'08 S60 2.5T stutter on start

[MOZKNBBUHD]

I've been trying to pin down this gremlin for a while now and it's starting to worry me going into the winter season. I've owned my 08 S60 2.5T for 18 months now and for the first 6 it was perfect, then intermittently it would have a small issue when it was starting. It wouldn't happen predictably and it wasn't the same thing every time, and it always ended up starting and running fine in the end so I didn't pay a whole lot of attention to it. Now that it's been getting colder it's started to crop up more and it's getting a bit more severe. What happens is that the engine starts to crank over and then it will either:

1. Start normally
2. Crank a bit longer than expected then start normally
or
3. Crank briefly, stop cranking, then immediately start cranking again and start normally

This sounded like a battery issue to me but I had it tested (resting voltage, 100A load voltage, starting voltage, and running voltage) and it passed every test twice on two different days. Right now to see if it's a low voltage/current issue with the battery I've got the trickle (.5A) charger hooked up and I left the car outside for the night to see if it stumbles in the morning. I'll try to see if I can catch it on video too, but if there's any other ideas floating around I'm definitely open. I'll try to take a look at the plugs tomorrow too because those are an unknown quantity. I also might have a drop in MPG but it might be more related to the cold than anything else.

I've got another thread on here about the transmission but I figured this warranted a different post.

[abscate]

Let's call this a cranking problem, to separate this from a starting problem ( where engine turns fine but doesn't start firing correctly or at all)..

You have done well being tuned into your beast as those symptoms narrow down to three things

Battery
Ignition switch / wiring
Starter motor.

Good cranking most of the time and battery testing eliminates the first
I eliminate the third based on reputation of Volvo starters ...not as good as eliminating battery but works for now.

I don't know if P2 ignition switches had the same problems as P80s, last generation , but that's where I would look.

Check to see if you have the white remote starter plug under your hood ..scour web for wiring diagram and post here

[oragex]

As above, leave the starter at the very end end.

Let us know how was it do it with that charger althought I would have rather do the test with a real battery booster. I would take this approach slowly, without rushing into replacing parts. For example, another user is having issues with a non cranking situation, and it may be tracked to a failed alarm siren module short which would drain the battery overnight, causing it to loose just enough power to not have enought juice for the starter. So better find the real issue first, as even a new battery would keep getting discharged which can quicjly damage even a new battery. Maybe make a test with the alarm, try to trigger it see if it sounds.

Don't discard also a close inspection of the cables around the alernator and the battery, while there.

There may also be an ingnition switch or immobiliser issue, but I'd leave these at the bottom of the list for now.

[MOZKNBBUHD]

Update: The car started fine this morning, but that's normal when it's sitting in the garage. It seems to be sensitive to the cold. I'll try to grab a video today of the issue.

[abscate]

How old is the battery - is it more than 5 years old?

[mrbrian200]

Throttle body. I'm almost certain. Intermittent problems become worse as the temperatures drop.
Internal nylon gear mechanism wears and the gear teeth start binding/sticking in the idle/part throttle range. At startup the throttle plate doesn't respond and it either doesn't open as far as the ECU is commanding or simply gets stuck at the idle position. It's running extremely rich until the engine warms a bit where the ECU would normally drop the idle/fuel to low idle and AFR for the actual amount of air getting in becomes 'appropriate'. If you kick the accelerator pedal in the seconds after starting sometimes you can unstick the mechanism and it will seem fine until the next cold start. Eventually it will get worse and have problems all the time, not just when cold.
Fortunately these Bosch throttle bodies after '01 or so are 'plug and play', and not too terribly expensive compared to the older Magneti Marelli ETMs that cost much more and required ECU programming if you needed to replace it.

Because the ECU doesn't tag the throttle body with an appropriate DTC seems there's 2 or three people that check in around this time of year who are having trouble diagnosing it. The Bosch throttle bodies are far less problematic than the MMs were, but by no means are they a perfect design that never ever fails.

[MOZKNBBUHD]

How old is the battery - is it more than 5 years old?

The battery is 4 years old, Everstart (I think, it's eversomething) Gold.

Throttle body could be. Is there an easy way to test it without having to get a new one?

[mrbrian200]

If you have a diagnostic tool with live data that shows the TPS/throttle plate position, from KPII while watching the sensor data cold start the engine. In the times it behaves normally the TPS will respond by opening part way during cranking then slowly close to between 0-7% as the engine warms. When the mechanism is binding you will see that the position doesn't open as much at startup, if at all, and a moderate press on the accelerator pedal less than about 1/4 down doesn't always/properly result in a corresponding throttle plate opening. Pressing the pedal further will often overpower the resistance in the mechanism and unstick it. On mine when I had a problem with this, the throttle plate would also at times stick part way open creating a lean idle condition. Whichever the case too rich or too lean it can sputter and run rough. Too rich becomes more of an issue on a cold engine because the ECU runs it rich during this time to begin with--if not enough air is getting in it's sort of a double whammy and begins to flood the engine.
If you determine the throttle body is not at fault, the next areas I would check would be the ignition coils which if weak/failing will be aggravated by the cool damp weather in early winter and spring. If those are ok I would move on the fuel injectors- one or more of them might be need to be replaced and getting sensitive to the cold, such as not responding to short pulses for idle or sticking open until the engine gets warm.
Failing cam and/or crank position sensors can become heat sensitive, but in my experience they tend to act up when hot, not cold.

[MOZKNBBUHD]

-snip for conciseness-

That's great info. I've got a bluetooth OBDII reader that can show the throttle position. I'll give it a check tomorrow in the morning when the car's in the warm garage then I'll park it outside and do it again.

[MOZKNBBUHD]

Alright, here's what I've got so far. It's coming through the Torque app with a bluetooth OBDII reader so I can't vouch for the accuracy, but on cold start from my garage (45-50 deg F) I got 20.7% throttle on fast idle, 16.7% after the revs dropped to 1000, and 13.7% once it was fully settled in. Once I got it warmed up it was at 12.5% at idle. When I stopped then restarted the car 40 minutes later I got 19.6% dropping to 13.3%. That sounds a lot higher than what you said but the amounts it's moving seem correct.