JayPoorJay

Hello Good People.

Been enjoying driving and working on my 928. Well, not driving too much due to winter, but thankfully we had a mild one.

Still!!! Lots to learn for sure and still don't know enough to be contributing here and helping others as I would like - but, anyroads,,, In time I guess.

Wondering if folks could help me with a thing...?

I recently changed out the orings for the underside of the throttle body and the lil rings of the shaft in the throttle body. For a while all was good. The idle started to creep up so I adjusted the air fuel mixture screw. That helped til it bottomed out, and no more. My idle sits around 8-850...

Today, digging around for the cause, I disconnected the second and smaller vac line that connects to the T (other goes to the ISV). It comes forward and connects to number 15 in the image below, then from there to the carbon canister thru the "shift valve", number 14?

Anyways, when I plug/block the hose from the T to the line that goes to the carbon canister maze and the oil inlet/neck just mentioned (with air mixture screw bottomed out) the idle DROPS to below 600 and is controllable again with the air mix screw. So, it's either the vap canister set up or the oil fill/crank breather stuff, right?

GURUS needed.... What do I need to do? And what needs looking at, repairing or replacing down stream of #15, the "control valve"? Am I even on the right track? There is a MARKED screw on the front of the control valve but I didn't think it wise to start monkeying around with that - without direction.

Is it time to start digging into the RH fender?

JayPoorJay

700$ for the #15 gizmo in the diagram? Is that for real?

Hmm. For the V12 5.3 Jaguar XJS, because we know (in most of USA) our cars will never be hyper scrutinized in inspections due to age,,, we simply (proudly) remove all of the vapor recovery stuff - makes life so much simpler.

Has anyone done that? I'm tempted. Maybe just figure out a way to route a bit O vac to crank case to save seals and what not...?

The Forgotten On

You can just bypass that metal valve (#15) and run the #14 valve on its own. It's a pretty common thing to do now due to that obscene price.

JayPoorJay

I'll do that, thanks. I've been reading a little more to get an understanding of the function of that valve and how it's controlled by venturi and variations in vac at the throttle body. Whether 15 is passive and how much etc, concerned me. I'll just hope that the temperature controlled switch at the back of the RH head is working properly and that it's controlling 14... I dunno. Proof will be if I can get control of the idle again, or not...? Anywho

I think it might be time to get into the RH fender/wheel well and see if the famous Y connection is cracked - and what else. Would that produce the extra and uncontrolled air?

Then, I've seen the Porken had bypassed it all and just connected two of the hoses in the RH fender and called it a day. But, call me dumb, I don't understand that exactly and how that doesn't result in lots of extra and uncontrolled air into the intake. I'm way behind in understanding the ins and outs of my 928. Just am.

Mrmerlin

send Porken an E mail and he might offer a better explanation of what he did.
Please post it here as many others could benefit from this idea

JayPoorJay

Right on, good suggestion... I just figured he was most likely BOMBED enough here. Folks asking questions and such. But yeah, okay...

The Forgotten On

What Porken did was to bypass the engine entirely and vent the tank to atmosphere like the ROW models did.

It just looped the 2 hardlines to make 1 long vent tube.

Personally I would just vent the tank to the airbox instead of the intake (like on the earlier cars like my 81).

That way it still vents the tank to the engine to burn the vapors, but wont cause the gas tank to collapse.

JayPoorJay

Understood ^^^

Still poking around. This is what I did/found.
Still have not had an opportunity to get into the fender.

The little gizmo, 15, doesn't hold vacuum. Partial, but not complete. I think that what I found was that with elbow and vac tube connected, at all times 15 allows air to be pulled thru the tube to component 14 when the car is at operating temperature = High Idle.

For 14, it is sealed, no air flow when cold. Once the car gets up to temp, open. If I remove the vac tube from the back of the unit it closes.

This is what I did (simple stuff really), and the car runs and idles MUCH better. Much. Much much less engine roughness running,,, and at idle.

I sealed the small vac tube controlling 15. Hose going to 14 is wide open. Sealed/plugged the line coming from the T to item 15, and sealed/plugged the vac to the crank case at the oil filler neck. That's it. With this, with the idle mixture screw turned all the way in, I get about 600 rpms. With idle screw turned out one full turn, about 650 - feels like a sweet spot.

I'm going to get into the fender to see if the Y connector is cracked and allowing more or too much air into 14 and the T in the intake plenum via that vac tube - but honestly,,, I don't know what else to do. It's not right, but runs good. I cannot afford to replace item 15... craZy money.

I'm all ears...

Fly911

Since I got my '85 928S about 14 months ago, I have tried to find the source of my high idle (1400 rpm on warm engine). So based on this thread I took out the charcoal canister, disconnected items 12, 13, 14, and 15 in the vacuum diagram (the cats and air pump has been out for a year...), and re-routed the tank vent lines to where the cat air return line connected to the air intake (on the right side of the air filter lid/elbow). Plugged the hose from the TB to item 15, but still have the vacuum connected to the oil filler neck. No change in idle speed. I have previously changed every rubber hose and vacuum lines in the system, and planed the mating surfaces between the intake runners and the cylinder heads. Still high idle... I have also swapped LH ECU with another '85, and same high idle. I have also tried to change both the EZF and LH chips (Porken), with no difference in idle speed. I have changed the ISV and every sensor on the engine (including MAF), and checked every signal into the two ECU connectors.

If there is no vacuum leak, and every sensor signal is confirmed working correctly into the two ECUs, the TB valve confirmed fully closed, the idle screw fully in, what could cause the high idle?

mj1pate

Tank vapor recovery does not necessarily have to be directed into the atmosphere. I hesitate to contradict Ken. He is an engineer. I’m just a computer guy. If you eliminate that gold plated aluminum valve, which leaks vacuum by this time for most of our cars, and continue recovering vapors via your intake vacuum, you’re likely to find that your fuel tank succumbs to vacuum and your fuel pump struggles. Vacuum for vapor recovery (while eliminating the golden valve) is now sourced from what used to be a blinded port in the pass side of my air filter housing. Vapor recovery without extreme tank vacuum. I purchased an online mini filter in the vacuum path to the air cleaner since tank vapors arrive via a charcoal canister. Search threads started by me and you’ll probably see the implementation. Good luck with this

hwyengr

Did you test the swapped LH with the new ISV? There have been reports of the idle circuitry burning out by trying to drive a seized valve, so if you put the known good LH in with the bad valve, idle would still be high, and then if you put your bad LH back in and then swapped the valve, it might not be controlling it right.

But that’s just a wild-*** guess.

JayPoorJay

This might sound like a bonehead thing to say, but just for S&G, completely remove and PLUG the line going to the oil fill neck. Lots can get drawn in thru that, which surprised me, but it was, and with the air mix screw bottomed out (and the other things I mentioned) idle went to 600 or a bit above... I had control again.

mj1pate

here is the thread I was referring to. My apologies about the small pictures.
https://rennlist.com/forums/928-foru...situation.html

JayPoorJay

Good stuff. Thank you!

Fly911

I managed to pinch the rubber hose from the ISV with a long and thin pair of pliers, and the idle dropped from 1400 to 800. I will take the ISV out and check it, maybe adjust the spring load in the mechanism...(?)