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Vacuum Leak at Throttle Body 700 1989

posted by  Dorsal900 on  Mon Apr 17 20:47 CST 2017
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last visit: Fri Oct 4 15:53 CST 2013
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I'm trying to solve a rich mixture problem with my 1989 Volvo 740 Turbo; car has been in my family since new and has only 104,000 miles (in 27.5 years!), good body condition. Car is currently getting approximately 12.6 MPG in mixed driving, sooty tailpipe and spark plugs.

Previous relevant work:
1. New air filter and spark plugs (NGK Platinum) approximately 4000 miles ago. 2. Coolant Temperature Sensor and Coolant Gauge sensor replaced a few months ago, along with thermostat which had apparently failed (hard to tell with non-working gauge).

What I've done lately:
1. Cleaned MAF Sensor with CRC MAF Sensor Cleaner. Made car run slightly smoother.
2. Sucked on vacuum hose connected to fuel pressure regulator - holds vacuum, not leaking fuel through diaphragm.
3. Visually checked all vacuum hoses, all are OK.
4. Borrowed a smoke tester, hooked it to the brake booster vacuum line (plugged the intake hose just downstream of the MAF) and found that smoke was coming out at the Compressor Bypass Valve (CBV) and through the spring on front of the throttle body! (see pic)

The leak at the CBV may be due to one of the two large hoses being slightly loose; the valve is very oily/dirty and I don't see any cracks in the body of it, so I will remove it, clean it up, reinstall and tighten the hoses. Hopefully the hoses will still be usable, as it's probably hard to find replacements.

The throttle body leak "through the spring" may actually be due to an ovaled out hole where the shaft goes through the throttle body. Has anyone seen this before? Is there a straightforward solution for this? I know on the older Volvos with SU carbs, this was a problem that could be solved by drilling out the hole and installing an appropriate bronze bushing. Perhaps a used part is a better solution?

Bill
1989 Volvo 740 Turbo
1970 Volvo 1800E









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    Vacuum Leak at Throttle Body 700 1989

    uid 40935's pic
    posted by  Goatman on  Fri May 5 22:55 CST 2017
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    last visit: Sat Jul 26 20:30 CST 2014
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    Hi,


    Running rich isn’t generally a sign of an air leak, the mixture will lean out if this is the case.

    Have you checked the fuel pressure regulator vacuum hose for fuel?


    Goatman








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      Vacuum Leak at Throttle Body 700 1989

      posted by  Dorsal900 on  Sun May 7 19:06 CST 2017
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      last visit: Fri Oct 4 15:53 CST 2013
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      Actually, it can work both ways: an unmetered air leak fools the oxygen sensor into believing that the mixture is too lean (more oxygen present in the exhaust than there should be) which causes the ECU to enrich the mixture.








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        Vacuum Leak at Throttle Body 700 1989

        uid 40935's pic
        posted by  Goatman on  Sun May 7 21:21 CST 2017
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        last visit: Sat Jul 26 20:30 CST 2014
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        Hi,


        An unmetered air leak will lean the mixture out because less air passes the air flow sensor.

        The oxygen sensor system does not even kick in until operating temperature is achieved, and would certainly not enrichen the mixture because of ‘‘extra air’’. That’s logically wrong.

        Extra air will be completely burned because of the rich mixture, and the oxygen sensor would never have any chance of ‘‘receiving’’ extra oxygen because of the air leak.


        Goatman








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          Vacuum Leak at Throttle Body 700 1989

          posted by  Dorsal900 on  Mon May 15 09:36 CST 2017
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          last visit: Fri Oct 4 15:53 CST 2013
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          There is much discussion on this subject online, suggest that you do a Google search for "unmetered air makes car run too rich": https://www.google.com/search?q=unmetered+air+makes+car+run+too+rich&ie=utf-8&oe=utf-8

          Here is at least one example of opinion going the other way: https://rennlist.com/forums/993-forum/432915-does-a-leaky-intake-cause-lean-or-rich-running-in-a-na-car.html








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    Vacuum Leak at Throttle Body 700 1989

    posted by  Dorsal900 on  Fri Apr 28 14:35 CST 2017
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    last visit: Fri Oct 4 15:53 CST 2013
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    I still do not have a solution for repairing the throttle body, but maybe this isn't such a large leak. I installed a new oxygen sensor (Bosch #13957) and set about to setting the base idle setting using a test LED (made from parts I had on hand) plugged into the mixture test point (Bosch LH 2.2 injection system) and an ANALOG voltmeter connected to the output of the oxygen sensor - you want to be able to see the needle swing back and forth, which is not possible on a digital meter. Initially, the LED failed to light, so I tried to set the MAF sensor adjustment screw to get a fairly even swing between 0.2 and 0.8 Volts. After turning the screw 9-1/2 turns counterclockwise (leaner setting) all of a sudden the light started blinking at about a 50% duty cycle - success!

    Car is now running much better, and I assume will use a lot less gasoline. My guess is that someone enriched the mixture severely to make up for an unknown intake air leak. What was news to me was that the mixture adjustment is not just a 360-degree setting, but can be adjusted over a very wide range - int this case 9-1/2 turns.








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      Vacuum Leak at Throttle Body 700 1989

      posted by  Nahtanha on  Thu May 11 14:57 CST 2017
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      last visit: Tue May 20 10:10 CST 2014
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      Place a thin nylon washer between the shaft but and the throttle body shaft. Your AMM basic reading (Ohm) should have a basic setting around 480k.
      Dirty fuel filter is also a big problem with your mileage. Have you tested your fuel pressure regulator? When you pull the vacuum hose off of the regulator, there should be NO gasoline present.







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        Vacuum Leak at Throttle Body 700 1989

        posted by  Dorsal900 on  Sun May 14 08:07 CST 2017
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        last visit: Fri Oct 4 15:53 CST 2013
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        You wrote: "Place a thin nylon washer between the shaft but and the throttle body shaft." Not following your response - where to put the washer?

        Already went through the fuel pressure regulator test, no gas present. Car is now running better than it has for years and appears to be getting better gas mileage based n the gauge, but I haven't refilled the tank yet.








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      Air Mass Meter 700 1989

      uid 27's pic
      posted by  Art Benstein on  Fri Apr 28 18:30 CST 2017
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      last visit: Fri Oct 17 20:10 CST 2014
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      I agree. The smoke was a red herring. Not having practical experience with smoke testing will send you on those chases.

      Interesting that you decided to use MAF cleaner on your AMM. Many have tried this and ruined it. One of the vulnerable items is the seal for keeping that 15-turn trimmer potentiometer clean. If you measure the voltage at the trimpot wire, you can have a visual reference (between 0 and 2.7V) for where you've set that. The ECU reads this voltage to combine with the AMM's output in setting the mixture.

      The clue your trouble wasn't the TB shaft was the horrendous MPG. Nothing so minor as a TB shaft leak would double your fuel consumption.
      --
      Art Benstein near Baltimore

      If lawyers are disbarred and clergymen defrocked, doesn't it follow that electricians can be delighted, musicians denoted, cowboys deranged, models deposed, tree surgeons debarked, and dry cleaners depressed?








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        Air Mass Meter 700 1989

        posted by  Dorsal900 on  Wed May 3 18:33 CST 2017
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        last visit: Fri Oct 4 15:53 CST 2013
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        Re MAF cleaner: not so interesting, as this is what the part is called:
        "Volvo Mass Air Flow Sensor - Bosch 0986280102".

        I asked my brother is anyone else worked on the car while he had it - but of course! He brought it to a shop near his house and it was barely running at the time. They did something (I suspect enrichening the mixture severely) that made it run "better", but didn't tell him what they did.








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    Vacuum Leak at Throttle Body 700 1989

    posted by  TONY H on  Wed Apr 19 17:24 CST 2017
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    last visit: Fri Oct 17 16:39 CST 2014
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    Sorry I cannot offer any good advice. I cannot believe a Throttle body with only 100K+ on it has worn out the Butterfly shaft holes. Not sure why you got smoke thru the throttle body but I think I would look elsewhere for the rich condition.

    Any cracks in the Large Bellows from MAF to Throttle body. They do get cracks in them which throws off the fuel to air mixture
    --
    '75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.lyse.net/brox/TonyPage4.html http://cleanflametrap.com/tony/








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      Vacuum Leak at Throttle Body 700 1989

      posted by  Dorsal900 on  Wed Apr 19 18:46 CST 2017
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      last visit: Fri Oct 4 15:53 CST 2013
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      Tony H: Thanks for your reply. However, the picture clearly shows smoke coming out of the throttle body spring, and the shaft is what is under the spring. I checked and did not see any leaks in the large rubber bellows, which is what I expected to find.

      I did tighten up the Compressor Bypass Valve (CBV) screws, four of them, compressing the gaskets - this may be what was leaking there.







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