Revs drop slowly when changing gear ... why?

[billjam]

When changing gear, my revs take forever to drop down to idle. (Forever = 2 to 3 seconds.) Not conducive to fast or smooth gear changes!
Idle speed is now also higher (at about 1100-1200).

I have been pointed at the vacuum control valve (the "gold" plated one in the center of pic). My other car doesn't have this problem so I swapped the valves over and it made no difference at all.
Yes - I have checked that the throttle is fully closing and isn't restricted or stiff in its operation.

This condition gradually developed over a period of a month or so a couple of months back. Even though it is now winter (in Western Australia), daytime temperatures are still 60-70F so I don't think that the cold(er) weather is a factor.

The car has also developed strange cold starting behaviour which might be related.
If I crank motor continuously, it takes forever (5 to 10 seconds) before it crawls into life. I have found that if I just give it a short burst on the starter (about 1 second), I can just catch it firing weakly and bring it up to idle with a little gas.
I have checked the fuel pressure accumulator and it holds pressure OK.
It always fires up immediately when warm and also on about one out of ten cold starts. Don't you just love intermittent faults!

Any hints for solving one or both problems will be rewarded with a smilie.

[mb911]

vacum leak(AAR?) or springs in the dizzy worn or broken giving too much advance

[billjam]

I'll check this. What's AAR - auto advance & retard?

[mb911]

Originally Posted by billjam

I'll check this. What's AAR - auto advance & retard?

aux,air,regulator= AAR+ vacum leak while warming up then closes when warmed and should not leak

[RarlyL8]

Check/tighten your intake manifold bolts (17/18psi). Symptoms of vacuum leak. Check hoses next.

[billjam]

Originally Posted by mb911

aux,air,regulator= AAR+ vacum leak while warming up then closes when warmed and should not leak

Ben,
Looking at the hose connection plan in the manual, this part looks like it allows air to pass from IC to inlet manifold under certain conditions.
I presume this is only when engine is cold? I can understand how this might cause revs to stay high if it was stuck open.
I have another car to swap parts from so I'll swap AARs tomorrow to see if this fixes the problem.
While it would be nice to identify the problem, my heart is hoping that it is a leaky hose somewhere rather than a $974 part.

[billjam]

Originally Posted by RarlyL8

Check/tighten your intake manifold bolts (17/18psi). Symptoms of vacuum leak. Check hoses next.

Thanks Rarly, will do.
By the way, I assume you mean 17/18 ft-lbs

[NathanUK]

As kind of stated you might want to check that your vacuum hoses are connected to the dizzy at both ends and that the timing is set correctly.

[billjam]

Originally Posted by RarlyL8

There will be no harm by removing the throttle bypass valve (mine's been gone for 6 years) but the car will drive differently. The RPMs will drop quicker any time the clutch is pressed. I modulate the throttle purposely to keep the revs up between shifts. You don't have to, I just like it better that way.
Bottom line - if you don't like the way it drives then put it back. Everything CIS is reversible.

I read this on the current BOV Vac Module Delete thread.
Along with other comments here, it spurred me into action tonight to sort out my suspect throttle bypass valve (AKA AAR).
I was going to swap it with one from another car but based on Rarly's experience above, I decided to replace it with a bit of broomstick instead!
The difference is amazing. Revs drop just nicely instead of hanging there at a zillion rpm for what seems like minutes.
After I had test-run my wooden AAR, I went back in and checked the tension on the inlet manifold bolts as suggested by Rarly and found that all were very light. To get up to 18ft-lb I had to go between half and one turn on all bolts. This didn't make any further difference to driveability but it clearly indicates that this is a potential trouble spot.
I couldn't see or hear any air leaks and all other hoses look OK so it seems that problem is fixed.

Don't panic - I will replace the broomstick with something more appropriate now that I know the solution.
Thanks for your help,
Bill

[RarlyL8]

(17/18psi)

Yes, units should be ft/lb. I've got boost on the brain, ha!

Interesting idea with the broomstick. I just used large bolts.
Although it is a throttle bypass valve in function, what you have removed there is the AAR, not the throttle bypass valve, refered to previously as the vacuum control valve. The throttle bypass valve is the goldish canister attached to the BOV and intake manifold.
Put the AAR back on and see if your problem still exists.

[DSPTurtle]

Has anyone here used nylon lock nuts to keep the manifold nuts from loosening up? I am putting my manifold on tonight and am thinking that this might be a good idea since it seems to be a common occurence. Any downside?

[billjam]

Originally Posted by DSPTurtle

Has anyone here used nylon lock nuts to keep the manifold nuts from loosening up? I am putting my manifold on tonight and am thinking that this might be a good idea since it seems to be a common occurence. Any downside?

DSP, I don't believe that any loosening of the manifold nuts is related to them actually unwinding. Nyloc nuts won't help, but they won't hurt either. (They are heavier though, so there could be an adverse effect on performance!)
I think the problem here it is more to do with gradual compression of gaskets and the phenolic injector blocks than nuts coming loose.

[DSPTurtle]

Ah ha Bill... that makes better sense. I think I may go with the nylocks and use aluminum washers to offset the weight gain

[billjam]

Originally Posted by RarlyL8

Although it is a throttle bypass valve in function, what you have removed there is the AAR, not the throttle bypass valve, refered to previously as the vacuum control valve. The throttle bypass valve is the goldish canister attached to the BOV and intake manifold.
Put the AAR back on and see if your problem still exists.

I am sure you guys are just trying to confuse a turbo newbie!
Everyone seems to have different names for these parts ...


PART A is called ...
Throttle bypass valve (by Rarly & others here) *
Decel valve (Pelican Parts)
Vacuum Control Valve (Porsche manual, page 26.37)
Aux air valve (Porsche manual, page 25.18)

PART B is called ...
Aux air regulator (Pelican Parts)
Throttle bypass valve (Porsche manual, page 26.37) *
Aux air valve (Porsche manual, page 25.10)

Just for the record, I have removed PART B and it solved my problem of the engine holding its revs for too long on gear changes. It doesn't drop instantly, but it is heaps better than it was.
PART A is still in place. I swapped this part with one from another car last week and it didn't make any difference. Maybe they are both not working.

It seems that PART A can also be scrapped without too much drama. Re-reading previous posts, it seems like deleting this one will also cause revs to drop off even faster. I'll try this in a few days and post results.
Looking at Rarly's engine, it seems that both A and B have been deleted - correct?

I would love to get a clear description of all the parts that control our engines (what do they actually do and why do they do it). Can anyone direct me to 930 Engine Management 101?
Bill

[DSPTurtle]

Careful... "A" was never refferred to as a throttle bypass valve. "A" is and always will be the decel valve. B is simply an idle adjustment for when the engine is cold. BMW and Mercedes call them ICV's on the new fangled fancy injection them thar folks use. Basically is allows air to bypass the throttle plate thereby increasing the revs of the engine. It is like a mini throttle plate that is controlled by electronics. The way I best understood it was like a manual choke on an old engine. And yes, it is really expensive. The folks that get rid of them are the folks that don't live in cold climates and they just hold the accelerator open a little more until the car warms up and that valve woudl have closed anyway.
As for A... someone else will have to explain why the he!! that rusty piece of crap is on our engines in the first place. Although it seems to be like an old school carburator dash pot... not allowing the throttle to snap closed and instead easing the motor back down to idle. I just wish the plating on it was a little better and Bosch did not require a second mortgage for a new one.

[RarlyL8]

Hehe, yes the name game is confusing. A warm up regulator (WUR) is actually a fuel pressure control regulator in function with warm up as but one of the many jobs it performs. Piss poor naming scheme.

I would not be too quick to blame the problem on "B" the AAR. You did 2 things at once when you tightened the intake bolts. Put the AAR back on and see what happens.

There are half a dozen devices on the 930 CIS that bypass the throttle. The AAR, AAV, BOV, Decel Valve, and the various thermo time switches and valves. It's a freakin' vacuum leak waiting to happen.

As for "A" the throttle bypass or decel valve, that device is in place to route air around the throttle body during the high vacuum event of abruptly lifting the throttle. It does this to prevent the engine from stumbling.

Your right foot can take the place of all of these devices. They all allow the car to be easier to drive under different conditions. When I removed all that stuff I lived in Illinois. It gets pretty cold there in the winter. No problems starting and driving the car. All that is on my engine is the CSV (cold start valve), Fuel Head and WUR.

[billjam]

Originally Posted by DSPTurtle

Careful... "A" was never refferred to as a throttle bypass valve. "A" is and always will be the decel valve. ....

As for A... someone else will have to explain why the he!! that rusty piece of crap is on our engines in the first place. Although it seems to be like an old school carburator dash pot... not allowing the throttle to snap closed and instead easing the motor back down to idle.

What has confused this issue for me is that the Porsche manual (page 26-37) calls this a throttle bypass valve on the hose connection diagram - typo maybe?

The second part of your reply raises another issue. When some ICs are fitted, they replace the BOV housing and the attached decel valve. I had a B6 IC on this car some months back and it didn't seem to be a major problem with throttle shut-off. It was abrupt, and I learned to drive with it, but it did make for interesting sphincter clenching moments during sudden throttle-off situations on bends!
I hadn't really thought about the effect of this valve before because when I fitted the IC, I took so much stuff out that I didn't really identify the change in throttle action with the absent decel valve - *****, I didn't even know how to spell decel valve then!! It's definitely going to get plugged soon, even if its just to see what happens.
I am currently rebuilding the engine in my #2 car (fixing oil leaks, stiff throwout bearing and about a million dollars worth of other "while I'm in there" stuff) and the B6 IC will end up on that car, along with other improvements I am picking up from this forum.

[billjam]

Originally Posted by RarlyL8

I would not be too quick to blame the problem on "B" the AAR. You did 2 things at once when you tightened the intake bolts. Put the AAR back on and see what happens.

I've already learnt that lesson - don't change more than one thing at a time!
I actually blocked the AAR and test run and got a favourable result.
THEN I went back in and checked the manifold nuts as you had suggested. They weren't tight, but don't seem to have been leaking because nothing changed after I did that.

As I mentioned in last post, I will block the decel valve as well, even if it is just to see what effect it has.

Thanks again.

[billjam]

Just to put this issue to bed, I can confirm definitively, positively and absolutely that it is the DECEL VALVE that is the cause of revs not dropping very fast on up-shift (as Rarly and others already knew!!).
I confirmed this to myself by clamping the vacuum line to the decel valve with a pair of vice grips and the result was instant. No more hanging revs while I'm trying to get into the next gear.
My exercise with the broomstick in the AAR circuit seemed to help, but had no where near the same effect as disabling the decel valve. With both valves disabled, idle revs when cold were about 500 so I retired the broomstick and put the AAR back and everthing is just fine.
Once I've made up a special bracket to support the vicegrips, the job will be complete (it looks a bit ordinary with them just hanging on the hose).

[RarlyL8]

If you must leave the decel valve mounted on your engine at least put a rubber nipple on the vac port feeding it and one the port on the face of the valve.
I can see a set of vice grips going through your engine fan ...