No RPM drop when removing oil cap after engine rebuild
 

I just got the car back from an engine & tranny rebuild and it runs good.......However the high idle when cold last to long (10 min) and then will hunt for about a second before settling in at 950. I pinched the vacuum retard line and got a good increase in RPM. I remove oil cap and nothing:confused:. I am thinking that there is a massive air leak and the co was set to compensate? Could the AAR valve cause this? How about the oil tank breather lines? All fuel pressures are spot on. New vacuum lines and intake boots were installed. Before rebuild it did drop 150 rpm with oil cap removed. The reason for rebuild was broken head studs and broken valve spring. Not sure where to start looking. This is a us 1983 911 sc.

pmead,
Do a search for smoke or vacuum test to find air/ vacuum leaks.
Introducing smoke or lightly pressurising the engine after spraying a soapy solution around the engine using the exhaust on a vacuum will help pinpoint these for you.
Tried and testing method for finding these leaks.
Cheers
Bill

There is an air leak. If massive (all relative) car would start from the cold start injection fuel then immediately die with a backfire through the intake. (My experience).

Side commentary: Did you wait until the car was pretty warm (after the ten minutes) to test the oil cap rpm drop? I don't think it will drop when first warming up. However, your symptoms indicate air leak so you may get a drop but it will not be much (assumption)

Bill's response makes sense. Are you the person who should be doing the troubleshooting work?

until the motor is broken in & the rings seated fully there will be a great deal more crankcase pressure due to the ring blow by. this may be the case, so drive it some continue the break in and see if it improves.

also, oil breather cans can be over whelmed at the outset after a rebuild due to this increased crank case pressure.

Bob, I checked the oil cap after a 20 mile drive so car was up to temp. This is my 3rd sc and all would drop rpm's with the cap removed, In fact, if I was uninformed I would probably be happy with the car. I will have to go through the process of finding the air leak. I was just trying to luck out by seeing if there was a common area to look after the engine install. My engine builder is 4 hrs away so I will be the one finding the problem. The guy I used for the engine/tranny work is very well respected in the Porsche community near Harrisburg PA. I was pleased with his engine rebuild knowledge but I am starting to question his trouble shooting of the CIS. I will look at the post for the vacuum cleaner method. The starting fluid test shows nothing. I assumed that it would be a major air leak for the cap test not to work.

You have a leak(s).

Did you spray the fluid at the base of the intake runners where meeting the heads?

I have been using brake cleaner. It's less volatile and has a plastic squirty thing.

The brake booster hose has a lot of linear feet you do not see from the engine compartment.

Hopefully not hijacking, but asking a helpful question: would capping off the port that goes to the brake booster(not during driving,) be a good way to test if that was the culprit?

Cap the port, then remove the oil cap?

Did you do the rebuilt yourself?

From your write-up it sounds like someone else did the work? If true I would start there and ask questions.

Cheers,
Ingo

I cannot see a reason why not. Cap it off on the engine side for testing.

If you can get to the hose that leads to the brake booster, suck on it and you should be able to sense some form of vacuum.

I just r&r'd my whole CIS and I've found another vac leak! It's small because I can remove the oil cap and the revs drop.

It's the vacuum venturi device that comes off the left side of the airbox and feeds to the brake booster. It appears that there is a rubber o-ring inside this NLA part. No doubt cooked.

Just thought I'd mention it because it's not really something that's mentioned when we chat about air leaks. Apologies not trying to hijack but I thought I'd mention this as another possible source of leaks.

Hopefully McMaster Carr has an o-ring for it.

Thanks for the responses. I did not get to do any searching today. I drove the car to work today and I am amazed how well it runs with a vacuum leak. I did test the idle air adjust screw and the car dies as it should when I turn the screw clockwise. (with a major air leak I think the car would idle slow and rough) The only symptom I have other than the oil cap is when I let off off the accelerator after a hard pull and let it return to idle I get a momentary rpm drop to 750 right before it settles in at 950. Cold start and warm starts are ok. I am thinking of just driving it and seeing if it gets worse.:confused:

I am well aware of the opportunity for leaks in the CIS system and agree with many who have posted above on where to check. especially after a reinstall of the system onto a rebuilt motor, lots of lil things can be tweaked n upset. I also submit to the court that as I note above a new motor, pre ring set creates a ton of crankcase pressure at the outset while breaking in is underway. I elude to over whelming an oil catch can after a rebuild when carbs are in place above. this increased crankcase pressure that is vented into the airbox with the CIS system can be causing a lean condition by actually increasing induction pressures? also, if the crankcase pressure is higher than normal removing the cap may not affect the idle as much in a broken in motor.

I had a freshly rebuilt motor with all new everything and a motor & compartment you could eat from. 1st time out after dyno tune & break in my oil return can which had always been sufficient pre rebuild was way under prepared.. it blew out and all the previous winter's work on pretty was drenched in oil.... the pressured created are very high and could skew an older, simple injection system like the CIS until broken in at intake. in fact, if you've over filled the oil at all you may have drenched your airbox...

The oil level is a little below the full mark on the dip stick with the engine warm and running, so I think I am ok there. I am driving the car to Hershey this weekend 150 miles each way. I will see what the results are after the trip. I will do an oil/filter change after the trip and check valve adjustment as well.

the drop to 750 then back up to 950 says you are too rich.
if you remove the cap when rich the idle may not drop, or like my 930 it will increase.

there should also be a filter/restrictor in the hose form the tank to the rubber boot on the AFM, make sure that is clear.

the way I use to set the mixture on my 77s was to rev the engine and let the RPM drop. I would adjust it rich until it did like yours does, then back it off until it did not do that,
I would also let it idle for a while and make sure it did not start to surge.
when I got the LM2 and could actually check the mixture it was right were it should be.

Thanks I did not know mixture would have anything to do with the cap test. I will lean it out a little and adjust idle accordingly and see if I can lose the rpm drop. It takes less than a second for the rpm to settle. (I will go small amounts on the mixture screw) I may not get to it tonight unless it stops raining in slower lower Delaware

I just noticed my engine builder disconnected my decel valve! I called and he said the car Idle was erratic and unpredictable but after disconnecting the valve things got better. This probable explains my small rpm dip before idle? I still drove the car for about 20 miles stopping and tweeking mixture/idle. I think I have it dialed in very close now. Rock steady idle, good cold start, hot start, and only a slight quick rpm dip before idle. Still no rpm drop when oil cap removed.

Do you have a timing light? I would like to see where that sits.

Tweaking AF is not knowing. If T77 says dropping is a sign of rich, I would focus on this.

I checked timing with both vacs disconnected/plugged. 5btdc So base timing is set correct. Total advance is 32. I am wondering if my vacuum retard is not working correctly or just dirty sluggish dizzy.

CIS troubleshooting........
 

Phil,

If you have not tested and confirmed the absence of unmetered air going into the system, you are hoping everything is OK which could be contrary to the fact. And by tinkering the mixture setting and having a vacuum leak some where make the troubleshooting more challenging. Test and verify the absence of unmetered air, then proceed forward.

Tony

Tony always confuses people with the facts.:)

You cannot discount false air with certainty by any stretch.

Please do this. Let's say six hours of work (Bob hours cause I am old and slow). Nail it with certainty. With extreme prejudice.

That said, your timing is absolutely incorrect. 5 btdc is correct with retard hose plugged. Advance hose does nothing until off idle. It does not need to be plugged. 32 degrees max advance is impossible with an 83 SC. 18-22 is what you should see at 3,500 hoses connected (hell at 2700). They max out quickly and they are wimpy.

Double check your dist hoses. Advance is hooked in the rear of throttle body. Up high above the throttle plate. Retard plugs in behind the air cleaner cover on the driver's side. Down low and below the throttle body. The advance hose hooks on the driver's side of the vacuum pod. Retard port is closer to the middle of the car