Idle issue on my Golf

[mikeonabike]

Hey guys, Ive got a Mk2 8v GTI which seems to be having real issues idling, which seem to have different characteristics depending on the temperature of the engine.
Basically when it first starts it idles at about 1200 which is OK, but its not totally smooth (it doesnt stay at a constant RPM). Then after a couple of minutes (presumably after its off the choke) the revs seem to bounce between about 400-500RPM, sometimes stalls and always turns the battery light on when it gets down to 400. Then as it gets some proper temperature the revs slowly climb up to 800 and it begins to be able to hold a more constant RPM (still not constant but only changing by about 50RPM).
So once its up to temperature its ok, but still a bit annoying because every second or so when the revs drop the entire car judders, and when its just come of the choke it feels like the cars going to shake its self apart.

Is this just a case of adjusting the idle mixture? And if so can I do it myself? Or is it more serious like a fuel feed problem?

Any help would be great,
Mike.

[rubjonny]

I'll take a wild stab in the dark and guess you have a post 87 digifant 8v gti, if so check the idle switch clicks and the wiring to it ok

[mikeonabike]

Its actually an 84 model but Ill give that a try anyway

[Ben Lessani]

Set your CO properly, then check your WUR - you'll need a fuel pressure gauge to setup a K-Jet though!

[rubjonny]

ok some of the earlier models had an idle switch so check that, but high idle on k-jet models usually down to air leaks or not set up properly.  check all the vac piping is connected and not cracked, pull all the inlet pipes off and check carefully in all the folds for splits.  if all that checks out try bringing it down with the idle screw, i had an issue on my 8v where the screw kept winding itself out due to a worn o-ring!

[Ben Lessani]

Difficulty starting engine only when cold

This is one of the most common problems with the K-Jet system. What happens is you turn the key and the starter motor engages and the engine turns over, but the engine just won’t fire up.

Here are some common causes of difficult cold-starting:

1) Control pressure too high (Control Pressure Regulator) The control pressure changes as the engine warms up. The pressure is created by the Control Pressure Regulator (CPR). The CPR has a lower pressure on the control plunger in the fuel distributor when the engine is cold compared to when the engine is warm. The lower pressure allows the control plunger to let more fuel into the engine when the engine is cold. If the control pressure is too high, the air/fuel mixture will not have enough fuel, and thus the engine will not start.

The common symptom with a bad control pressure regulator is that the engine takes about 20 tries to start, each time idling for a little longer after it starts. Basically, you turn the key, the rpms go up to about 1500 - 2000 when the cold start injector fires, and then the car dies because it is not getting enough fuel after the cold start injector shuts off. Also, a bad control pressure regulator can make the engine almost die when you shift in to neutral (auto trans.) or depress the clutch pedal (manual trans.)

Fixing this often requires a new control pressure regulator (a fairly expensive part). However, many people have achieved success by cleaning out their control pressure regulators. The come apart fairly easily. First though, the CPR must be removed from the car, which usually requires removal of the intake manifold first. However, I have replaced the CPR on a N/A 240 without removing the intake manifold. I removed the distributor cap, the intake manifold support bracket, and the rubber bellows, and I had enough room to move. for turbo models, this probably could not be done.

The CPR is held to the engine by two hex bolts, and is held together by four flat-head-screwdriver bolts. Be careful! The CPR contains several springs and other fragile components. Take it apart indoors in a clean environment where you can keep track of all the little pieces. When it comes apart, there are three metal pieces held together by two gaskets in a sort of sandwich. If your gaskets are ruined (hopefully not) you can replace them with that gasket maker stuff.

Once you have it apart into these pieces, you’ll need to remove the 10mm nut on the heating element and carefully bend the heating element out of the way by bending the two wires that attach to it. Now, use a small flathead screwdriver and remove the four bolts that hold down a circular diaphragm. Be careful!. Once you have that taken apart, get out your carb cleaner and spray out the insides of the thing as best you can. Now, put it back together, making sure that everything goes back the same way it did before.

2) Auxiliary air valve or idle air motor not working or sticking: depending on which one of these devices you have, the symptoms may be different. I’ve never had a bad auxiliary air valve myself, but if I did, I would probably have just replaced it with a “new” one from a junkyard car. If the auxiliary air valve sticks in the open position, the car will idle very high, about 2500 rpms. If it sticks in the closed position, the car won’t idle at all, but may run if you keep your foot on the accelerator to let air into the engine.

A sticking idle air motor will often cause a “surging” when driving - your car will accelerate when it’s not supposed to - and/or a “hunting” idle, where your car just can’t seem to find the right idle. A hunting idle might range from 500 to 2000 rpms, and just keep going up and down.

An idle air motor that just plain won’t work can be detected by trying to start the car and having another person “feel” the idle air motor. It vibrates (buzzes) when it is running. You can also hook it up to a battery or booster pack to test it. Supposedly the middle terminal on the plug is positive and the two outside terminals are negative, but I have been unable to verify this other than just hearsay. An idle air motor that is REALLY dead will actually emit white smoke when you try to use it.

The idle air motor can be cleaned out by removing it from the car completely and spraying it out with carb cleaner. Wear gloves and goggles! This stuff can burn the bejeezus out of your skin and eyes! Spray a bunch of carb cleaner in it and plug both of the ends. Now, shake it up as much as you can - up and down and roll it between your hands like you were trying to make a long snake out of Play-doh like in kindergarten. When you roll it, you should be able to hear it clicking back and forth. If you can’t hear anything moving in there, keep cleaning it or get a new motor. Make sure to shake out all of the excess carb cleaner and let the thing dry for a bit before you re-install it.

3) Problem with cold start injector / thermal time switch circuit The cold start injector provides a brief blast of fuel only when the starter motor is running (when the key is in position III). It operates based on info from a thermal time switch, which tells the cold start injector when the engine is cold. If the engine is warm, the thermal time switch breaks the circuit and the cold start injector doesn’t fire.

Testing the cold start injector is fairly easy. Just remove it from the intake manifold (and make sure to bolt down the ground wires again). Now have a friend start try to start the car while you hold the cold start injector in a small bottle or napkin or some such. If it sprays out fuel, you’re good to go. If it doesn’t, then you’ve got a problem.

The best way to try and solve this problem of whether it’s the cold start injector or the thermal time switch causing the problem is to get out your voltage meter and hook it up across the two little plugs in the plug that hooks up to the cold start injector. If, when you start the car, you get 12 volts across the cold start injector plug, then the thermal time switch is working and the cold start injector is faulty. If you don’t get 12 volts across the plug, then the thermal time switch needs to be replaced.