Having trouble with Bosch K-Jetronic CIS

[Eiron]

My son's '85 Saab has the mechanical Bosch K-Jetronic fuel injection system, also known as "Constant Injection System" for the way the fuel is continuously recirculated from the tank to the fuel distributor & back. It has no fuel rail, but instead relies on the fuel distibutor (about the size & shape of a hockey puck) to sequentially flow fuel to each mechanical spray nozzle (the actual "injectors" sticking down into the cylinders with the plugs & intake/exhaust valves).

Does anyone have experience with these things? I think ours went bad, but I'm stumped on how to tell & I don't want to just start swapping out expensive parts. I looked at getting a fuel pressure tester at the local auto parts store, but it looks like it only works with fuel rail systems (it has a schraeder attachment for a fuel rail, & the CIS system doesn't have anything like that).

Any help would be greatly appreciated.

 

[cryptokid]

continous injection systems like youre k-jet are known as cis for the reason that they continously inject fuel into the engine. the injectors never turn on or off like a conventional system. the distro just adjusts pressure and flow according to the demands on the engine. sort of like a carby does. youre cis does indeed recirculate fuel back to the tank, but do does most other fuel injection systems.

anyways, if you mount a fuel pressure tester on the line going from the pump to the fuel distro, it should work for testing fuel pressure.

whay do you suspect the distro is bad? does the car not start anymore? usually if the dirtro goes bad, its from water in the gas causing rust in the distro's inners. although yours may just need to be cleaned up. the air flow plunger can somtimes stick causing a nostart condition, or if it does start, it wont run very good at all. its pretty easy to pull the whole assembly out, clean it up and throw it back in.

also, between the distro and the intake tube there is a small hole, and if you put a 3mm allen wrench in this hole youll find an adjustment screw. this screw is used to adjust the air/fuel rato. you need a fairly long allen wrench to get to it, atleast 4-5 inches. what i did was to mig weld a allen wrench onto the end of a sacrificial screwdriver. it worked great and cost alot less than a factory adjustment allen wrench does.

 

[Eiron]

Hi cryptokid,

Thanks for the clarification on the system’s operation. Yes, the car won’t start any more. It will fire up, but it only runs for a second or two & then dies. It has just over 150k miles on it. The fuel filter/O2 sensor/wires/plugs/cap/rotor were all replaced about 4k miles ago. I also de-tarred the throttle body, aux air valve, cold-start injector & idle screw port at that time, & ran a can of BG 44K thru the system. We bought the car in November, did all the tune-up work in December & it’s been running fine up to this point.

Here’s what happened: it started as a slight hesitation under load, then became light bucking, turning into heavy bucking, & then simply shutting down (all in the space of about 8 miles/20 min). After a few minutes of rest, it started right up & drove another 1/2 mile before starting to buck again & then shutting down for good.

Both the pre-pump & main pump work, & all the voltages appear good. I verified pressurized fuel all the way up to the fuel distributor input by loosening the fittings along the way up to that point. I also removed what I think is the fuel return line (it’s the middle (& largest) of three fittings on the side of the fuel distributor, & marked with an exit arrow) & ran the pump with a jumper wire. No fuel came out of the fuel distributor, but it did gurgle out of the detached line like it was back-flowing. I pulled out the injectors & placed them in a bowl, then had my son jumper the relay while I pulled up on the airflow plate’s center bolt with pliers. Nothing came out of the injectors. Someone later told me I needed to cover the injector holes before doing this. I haven’t checked the cold-start injector since I cleaned it out in December. I don’t yet have a K-Jet fuel pressure tester, but I’m ready to buy one if it’ll help me find the problem.

Thanks!

 

[tom slick]

there are a couple of books written specifically on the CIS system, it might be a worthwile investment.

 

[Alan]

You may want to change the fuel filter if it's questionable.My 79 BMW has pretty much the same Bosch K-Jet system.There is a mixture screw usually on the fuel distributer block that can effect idle.

 

[porterdog]

CIS is a good system; simple, reliable, relatively easy to service, but you do need the special tools to work on it. As I recall, I got my CIS pressure meter from JC Whitney for about $50; I've used it on at least 8 cars so far, so that's cheap!

Given the fact that it starts and runs for a few and then dies I'd say your cold start valve is working fine- it only squirts during cranking anyway.

The issue is the lack of flow from the injectors with the pump jumpered and the plate raised. On many of these CIS cars Bosch installed a screen on the inlet to the fuel distrubutor that never got removed. I guess to protect the FD from any swarf in the system; these were supposed to have been removed at the first dealer service, but you know how that goes so I'd check it first.

Despite the fact that you have fuel flow in the supply line, you may have a pump problem as the CIS injectors won't crack open until there's a certain pressure behind them. If you're lower than that, well, there you go

.

Another possible culprit is the.... (****, what is that thing called...) differential pressure regulator. It's a small electronic box (size of a zippo, approximately) screwed to the side of the FD. Take that off and shoot brake clean into one orifice to ensure that it's open all the way through (safety glasses must be worn). If it's clear then it shouldn't prevent it from running.

Finally, the FD's do go bad (and are ugly expensive, too...). Take the metering head off and make sure the piston that cryptokid mentioned is free to float.

There is a good CIS book available by Probst which I highly recommend; it explains everything.

Best,
Robert
(CIS cars owned/worked on:
79 rabbit,
85 jetta,
87 golf
87 GTI,
86 audi 4000q,
87 audi 4000q,
and a couple old 911's)

 

[Eiron]

tom slick,
Do you have a favorite publication you'd recommend? I need something that has a good trouble-shooting section.

Alan,
Well, I changed the fuel filter less than 5k miles before this happened, & it's a pretty big one. About the size of a softball. When I pulled out both pumps to check 'em for operation, both of their debris screens were still intact, so it doesn't appear as though much could've gotten thru. And these filters are supposed to last 30k miles. Is there something that could pass thru the pumps' screens & still clog the filter this quickly?

Could the mixture screw change its setting on its own? The car went from fine (the same condition it had been in for the previous 5 mos) to dead in about 8 miles/20 min. Is there a lock which might've come loose?

porterdog,
Thanks for the tips!

Yes, I'm ready to buy the JCWhitney tester (now $66 w/shipping, or $71 if I need it in less than two weeks), but money's tight & I wanted to perform all the "free" work first.

Is the inlet screen set inside the fitting between the tank supply hose & the FD? (Isn't there another "inlet" fitting that feeds the four injector lines?) If the pressure regulator is the thing with the shims, I was going to pull that out & clean it when I pull the whole FD out this weekend. Any good sources for replacement FDs? I figgerd they were pretty expensive, so I hope that's not it.


Thanks for all the help guys!

 

[Alan]

Be sure to also check for vacuum leaks.Just today I forgot to securely reinstall the dipstick into the tube,sure enough the car wouldn't idle for crap.

 

[porterdog]

quote:

---

Originally posted by Eiron:
Is the inlet screen set inside the fitting between the tank supply hose & the FD? (Isn't there another "inlet" fitting that feeds the four injector lines?) If the pressure regulator is the thing with the shims, I was going to pull that out & clean it when I pull the whole FD out this weekend. Any good sources for replacement FDs? I figgerd they were pretty expensive, so I hope that's not it.

---

As I recall, the screen filter is in the main IN line to the FD. The DPR that I am familiar with has no shims, but there were several different iterations of CIS; does yours have a knock sensor and/or O2 sensor? I can put up a pic of a DPR later if you need. As far as used, find a good Saab scrapper and see what he's got, or look to find a VW cross reference as they may have more installed units available at the junkyard.

While vacuum leaks can definately cause MAJOR CIS problems, a cracked inlet hose won't prevent your injectors from spraying with the pump running and the plate lifted.

Robert