931 running problem
 

My 931 has an idle problem, when it is warm it idles real high 1200-1700 rpm, the C.O. has been checked and it is ok, there does not appear to be any air leaking in and the injection system has been checked over.
On the throttle body there is a small bore hose, it runs under the throttle body and then connects to large vacuum unit that is mounted on the inlet manifold, it has a large bore 1/2" hose going into the opposite side from the small bore hose, when the small bore hose is disconnected and both ends are sealed up the car runs fine, it idles smoothly.
The vacuum unit is about 4" in diameter, it is not the cold start valve but is mounted near to it, i have not been able to establish if it has an electrical feed.
The car is a series 2 and has the DITC ignitopn fitted to it, the distributor has not got a vacuum unit fitted, could this be what it is?
Any help would be real appreciated as this is spoiling and real good car

Thanx

Doc

Hi Doc,

I'm printing your post out so I can take it out to wife's car & have a look.

drew1

Hi Doc,

That big round thing is called a Decel valve in the factory manual. The vacuum diagram shows that the two big, 1/2", lines are parallel to the aux air regulator lines, by the series parallel hookup consideration.

My wife's car does the same. I had to put a distributor with the guts in it and an MSD box on it, when the digital unit under the dash went bad. I have to run the timing high to get any kind of power. I'm "T"ed into the small line on the trottle body to get a signal to the distributor diaphrams.

What is the purpose of the decel valve? If it is to let more air in on an increase in vacuum, caused by throttle closing, the this valve is probably OK, at least on yours. If the diaphram is busted it seems like the valve would still bypass the throttle when the little line is unhooked from it.

The Haynes 924 manual gives a method of checking this valve. Unhook the hose from the intake duct, not manifold, put finger over valve port & have somebody rev motor to 3500 & then take foot off. The port should pull a vacuum as motor drops to idle.



drew1

fast idle means the warm up regualtor is jacked.

It is not putting extra fuel into the engine while warm, but the aux air valve IS working, making it fast idle because there is no fuel being dumped in by the warm up reg to take down the fast idle.

The aux air valve is like pressing the gas pedal down a little bit. It takes the idle up UNLESS you up the fuel to take the idle down.
Its like flooding your car, if you add gas, teh idle goes down and stalls the car, it doesn't stay the same.

i don't think 931s have decel valves in any year, i think that is the aux air valve that he is talkign about but i ain't sure.

Hi Guys,

we have managed to eliminate the aux air valve, this unit is mounted real close to it though.
I have looked through the Haynes manual and i caa see the reference to the decel valve, however my car does not have EGR system fitted to it (it is european spec).

Any ideas.


Regards

Doc

But back to the fast idle problem, does it matter what this peice is?

Hi Guys

I apologise, this unit is a form of decel unit,the european spec cars seem to be very different to cars that have EGR fitted.

Thanks for your help

Doc

Hi Doc,

Try checking Decel Valve as in my reply above. If it pulls vacuum upon dropping to idle but otherwise no vacuum it should be OK. If Aux Air Valve is good, it seems to me problem could be a leak in the small line from throttle to Decel Valve.

drew1