Carl Rose

Hi everyone,

My apologies for posting this here, but you guys seem incredibly knowledgible about Bosch K-Jetronic Lambda systems. The Ferrari guys are great, but we can't seem to figure this one out.

Symptoms: over period of about 3 months, noted idle speed would not return completely to baseline & idle slighlty irregular with "pops" in exhaust. Revs freely to 7K, doesn't seem to have lost power. But this is a "new" development since purchase of car 6mo previous.

To date:

1. Compression tests fine, camshafts properly aligned.
2. Ignition system tests fine, replaced plugs/extenders/wires/caps/rotors. Prophylactically switched computer with another to no effect. Both TDC & RPM sensors tested fine.
3. FI system: replaced thermo-time switch, all injectors, all intake rubber hoses (do not think air leak), fuel filter, )-2 sensor, reseated manifold/plenum. Exchanged fuel distributor & ECU to no effect. The O-2 sensor protection relay (w/fuse, actually a 928 part) is OK. O-2 sensor oscillate correclty. TPS sensor clicks and reads properly through travel. Fuel pressures control cold/hot and system/residual pressures OK. Strange 900ohm resistance to ground initially decreasing to 45 cleaning all connections & grounds I can find. Fusebox looks good no hot relays.
4. Exhaust: replaced all donut gaskets (no air leaks), no loose baffles in muffler, converter mesh looks OK.

Removing O-2 sensor from circuit (running open-loop) smooths idle slightly. Richening baseline mixture improves but does not eradicate symptoms (i.e. don't think it's a baseline mixture issue). Have checked baseline CO with Gastester but below 1.0% (factory setting 0.7-0.9% pre-cat) seems to be stumbling due to lean mixture.

For now, we have disconnected O-2 sensor (converted to K-Jetronic without lambda) set mixture to best running, and run suplementary ground (now 6 ohms). Sortof a band-aid but hasn't properly fixed problem. The lamda systems

At this point I am considering replacing: frequency valve (rationale fuel flow vs actual duty cycle might be off), WUR, and no other great thoughts. Does this sound familiar to anyone? Any and all input (prior to pushing off cliff or selling!) greatly appreciated.

Thanks for your time,

Carl

PS: My father opined I should sell and purchase 911 (which I intend in addition) but quite vexing that I cannot seem to solve over past 6 months.

Dennis Wilson

Carl,

An internally leaking deceleration valve (if equipped) would cause the symptoms you described. Also a sticky/misadjusted accelerator cable could be holding the throttle plate slightly open and cause the high idle and slightly rich down throttle.

Dennis

Steve J.

Carl - Check the frequency valve - maybe stuck open or closed, or no voltage to operate it. Sounds like you've covered about everything else. Stock CIS 928's did not have closed-loop lambda control, but it was sometimes added to euro models when federalizing to meet US emission requirements. My '78 928 euro has the Neutronics Bosch K-Jetronic Upgrade Kit. I would like to see photos of the Ferrari CIS.

docmirror

Did you do the cold start enrichment circuit test? if not, from cold engine, start and do not press pedals. Idle should be 2200-2800 while temp at sensor is below about 70C. Drive car until fully warm and do the cold start valve test. If the circuit tests okay, check the solenoids for intermittant leaks. I think you have the same setup as I do on the 308QV, depending on year. There should be two solenoids, one for fuel, and one for bypass air. If the fuel injector solenoid is leaky, it will cause a hunting idle, and popping in the exhaust.

You mentioned you think it's lean. On what do you base that? What are the color of the plugs after problem idle? If you have a device called a "Colortune" you can tell the mixture is either rich or lean. There are many vacuum lines running under and around the plenum. I know you checked them for condition, but go back and check for routing and connection location. A handy-vac pump is good for testing these lines.

The freq valve is pretty reliable, but could be the problem too. They are all the same for the K-Jet I think, so if you find one off another K-Jet car, try it and see. K-Jet came on many cars from the '80s

Carl Rose

Thanks guys for good input. To follow:
* accelerator cable is free (not sticking). These cars also have a notorious "floor mat cruise-control" feature...!
* FV is buzzing properly when running, and disconnecting causes engine to falter. My thoughts on replacing: perhaps the DC shown (interestingly, about 72-72% on 328s) is reflecting electrical input signal & not actual valve operation. Do you see FVs fail often?
* The CSV (cold start injector) isn't leaking and only spraying when activated by TTS. The engine does have a thermostatically controlled valve system (one for each manifold & a central dome-shaped device) to allow recirculation of air into the intake when warming up (emissions-related). None of these are leaking (verified by my vacuum pump). System is almost identical to late (84-85) 308 QV cars; it's actually a conglomeration of K & KE-3 Jetronic.
* Oh, forgot to mention: disconnecting fuel safety switch doesn't influence.
* Vacuum lines under plenum actually replaced (again courtesy hose from friendly Porsche dealer) which didn't change symptoms (thus doubt leaking). Lifted & re-sealed intake & plenum.
* Considering car is running "lean' as richening mixture with CO adjustment screw leads to slightly smoother running & sulfur smell from exhaust.

I've tried to be extremely thorough with this but, as noted previously, as unsure which direction to go next. And since it was running properly before - and something spontaneously changed - realize I haven't isolated the problem. Suspecting also might be something relatively simple that, when repaired, will immediately correct. Fortunately this is not intermittant.

Once again, thanks for everyone's input. Any and all furhter suggestions appreciated.

Carl

hupp

Sounds like unmeasured air is getting into the intake, thus screwing up you idle and mixture (leaning it out). You mentioned the reseal of the manifold and that the hoses have been replaced, but have you checked all other potential air leak points?

docmirror

Yep, this has all the elements of a vacuum leak. I say again, go back to the vacuum hoses and recheck not the condition, but the locations too. I had a F-car mechanic put one hose on the wrong side of a valve, and really changed the car.

Vacuum leaks at the mainfold can be discovered with a can of carb cleaner. Sitting at some idle point, spritz the manifold-head junction and anywhere else that you worked. If the idle goes up quickly, you've found a leak. Spritzing around the plenum in general won't really help. Use it as an isolation tool.

SharkSkin

Another way to check for vac leaks if you're worried about carb cleaner on painted engine parts, is to use an unlit propane torch to spray raw propane gas on possible leak points. Do this outdoors or in a well-ventilated area, and make sure you have a halon extinguisher handy whichever method you try!

hupp

While we're on the subject of CIS performance:

Do any of you CIS folks notice odors emitting from the exhaust? I've been tweaking the mixture adjustment and the smell continues no matter what. Just wondering if anyone else has a smelly exhaust.

Carl Rose

OK shall try the propane torch bit (can't bring myself to spritz carb cleaner on immaculate engine!) Will WD-40 work as well? Vacuum leak was one of my first thoughts but...goodness, Have been through almost *every* hose in there and disconnected (and plugged) the ones I couldn't get to. Have also seen (in Bosch book) folks "pressurizing" intake with compressed air. Does this technique work?

Thanks yet again for everyone's sugegstions.

Carl

PS: Can errant fuel pump cause these symptoms? Reason I discounted was revs to 7K just fine.

hupp

Could also try the soapy water technique. Dab it on the areas of interest and see if it gets sucked in.

SharkSkin

WD40 will probably work OK too, but leaves a mess.