3.6 cold start problem.

Hello Rennlisters! This forum has been a huge help before, with many problems of my previous projects, 997 4s and carrera 3.2. My current problems is with 996.2 3.6 C4 Tip. Feels like I have searched the whole internet, with no luck, so I'm turning to you guys. Just purchased the car as a slight project, fully running but very mistreated car. Engine is once rebuilt, and there is no signs of score or chain problems. It started to have a cold start problem when standig overnight. At first it started well, and when immediately on a drive it would sputter a bit but usually went away if throttle lle was avoided. Then it startet to get worse, misfiring and rattling and also failure indication from oil pressure, cauge went from 5 to 0 and. There is anything that would point to an actual fault with oil pressure, its rather the sensor, or bad connection. Temp sensor shows also correct reading, o2 too. I don't get any codes on generic obd reader. (durametric on its way) Ignition switch has the typical 996 problem, that I have to bring it back a bit from starting position to get headlights and wipers powered. I changed, bosch plugs and new oem coils and the problem still persist. Tested also maf and all o2 sensor unplugged, same story. Vacuum leak test done engine running at 3000rpm with brake cleaner spray, no findings. Previos owner also stated that he has replaced AOS. E-gas calibration also carried. The colder it gets the worse it is to get running. But once warm, runs like a charm and all the power is there. Summary, hard to start on cold and once you get it running it would stay on idle nicely, but dies at rpm drop, and sometimes even just pushing the throttle at all. Even coler it won't even reach idle until stalling. Depending on the ambient temp it settles when I let idle and approx after 30sec to 1 min it runs normally as in video. MAF is disconnected, thats why all the lights. Youtube link to video ​​I would really appreciate a little help with my problem. Thanks in advance! Juuso

You mention a bunch of electrical stuff, but nothing concerning fuel. Have you checked the injectors? Fuel filter(s)? Fuel pump/rail/regulator/pressure?

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I haven't put lot of focus to fuel, since I believe it would affect behavior with warm engine too. For the lack of fuel pressure I have tried to prime the system before start, (ign. on - off several times) but no luck on that too. Todays update: Cold start after standing over night, maf etc. plugged. I't started on second try, on 3rd try it could handle very gently throttle, and around 2500rpm it starts to spatter and immediatly the oil pressure cauge drops to zero, I'm able to hear minor chain like rattle too, I was able to repeat that two times. After that I started again an let it run on idle for to minutes, and again problems gone, no rattle, sputter nor oil cauge dropping to zero. Two options in my mind. Fresh ideas welcome! 1. Earth problem, and the oil pressure cauge is just as sign, and the rattle has nothing to do with it. 2. Problem with oil pressure, and lack of oil pressure affecting timing? Could bad oil do this, also the oil level is bit low too. I'm going to do engine flush and new oils next.

Properly running engine needs all of these functioning properly - fuel, spark and compression so I'm with dporto here - it's likely an issue with fuel delivery but i wouldn't rule out a possibility with compression as well. You say that there's no sign of scoring or chains - how's that confirmed? I would wait for Durametric before driving that car because because that could reveal some useful information and not to possible make thing situation any worse - considering that it's getting colder here day by day

An easy way to test oil pressure sender unit is to turn on ignition but don't start the engine. If it reads over 0 then the sender unit is faulty and needs a replacement (or cleaning the terminals).

Quotes source posts: 18355551, 18355551, 18355551

Engine runs in a "open loop" mode when it's just started which means that it runs in rich condition (ie. more fuel). When warm it switches to closed loop so in theory it could be an issue with fuel. Check fuel trim values when you get Durametric. Not an expert on this but could it be cam deviation (ie. problem with those wear pads) - timing will be off if the deviation is too much out of spec and there's a bit of variation between cold and warm engine. Again something the check with Durametric. If it's just a bit too low then no - remember if there's oil in the dip stick at all there's over 8 ltr of oil in the engine when the engine is cold (or stayed over night). Measure the oil every morning and if it's in the middle you are ok. Too much oil could be an issue so if it's at top in the morning then there's too much of an oil and that could cause a problem.

This may not fit your issue howver i was dealing with a cold start misfire issue this summer and went through everything before I did a Vacuum check... My issue turned out to be a vacuum leak on my UAOS. Once that cleared the car is running great. Though you may have a different issue i would do a smoke test to rule that out. I made mine with baby oil, mason jar, soldering iron, and some hoses. Once I pumped air into that system I found my problem immediately. Good luck!

Pull off the connector from the engine coolant temperature sensor when the engine is cold and then start. Does it statt easier now?

I would double check possible vacuum leaks with smoke also. Reasoning is that (after watching that video) is that when you run your engine with higher revs maf etc compensates that by pushing more air into engine that can mask a vacuum leak. Now when idling engine gets too much air because of that vacuum leak and it air/fuel mixture is incorrect and that will cause stalling... maybe a major vacuum leak somewhere? - just a though.

Carefully check your oil fill pipe for cracks. Make sure the oil filler cap is on tight.

This one is solved. Bad ground. After further checks of all the groundpoints on both valvecovers and the thick one between engine and chassi, all of them had junk so I cleanded them, now it runs like a charm hot n cold. ​​​​​​Hopefully someone finds help from this.