Bosch K Jetronic
- Last Updated: 22 February 2012 22 February 2012
- Created: 19 December 2007 19 December 2007
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Bosch K Jetronic
The Bosch K Jetronic, or continuous injection system (CIS), is a mechanically-based fuel injection system that was used on the 911, 911 Turbo, 924, 924 Turbo, and 928 models during the 1970's and into the early 1990's. As these systems age, there are several components you can change to tune up the system, which is what we'll cover, using a 911SC as the example. The components we'll look at are the in-tank fuel filter, the regular fuel filter, and the fuel injectors. Note that CIS fuel injectors have a non-replaceable filter inside the fuel injector.
CIS-equiped 911's have one fuel filter in the gas tank and one in the engine compartment. Normally, the primary fuel filter in the engine compartment is the one that gets changed with some regularity. The fuel tank filter, on the other hand, is out of sight under the gas tank. Porsche points out in the factory service manual that many CIS-related problems originate at this filter. However, the reason you don't hear much about this today, is that unleaded gas is actually pretty clean as compared to leaded gas, so this particular filter doesn't gum up anywhere near as often as it may have in the mid-1970's when CIS was introduced. On our 911SC, the removed filter was actually in good shape, although it did have some debris on it. So, unless you believe it's causing a problem, and there's only been unleaded gas in the tank, I'd leave it alone. However, here's how to change it:
You'll need to pick up both a filter (around $20) and a new o-ring (around $3). If your Porsche isn't lowered, you can probably change the filter without any jacking. If it is lowered, you can jack up one side just enough to remove a front wheel, which will give you access to the underside of the gas tank. The fuel filter requires a 22mm hex tool to remove/install it. If you don't have one and can't find one (forget Sears and Home Depot), go to the corner hardware store, and buy a 19mm bolt (which has a 22mm head) and nut. The "tool" is simply the nut cinched up to the head of the bolt, saw off the remainder of the bolt, tap on the base threads so the nut can't come off, and voila!
Before removing the fuel filter from the bottom of the gas tank, you should probably run the gas down so there is as little gas as possible in the tank. And since you'll be draining whatever is in the tank, the normal safety cautions about sparks, matches, cigarettes, ventilation, etc. apply here. Using your 22mm hex tool, loosen the filter and drain the gas. The new o-ring goes on the base of the new filter, the filter goes in, and it's tightened to about 8nm's. Add gas, check to be sure there's no leakage, and you're done. Check the drained gas for evidence of gas tank deterioration, i.e., metal flakes or other suspicious particles.
You should already be replacing your main fuel filter with regularity, according to your owners handbook. If you're going to replace your fuel injectors, replace the fuel filter at the same time.
For a 911 fuel injector refresh, you'll need to buy 6 fuel injectors (around $23 each) and 6 injector o-rings (a buck and change each). If any of the injector shells, which go between the intake runner and the injector, come out, replace them too. The injector shells and o-rings are available from Porsche for around $4 per shell/o-ring combo.
First your engine needs to be cold, that is, let it sit overnight. Take the proper safety precautions when working around gasoline - nothing to provide sparks, no cigarettes, good ventilation, etc. On the 1981 911SC, the lines from the fuel distributor to each injector are metal. Prior to this, they were plastic. If you have the plastic lines, use a 12mm and 14mm wrench to separate the line from the injector before pulling the injector out. If you have metal lines, you can try leaving the lines attached. As the 20 year old o-rings on the injectors can be quite hard, even with some WD40 around the grommet, you may want to separate the metal line from the injector and use vice grips on the injector base to pull them out (note: don't mess up the threads). If the lines and injectors don't want to separate, use some PB B'last on the threads and let it sit overnight. You only need to pull the injector out far enough to get the o-ring out of it's groove, and at that point it easily lifts out.
Before putting the new injectors in, check the injector shells to ensure you didn't pull any partially out. Correct position is to sit below the notched edge of the intake runner.
Putting the new injectors in is quite a bit easier than getting them out. First, clean each groove where the new o-ring will seat with Q-tips and the appropriate caked-on-muck cleaner. Then put the new o-ring on each injector, and apply some silicone grease (3M Silicone Paste or Dow 111 are two silicone greases. Do not use silcone sealer) to the rubber o-ring. Ease the new injector into the port and push on it until you feel the o-ring snap into place. Reattach the injection lines, and burp the system. To burp the fuel system, remove the air cleaner cover and filter, turn the ignition on, but do not start the engine, lift the CIS plate up about 5mm to 10mm and hold it for 5 to 10 seconds. In addition to the Bosch CD ignition's high pitch, you'll be able to hear the injectors humming, which means the burping was successful. Turn off the ignition and put the air cleaner and cover back on. Before you start the engine, you might want to have a fire extinguisher handy, just in case. Start the engine up, and inspect for leaking gas, particularly at the junction of the fuel line and the injector. Assuming all is well, you're done!
Written by Bill Gregory for the "Challenge", monthly publication of the Connecticut Valley Region, Porsche Club of America.