thrasher

Hi, I have a 2005 997.1 S (3.8).

About 2 months ago, after not driving the car a while (traveling) I started having a problem that goes like this:

• Car starts and runs fine.
• After driving ~10 minutes, I feel a shuddering, loss of power and the CEL illuminates.
• If I give the car more gas the CEL flashes yellow and the shuddering intensifies.
• If I key off and restart, the car is fine, but only for a short while.
• When I check the codes, there are misfire faults on all 6 cylinders
• Sometimes when this happened, I swerved the car back and forth to "stir" the tank and the issue would go away for a while.

Initial diagnosis was old fuel, so I drained it and refueled. No change.

2nd diagnosis was clogged/failing fuel delivery. So, at the advice of a mechanic, I replaced the fuel pump assembly with new OEM.
I drove the car yesterday, and same thing.

• It was fine for 10 min (about 5 miles), then the same issue.
• I parked the car and it sat for about 1 hour.
• I restarted it and drove home (about 12-15 min), keeping revs < 2500 and the problem did not recur.
• No CEL, but pending codes, again misfires on both banks

Coil packs and plugs have <5k miles on them

Although I'm not getting any specific codes beyond P0300 (or 301-6 for each cylinder)

So, the leading candidates from my searches say

1. Crankshaft sensor
2. MAF sensor
3. Coolant temp sensor

Any advice on how to narrow this down?

Misfires on both banks; have seen all six cylinders at times.

scottrx7tt1

could be a bad fuel pump. Id test the pressure

Carreralicious

I had a similar issue and it turned out to be a bad evap purge valve (located in the engine bay). My Indy changed it out and the problem went away. I think my issue was that I used too much fuel injector cleaner (multiple times in a row) and it dislodged so much crap that it clogged the evap purge valve.

thrasher

Thanks scottrx7tt1, but the chances of both my previous pump and a brand new OE pump having the same issue seem very unlikely.

Carreralicious - I'll look that part up and see if I can test/replace it. My car sits far too much since I wore remotely and also travel for months at a time, so maybe bad fuel/crud (ethanol residue?)

DC911S

Have you ever cleaned your throttle body? They can get gunked up a bit. I cleaned mine in both the 997.1 and the 997.2. Its easy to take off, get some good TB cleaner and spray it good and wipe off all of the junk. And if it's never been cleaned, you will see a ridge of gunk near the TB plate inside and outside. Then redo the TB calibration adaptation sequence.

thrasher

DC911S I have not, but I will take a look. What is the best chemical to clean it with? I wiped the dust out of there the last time I had the airbox out, but that's been a couple of years now.

ajayabb

They make throttle body cleaner

scottrx7tt1

ahh, yeah, i missed that part that you had the pump replaced..... my bad

thrasher

ajayabb - Right, just asking for opinions on which "they" is best.

scottrx7tt1 - no worries!

DC911S

I use CRC products for TB cleaning...
CRC TB cleaner....

HartyWags

don't the 997.2 have 2 fuel pumps, one from the tank to bring the fuel up and another High pressure pump for injection. Did you replace both?

thrasher

Ok, I pulled the MAF and it looks brand new. I cleaned it anyway, but most likely not the culprit. I also looked at the throttle plate and it was silver and shiny on both sides and the entire intake path was spotless.

I could mention that the car had a complete engine rebuild 5,000 miles ago after the IMS grenaded with a previous owner, so it's likely the mechanics replaced the MAF and also tidied everything up. I've had the car since and never had a single issue.

wires are immaculate, but cleaned for good measure (inside and out)

throttle plate also virtually spotless

thrasher

This is an early 997.1 (2005), not DFI. It has a single pump and yes, I replaced it with new OEM.

thrasher

I don't have any of the codes for the evap valve, nor the other symptoms you had. I don't have hard start, rough idle, or any issues until after the condition appears in 10 min, and then all symptoms go away after shuttting ignition off, then restarting. I pulled the connection to the valve and it appears to be holding vacuum (not leaking) with the car off. I believe they fail open/leaking

Petza914

With misfires on all 6 cylinders, it has to be something common to both banks. How does it drive if you unplug the MAF? That will use middle of the road values for fueling instead of the measured ones. The other thing could be the coolant temp sensor since you say it happens after 10 minutes, which is about how long it takes these cars to warm up.

A bad brake pedal switch will also exhibit this driving behavior, but not usually the CELs. It will just fell like the gas pedal is dead and then a few seconds later come back to life, so that sounds different.

Wouldn't hurt to know camshaft deviation numbers and mean fuel trim values if you have a scan tool that will let you see those live values.

Start with the MAF.