sparky57

My mechanic is in the process of troubleshooting a rough idle problem in my 73.5 911T. 73.5's have the first version of the CIS injection system. He's tried several WUR's and a has also tried a different Accumulator. No improvement. Ran a smoke test as well to check for vacuum leaks. Nothing. Tested all fuel and vaccuum pressures and all are in spec.

I test drove the car today from cold and the engine felt like it was loading up while idling at a stop light. Also it wouldn't stay running without the throttle lever pulled up to raise the revs. Even after engine warmed up, the idle was rough. I managed to take it for a short sprint on the freeway and it felt strong. One thing I noticed was the tach needle would drop pretty fast between shifts. Almost like fuel was being cutoff between shifts? Don't know what would cause that. Perhaps a clue to the problem?

Looking for help from you CIS experts on the forum. Thanks!

theiceman

well the first thing I would ask is did you put a set of fuel pressure gauges on it and how does it compare to spec ?
system pressure ? control pressure ? does the profile of pressure build compare with spec for that WUR number ?

sparky57

Yes, he checked ALL the fuel pressures and he stated they are all within spec. Good to know that the pressures vary for different WUR numbers. Not sure he knows that, so I'll bring that up. Is there an online resource with pressure profiles for specific WUR numbers?

theiceman

im not sure about online. Even within my Sc I think here were 2 different WURs used and the profiles are different in the Bentley

There is a guy on Pelican named Tony who is a guru of this stuff, he will ask you for your numbers for pressures and the number on your WUR , as well as the resistance you measured on the WUR with a meter.
he is the man to check with for this stuff.

pu911rsr

Check the gasket on the cap for oil tank, seems to me that could cause issues with idle stability of CIS cars.

Phil

sparky57

Thanks Phil. Will check it.

Steve Weiner-Rennsport Systems

Is it rich? is it lean? Did he check CO or AFR at idle?

Did he ensure there are zero vacuum leaks?

Did he check and make certain the distributor and the rest of the ignition system is working perfectly?

sparky57

Hi Steve,

He said he adjusted the fuel mixture until he got what he felt was the best performance. Then he measured the CO level at idle and surprisingly it measured 9%! Very high CO level. Any idea what would cause such a high CO level, if there are no vacuum leaks? The FD is not the original 73.5 version, instead it's an FD from a later 2.7L 911. He says they are completely interchangeable though. True?

He ran a smoke test and says there were zero leaks. I'll make sure he's checked the distrbutor and the rest of the ignition system.

Thanks,
Steve

Steve Weiner-Rennsport Systems

Hi Steve,

9% is FAR FAR too rich and will result in heavy fuel dilution of the oil. At that CO, your AFR is 11.16:1 which results in failed rod bearings & scuffed pistons if not addressed. You need to be at 2.5%.

FD's are not really interchangeable, however there are no more new ones so sometimes we must improvise. I would have your 2.4 one rebuilt (CIS Flowtech) as thats the only way it will run properly. Further, the WUR MUST match the FD in use as you already know.

sparky57

Thanks Steve. I think he knows that the CO was way too rich. But I guess that's where it is running best, so something obviously isn't right. Smoke test shows no leaks and fuel pressures are all correct, so he's a bit stumped. Regarding the FD's and WUR's, so sounds like a 2.4 FD can't be used with a 2.7 WUR and vice versa. A prior mechanic swapped out the original 2.4 FD with a 2.7 FD. Will a 2.4 engine run fine with a 2.7 FD and 2.7 WUR combination, assuming fuel pressures are correct and no leaks? Or will a 2.4 engine run well ONLY with a 2.4 FD and 2.4 WUR? Where can I access information on the correct FD and WUR combinations?

Thanks,,
Steve

Steve Weiner-Rennsport Systems

While 2.7 FD & WUR's interchange, the 2.4's do not and thats why I recommended having yours rebuilt.

I use my old Bosch books to determine some of the FD & WUR data, the rest comes from long-term experience. I don't know whats out there thats accurate on these old cars since most books are long out of print.

sparky57

Well, my mechanic got the CO level down to 3.0%. He used a WUR from a 74 911. He said he was able to adjust the bi metallic spring inside of it to dial in the correct pressure. Since my FD is a 2.7 version (006 part number?), he thinks the 74 WUR is working well with it. He mentioned something about early WUR's not having an extra vacuum line connection/port that the later WUR's have. Anyway, idle is much more stable now, though when the engine is fully warm there is still some very slight variation in the idle speed. I'll try to post a video in the next few days.

Jadz928

Deleted

theiceman

wow ... talk about your wrong forum, not to mention 3 years old .... a 928 ??? that has to be a first for our " wrong forum" list.