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-   -   No Throttle Body Vacuum at Idle (http://forums.pelicanparts.com/porsche-911-technical-forum/197619-no-throttle-body-vacuum-idle.html)

TurboBert 12-19-2004 11:34 PM

No Throttle Body Vacuum at Idle
 
At idle, I have good manifold vacuum but no throttle body vacuum (measured at both the distributor and upstream of the thermo valve, directly at the throttle body).

Anybody know what would cause this?

Here's what I've checked:
1. I have checked for vacuum leaks - none that I can find using an ultrasonic sniffer (has never failed me in the past).
2. The thermo valve is working properly (opens when warm).
3. Manifold pressure (checked at brake booster line) is within spec.


Background:
My distributor doesn't advance enough (advances only 15 degrees at 4000). I took out the distributor and checked and it's fine. Weights and springs are good. Vacuum advance on the distributor manually tested fine.

The car accelerates and idles perfectly when I set idle timing to 10 degrees BTDC - spec is 5 degress ATDC. This makes sense because a vacuum would pull this back to the spec value (when warm idle vacuum retard ignition 15 degrees).

Thanks.

dean 12-20-2004 05:32 AM

There are usually 2 ports on the TB. One above the throttle plate and one below. You must be on the one above.

The 930 has a vacume retard not a vacume advance. So it retards with vacume.

Dean

TurboBert 12-20-2004 09:48 AM

Nope, I'm connected below the plate at the base of the throttle body.

Re-reading my initial post, I realize it sounds a little confusing. Basically what I am saying is that without the vacuum at idle to retard ignition, you end up setting the wrong timing and missing out on 15 degrees worth of centrifugal advance (weights) at higher rpms.

beepbeep 12-20-2004 10:13 AM

Quote:

Originally posted by TurboBert
Nope, I'm connected below the plate at the base of the throttle body.

Re-reading my initial post, I realize it sounds a little confusing. Basically what I am saying is that without the vacuum at idle to retard ignition, you end up setting the wrong timing and missing out on 15 degrees worth of centrifugal advance (weights) at higher rpms.

I heard somewhere that timing should be dialed-in with vacuum-hose removed? I will check manual on this.

aturboman 12-20-2004 12:51 PM

Check to see if the port is plugged in the throttle housing. The hole is very small and I've seen one clogged from coked oil. Some of the ports are also covered by the side of the butterfly and are only exposed as the butterfly is just cracked open. The only way you can see this is if you remove the throttle body and look from under the butterfly. I know there were alot of variations of throttle bodies based and year and market. I have a Euro throttle body and California version both from a 79 turbo, they have vastly different porting although they appear similar from the outside.The factory book is also incomplete (incorrect) on the routing of some vaccum lines.

TurboBert 12-20-2004 12:53 PM

Goran,
Yes, you are right the manual says to have it removed, so my comments are incorrect with regard to lack of vacuum being the problem with getting full advance.

Anyway, it looks like I still have two problems:

1. I have no idle vacuum, so I get no ignition retard at idle. That probably wreaks havoc when I adjust idle speed and CO levels.

2. Distributor doesn't advance the specified 29 degrees at >4000 rpm. I took it apart and cleaned it and it's in perfect condition. The car only has 23k miles, so it seems strange. I also did Wayne's trick using only the starter to check premature mechanical advance - the springs aren't the problem.


aturboman,
Yes, I verified that there is no blockage into the throttle body. There was some slight resistance, but not enough to stop a vacuum. maybe I'll squirt some carb cleaner into it and see what happens.

dean 12-20-2004 01:00 PM

So you are saying your mech. advance doesn't work or you can't twist the dist to set the timing?

I always thought the way Porsche did the vacume retard sucked anyway so I wouldn't worry about that. IMO

Dean

TurboBert 12-20-2004 01:05 PM

Mechanical advance works but I only get 15 degrees total advance.

The car accelerates poorly when it's set to the factory setting of 5 degrees ATDC. I have to set it to 10 degrees BTDC to get the car to run right.

dean 12-20-2004 01:14 PM

Here is what I would do.

Disconnect the vacume retard line. Rev the motor to 4000 and set the timeing to 29 degs. I think your dist might need to be indexed one tooth. And then I would make sure the mech. advance matches the advance graph in the manual. Then you can screw with the vacume retard if you want.

Dean

TurboBert 12-20-2004 01:24 PM

Dean,
When I set the timing at idle to 10 degrees BTDC then I get 29 degrees advance at 4000 rpm. I haven't checked the advance profile against the book, but I'll probably do that.

TurboBert 12-20-2004 05:42 PM

OK, I solved one problem. Aturboman was right, the throttle body port had partial blockage. I sprayed copius amounts of carburetor cleaner into the vacuum hoses and blew it back through the throttle body. It took a while, but I now have full vacuum.


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