You need a smoke tester, it's worth buying if you own a shop [I do]. Contact a shop locally and ask how much to do it. It literally takes a couple minutes, but I would assume the shop would charge at lease .5-1.0 hours to do that.
I built my own smoke tester and did a smoke test. No vacuum leaks! I use a test port on the intake boot to bring the smoke into the intake system, when reinstalling the plug in the test port I used some sealant and after doing this the idle rpm dropped so I guess there was a vacuum leak there.
But the engine still idled very rough. It constantly dropped rpms, then the ECU kicks in and brings the rpms back up. This repeats itself.
BUT! all of a sudden this was gone, the engine idled around 950rpms and started smoothly. It did hold back above 4000rpm when driving. When I went to adjust the ignition I noticed the ignition was severly retarded, and when I turned the distributor to bring it back it would retard again after a while. This led me to the knock sensor, which was bad. I replaced this (and torqued it down properly) and now the ignition was adjustable. I brought the engine up to temperature, connected my multimeter and stroboscope and got adjusting. I got the DPR current to 'search' between 4 and 6 mA, and the ignition to 6° before TDC. Idle rpms where too high now, but I brought these down by adjusting the idle air bypass screw. After every adjustment of this screw I checked and adjusted mixture and ignition again.
Finally I got the idle rpms to sit around 950rpm, the mixture at 5mA +/-1 and the ignition at 6° before TDC. The exact specs as given by the Bentley manual. Throttle respons was great. I tightend down the distributor, checked the ignition one last time and removed all the test equipement. I shut off the engine.
I cleared out my driveway with the intention of doing a test drive, so half an hour later I come back to the car to do a test drive. All of a sudden the car is very hard to start, and when it starts the idle problem is back. Throttle response is still great, but below about 2000rpm the car runs extremely rough. The problem doesn't go away when the engine warms up, oil temp was 96°C so the engine was warm.
I did an electrical check of the knock sensor system as explained in the Bentley manual and everything came back fine. Hall sensor on the distributor recieves power, and the ignition module is fine according to the electrical tests in the Bentley manual. There was some oil in the hall sensor connector, but cleaning this out did not eliminate the problem.
Another update: I'm now quite positive the problem can be found somewhere in the ignition system. It's almost certainly something electrical anyway.
I did some checks of the ignition system as described in the bentley manual. When the ignition is switched on, the voltage between pin 1 and pin 15 on the coil should jump to at least 2V and then drop back to 0V. It jumps to about 1.5V. When pin 12 of the knock sensor module connector is briefly connected to ground, the same thing should happen. The voltage jumps to about 1.5V but doesn't go back to 0V. It stays floating between 0.6V and 0.8V. The bentley manual tells me this points to a faulty coil or ignition module. I measured primary and secondary resistance on the coil:
Primary: should be 0.8-0.6 ohm, I measured 1 ohm
Secondary: should be 6500-8500 ohm, I measured 7500 ohm
So basicly, secondary resistance is within spec, primary isn't. However a multimeter isn't accurate enough to measure such small resistance, so I believe the coil is fine.
This led me to believe the ignition module was faulty, since it's only 30€ for a new one I replaced it. I ripped the old one open and found a stain on the board:
I don't know if there was supposed to be a component there, and if so which. I also don't know if this was the cause of the roug idling and the strange engine behavior. But all I know is that the module was due for replacement anyway, judging from that stain. Which makes me happy because untill now I haven't replaced anything that didn't have to be replaced, and I want to keep it that way!
Replacing the ignition module fixed the rough idle issue. It was a long road and I've learnt a lot about the K-jet system, but I'm happy I kept searching and didn't bring the car to a shop. Knowledge about these systems is valuable.
In the end the original problem was probably a combination of bad injectors, bad knock sensor and an ignition module which acted up from time to time. The injectors were probably okay at first, but the sheer force which was needed to pull them out when I replaced the O-rings probably destroyed them. The knock sensor was bad but I think it worked half of the time. Same goes for the ignition module.
I'm happy I didn't replace things which were not due for replacement. It took me longer to find the fault but cost me less money.
Now I'm off to have fun with my 16V :laugh:
You have a great attitude towards your journey. You did a lot of testing that helped you go towards your issue. Even some of the tests that did not give you the results you wanted, those test will help you in the future about most any fuel injected car [CIS or EFI].
Ok I can tell you from my experience and having almost the same issue and doing just about everything you’ve done. I found that my fuel distributor is bad it was leaking internally causing all of my issues. If you’ve got a spare I would throw that in and see if that fixes the issue but make sure it is exactly the same as the one you have because they’re not all the same good luck
A forum community dedicated to all Volkswagen owners and enthusiasts. Come join the discussion about performance, builds, modifications, classifieds, troubleshooting, maintenance, new releases, and more!