Oxygen sensor, when your exhaust gets hot, puts out a small voltage signal. This makes a computer send a different signal to the frequency valve, which is the buzzing thing on the side of the fuel distributor. You can disconnect the oxy sensor and hook it up to a voltmeter, and rev the engine a bit, and see if you are getting a varying .5-1 Volt from it. If you are, at least it is working somewhat. If you aren't getting anything then it's bad.

Transfer pumps can become intermittant before they die entirely. They have about a 10 year lifespan before I consider them suspect. If yours has never been changed I would just do it now and get it over with. Me in my 85 Golf, I carry a spare with the extra parts I have stuffed in/around my spare tire.

I'd get the Bentley book, Actually, since your car is very much similar to the 84 models you could use the 80-84 Rabbit/Jetta/Scirocco book, if you find one of those cheaper than the 85-93 Cabrio/Scirocco book.

-Andy

Fri Jun 02, 2006 10:17 pm

I own the same exact car...went through the same deal, changed all the sencors with new ones, and did the oxy sen. and the cat convert. alot of money my friend you know what the culpert was.

for me it was the Fuel Pressure Regulator, hard start, putters a bit first when going yea...save you self the hassle, try that first!

Magiceddie88

Sat Jun 03, 2006 7:06 am

yep, that was the second thing I mentioned in my first response - although I called it the "warm up regulator", I've seen them get plugged up or otherwise stop functioning on a few cars over the years. (Fortunately on my own Golf I have the more advanced CIS system with a differential pressure regulator and a temperature sensor)

-Andy