Rought idle, poor performance when cold.
 

I am new to 928s and trying to learn my way around these engines. I have an 86 model that fires right up when cold, but runs really rough....it also is lacking in acceleration when cold. But after running for a few minutes, the idle gets better and the acceleration is good. I know it is a misfiring cylinder, but can't decide why. I already replaced the plugs, so my next guess would be the plug wires, but don't understand why the temp of the engine would make a wire start working properly. I also have the same thoughts about the injectors.

How should I go about troubleshooting this problem....or is there a common issue with these cars that may cause this problem?

Thanks

first thig i would do is run the car in the dark with the hood open,
look for sparks from the ignition wires.
after this remove both of the coil wires look for cuts in the insulation and corroded ends.
You will probably need a new set of wires, best to get the Berus from Roger, yes they are expensive but should last another 10 years

No arcs, wires look good. I would think if it were wires, it was be a constant issue, not one only when the engine is cold.

Temperature sensors, especially Temp II where the radiator hoses connect.

Is there any way to test these sensors?

I own an '86 also and you have three temp. sensors. The temp II as mentioned above, a time temp switch on the front of the intake, and an air temp sensor attatched to the bottom of the air box. They are all under $40 and easy to replace. You also need to check all of the vacuum lines. Broken or rotted vac lines can cause a lot of issues. Check the condition of your couplers on the intake also and be sure the are all tight.

So you are saying I should just replace all the sensors and not test them? Seems like a waste of money on a hunch that might be the problem.
I have ordered a silicon vacuum kit and plan on replacing all the vacuum lines.

You can test each sensor by mearsuring resisitance between the terminals. Specs are on page 24-17 of the workshop manual ( a must have tool) I believe. Not saying to just throw money at the car but based on my experience with older Porsche's in general if the sensors are original many times they will need replacing. My '86 only had 65,000 miles on it when I purchased it from the 2nd owner. It ran rough. When I started going through things many of the sensors tested bad, plugs wires were bad and vacuum lines etc were either rotted or loose just due to age. Do you have any maintenence records from the previous owner? It really helps to know what/when certain maintenence was done. Especially timing belt and water pump. :)

No records :( But working on catching up on it all. I plan on doing the timing belt and WP over the weekend.
I found 2 sensors in the water jackets on the front of the motor. But neither of them look like the ones in the manual, well at least they don't have the same connector. Mine have 2 wires with OEM spade connectors on them....not the typical snap on Bosch connector with the wire lock.
It appears the manual I have is for the older motor, not the one I have. Mine does not have the big alien looking intake....it has the intake with the large rectangular boxes on each side that cover the valve cover and spark plugs. 5.0L

Thats the US 85/86 often called organ pipe intake.

Post some pictures once you get the covers off on the timing belt.

I also recommend using Jack Kelly's guide, about $15 on ebay.

There seems to be several guides on the different forums, for free....how much better can it be to pay for it.

Agood1,

If you have not tried the quicky fix yet get a pint of Techron and dump it in your gas tank, it can't hurt.
Techron - Wikipedia, the free encyclopedia


And, check out Mr. Nichol's 928 Tips site, I found it very helpful when I owned a few sharks over the years.

Where can you find these sensors for under $40? I have looked all over including some commercial parts suppliers and cannot find them for this price. Also, mine do not have the typical Bosch style plug on them...they have 2 spade type terminals. Most of the sensors I have found have the Bosch style plug.

I purchased mine from Pelican.The coolant temp sensor is the only one that should have two spade terminals. The rest all have the Bosch style connectors unless they were broken off(at least on my last two sharks they did). I am assuming you have the 32V, US car.
Temp Sensor II 928-606-126-00 $28.25 (located to the left of the oil fill)
Termo switch 928-606-217-00 $38.00 (located a little right of center on the front intake runner)
Air temp sensor 928-606-143-00 $26.25 (screwed int the bottom right of the air box)
Cooloant temp sensor (for gauge) 928-606-201-01 $39.50(right of the oil fll)

Good information. I have ordered the parts, so we'll see if it corrects the problem. I am not going to replace the sensor for the gauge, since the gauge appears to work fine.
Thanks again

Hope it works to replace the sensors. My '86 is running great since I changed them. If not it could also be the Idle Control Valve. My 911 ran a little rough when cold and that was the problem but a little more pricey($155.00). It sounds silly but also check the O-ring on your oil fill cap. If oil is bad it can cause a rough idle too. Good Luck.SmileWavy