Troubleshooting Stock 83' Turbo
 

I'm looking for a little advice in troubleshooting my 1983 911 Turbo.

The car starts and runs fine. It runs smoothly when accelerating at anything above just barely closed throttle (just pressing very lightly on pedal). It idles smoothly (foot off of the pedal). The car runs strong while accelerating and seems to have no issues when run hard, just when cruising at constant speeds (at low RPMs) around town.

I am thinking that it feels like a very slight miss while cruising at constant speed at anything less than approximately 3000 RPM. It seems like much less surging/bucking at the higher RPMs. It is annoying, but it does go away immediately if the throttle pedal is pressed slightly harder.

I think it may be occurring where the vacuum cans on the distributor may be transitioning, but I am uncertain how the two vacuum cans work together.

If this makes sense to anyone else, please let me know what your thoughts are regarding this running condition.

The engine is stock with all the hoses/air pump installed. The hoses are tight, and I have found no vacuum leaks that I have not addressed. I have been living with this condition for quite some time (probably since I have owned the car 17+ years).

Thanks for looking, and all suggestions are welcome.

Have you tried adjusting the idle mixture screw? What you are describing sounds like it is slightly lean.

Check ignition timing. Mine had very similar symptoms and it turned out to be too much ignition advance. The distributor hold down nut was loose which allowed the timing to drift out of spec.

You should invest on wide band airfuelratio gauge and possibly cis pressure gauge set. Sounds afr related so an easy test would be a small adjustment on the idle co screw like brian suggested.

The car does not surge or "hunt" at idle. This was one of my thoughts as well, and I did adjust the CO idle mixture screw in tiny increments until it seems to idle the best without lean surging.

I know my distributor lock down nut is tight as I did check to see if I could rotate the distributor and could not. Its right at 29deg BTDC at 4000 RPM with vacuum hoses off. I suppose I need to check it at idle with the hoses connected, and see where it is at and how it transitions.

I suppose I will find out soon, but which side of the vacuum can is for retard and which side is for advance? Unfortunately, my workshop manuals don't seem to cover my car very well since it is not a California or a USA car.

Thanks, and I guess it may be time to get out the CIS pressure gauges, but I really don't feel that fuel pressure is the issue here.

Please keep up the great suggestions.

Oscillating idle would be a rich condition. You don't need gages to verify a lean surge. If it is slightly lean the surge will be subtle at steady cruise from 2200rmp-2800rpm. To verify this is not the problem adjust your idle mixture screw until the idle begins to oscillate then back off just so it stops. Drive the car and see how it responds. Each engine has a sweet spot. You can use the gages to track where you are which is good data.

No, it does NOT surge/oscillate at idle.

Now, the slight surge at steady state cruise from approx 2,000 - 3,000 RPM does sound more like what the car is doing.

I did, however, find a vacuum leak whilst poking around today and checking the timing which is just as the book says, 29 deg BTDC at 4,000 RPM (vac hoses disconnected) and right at 0 deg BTDC at 1,000 RPM (hoses connected).

My vacuum leak seems to be at the pop-off Valve housing (930.110.056.02) on the cover end closest to the throttle valve (furthest from the rear bumper). If I place one of my fingers right at the joint, the sound of the leak changes, so I am guessing that the gasket (930.110.149.00) needs to be replaced. Does anyone know if I can simply change out the gasket or is this under spring pressure from the pop-off pressure spring?

No one said your engine was oscillating at idle, that would indicate a rich condition. You have symptoms of a lean condition. A lean condition can also be caused by vac leaks.

Gotcha, (I had it in my head the other way around and after re-reading what you wrote, you were suggesting to richen the idle mixture slightly and see if the slight surging 2,000 - 3,000 RPM goes away)

Right now, the mixture screw is adjusted so that if I turn the screw any further clockwise (richer), it will occasionally oscillate "hunt". The slight surge at steady state cruise remains. It is slight, but is there. I'm assuming that this is probably due to at the very least in part due to the vacuum leak that I located today.


Now, do you know if the pop-off valve housing (1) would need to be removed from the car in order to replace the gasket(7) on the cover (6)?

http://forums.pelicanparts.com/uploa...1419219184.jpg

Or, if the cover is under pressure from the spring, can both covers be removed and replace both gaskets and leave the pop-off valve in place? (or am I making too big of deal out of removing the housing?)

You can change the wax gaskets with the BOV in place, there is a thread on it somewhere. The springs will push both covers out so be aware of that. I just remove the BOV that way you can check all the lines and fittings for issues. Sounds like when you get the vac leak fixed the surge will go away. Please let us know how it goes.

The gaskets are wax?

Do you know of any other gaskets in the intake that are prone to leaking after 31+ years? (I know it is a loaded question, but the only dumb question is one that is not asked.)

Thanks for your help so far. I'll keep plugging away at it and update my post when I get some parts.

One often overlooked area is the injection blocks and intake manifold. Stick a wrench on and see if you can hand tighten. Having them loose or cracked blocks can cause small annoying issues such as you describe.

I had the same issue in my 89. I did all the checks for vacuum leaks and tightend the injector blocks. All of those helped but what made it go away completely was disconnecting my oxygen sensor. I had a one point rise and decrease fluctuation on my AFR gauge while cruising at highway speeds until I disconnected the oxygen sensor and it went away. I do agree with one of the previous posters in getting a a/f Meter. if disconnecting the oxygen sensor fixes your issue, I would suggest getting your car checked to make sure you have the proper air fuel mixture if you decide to leave it that way. I disconnected mine and never looked back.

Thank you, but alas, my car is an 83 (Euro) and has no O2 sensor. The only AFR gauge that I have is stationary and cheap (gunson gastester) so I can't really see live data under driving conditions or engine load (maybe I need to bite the bullet and get one, but haven't done so yet).

intake air leaks making it run lean. mine did that too but i verified it was lean with an LM2.

As far as I can tell, the nuts are hand tight (and I didn't want to pull any studs or crack any injection blocks).

These are what you are referring to? (and I had read before that they do and will crack)

http://forums.pelicanparts.com/uploa...1419273623.jpg

930.110.141.03 flange (7), qty 6
930.110.197.10 gasket (8), qty 12

I may just get these for piece of mind (while i'm in there), but it's sounding more and more like it will be easier to do this work by dropping the engine.

Those nuts need to much more than hand tight... I don't know what the torque specification for them is with the original plastic bakelite injector blocks.

I put aluminum injector blocks with PMO heat insulators and gaskets in mine and tightened the intake manifold nuts to 18 ft. lb.
No leaks are happening.

IIRC the factory-style blocks are also 18 lb/ft.

I use hand tight as the bench test to see if the nuts are loose. If you can hand tighten one then it is time to get out the torque wrench and check them all.

I finally got around to addressing this issue today and both of the "wax" gaskets were disintegrated. The spring end was really gone and the piston end was split in many pieces with large gaps. I had to reset idle screw (a lot) and mixture (some) to get the car to idle right without popping out the exhaust. I will now move to using the wide-band O2 meter for further testing.

I'm thinking it will run better with these leaks fixed, but need to charge the battery before taking it out for much of a drive.

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