How a broken motor mount can set a MAF code...

Discussion in 'General Automotive Tech' started by joe_padavano, May 28, 2016.

  1. joe_padavano

    joe_padavano Well-Known Member

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    It pains me to admit that in addition to my Oldsmobiles, I have a 2002 V40 wagon. Long story, not my first choice, but it's what we have.

    The Volvo had been running poorly for over a month now. As usual, this always happens right before I need to get emissions testing. (Memo to self: get rid of all cars less than 25 yr old...) Symptoms have been difficult to start either hot or cold, stalling (requiring two-foot driving at low speeds), lack of power, and poor gas mileage. CEL comes on all the time and codes have been MAF sensor, fuel trim pegged at the stops, O2 sensor, and misfire.

    I had replaced the notorious Volvo fuel pressure regulator a couple of years ago after another no-start problem. I had also replaced the idle air control valve at the same time. The new problem kept throwing a MAF error code, but the car ran fine at high speed; the problem was only at idle and low speed, mainly within the first few minutes of startup. I bought a new IAC anyway, in addition to a new MAF, based on the code and web posts on Volvo sites about dirty MAF sensors. The car does have 125K miles on it, after all. I also was thinking this might be a sticky injector issue, since I've never cleaned the injectors.

    Yesterday, since it has finally stopped raining, I pulled the plastic cover over the IAC and throttle body (do not get me started about stupid plastic engine covers...) to replace the IAC, and that's when I found a broken hose. Turns out that unlike every other EFI car in the world, where the IAC is fed by a bypass passage in the throttle body, Volvo decided to run a separate intake hose to the IAC from the intercooler outlet. Keep in mind that this is a transverse engine. The main intake pipe from the intercooler to the throttle body is long and U-shaped, allowing plenty of flexibility to absorb engine rocking. This bypass hose to the IAC is short and stiff, running directly back from the intercooler to the IAC valve on the intake. It's about 8" long and is nearly straight - there's a slight S-bend in it. It had broken completely right at the nipple on the IAC, which is naturally under the throttle body and hidden by the stupid plastic cover. I had suspected the motor mounts were bad, since I routinely got a clunking noise on bumps and on/off the throttle. Clearly, my binary throttle driving style has regularly rocked the engine and strained the hose, though why there wasn't extra strain relief designed in remains a mystery. Naturally this hose is THAT MUCH larger in diameter than normal heater hose, so I had to order one. Fortunately the break was close enough to the end that I could reinstall the old one for now and stretch it to reach. So long as I don't hit the throttle too hard, it should stay on. Motor mounts are on the way also.

    Car now runs like it used to , but I remain a little confused as to why this broken hose caused the driveabilty problem. As far as the IAC is concerned, the broken hose should have no effect other than to allow the IAC to draw unfiltered instead of filtered air. Similarly, since the leak in the main inlet tube is upstream of the MAF, it shouldn't have any effect at low speeds from that side either. Obviously the leak was bleeding boost pressure, so I understand the loss of power. Since the rough running was primarily within the first couple of minutes of startup, all I can surmise is that the EFI must use a speed/density logic while in open loop mode, waiting for the O2 sensors and MAF to heat up. That would explain both the poor running at startup and the improved running after a few minutes.
     
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  2. ModelT1

    ModelT1 Still Lost in the 50's

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    A book about a Volvo! Why are all cars put together so stupid? :slap:
     
  3. BlueVista

    BlueVista Well-Known Member Charter Member

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    Now all the Volvo wagon people will be trying to get a hold of Sven at the same time to get their motor mounts checked.:D
     
  4. Silvertwinkiehobo

    Silvertwinkiehobo "Everything that breaks starts with 'F.'"

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    Yeah, those stupid Volvo mounts. Had a good customer's older sedan have the same problem. Had to order the hose (which, luckily, comes in a bag of lots of hose), so until the hose showed its face, I deftly made a hose extension out of a piece of tubing and shoved it in the nipple, then into the air hose, to temporarily make the driveability headaches go away, even though the CEL was still on. And it affects the MAF because the IAC is ultimately downstream of the MAF and throttle body. After all, that's how the IAC works--it bypasses a measure of air past the throttle body to control the idle.
     
  5. joe_padavano

    joe_padavano Well-Known Member

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    I misspoke above. The MAF is BEFORE the turbo, so yes, the bypass hose (and resulting leak) is after the MAF. Sorry for the mis-information.
     
  6. Silvertwinkiehobo

    Silvertwinkiehobo "Everything that breaks starts with 'F.'"

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    Hey, no problem. If nobody speaks up, nobody learns anything. I just hope I didn't offend you by insulting you; if I did, I do apologize.
     
  7. joe_padavano

    joe_padavano Well-Known Member

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    No apology necessary. I want to be corrected when I make a mistake, especially if it's a dumb-@$$ mistake. That's the only way we learn. You certainly need to try harder than that to offend me. :cheers:
     
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  8. Silvertwinkiehobo

    Silvertwinkiehobo "Everything that breaks starts with 'F.'"

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    OK, yer ugly and yer mama dresses ya funny....
     
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  9. joe_padavano

    joe_padavano Well-Known Member

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    Tell me something I don't know...

    I think one of the worst insults I ever received was when someone referred to my 1969 Hurst/Olds as a GTO Judge. :eek:
     
  10. OldFox

    OldFox Curmudgeon

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    You do realize that statement dates you.
     
  11. ModelT1

    ModelT1 Still Lost in the 50's

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    Glad i nevur mak misteaks!:biglaugh:

    and yer mama did dress ya funny.... I saw pitchers.:)
     
  12. Silvertwinkiehobo

    Silvertwinkiehobo "Everything that breaks starts with 'F.'"

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    What's funny about your post is that I don't know if you're referring to him...or me....:rednose:
     
  13. OldFox

    OldFox Curmudgeon

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    Well, as Clem Kadiddlehopper used to say, "There goes another flock of em."
     
  14. Silvertwinkiehobo

    Silvertwinkiehobo "Everything that breaks starts with 'F.'"

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    Oh, yeah. God, it's been such a long time. The last time I saw Mr. Skelton on TV was a special on, I think, HBO when I was in high school. Mom and Dad were both tickled pink I was finding so much laughter from him! Thank you for the memory!
     
  15. joe_padavano

    joe_padavano Well-Known Member

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    This thread has now completely left the pavement, crossed the median, become airborne, and is about to come crashing down into a bridge abutment.
     

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