hupp

Bill,

Sorry if this question is redundant, but....Did you actually swap the injector or just the wires?

One possible explanation for the "after-warm-up" miss (that can be quickly ruled out depending on your answer to the question above) is that the injector performs poorly when idle fuel flow demand drops after warm up. Fuel demand is up when the shark is cold so the higher flow could be masking the problem.

My shark has a similar issue and upon inspection, one injector has a very poor spray pattern when fuel flow is low but improves after fuel flow increase by just a small amount.

Tony

Just a thought to add to the other idea mentioned. Im at a loss others wise. Any issue i have with idle smoothness gets more pronounced when the engine is warmed up. It doesnt surge or anything...just a wee bit of lumpyness to it. No biggy really.

Bill Ball

Tony:

That has been my theory too. Originally I mistakenly thought the bypass dumped air into that area, but instead it just allows recirculation out. I guess it could still lean the area. Certainly, it can disturb flow either way. So, I thought it would be best to move the bypass more towards the center of the rear section of the manifold plate. Haven't done that yet. I did put a plate under the bypass that restricted it and directed its draw towards the center. That made no difference. It's certianly very coincidental that the bypass is right next to cylinder 7, which is consitently the dead hole, although it's not much further from 8.

I don't think another person has made this observation. Most idle issues have been tracked down to vacuum leaks. You don't have the same dead hole at idle issue, do you?

It has absolutely no impact on the car off-idle, and the car performs perfectly on the road, so it is a backburner item for me. It's striking how well the simple box manifold works. I'm still getting better gas mileage than before the kit.

Bill Ball

I swapped the injectors - no change. New plugs and wires - no change.

I see Tony reports some "lumpiness" too. That's it. S4s should not have a lumpy idle. Mine was previously butter smooth. Sometimes when really warm, it is a regular miss and can be shown to be defintely be a dead hole at #7. I'm eyeing the bypass, as Tony suggests, but am not sure of a good way to test the theory short of moving the inlet.

Bill51sdr

Um, actually Bill, I did make this observation both here and to you at Sharks at the Lake. I'm pretty certain you have a vacuum leak near #7 and it may well be that bypass pipe. A miss that goes away off-idle is almost always a vacuum leak, in my experience. It may be a flaw in that manifold, you will find out when you pull it...

Bill Ball

Tony, et al:

I'm not sure I understand the bypass. At idle, I've got high vacuum, so what is the bypass doing? I was told it prevents boost at idle when you don't need it. What boost? If there were no bypass, what would happen?

Bill51sdr

I believe boost at idle would severely lean out ALL cylinders, resulting in a very rough, loping idle... very undesireable circumstance

racerx7

I agree 100% with Dennis. A miss at
low rpm is most likley some kind of
leak (intake, vacuum, head gasket, what ever) as the rpm go up the
amount of air leaking becames insignificant and the car runs normal.

On a large powerful car like a v8 928 I don't even know if notice a miss on one cylinder if you where note looking/listening for it.

Bill Ball

Hi Bill:

I meant other Supermodel owners. I've suspected the bypass could be involved since I identified it as dead #7. I can find no evidence of a vacuum leak anywhere.

Bill Ball

I did a compression test but not leakdown. All the cylinders on that side were 200-210. If I pull the #7 plug wire and/or injector lead, the idle does not change but the off-idle performance becomes terrible. Pull any other cylinder plug or injector wire and the idle drops and roughens.

OK, I have a leakdown tester and will do it this weekend, but I have no reason to believe the problem is internal. The car is a beast otherwise and has run 100 miles at 145 MPH with miles and miles at 165-170 twice already this summer.

Bill51sdr

Ok... If you recall, we discussed the possibility that the leak on #7, if not caused by the bypass tube, may be an improperly sealed intake port (7), inside the valley where the intake & port meet, inaccessable to any typical external vac leak checks. Like I said, you may have to remove the manifold to investigate this. I hope it turns out to be something much more simple

Bill51sdr

I wouldn't waste my time Bill. The problem goes away off-idle. It's a vacuum leak.

SharkSkin

I don't know if this can be done with the "Keel" manifold... but if there is even the tiniest access route, say near the water bridge or bell housing, maybe you could get some 1/8" i.d. copper tubing, kludge it onto the end of a propane torch, and spray propane gas(sparingly) around the valley side of the #7 intake port at idle to see if anything changes....

Robert_H

Hey Bill,

There is a guy located in Alamo who has always helped me out with problems on my 928. He even has a lift. He's a good guy, name is Bill... wait, sorry.

You're screwed.

Rob

Bill Ball

Rob:

LOL. Fortunately I am getting good suggestions here. This is one of those things where it helps to have somebody checking your work. I've probably overlooked something that would be obvious if you saw it, like when I put a single filamament bulb in a dual filament socket.

BTW, Sunday is lift day. The family is away and Brian is coming over for a look-see and oil change. Come on by any time after 10am.