Rough idle - need help

[djnolan]

The 85.5 Auto has been running rough after warm-up for some time. I have been thru the diagnostics on Clark's and can't find the cause. Some days it runs fine and some days after about a mile of warming up, it starts running rough and a very distinct continuous pulsing thru the exhaust (pow pow pow) is heard at idle on all cylinders.

Things I have replaced: Cap, rotor, ignition wires, plugs, fuel pressure regulator, throttle switch sensor.

What else could be causing this?

[Dave Colangelo]

I would take apart the throttle housing and clean it up really well. Get in back to shiny metal, even the small passageway around the valve. I would also replace the fuel filter, and air filter. The air filter can make a big difference if it is even only a bit clogged. If the issue persists I would test your fuel pump's flow rate. This can be done by removing the nut at the end of the fuel rail, attaching a hose, and bypassing the DME relay to run the pump. I forget exactly what the rate should be but if you run it into a measuring cup and time it you can see what you are getting. The haynes manual has the numbers listed somewhere.

Regards
Dave

[djnolan]

Air filter is a K&N recently cleaned and the throttle body was disassembled and cleaned as part of the throttle switch replacement process.

I would think the exhaust sound (pow pow pow) provides a clue as to whether the problem is timing, fuel or ignition related. If so, I don't know how to interpret the sound. Any thoughts?

[iplagolf]

Start the car. Take a handkerchief or thin material, cover the end of the tail pipe and see if it is blowing the material out or blowing it out and sucking it in and blowing it out. If it's the second, you have a bad exhaust valve. Just a thought.

[iplagolf]

Something else. Have you checked the timing? Get it to top dead center and make sure the lines line up. Something in the front of the engine could have slipped or is slipping.

[djnolan]

No it is not, it is just a short exhaust pulse. It also pops under acceleration. It has got to be ignition or fuel, just don't know which one.

[iplagolf]

OK, but I'd still try the tail pipe test. Just one more thing that you will definitely know is not the problem. I don't know what a pulse is, but you said a pop. To me that's a detonation in or next to the exhaust system which is the combustion chamber. There could be a problem with the engine running too rich. Un-burnt fuel leaving the chamber and detonating in the exhaust. Maybe a weak spark. It will do it at idle or running. Just try'n to help out.

[944Toon]

Temperature sensor and / or connection perhaps? If thinks engine is still cold could cause over rich mixture?

[Gawernator]

Yeah, and how old is the O2 sensor?

[djnolan]

Had to run a few errands...

The O2 sensor is about 4 years/40k miles old. When I unplug it there is a barely noticeable change in the idle that tells me it is working.

Regarding the exhaust note at idle, it is not a bang bang bang as in a backfire, and it is not a mum mum mum as in a normal idle. it is somewhere in between and it is crisp and all four cylinders sound the same. it doesn't seem to be running overly rich per the smell of the exhaust. And it is not sucking in like a bad valve would. And it pops under moderate throttle.

Could be the DME temp sensor. Could it be the coil? I suppose it could be a crank sensor or even the DME. It only happens when warm.

I wish I had an engine analyzer to hook up to it...

Instead I have you guys and all of the replacement parts I can buy.

[Gawernator]

If your DME temp sensor is original, might be good to replace. It's not expensive

[djnolan]

My FLAPS had a DME temp sensor in stock so I will replace it. I also picked up a coil just in case. We shall see if this fixes it.

[Gawernator]

Who had the Bosch in stock?!

[djnolan]

So far so good with the DME temp sensor replaced. We shall see...

[968 GUY]

I have seen FLAPS several times and the only thing I can come up with is F...Local Auto Parts Store.
Please clear this up for me.

[Gawernator]

Favorite local auto parts store. All the ones around me only have domestic manufacturer parts mostly. Stuff like belts they sometimes have. Most things I would have to have them order. So I'm in awe that your FLAPS had an OEM Bosch DME temp sensor in stock lol.

[djnolan]

F is for Friendly I have heard. It is a a local import place but I prefer Pelican if I am not in a hurry. More assured of getting fresh and correct parts.

Anyway, replacing the DME temp sensor seems to have done the trick. Car running much better. Now If I could figure out what the whining is all about. Belts to tight? Balance shaft bearing?

[Tcrate]

Have you tried pulling spark plugs. reading the ceramic color might give some insight to whether this is an engine management issues (all plugs read the same) or if just one cylinder has an issue (one plug color different from the rest). Black is rich or oil blowby, white is lean and tan is good. Might start there.

[djnolan]

I take back everything. After a few flawless runs the problem is back. I think it has something to do with driving it to work. I didn't get a mile from home when it started to miss again. I stopped by a local foreign repair place and asked their opinion. They don't worked on porsches but advised to replace the coil. I immediately replaced the coil and it didn't help at all.

[968 GUY]

I always refer back to searching the forum and clarks-garage.com