Anyone Using Control Pressures Outside of Range?
 

In another thread I am documenting start and run peoblems and WUR adjustment. Despite CPs within spec I am still having problems @ idle. Popping and banging can indicate several things, but one is a too lean mixture.

Since I have a modded engine I am wondering if the chart for my MY no longer applies. Can this be possible? Earlier tests w/ lower CPs produced better running until the mix became too rich @ higher engine temps.

Does anyone have experience w/ running their CIS engines @ control pressures other than specified? What CPs did you use?

I had mine in spec, but still had a cold start/stall issue. I recently shortned the pin in the WUR a bit (.005") and eliminated the problem. I did also need to reset idle bypass and throttle stop as well as the CO adjustment on the FD to get it to behave again. Obviously I changed a bunch of things that are at cross purposes. Car starts and runs great, so not too far off.

I have not yet rechecked the CPs, but I plan to and also plan to put an O2 sensor on it with my new h/e that has a bung welded into it. This is the only real way to know, the CP is somewhat arbitrary, especially with a modded engine.

If you are too rich at high temps that means that your WUR is not shutting down enough flow. You might have to sacrifice some cold running to get good hot running. The basic idea of the CIS system is to measure airflow, so it is pretty foolproof. If your mods include cams that cause a lot of reversion into the intake this could be the cause of the lean idle condition. I would need to think this through, but I'm wondering if you could do something with the FPR to cheat the system.

Thanks for that, Art.

Anyone else?

i think that after a while, 20- 30 years of service, the spring in the WUR just gets "tired" and they just need to be replaced. i dont know what all you have done to your 2.7, but as long as the rest of the cis is stcok you should use a stock WUR. i was thinking about all the work you must be doing on these WUR's while i was enjoying my smoooooth running 2.7:D
as far as adjusting or checking them, dont forget it will make a difference on a vacuum control WUR if vacuum is applied or not.
if the car is off and FP is running and you set the CP then add vacuum, the CP will increase. the vacuum on mine changes the CP by 1bar.

i was thinking about this on the way home yesterday. if the CP is outside spec, you may run out of mechanical adjustment with the mixture screw. also, the plate in the AFM may be in the wrong part of the "bowl" to properly meter the air. for instance, if the CP is too hi, it may lean out at hi RPM because the plate is physically still in the mid RPM section of the bowl. its hard to explain on the computer.anyway, pg 26 in the bosch cis book shows the bowl and different operating regions.
whether you are at 3.4 or 3.8 bar warm, as long as you set the idle mixture correct, the AFM will determine mixture based on air intake....basically.

if you dont have the book i can send it.

Is there a procedure to get the plate back to 'the factory starting point'?

is the frequency valve still in the system and functioning? 80-83 had them.

Modified 930s commonly run higher than spec system pressures. The fuel head is shimmed to adjust. There are also a few companies that "modify" fuel heads for higher flow, part of which is to adjust each fuel cylinder out of spec high. You need an adjustable WUR to tune the system for proper operation once the flow/pressure has been changed.

Before you consider doing any of that I would invest an A/F gage and get some data. An adjustable WUR might be all you need to solve the problem.

T779S,

You are right. There is obviously a limit to this. I have a feeling, however, that my graph will prove to be not too far from the stock range. See my latest post today in my other thread re testing.

Thanks for the offer. I have both the Probst and Bosch books, and the Bentley, the one that came w/ the car from Porsche/Bosch, the Clymer, the Chilton's, another one I can't remember th author of, and John Weir's old one on VW tuning for the "Compleat Idiot".

Porsche_monkey,

The plate should always return to the point at which the top edge of the plate is even w/ the top of the shoulder of the cone in the sensor unit.

JW,

The frequency valve is long gone. I have read some posts online where people used the FV to tune their WURs. Alas, mine is gone. I have also read about people who put a 15 ohm variable resistor inline to tune the amount of time for the WUR to warm up.

RarlyL8,

I think I am very close to finally getting this thing to warm up and run correctly. Mark87Turbo has kindly offered me his AF meter. Once I get a good start/run cycle, I'm going to test and (if reqd) reset the CO. If I must, I am considering the programmable WUR. For now, all my WURs have been made adjustable per the "Upfixin'" mod.

See my oher thread for today's (quite good) test results.

Paul,

If it will help, I have a WUR from an 80 SC. I could loan it to you for testing purposes. I don't know the condition of it though - it was on the engine that I bought and rebuilt. I used the CIS off of my 82 and never saw the CIS on the 80 in action.

BTW, I knocked the pin in on my WUR trying to fix a cold start problem on my 82. It didn't work even though it brought my cold control pressures to within spec. It still ran great warm and cold though - it still takes 2 key turns to get it started.

However, I replaced the intake boots and the injector / injector sleeve o-rings during the rebuild. This may have solved the cold start issue but I don't know yet b/c I am still sorting out the exhaust and alternator.

Let me know if I can help. You are welcome to borrow any of the CIS components off of the 80.

Cheers,
Craig

Wow! Thank you for your generous offer. I am currently experimenting w/ an 80 that I got new. Like you, I'm not convinced that the charts are correct for a 30 YO engine, especially modified . I think I am getting close now. See my other thread for today's test results.