Multiple misfires - bad vacuum leak?

[skibo]

2004 XC70 - 242,000 miles

For the past few weeks I've been getting an occasional stumble when pulling hills in hot weather - i suspected plugs.

2 days ago I was under the hood and heard a loud hissing from under the intake manifold - sounded like a vacuum leak - I moved some hoses and it got louder and softer, but I could not see anything.

Today the wife brings it home and reports it is running very badly and the CEL is flashing.

I pulled codes, and see
P2187 - lean mixture
P0300 - general misfire
P0304 - misfire cylinder 4
P0305 - misfire cylinder 5

So it feels like the vacuum leak has suddenly gotten much worse - possibly because of me flexing the hoses. Where can I find a diagram that might help me track this down by feel - don't want to take off the manifold for this.

Thanks.

[Astro14]

For chasing down vacuum leaks, I use ether (starting fluid). IF the engine RPM increases when the fluid is sprayed, then you know that you've found the general area. It's not precision work, but it gets you in the ballpark. Be careful, ether is really flammable.

Alternatively, you can use a propane torch, unlit, and the propane (also a fuel) will cause the RPM to increase. I've not tried that method, but it seems a bit more precise than the ether, and perhaps a bit safer.

Garages use a smoke machine and pressurize the manifold to see where the smoke escapes. I've read that this can be done with a cigar. Which I might be willing to try just to enjoy the cigar...

[skibo]

Found the vacuum leak - obvious once I got in there with a flashlight - replaced the hose and got rid for the 'lean mixture' code - still getting the misfire codes.

[skibo]

Ended up being a single bad coil. Biggest lesson learned is that the misfire codes (0300, p301, 0302, 0303, 0304, 0305) are not accurate - I moved the coils around, cleared the codes and still got 0300 (random misfire) and 2 specific cylinders. It was only by unplugging the injectors one at a time that I was able to track down the one not firing - then I moved that coil again and the misfire followed. Went with the cheapest coil I could find just to get it running ($35 US) - may get a full set of Bosch's from FCP to avoid the next one.

[StuntmanMike]

Would a misfire always cause the CEL to come on? Mine has been stumbling under acceleration or on hills, and I assume it's a misfire. However, no CEL yet.

[skibo]

I've had that twice - miss and stumbling at heavy throttle - especially on very hot days (upper 80's counts as very hot here) - at 81k mile and again at 177k miles - no CEL either time - both times a set of new OEM plugs fixed it.

[Rene12]

Simplist way to find vacuum leak is make smoke test with pressure not more than 0.3bar

[Thang Vora]

Yes, lean mixture could be vacuum leak, if it too bad engine will shudder and stop. To confirm the miss fire move those ignition coils to difference cylinders, if they're miss fire on new cylinders, replace the coils, if it still miss fire on the same old cylinders, replace the spark plugs.