740 Why do I have a Saab valve on my car? And/or is this why I have no power under boost?

1991 740, B230FT, 16vT with Garrett T3. I did not build this car and I am fairly new to turbocharging, so please bear with me.

Lately my car has had some trouble with power. It drives almost exactly like a naturally aspirated 740. The compressor is spinning up, it's generating boost (currently at 10 psi) but there's only a teensy little bit of acceleration that dies off almost before you feel it. Prior to a few weeks ago it was just as quick as I could ask it to be.

After some searching and pondering, my first thought is that I have a boost leak somewhere so I put together a tester and hooked it up. The hoses hold pressure up to about 20 psi, and I couldn't find any leaks by sound or feel anywhere.

Unfortunately there was also a loud POP, which it turns out is from a Bosch valve, item # 0280142105. This seems to be a Saab overpressure valve, which seems to have taken the place of the external CBV that the T3 in the car came with. It was venting to atmosphere, so I guess it's sort of a BOV given the way it's plumbed.

In fact, the more I look at it the more I wonder if it was even hooked up to the correct port at all. The car has always had a strange noise off boost that sounded like really quiet compressor surge, but the previous owner said that it wasn't when I test drove it. This thing has some sort of vacuum diaphragm but pressure from that side wouldn't open up the valve at all as far as I can tell.

Is this a common modification? <strike>I don't remember ever seeing a reference to this being used on a Volvo before.</strike> It looks like I have an oddly numbered variant, I just found a bunch of threads describing very similar parts with final digits off by one or two. Can anyone who's used one of these tell/show me how it should be plumbed?

And since it apparently held pressure until today, what else should I be looking at to find my boost problem?

For now I guess I'll push the MBC down to zero pounds and drive it like I'm scared of it.

Thanks!

This might clear things up a bit for you.

http://forums.turbobricks.com/showthread.php?t=231607

Some more:

http://forums.turbobricks.com/showthread.php?t=157725
http://forums.turbobricks.com/showthread.php?t=137666

Okay, I am fairly sure my hoses aren't collapsing, as the car revs freely as high as I want, it just doesn't accelerate. And I don't have a torn intake coupler or hose, as far as I can tell. Accelerating slowly makes no difference.

Pretty sure my AMM is okay. I had some sketchy contacts on it, cleaned and tightened them. When the AMM was giving me trouble the car behaved completely differently, refusing to accelerate at all, bucking and generally being difficult.

From the pictures in those threads it looks like two of the three ports on the bypass valve were hooked up correctly, so dumping to atmosphere would just result in a rich spike on my car. I do remember occasionally smelling a little gasoline smell back when the car was running correctly, so it looks like that's my issue. I'm guessing that the surge-type noise I was hearing was the valve fluttering.

I'll have to make a junkyard run tomorrow and see if I can scrounge up a few replacements. Hopefully that's all this is.

Okay, I am fairly sure my hoses aren't collapsing, as the car revs freely as high as I want, it just doesn't accelerate. And I don't have a torn intake coupler or hose, as far as I can tell. Accelerating slowly makes no difference.

Pretty sure my AMM is okay. I had some sketchy contacts on it, cleaned and tightened them. When the AMM was giving me trouble the car behaved completely differently, refusing to accelerate at all, bucking and generally being difficult.

From the pictures in those threads it looks like two of the three ports on the bypass valve were hooked up correctly, so dumping to atmosphere would just result in a rich spike on my car. I do remember occasionally smelling a little gasoline smell back when the car was running correctly, so it looks like that's my issue. I'm guessing that the surge-type noise I was hearing was the valve fluttering.

I'll have to make a junkyard run tomorrow and see if I can scrounge up a few replacements. Hopefully that's all this is.

If it's a CBV venting to atmosphere, it will be open during cruising, sucking in unmetered air, unless the diaphragm pre-load is set high enough (if that model is adjustable).

Can you verify you are getting 10psi in the intake manifold, not pre-throttle body?

Canuckvolvo said:

If it's a CBV venting to atmosphere, it will be open during cruising, sucking in unmetered air, unless the diaphragm pre-load is set high enough (if that model is adjustable).

Can you verify you are getting 10psi in the intake manifold, not pre-throttle body?

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My boost/vac gauge is plumbed after the throttle body, into the same line that feeds the FPR on the rail. (I checked that the FPR is not leaking, just to be sure.) I assume that's what you mean?

That valve is not adjustable. If I had the money handy I would grab a Forge 007 but right now I'm limited to whatever I can find in the scrap yard or on ebay. Haven't had a chance to check the yard yet.

Was this a previous TB'ers car? Maybe some pictures of how things are setup. Sounds like you're on LH2.X?

Maybe for the same reason I have a Saab coolant cap on my car. Because it works for me.

doucheNozzle said:

Was this a previous TB'ers car? Maybe some pictures of how things are setup. Sounds like you're on LH2.X?

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Yeah, I know he was on here at least for a while because he originally listed the car for sale here a long ways back. I bought it from him directly since he's an old family friend.

It's LH2.4. Should have mentioned that before, sorry.

Here's the only picture I have handy besides a really blurry shot of the turbo ID tag. Anything in particular I should take a picture of that would help with diagnosis? http://i.imgur.com/HKrndvG.jpg

AdmiralSenn said:

Yeah, I know he was on here at least for a while because he originally listed the car for sale here a long ways back. I bought it from him directly since he's an old family friend.

It's LH2.4. Should have mentioned that before, sorry.

Here's the only picture I have handy besides a really blurry shot of the turbo ID tag. Anything in particular I should take a picture of that would help with diagnosis? http://i.imgur.com/HKrndvG.jpg

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That CBV needs to be plumbed back into the intake pre turbo.

Also that hose looks fairly kinked off, probably not helping.

Try this, pull the CBV and plug the port for now. Try accelerating and see if that improves (one thing, avoid suddenly snapping the throttle shut after being in boost as you'll have no pressure relief while its "bypassed".

Hank Scorpio said:

That CBV needs to be plumbed back into the intake pre turbo.

Also that hose looks fairly kinked off, probably not helping.

Try this, pull the CBV and plug the port for now. Try accelerating and see if that improves (one thing, avoid suddenly snapping the throttle shut after being in boost as you'll have no pressure relief while its "bypassed".

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Yeah, I'm going to look at some better plumbing stuff but I don't have any spare hoses to mess with so I'll have to do some measuring and order something online most likely.

I did get a replacement CBV of the same type, but the "better" model (ending in 110, supposedly built for ~10-15 pounds of boost rather than 7-10) that's supposed to be more durable. Haven't had a chance to run without it, but with the new one installed it definitely feels better. Instead of the faint surge I was hearing I get what you would expect from an open-port CBV, a nice BOV-like hiss on deceleration.

Not sure if the boost issue is fixed, as it rained tonight, so my drive was a little too scary to attempt any boost. It did seem like it pulled a little harder than it had been lately, but the tires started spinning before I could get any real sense of how it's performing, so I laid off for the night. I will drive it some tomorrow and see if the new CBV fixed the problem.

I do also have an issue with it not wanting to idle when cold and dying after letting off the gas after boost, which was a problem I had when I got the car that "fixed" itself. It seems that the CBV being open was helping it smooth out the idle, and now that I have the stronger model it's not open at idle any more. I am going to test the throttle switch and its alignment as I know it's been messed with at least once. The IAC does operate if I put the ECU in test mode, so I know that's working.


tl;dr: multiple issues now presenting symptoms, CBV was only part of it. Test drive inconclusive due to wet streets.

After a few more weeks of driving it with some new parts, here's where I stand.

Replaced AMM with known good rebuilt unit. No change.
Replaced CBV with higher-quality, non exploded CBV. CBV now functions although it is still venting to atmosphere. It does not feel like there is any vacuum on it at idle.

Car pops on initial throttle tip-in whether in gear or in neutral. I'm not sure if it's a backfire or something else, it sounds like someone smacking the end of a hollow tube. Maybe an intake backfire?

Car hesitates after these pops, and eventually engages and drives normally (as a naturally aspirated 16V) after a second or two. If I really step on it it will occasionally do the same thing at higher RPMs under load, but that's only happened once or twice and I can't seem to duplicate it.

TPS is adjusted correctly and idle motor appears to engage as it should. Pinching off the idle hose to the manifold does not affect the motor, I am looking for post-throttle body leaks but haven't found anything.

Unplugging the AMM and plugging it back in results in about a 600 RPM increase in idle speed and almost complete reduction of the hesitation and popping. I am not sure if this is normal or what this does to LH2.4 - I know unplugging the AMM puts it in limp-home mode but I don't know what effect reattaching it does.

I think I found a major boost leak and will be testing that today, but I don't want to apply boost until I fix this other issue.

Thanks!

If your AMM is in the stock place, a boost leak means a vacuum leak which means you **** ain't going to run correctly.

Fix both the boost leak and vent to atmosphere.

Well, I finally got some time to work on the car today. After spending about half an hour with the compressor hooked up to the turbo inlet hose and pushing 20-25 psi through the intake I was unable to find any boost leaks after all. The hose I suspected to be the problem is actually fine - it rubbed up against something and rubbed a small divot in the outer sheath, but it's not leaking. I will replace it soon but for now it's not the problem - I added a few layers of padded tape to keep it from rubbing any more. I did take all of the older looking hoses off and inspect them by flexing and twisting them but it turns out they're just faded, not cracked. Checked the throttle body, it's clean. Checked the FPR, it's not leaking.

I did end up pulling the fuse for the ECU to reset it and that seems to have fixed the stumbling problem. It now drives beautifully under any condition, no backfires, no hesitation. Idle is back to normal, throttle response is back to normal.

Still no power from the turbo though. Gauge reads 10 psi, dropping to around 8 after a second. I'm not even approaching high revs, the car runs completely out of steam before 6000 RPM. Previously I was getting some boost at around 3000.

I'm aware that the CBV venting to atmosphere is bad, but I did drive it like this for a few months straight with zero issues and it pulled as hard as I could have wanted. It's on the to-fix list but because of the way everything is plumbed I would pretty much have to redo the whole intake and the hot side of the intercooler, and I don't have the budget to blow on a ton of new hoses right now. If I move the AMM->Turbo hose forward to install a port for the CBV, it'll kink the hose pretty badly. It looks like everything is made from OEM Volvo hoses so I'm guessing I will eventually end up buying a bunch of hose lengths and bends from siliconeintakes.com to rebuild it.

Sorry for all the stupidity on my part, I'm trying to learn while diagnosing.

Have you <del>considered </del> CERTAIN the hose from AMM to turbo isn't collapsing under boost?

The pressure test won't show this collapse because it makes hoses expand with pressure.

When actually driving the opposite effect is happening: the turbo literally is sucking air out of the AMM into the turbo and an old oil soaked hose will distort and make a smaller opening, choking the boost. A cheap fix is to take a second hose clamp (or two) and place it between the AMM and the crankcase inlet hose stub. this helps hose stay round instead of squished under the intensity of boost.

Bensin said:

Have you <del>considered </del> CERTAIN the hose from AMM to turbo isn't collapsing under boost?

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I was fairly certain, but I tried the extra clamp trick anyway.

It did make a difference, although I don't have the level of power I had before, there is definitely a bit of boost there. I'll try adding a few more clamps.

I'm assuming that if the hose collapsing is my major problem, then I should expect an occasional backfire under load and code 1-1-3?

Also, my intake hose is the one with the heater for the crankcase vent system. Is that necessary and/or is there a better way to handle that? I'm trying to figure out if I can build a replacement hose from silicone parts.

Sounds like you have more than one problem. I don't have OBD on my 86 car, so I don't know codes. I don't know typical causes of backfire, but it makes me think ignition. Are your spark plug wires good? Distributor cap and rotor good?

my intake hose is the one with the heater for the crankcase vent system.

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?? Need some more clarification on this style, or even better, some photos.

By the way, the BOV is also stock for Volvo's with Garretts, it's just they way it's connected isn't standard.

Bensin said:

Sounds like you have more than one problem. I don't have OBD on my 86 car, so I don't know codes. I don't know typical causes of backfire, but it makes me think ignition. Are your spark plug wires good? Distributor cap and rotor good?

?? Need some more clarification on this style, or even better, some photos.

By the way, the BOV is also stock for Volvo's with Garretts, it's just they way it's connected isn't standard.

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http://www.ipdusa.com/products/7132/115019-air-intake-hose-740

That's the hose I have. It's got a two-wire plug in it to heat the air going to the crankcase vent. I read somewhere that it was to help de-ice the system in cold weather, but I don't know if that's entirely true or if removing it would be a problem.

Spark plug wires are in good shape, cap and rotor looked good when I first drove the car a few months ago although I will check them again next time I get to work on the car. I actually busted the coil wire when I first got the car and replaced it with a brand new one I had as a spare for my other 740, so I know that one at least isn't the problem.

The intake hose is definitely creased in a way that will encourage it to collapse and seems to be getting worse, so I'm putting together a parts list to replace it with something more durable. The only holdups are figuring out the crankcase vent setup and actually having the money to put it together. If I can just ignore the heater in that tube I can just add another regular port and be done.

The power loss problem seems to be almost entirely absent from a cold start and gets progressively worse over a long drive, which I think is consistent with the hose collapsing as it gets warmer. I feel pretty stupid for ignoring the possibility before.

I see, I have heard of this heater set-up but no experience with it. The only concern with it disconnected is what receives the signal? is disconnection going to change the ECU readings or LH system readings? I'm sure this has been covered on tbricks before (search google with "site:turbobricks.com [search words]")

Look up crankcase system or PCV. The hose simply starts at oil trap and runs to turbo hose; there is small y-joint of sorts that connects a small hose to manifold. But then the turbo may not be stock if the BOV vents into engine bay. I think maybe turbo is supposed to have the BOV built into it, which is usually called a CBV, compressor blow-off valve instead, does the same thing, different name.

Also try Google images for your engine, 16v is less common compared to 8v.

Don't worry about not knowing about hose collapse, turbocharging is a System, not simply a single part, by the end of this you likely know all about them!