Here ya go Rich...Let me know if you need more specifics or I can be of any help in your endeavor...

Jeff

I didn't get the whole page in the pic...but "F" is going to the LH control unit, and "E" to the automatic Transmission Modulator valve.

 

OK. When you figure it out, let me know. My 86' does the same thing. I have done a complete top refresh, replaced all the vac lines, plugs, wires, boots, o-rings, caps, rotors, MAF, Temp II, and the list goes on. Mine runs great but at idle, it wants to miss every once in a while. I can feel it and see the oil pressure guage jump just a tad when it misses. Sometimes it wants to flood when it sits for a while.....runs really rough....then it final settles. The only two things I have not replaced is the coils and battery. Last winter, I was having problems in the mornnig when the car was very cold....except when I kept my tickle charger connected. I know that the Ing. system requires a ton of juice. I am considering the coils next.......after I get the AC done.

 

Jeff T,

yup sounds like my problem too. Very intermittent. I will replace the idle stabilizer valve next.
For example yesterday after the car sat all weekend I started it up to go to work.
Ran very rough at idle and bucked st low speeds for most of the 12 mile trip. On the way home from work it ran perfect. No rough idle but a slight surging when creeping along at 1200 rpm. Also the exhaust smells a litlle like unburned gas or a car that is out of tune.

Rich

 

you guys think it could be a fuel pressure regulator, or damper issue ?? If NOT vacuum...fluctuating fuel pressure could also produce these symptoms

 

Sorry, don't know if this applies and I have very little authority in this answer in regards to a 928, but I had a bucking problem at low rpms with a BMW e36 a long time ago. Turned out to be a weak battery. Idiot dealer tried to tell me it was a bad alternator.

Rich, do you know how old the battery is?

 

I'm interested to learn what causes this, too. My '85 bucks when at idle, but only when cold. My vacuum lines and connectors were all refreshed last year, so I was assuming I needed to hit the idle stable-izer with some WD40. It starts fine, both hot and cold.

Let us know what you find out.

 

John,

I can't say how old the battery is. It came with the car when I purchased it last year.
Big Dave, I will post results once I change out the idle stabilizer valve. I found one for $113.00 online.
Another for example to the intermittent nature of this problem. Today car fired right up, smooth idle no bucking at slow speeds all the way to work.
I am leaning torward the ISV since when they stick that is when the bucking and rough idle occurs.

Rich

 

Oh yea. Mine really was acting up a few weeks ago. Wanted to run about 1200 RPMS in N. About 900 in D. Found that the Throttle position sensor wiring harness at the front of the engine had come loose....since it has been destroyed from the heat. Put fresh tape on it and cleared up the "miss" and idle problem for a couple of weeks.....then it started again. I have not rechecked the connection....but know that the heat lets the electrical tape expand, therefore, taking the pressure off the broken wiring harness. After I get the AC conversion completed, I am going to cut the broken connector off and straight wire the switch to remove this issue from re-occuring. How is you throttle position sensors connection? The one I am talking about is between the main wiring harness and the hookup at the swith....not the one that connects directly to the switch. Although the switch lets the computer know when your at WOT, it must also tell the computer something else.

 

Jeff T,

Funny you should mention the throttle position switch. Mine too was destroyed from 21 yrs. off heat. Crumpled into 100 peices right before my eyes when I tried to remove it.
It currently is held together with electrical tape that has come loose from engine heat. I know it affects the drivability of the car when it is lying on the hot manifold only protected by electrical tape.
I will replace it when I change out the ISV. The throttle position switch is available on line for $40.00.

Rich

 

Rich....Believe it or NOT...the new throttle switch (its NOT a throttle "position" switch BTW...it only indicates when the throttle plate is at idle...or WOT)...and the new one wil NOT come with the cable. The cable is NLA from Porsche either...SO...your best bet, is to talk with somebody like John Dozbush, or the Rixter, or perhaps Jim Bailey might be so kind as to pull a cable from one of their "used" throttle switches and help you out. If you'll notice...the cable in question, is "Connected" on both ends by harness connector clips. One to the throttle switch itself (which is the part you will receive if you buy the new one)...and the other end is connected to the engine harness, which feeds directly back to the LH fuel computer. Like I said...this switch basically tells the LH when to idle...or when you are at WOT....ALL other fuel management parameters, are covered by a myriad of sensors, most prominently the MAS So...You (like me)...need to find a used 928 parts vendor, that can be persuaded (generally MONEY will do the trick), to separate the short (about 12" connector to connector) harness from their Throttle switch, and sell it as a individual part..or hell, buy it WITH the switch if they will sell it that way. BUT..you can't buy this harness as a "new part" anyplace, that I'm aware of. Why don't you have you're old one (since its still working apprently) connected through the bracket at the coolant crossover manifold, thats meant for this connector to situate in ? That way at least, your "taped-together" connector, won't be laying on the intake manifold, and exposed to the heat thaat probably caused it to cook and break apart in the first place, (Ask me how I know this)... Regards to you Rich. And good luck my friend,

BTW...if you "really" decide to get creative with this issue...(I am assuming the conector end that connects to the switch "proper".. on the throttle housing)...is in good condition,... and the "other end"...that connects to the LH engine harness, is the broken / damaged end of the throttle switch harness, you can "easily" purchase and install a replacement GM 3 wire 'weatherproof" engine harness connector, and cut off the damaged end, then solder on the new 3 wire connector, and do the same thing to the harness side connector. Then, (although your factory "Porsche harness connector, has been permanently, ...and irrevocably, "modified" )...the "taped connector" issue ceases to be an issue. Kind of a "radical solution"...but it does "cure the concern.

 

Thanks Jeff,

good advice as always. I wish the 928 engine was as easy to work on as my 911.

Rich

 

Working on it isn't that bad...Its just the lack of "similar parts availability" compared to the 911 that makes the sharks so hard to maintain mechanically...However, you do have resources like Jim Bailey @ 928 INTL. (they sell boo-coo "used parts")...and like I said, John Dozbush is good in a pinch,...Rixter too. And Then you may get "lucky" on Ebay too. So, make the best of what you have, (if its working for now)...and check around...these harnesses can be had from guys parting cars, and the sources I mentioned above. You "might" try PMing Gary Knox...He sold me his pristine Climate Control Head Unit from his immaculate 88 S4 that got totalled... He "may" just have this switch/harness left in his inventory of parts from this Beautiful car he was forced to part out due to a terrible accident. Check the 928 classified here on the list, Gary had an ad there with his number for contact (don't know if he still has parts listed or not)...but my point is...there are always guys parting cars, and even though this part is no longer available "new"...if you beat the bushes hard enough, one WILL fall out for you. In the meantime...make the best of what you have to work with...and keep looking. My offer still stands, anytime I can be of service or help you Rich...say the word. I've always wanted to see that part of the country...and would love to shake hands with you personally,.

 

Jeff-alright then.

Maybe when Dave Lomas sends out the electric fans I have been waiting for we can do the install!

Rich

 

Wow this must be fairly recent, the last throttle switch I bought came with a cable. I didn't need it and gave it to Heinrich. Standard Bosch part that I got from 928 Intl.

 

Shane...when I spoke with Tom (NOT Jim)...at 928 INTL...he told me the electrical harness cable, was NOT included with the throttle switch, and "basically...if you ordered the switch..the switch was what you received..NOW...if you BUY a "USED" switch...this is exactly what I was referring to in my earlier post....They may, or may NOT, include the harness for this switch, depending on whether or not the switch being sold, has a harness that can...or "should" be included with the switch.
My advice, is to ask at the time of purchase whether or not the harness is included with the switch. In any event...if you need the harness, and buy the switch.....Do Not assume the harness connector comes with the switch, ASK. That way, you will at least know "upfront" whether or not the electrical harness is "included" with the switch or not... MY experience is ....ITS NOT. YMMV.