mcfisticuffs

Here it is again on a buddy's car - X51 40th AE, all factory. It's not the intake, tho it may still be intake leak related?

https://www.youtube.com/watch?v=Go6K...ature=youtu.be

He's replaced MAF - no improvement.

He's replaced the crank sensor - no improvement.

Dealer thought his was the flywheel - replaced - no improvement.

He has also replaced the AOS.

Plot thickens: It only does this on a cold start when it has been driven recently (first start of the day). The starts coming out of winter storage (not running for months) were fine.

DBJoe996

From the video...it is obvious! No gas! Or maybe the gas is old....I do have to say that sounds crappy. Know anyone that can put a Durametric on there? Short of doing all that I have put out there so far, I'm going for an some type of intake leak or vacuum leak. Have you also checked for a cracked oil filler tube? BTW, what maintenance has been done, i.e., plugs, coils, etc.

mcfisticuffs

Fresh gas, less than a week old. (in my video anyway, not sure why Nedster keeps his tank so empty.....) I use strickly Shell Premium (91 octane), with some PetroCanada Ultra94 for "spirited" drives where I'll be emptying the tank in a day and can refill with Shell.

I just did coils & plugs a month ago (it did this before and after coil/plug change).

Maintenance is quite up to date. Oil was changed prior to winter storage. Clutch / flywheel done 15k miles ago, along with IMSB and RMS. Waterpump, LN thermostat and coolant flush done last summer. Again, hesitation occurred prior to, and after, all this.

I'll check for intake / vacuum leaks and put a durametric on it on a cold start over the weekend, see if I can catch some logs of it in the act. Then maybe a TB cleaning...

A third sample from a friend: same issue, replaced injectors - no improvement.

Local Indy suggested DME or VC+ actuators.

Ahsai

So are we trying to diagnose multiple cars on the same thread? It may get very confusing.

For OP's car, if you suspect the SAI pump drawing too much current, you can just disconnect it and retry the cold start. Second Joe's suggestions, especially on intake leak.

When you use Durametric for a cold start, it will help if you log all four O2 sensor voltage, MAF, misfire counters, RKAT and FRA.

Nedster

I can assure you on my car, it wasn't because the gas is low The only thing i can add to the mystery is that if the car sits for a long time (over a month) it won't hesitate like that it will start just fine. I did all of the throttle cleaning, replaced MAF, new gas, no change. Not sure those three things ever fixed anything I am about to change the coolant temperature sensor as well with engine bay temp. sensor to see if that might be it as at least the coolant temp. value is important at startup.

An indy suggested it could be go as far up as the DME, or VarioCam actuators.

mcfisticuffs

After watching this video (https://rennlist.com/forums/996-foru...any-ideas.html) I can say my SAI pump has NEVER ran. Not once have I heard that vacuum noise.

I don't recall seeing any codes on the dura for it... the car is getting a new AOS on Tuesday so I will pull codes and track starting after that.

Ahsai

That's very unlikely because the DME watches the SAI system like a hawk. Normally it sounds like a small portable vacuum cleaner but when its bearings start to dry up, it will sound like a benchsaw.

mcfisticuffs

It makes no noise whatsoever.

Ahsai

Then it''s a mystery to me. Your car should have the same OBDII smog requirements...unless someone coded it out in a tune.

DBJoe996

It appears you have Durametric. You can trigger the SAI pump and see what happens. Agree with Ahsai...the DME monitors the SAI and control valves like a hawk. Anything not right and it triggers a CEL and code. Unless, like he suggests, you have a different "tune" in your DME. And, you are not getting any OBDII codes...right?

rymerc

Good luck. My 03' has a similar behavior, where on cold start it fires right up, but for 5 or 10 seconds if you try to drive off it will miss slightly just for a split second then be fine. If you rev above idle even for 5 seconds after startup it won't stutter on drive-off at all.

I've replaced coils, plugs, oil filler tube, checked all clamps/hoses/intake plumbing, cleaned the MAF and throttle body, and can't turn anything up. Watching with an obd2 reader all trims and values look normal. Does it at any ambient temp, only on a cold engine, on any gas I've tried.

Good. Luck.

Ahsai

One possibility is a leaky fuel injector. Another member "Sean" something (don't remember) had similar symptoms. Basically extra fuel leaked down to the cylinder when the car is parked and the rich mixture causes the cold start shudder.

mcfisticuffs

Update: It is NOT the AOS. Just picked it up from the dealer where it had its AOS replaced and it shuddered twice on cold start. I hope to have the dura on it this week.

That fuel injector theory is sounding good.