champagne944

Hello, my name is Jeff and I'm new to the forum and to Porsche in general.


Two weekends ago I bought my first Porsche, an early 85 944. I bought the car with "throttle issues" and I mistakenly thought it might be an easy fix.




Symptoms are as follows:

• When cold the car idles poorly but improves as it warms up
• Any throttle application when cold results in sputtering with the odd backfire
• Once warm the car revs very smoothly to approx. 1200rpm
• As the rpm increase the cars seems to have a slight miss between 2-3000rpm
• After 3000rpm the car smooths out and rpm increases
• Any aggressive throttle application results in harsh sputtering and missing along with an occasional backfire through exhaust or intake
• With gentle throttle application I can bring the car to high rpm but full throttle results in a bog down to idle while the throttle is wide open

Parts replaced by previous owner:

• Fuel pump
• Fuel filter
• Fuel pressure regulator
• Coil
• Rotor (cap appears new as well
• Plugs
• Plug wires
• Throttle sensor
• O2 sensor
• Catalytic convertor
• DME temp sensor

I have tested the afm (both temp sensor and potentiometer) and they are functioning correctly. I have checked several other things but I wont mention them in case I checked them incorrectly and I don't want to rule out any possibilities. Another issue that I'm not sure is related; my tach reads incorrectly. It dips down to 700rpm when the car sounds closer to 2500. It then climbs to 2000 rpm but the motor sounds closer to 6000.


Any ideas or input would be much appreciated. I've spent hours searching forums and have found nothing that explains all these issues.


Thanks, Jeff

Tiger03447

If it idles poorly the Idle air control valve might be dirty inside. This lets in enough air at idle to allow the engine to run, bypassing the throttle blade in the intake. this may be partially plugged or blocked with debris. I think it is vacuum operated so put a mightyvac hand pump on it and see if it will hold vacuum. If not, it probably has a tear in the diaphragm and needs replacement. Shop around..I think that new ones are kinda pricey. Let us know how you make out with this. There are other things that affect the idle and air fuel mixtures...as well.

mel_t_vin

I'd replace the DME coolant temp sensor.

When you tested the AFM, did you measure output voltage through the full operation of the "barn door"? Regardless, you could still have a faulty AFM.

mel_t_vin

Jeff mentioned he had an early '85...ergo no ICV [idle control valve]. Correct me if I'm wrong...his car would have an AAV [auxiliary air valve].

In any case, my money is still on the DME temp sensor or the AFM.

mytrplseven

+1 on DME temp sensor

Tiger03447

I'd go with that also..however, he stated that it ran better once it warmed up. so I kinda ruled that out..am I missing something? If it's cold it would generate no voltage..or not let any through.switch open..once it warms up, the switch closes and it runs fine? Or does it work backwards from this idea? Thanks.

Humboldtgrin

To me it sounds like bad wires even thou they were replaced. Or a broken spark plug. Check to, see if you broke the porcelain on the upper part of the spark plug on install. It happens. That will cause this same sympom.

champagne944

Thanks for the replies!

Plug wires have not been replaced, I was wondering about that myself. Plugs themselves are brand new again. I've had them in and out a couple of times

I did check output voltage through the full motion of the door. Everything was in spec

Would the dme temp sensor cause poor running even after its warmed up?

I played with the mixture screw in the afm a little. I returned it to where it was because it only ran worse

Humboldtgrin

Replace your spark plug wires then. That would be where I would start. That's what the problem sounds like to me.

Humboldtgrin

The temp sensor would cause a different, usually no start situation due to telling the computer it's realy cold out and to super richer the air fuel ratio making it to rich to start.
The taddle tale was when you said you appied load and it starts to miss. The spark takes path of least resistance. When you full throttle more air goes into the cylinder, the more air the more mass that is being compressed. It's easier for the spark to jump from a bad wire to ground or to another wire where the cylinder is pulling air in creating a vaccum then to jump across the spark plug with all the compressed air in the cylinder. In a vaccum electricity has so little resistance so it can travel easier then if there are a lot of molecules it has to jump across. More molecule the more resistance. So the more air into the cylinders the more resistance to the spark. Hence you have bad plug wires.

champagne944

Makes sense to me. I'll buy new wires tomorrow and let you know how it goes. Thanks again

champagne944

Alrighty got some news about the car. So I finally got new wires today. Paid $232, thought that was ridiculous. Anyway I tried not to get my hopes up. Swapped all the wires out, heart thumping, get in the car, turn the key and... nothing. By nothing I mean the starter doesn't even engage. The only thing that changes is the electric fuel pump runs. Wtf. I haven't touched the car since Saturday when I started it multiple times while troubleshooting. This is absolutely ridiculous. I would appreciate some ideas or insight on this. Is there something electronic that could be causing all the other issues and now it completely **** the bed? Is that even possible? Thanks again

mel_t_vin

How old is the battery? What's the voltage drop across the battery terminals, as it sits in your driveway?

mytrplseven

You may have two things going on now. Sounds like the battery voltage is on it's last legs or just run down from trying to start the car. I still feel the initial issue is the DME temp sensor. It's a quick to fix, inexpensive repair that you should do so you stop chasing your tail. Get the battery fully recharged or replaced if it's over 6 years old (at least have it checked out at an auto parts store to be sure).

konakat

The tach having the wrong reading seems like an obvious case of bad reference sensors. It may just be the wiring or connectors since they seem to be somewhat intermittent. The connectors are attached to a bracket on the back of the head. Take them apart, clean the contacts, and make sure they aren't falling apart as that is a common problem. There is also some sort of TSB about electrical noise interfering with the sensors that someone else probably knows more about.