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Discussion Starter · #1 ·
Not exactly a miss. Over the past few months my fuel economy has been getting worse. It seems like it started around when my oil filter housing went bad (leaking). Now the engine stumbles when holding a constant speed between 1500 and 3000 RPMs. There are no codes yet. Idle is good except for a few stumbles when cold and high idle. Once the RPMs drop it idles smooth. Acceleration is good. The long term fuel trim is running higher than normal at +7. Occasionally it drops to around +3 and there is less stumbling.

The plugs and coil packs were replaced about 30K miles ago with Volvo plugs and Bosch coil packs. The FPS was replaced a couple months ago (Bosch) when the long term fuel trim was running low -5. The MAF has been cleaned and I also tried an old spare MAF. The long term fuel trim started running + right after changing the FPS but seems to be increasing. The according to DashCommand the fuel pressure is relatively stable around 44 psi. I haven't been able to find any vacuum leaks yet. I'll plan to inspect he plugs and coil packs to see if they will tell me anything.

Any ideas?

Thanks
Stan
 

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So your PCV Box is bad. That is most likely the cause. It allows unmetered air to enter the engine, usually through the cam seals (thats the squealing noise generally associated with a failed diaphragm in the PCV). Crankcase vacuum will be very high. If crankcase pressure is more than -.5 in. of water then the PCV is faulty.
I've also seen a few faulty injectors on those vehicles.
 

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Discussion Starter · #3 ·
Thanks but I have already replaced the PCV box (oil filter housing) back in January when it was leaking.

I've been wondering about the injectors. Do you know of a good way to test them?
 

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I'd check the 5 intake runner o-rings, plus the big one right before the manifold. Maybe wipe them clean and grease them. If that works, then you could order new ones.
 

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Discussion Starter · #5 ·
Ahhh! Good idea. On my previous V50 the screws came loose and one actually fell out. Whatever the problem is, it is somewhat intermittent.
 

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So your PCV Box is bad. That is most likely the cause. It allows unmetered air to enter the engine, usually through the cam seals (thats the squealing noise generally associated with a failed diaphragm in the PCV). Crankcase vacuum will be very high. If crankcase pressure is more than -.5 in. of water then the PCV is faulty.
I've also seen a few faulty injectors on those vehicles.
Where did you get this number? Reason I ask is at idle there is definitely more the -0.5 in. H2O vacuum in the intake manifold alone, and I have found a high vacuum in my crankcase at idle as well, evidenced by the whoosh of the oil filler cap AND relief by the dipstick, but couldn't find any figures on why that would be. I replaced the diaphragm to no avail.
 

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2007 Volvo V50 T5 AWD M66
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If you just replaced the rubber it might be clogged internally.
 

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I am diagnosing something similar. My LT trims were in the -8 area.

I started with a stage 0, replaced all the vacuum lines I could, inspected the others, and clamped them all. I also cleaned the throttle, and bought a new cone filter, and removed inspected and cleaned the intake, including the green o-rings.

Actually let me back up. I didn't start getting the negative fuel trims until I replaced the vacuum line in front of the engine, then my fuel trims changed. I knew beforehand that I had some air recirculating somewhere because the engine ran like ****.

Anyways fast forward to completing stage 0, no change, I actually unplugged the battery for a week to do some interior work, and since then the LT trims have been in the 1.6 area (I don't remember if it's + or - I can check in an hour or so).

So fuel trims in check, however I still have ****ty mpg!! (22-24 mpg... 65% hw, 35% city) Still no codes. I do notice my mpgs go to **** when in traffic, usually slow enough where the torque converter doesn't engage and the engine can't go into ULEV mode, it's anywhere from 12-28mpg on the dash reader during that time. My next step it to do spark plugs, it's time anyways.

I dunno, it may be similar, but my recommendation for you... maybe reset the ECU and let it relearn? You may have more luck than me in getting better mpg.

Good luck!

Chris
 

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Discussion Starter · #10 ·
I'm still chasing mine too. I pulled the plugs to inspect. They look good as do the coil packs. I re-seated connectors on the cam position sensors and made sure they were tight. Inspected all I could before loosing daylight. Didn't find anything and still has the surging and poor mpg.

The vacuum gauge behavior puzzles me a bit. Holding the accelerator steady in a flat section of road, 2nd or 3rd gear, 2500 RPMs; the vacuum decreases slightly with the engine surge. It is like the throttle plate is opening a little even though I haven't moved the accelerator. The ECM must be controlling it. So is the ECM getting bad info and activating the throttle, the throttle body misbehaving or un-metered air (vacuum) leak?

Possibly a bad injector? TCV?

I am likely going to buy a bunch of parts and start replacing.
 

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Discussion Starter · #11 ·
Throttle body... I went to the bone yard and pulled some parts. I grabbed a brake vacuum hose and a throttle body off a 2007 T5. It took a few tries to find the brake vacuum hose with a good valve. I wasn't sure if mine was bad at the time but on a hunch I figured it was worth a try. Sure enough, mine was bad. The replacement did improve the feel of the engine but I still had the surging.

The next day I cleaned the replacement throttle body and installed it. I was surprised how much dirtier mine was. There was a brown coating on the back side (intake plenum side). I also noticed some oil residue at the bottom of the pipe coming from the intercooler. Anyway, what a different the throttle body made. Idle is very smooth, acceleration is strong and the surging is gone.
IMG-2978.jpg

The long term fuel trim is still running a bit high and screwing with my fuel economy. There may be some un-metered air getting in somewhere. I'd also like to clean out my intercooler and pipes to see how much is in there and to improve efficiency.
 

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Yeah, here's a pic of my TB when i cleaned it back in October last year, mine started running alot better too, but still no significant mpg increase.

View attachment 53909

I did manage to get a 26.1mpg drive to houston a few weeks ago, first time that's happened in a long time... so maybe things just need to calibrate. I did the calculation of the mpg manually.

Also, i don't understand how to not make my images attachments, any1 want to pm me the secret? :confused:
 
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