Cold start driving problems. lean missfire.

[nick-nismo]

this side of the forum has been super dead for a while now so ill post this up and try and get the discussion going,

so recently when i start my car up, either mid day or in the mornings, and i try and drive weird stuff happens.
ill be cruising on a flat road with the most minimal amount of load at say 2500 - 3500 rpm and the car will start to sputter and hiccup, just before it sputters/missfires ill notice the wideband gets very lean. 17+ then it gets off the scale and the missfire happens, then it will bounce back to stoich, then back to lean / missfire. Also while the car is cold it feels like it is under a lot of load even with the least amount of pedal, car will be building boost like at WOT but at under 10% throttle.

ONLY when my car is not fully warmed up, (below the half way mark on stock temp gauge)
Note that when the car is warmed up and has switched from cold start maps this will not happen at all. also this happened right around the time i switched from 5w-30 to 5w-40?? also car has slight exhaust leaks right around turbo which close up when hot enough.

so i think it is the ecu coolant temp sensor,
any imput before i buy a new one?

nistuned, basically oem cold start maps

[Aamp]

I'm having the same symptoms (except no turbo, and my car is stalling too) on a bone stock 1990 NA GTS that doesn't get used nearly as much use as I'd like (54K). It's always been sluggish and feeling like it's labouring until warmed up. Then it progressed to stumbling/cutting out, now it's actually stalling often at 1/4 temperature on the gauge. I suspect(ed) the coolant temp sensor too, but one time I was able re-start and get it running/idling fine just by giving it some throttle while turning it over, then because I had the hood open, I closed it, and it died. I could repeatedly restart and kill the engine simply by closing the hood until it reached temperature. I've tried alternately tapping on the sensors that I could see with a wrench on another cold start and nothing happened, it didn't stall. Pretty sure I have an exhaust leak somewhere up around the front too. Once at 1/2 temp on the gauge, car's fine and everything's normal.

Just thought I'd share in case whatever it is, can maybe be disturbed by a physical shock, (sound?) or impact of say closing the hood from a low height. But in my case it's past the point of stumbling and is stalling (lean? knock? sound? something loose?). I don't know what's inside any of these sensors other than some fine wires in the MAF.. Maybe this helps, maybe it's irrelevant but I'd like to see what anyone has to say regarding the OP's issue since it might help with my similar sort of issue too.

[Skym]

When my car did that (stuttering) it was due to a airleak just behind MAF (MAF gets 2x airsources instead of one). Used to do it when exiting motorway. Ended up replacing intake pipe as it had cracked under intake hose clamp on MAF due to age, etc (common problem). Also cracks inbetween ribs which it was starting to do. It's similar to coilpack missing, so can be easily mistaken for that.

Other is intake airleak before MAF that makes MAF max out and hit boost cut (fuel cut) on ECU. Happens with clamp onto adaptor type of airfilters.

Coilpacks intermittently missing would be another. Checking coilpacks on bench via coilpack testing machine or multimeter should help to find the weak coilpacks.

Exhaust leak could be a cause. Faulty coolant sensor would activate coolant temp sensor error code on ECU.

[Frankyy]

my car does the same, as soon as I get to 150 F it's fine everything goes away and it drives like a charm, just like you guys, sounds like there's a little exhaust leak just a tick, I re-did everything and nothing seems to be leaking, I have no leaks anywhere in my piping

coolant and air temp sensors are brand new, going for a tune in about a week and a half maybe it's a Japanese thing with the good fuel they have there and the base tune they put in the ECU

[MattiasJacobs]

Probably your flex pipe like mine

[nick-nismo]

i reallly dont think its an air leak, but i will do a boost leak test to see.
gonna check ecu for codes

edit: got code 21. but i have ls2 coils so all normal there

[Skym]

Is this with the same ECU you had the high idle problems with???? Or is this with the replacement ECU????

When engine is cold, stock ECU runs engine rich, pulls ignition timing to suit, idles engine higher, ignores o2 sensor input, not much else.

Have you tried doing a datalog of sensors with Nistune while driving (someone else driving while you datalog sensors). Try MAF, o2 sensor, ignition timing.

With W40 oil it should make engine slightly sluggish if clearances don't allow for that thickness oil. Need to look at oil pressure, oil temps to confrim if it's the correct oil. Oil pressure check is done at around 85 degrees C with oil temps and should match factory figures if using stock oil pump.