Big Dave

I had previously posted a question regarding my 85's poor idling when cold. I had thought that it was because of a temp sensor, but the idling problem remains even when the car is at operating temps.

When cold, the car stalls unless I'm giving it gas. Plus, there is noticeable hesitation while cold...it seems like there's no power (while the car is still cold) until I get over 2000 rpm or so. Again, this is only when the car is first started while cold.

When the engine is nice and warm, the stalling problem goes away, but I've now noticed that the idle is irregular. Actually, I should say "abnormal" since the idle fluctuates up and down in a wave-like sort of regular manner by about 100 or so rpm--enough that I can hear and feel it.

When warm, I'm idling at about 400 rpm while in gear and about 800 rpm while in neutral.

Driving while warm feels normal, but I can't tell if I've been experiencing power loss at mid-rpm levels. I'll have to try some acceleration runs on the way home tonight.

A few weeks ago, the entire intake manifold was removed and all of the rubber sleeves replaced. Several vacuum lines were loose and replaced (thanks Dozman and Dennis Sz). My gas mileage (city) is good and consistent at about 16mpg. I don't hear any whistles from leaks like I did before the sleeves were replaced. Fuel injectors were taken off of one side and put back on with a slight coating of oil on their rings.

How common is it that an intake manifold with all new sleeves will still have a leak large enough to cause this (i.e. not seated properly) but not be audible (unless I don't recognize the sound)?

I'm going nuts trying to figure this one out. Any help is appreciated....including a list of possible culprits.

Greg86andahalf

Dave,

Before going in too many different directions, you should pressurize the intake system with the innertube method spelled out in the archives. Eliminate the possibility of intake and vacuum leaks. Once false air is eliminated, let's see where you are. Otherwise, you may be chasing around the problem.

Let us know.

Greg

Bernie

I agree with Greg here.
It is really important to get that motor airtight.
False air and vacuum leaks on my '85 were frustrating but I narrowed it down and now it is rock solid.
The innertube trick and a MityVac to test various vacuum lines is essential in tracking down these culprits.
I would start with a very good visual inspection to make sure nothing has popped off anywhere. I found my first vacuum leak from a thin plastic vacuum line that ran from the airpump actuator to the intake. It took about 45minutes to gingerly put it back on the intake with needlenose plyers but i finally got it on.
It makes no sense to run around components until you are sure it is not just an annoying leak somewhere. Cracks in these lines is also common. I found 2 cracked lines and a cracked rubber elbow on my search.
And guess what? I'm not finished yet!!!! I still need to replace leaking cam bore seals and I am replacing my intake with a freshly powdercoated unit. More tightness to come.

Hope this helps

Thom1

Hi Dave,

The innertube piece pressurization trick worked to find vacuum leaks for me after I re-installed the heads. I replaced the inflexible/cooked vacuum elbows with vacuum hose and new connectors. I found some formed hoses that were non-Porsche to cut down to replace brittle hoses. I am not really sure if a leaking oil filler neck amounts to a critical vacuum leak. Maybe someone else knows how deep you have to seal the top of the engine area. See some excerpts from one of my essentials write-ups for ideas about rubber to replace.

Thom’s Cam Cover and Intake Service and Painting Essentials:

Thoroughly clean the engine first, without removing any factory stickers.

1986 model recommended replacement parts:
Rubber Intake Manifold (Intake) Sleeves
Rubber Vacuum Elbows
Intake Sleeve Hose Clamps – New ones look better
Rubber Intake Hoses – Most of them are formed.
2 O-Rings at air flow meter
Cam Cover Breather Hoses (on one side)
Check Valves at chain tensioners
Crush Washers for check valves and tensioners
Thermostat
Thermostat O-ring
Fuel Injector O-rings
Oil Filler Neck Baffle
Oil Filler Cap O-ring
Cam Cover Gasket Set – May include the following – Verify contents:
Camshaft Seals
Camshaft cover bolt Seals
Cam Cover Gaskets
Cam Cover Seals for plugs (on one side)
Cam Cover Plugs (2 on one side)
Cam Cover Breather Hoses (2 on one side)
Spark plug hole Sealing Rings
Intake Gaskets

If removing cams, for the chain tensioner service add:
Chains
Tensioners – only if deeply grooved (in my opinion)
Loctite 574

Note: When I removed my intakes I had to replace all the rubber up there. This includes vacuum sleeves, vacuum elbows, and hoses. The rubber has been cooked so much by now that it is stuck in an inflexible shape, and it will leak after it is disturbed.

Warning: My washers, beneath the cover bolt heads, had a cushion of rubbery looking substance stuck beneath them about 1/16th inch thick. Upon investigation, none of the big 3 could find a part # or reference to this part/substance. Other 928 owners told me that a spacer is required for my model to prevent leakage at the cover gasket. Therefore, if your washers have spacers stuck to them, leave the spacers alone.

Note: The Allen Head bolts may be very stubborn. Have a good 3/8 inch socket type of Allen wrench on hand.
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Big Dave

Thanks for the suggestions. I'll try that test just as soon as I can get some free time in the garage. Finding the leak may be easier than finding the time!