Carl Fausett

I get a fair number of questions about "tuning" the mechanical injectors of a CIS system - thought I'd show you somthing interesting here that just happened.

I had a 16v K-Jetronic with questionable injectors that was running oddly in the shop. A study of a couple of the injectors showed poor spray patterns - so I decided to order up a whole new set of 8.

Installed them, and the car ran worse. Pulled the new injectors and sent them off to Marren Injectors to have them flow-tested - and the results are shown below.

You can see that at a constant test pressure of 60 psi that the CC's of flow from one injector to another would vary by as much as 30%. Not acceptable.
Fortunately, I also shipped along about 20 other used injectors to have them flowed too. Eight were junk, the rest OK.

Q: How do you "tune" CIS injectors? A: You can't. Not adjustable. BUT, you can flow test a bunch of them and try to pick 8 that are within 10% of each other to use. That's what we did and the car runs great.

Marren charges $10 an injector just to test the spray pattern, opening and closing pressure to tell you if it is OK or junk; and $16 per injector to provide a computer report to you of what each injector flowed. I had a 3-day turn-around time. Their phone number is on the picture below.

One more thing: see that injector that flowed 265 cc's? Rich is good, right? Wrong. A number that high above spec indicates a weak sprung in the valve within the injector.

OK - two more things: disregard the row on the report that says "dirty"
They cannot back-flush a mechanical injector, so the computer will say it's dirty all the time on these reports.

Carl Fausett

Trying to imbed the picture again...

Scott M.

Carl;

Curious, whom did you get the new injectors from??? Not trying to slam anyone, just curious if they were an OEM, Porsche or aftermarket unit.

Scott

Carl Fausett

They were all Bosch OEM units purchased from a very reputable Porsche dealer. The R&D has gone into electronic injectors for some time now - the manufacturing tolerances and quality Control may be higher on the electronics - these mechanical injectors are far more basic, and whil it is tru the do not have to cycle at all so they can optimize the atomization and flow better than an electronic - there has been no new developments in mechanical injectors for quite a while.

Carl Fausett

They were all Bosch OEM units purchased from a very reputable Porsche dealer. The R&D has gone into electronic injectors for some time now - the manufacturing tolerances and quality Control may be higher on the electronics - these mechanical injectors are far more basic, and while it is tru they do not have to cycle at all so they can optimize the atomization and flow better than an electronic - there has been no new developments in mechanical injectors for quite a while.

jpitman2

Very interesting stuff. I replaced 4 of mine after cleaning and testing- 4 were marked as poor leakdown or opening. Curious about your remark re back flushing - my wrench says he has the gear to lift the pintle off its seat and back flush to clean the internal filter. The proof is in the pudding - it runs like a swiss watch hot or cold. In the airplane industry there are people who will flow match injectors to cylinders with the aim of reaching identical EGT so the mixture can be safely leaned out without tipping any one cylinder into burning valves, which is normally the limiting factor.
jp 83 Euro S AT 48k

Randy V

Good stuff, Carl.

I had problems with injectors that appeared to have a good spray pattern, but flow testing indicated they were way out of spec.

Interesting that new injectors off the shelf tested bad.

Can I call you a CIS guru? I've got a recurring problem with my '79 that no one has been able to diagnose. Didn't want to hijack this thread.

Jim bailey - 928 International

I wonder how they would flow at their normal operating pressure of 80 LBs ? the spring and pintle arraingement is not supposed to meter the spray but only crack open above the threashold pressure at which time it should be wide open . According to the WSM the CIS injectors "open at 3.0 - 4.1 BAR " !!! Note that 4.1 bar IS 60 LBS so the above mentioned "test" sucks when done at 60 lbs as some injectors may just be opening .

Dennis Wilson

This may be why the later model Mercedes, Audi and Porsches with CIS had individual external adjustments on the distributor for each injector. My 81 931 also has this set up. BTW the CIS Mercedes V-8 has an adjustable distributor that appears to be a direct bolt on to our early 928's with minimal replumbing due to the frequency valve. At $900 for a rebuilt, I'll let someone else try it out.

Dennis

BC

This CIS stuff is great. My "new" 78 seems to have some sort of on-off switch in the power band. 1-3k - OFF, and then 3-4.5k - ON - VERY ON, and then it dies at 4.5k. I can bare;y get to 5500 in the higher gears. I was thinking the CIS was a bit lean - the spark plugs are a light tan - seems in spec to me.

Any ideas Carl?

What is the throttle response supposed to be? Quick or delayed?

Carl Fausett

Brandon - the throttle response to the CIS system is very snappy and responsive when it is working correctly. There are no delays. When you say that it comes in hard in the mid-range yet dies out at the upper end, that can be a weak pump, low volume, old filter, clogged inlet screen, I can think of several things. You need to get a air-fuel ratio measuremeant on your car - it does sound lean to me too - but without a Air/Fuel tool or measuremeant your just playing. Several ways to get it done: Buy a $100 Grunson CO tester used on eBay; go to the dyno and have your Air/Fuel ratio checked across the RPM band, or install a wide-band sensor and gauge in your car temporarilly or permanently.

Jim Bailey - You are exactly right... Porsche does say the opening pressure for those injectors can be as much as 60 psi. Bosch says they open at 45 psi in their manuals... wonder what the difference is. On those that I have tested, they all opened at 45 psi on the nose with little variation. I run my system at 85 psi, I can test them at 70 to be sure we get away from the "just openiong at 60" point.


Jpitman - yeah, I have had a couple of different injector cleaners tell me that either its IMPOSSIBLE to clean and backflush the inside of the CIS mechanical injector, or it CAN be done, but they do not have the equipment. Who's the guy you used? I'll try him... I'd love to find someone who can. It has been recommended to me to go to a diesel injector cleaning house as many diesel injectors are mechanical and they might have the equipment for it.

Randy V - I'll take a stab at your intermittent problem... what's up? PM me if you want. Or, my 928 Tech support toll-free number is 877-for-928M - give me a call.

BC

Thanks Carl. I'll look through the reciepts on the car - it could be any one of those things. It also didn't sruge with the old East coast gas in the car and now does with the crap ox- gas we get in the People's Socialist State of California.

jpitman2

Carl,
I think I am a bit far away for my guy to do your injectors - down in Oz. Do Bosch list CIS injector cleaning kits?
jp 83 Euro S AT 48k

Carl Fausett

jpitman - not to my knowledge.

SteveG

Carl: Great ino.

All: http://www.cruzinperformance.com/

No affiliation. I used Rich above. Graphs, cleans $11.00 ea, + S/H.