Rough idle after long drive

[Dano944]

After driving my 944 on a long drive, then stopping the car, when I restart to continue driving, the engine idles very rough, almost as though it’s mis-firing. I need to give it gas then it runs smooth again before I continue driving. Up to this point the car runs fine, no issues. Any ideas?

[Jfrahm]

Check the plugs when it's misfiring. See if they are wet, black, etc.
Sounds like excess fuel that is manageable when driving or evaporates if the car sits for a while.
Maybe a leaking injector, FPR, damper, etc.

[Dano944]

Originally Posted by Jfrahm

Check the plugs when it's misfiring. See if they are wet, black, etc.
Sounds like excess fuel that is manageable when driving or evaporates if the car sits for a while.
Maybe a leaking injector, FPR, damper, etc.

Thanks, good suggestions. If the plugs were fouled, wouldn’t they be missing all the time?
A fuel issue might be the answer. I’ll have the shop look into that because I’m not qualified.

[walfreyydo]

Could also be faulty DME temp sensor (blue top one). Tells the DME the engine temp and richens the mixture when cold. Could be giving a false cold reading, thereby increasing the fuel more than it should be.

Other possibilities: cap and rotor, plug wires

[Jfrahm]

Yes it could be a DME temp sensor as well. Could be other things.

The plugs might be sooty and black but still sparking, it's possible to run very rich and still run, just not very well.

[Dano944]

Originally Posted by Jfrahm

Yes it could be a DME temp sensor as well. Could be other things.

The plugs might be sooty and black but still sparking, it's possible to run very rich and still run, just not very well.

I thought about the DME Temp sensor. For just a few $$, it might be worth replacing it to see what happens. If I replace the Temp Sensor, do I need to drain the coolant?

[Jfrahm]

IMO a hail mary but whatever. In my experience they work fine or open up at particular temps. Maybe that's what you are seeing.

if you are hesitant to look at a spark plug I do not recommend changing a temp sensor.

[wwdwgs]

I didn't drain the coolant when I replaced DME temp sensor. I put a rag below the work area to absorb whatever coolant may slip out.
I still don't understand whey Porsche installed two temp sensors - one for DME,, another for your viewing pleasure (temp gauge).

[walfreyydo]

Originally Posted by wwdwgs

I didn't drain the coolant when I replaced DME temp sensor. I put a rag below the work area to absorb whatever coolant may slip out.
I still don't understand whey Porsche installed two temp sensors - one for DME,, another for your viewing pleasure (temp gauge).

Because the sensors back then dont allow to run two feeds off of one sensor. It splits the resistance between the two.

[wwdwgs]

Couldn't DME "multiply" that value by two? it's a calculator, after all. It was a rhetorical question, don't answer it.

[walfreyydo]

Originally Posted by wwdwgs

Couldn't DME "multiply" that value by two? it's a calculator, after all. It was a rhetorical question, don't answer it.

Something to do with being subjected to two bias/pullup resistors (gauge and ecu).

More info here (the thread concerns why you cant run ecu and gauge off same temp sensor):
https://www.msextra.com/forums/viewtopic.php?t=80344

[Monkey Wrench]

if you put two resistors in series you can add the resistance,
if you put them in parallel you can half the resistance , If they are the same value of resistor. if you have two resistors in parallel of different value than the math gets a little more complicated. in general one conducts more than the other and the load may have some effect. a temp sensor is in essence a variable resistor.

you might have the mechanic check the plugs and also do a leakdown test, this is a way of knowing if you have a injector that wont shut off.

a stuck open injector may cause it to blow a lot of white smoke upon startup as it's leaking raw gas into one of the bores, It may recover itself after it pukes out some white smoke. thats bad for the engine.

if you pull the plugs immediately when its running rough after a restart and one is all fouled that's a sign but if you run it and then check you may find they look ok as they can clear themselves.

when the car shuts off the fuel rail should hold pressure, a leaking injector may cause the gas pressure to leak down and soak one cylinder.

If your mechanic finds that he may replace the injectors,
Near the fuel pump there is a fuel line check valve, it may also leak and that can also cause a leak down.

also there have been fires caused by ruptured lines, some burn up because the fuel lines go overtop of the hot manifold. mine is an early 85 with a metal tank.. Id suggest replacing the fuel line , the one connected to the tank, mine leaked and it spilled a whole tank of fuel so don't let that stuff go. check it.

all the lines and fittings add up if you go OEM so I'm going to consider changing them over to a different type.

ones made after early 85 and 16 valve cars have different fuel line routing and probably a plastic tank. the whole system was changed significantly.

[Dano944]

Thanks Monkey Wrench. I found yet another independent Porsche repair shop that I feel good about. I visited the shop on Saturday when they were closed, but open for overflow, and the owner welcomed me and spoke to me in great detail about 944’s. Another shop I visited on Saturday said “We’re closed and we only work on 911’s”. I had the DME Temp Sensor replaced in 2022, but for minimal $$ I may get that replaced just to see what happens. I learned a long time ago…try fixing the easy stuff first before huge major steps. I mean, why not just rebuild the engine as a preventative measure??

[wwdwgs]

yes Dano944, start with "low" items: 911, 912, 914, 924, 928, 944, 951, 968 etc ))

[Monkey Wrench]

It's easier to rebuild an engine than ask a mechanic to "please remove all the unnecessary electronic gadgetry" from a new car ;-)

[Dano944]

Originally Posted by Monkey Wrench

It's easier to rebuild an engine than ask a mechanic to "please remove all the unnecessary electronic gadgetry" from a new car ;-)

Ha! Yes, but after rebuilding an engine, won’t mechanic need to rehook all the “unnecessary” gadgetry?😆

[garysoul]

its Could be the DME

[garysoul]

Originally Posted by Dano944

Ha! Yes, but after rebuilding an engine, won’t mechanic need to rehook all the “unnecessary” gadgetry?😆

mechanic dont need to