Loss of boost/power at high rpm - Saab 9-5 content

Still trying to track down this issue on my '01 Saab 9-5 wagon. It's the 2.3L turbo, 5spd. OK, the phrase "trying to track down" is a bit of a stretch...I've mostly just been living with it and have done much with it.

I drive more towards fuel economy than sport, so I'm usually rather easy on the gas. Under normal driving, the car acts fine. It accelerates normally, I get strong boost from the turbo. But if I let the revs climb under part throttle, I usually get a drop off in boost as the revs push beyond 3200rpm or so, with a corresponding loss of acceleration. Tried cleaning the boost control valve, but it didn't help. This morning, I decided to try to do a few flat out acceleration runs just to see if the problem is worse. It is. I get strong boost up to roughly 3200 or 3300 rpm and the car feels great. Then suddenly the boost drops way off. The engine gave a very brief but pronounced sputter and acceleration slowed to a crawl. It was more noticable in 2nd than 1st, and even more in 3rd (I let off the gas in 3rd).

As I said, the typically only shows over 3k rpm, and gets worse the harder you hit the throttle. I have a CEL for a "slow to respond" cat. Could the cat be partly clogged and therefore causing high rpm issues?

Yes.

I'd also make sure the fuel filter is not plugged, that there isn't a dead cat in the air filter, and that the coil output is healthy.

Thanks. I'm pretty sure I pulled the dead cat out of the air filter a few weeks ago, it's in the hatch of my wife's car...

I should mention that I'm not getting the fuel mileage I would have expected from the car. I average about 26mpg. My commute is mostly 50-55mph country road cruising, ideal for good gas mileage. May be unrelated, but thought it's worth mentioning.

If you do find that the cat is to blame and you have the money I stongly recomend this as a replacement:

3in Downpipe V2 for 9-5 4cyl

It is pricey but worth it. I put one on my 9-3 when the flex pipe broke. It is a shame that it is hidden under the car because it is a very pretty thing.

I do not have any connection to the above place other than a very satisfied customer.

I had a similar situation after a water pump replacement. Can't recall what my CEL code was, though. Had an experienced SAAB technician go over it with particular attention to the vacuum line routing. He corrected a couple of misrouted lines and voila, back to her old self. I would have never found it on my own, nothing was obbviously out of place underhood.

Have you replaced the weak factory diverter valve or tried a new BPC valve? If either has a failed diaphragm boost will leak as it builds past a certain point.

If your cat is toast, spring for one of these bad boys... http://www.genuinesaab.com/catalog/product_info.php?cPath=47_53&products_id=407&osCsid=5f45a51db26ee33db923a4cafc57a97e

Any way to relatively easily rule one of the possibilities in/out, before I start throwing parts at it? I took the BPC valve off and used carb cleaner to try cleaning it. Not sure if this is supposed to happen, but when I sprayed into the middle tube, it shot out the bottom tube.

I'm getting good boost at low rpm. It goes easily into the yellow, and even sometimes into the red. But once I pass that 3-3.2k rpm mark, the boost just falls off, as does acceleration. And the hiccup under full throttle was odd.

Disconnect the cat, go for a drive?

Klayfish wrote: Any way to relatively easily rule one of the possibilities in/out, before I start throwing parts at it? I took the BPC valve off and used carb cleaner to try cleaning it. Not sure if this is supposed to happen, but when I sprayed into the middle tube, it shot out the bottom tube. I'm getting good boost at low rpm. It goes easily into the yellow, and even sometimes into the red. But once I pass that 3-3.2k rpm mark, the boost just falls off, as does acceleration. And the hiccup under full throttle was odd.

If your car makes a sound sort of like a beached whale under part throttle the diverter valve, aka "hooter" is bad. Forge make tunable/rebuildable replacements with a metal piston for about $140 that are much more robust, and Chinese knock-offs of the Forge unit are pretty common on eBay for half the price or less.

You can bypass the BPC valve by swapping the hoses to verify if it is working or not. New ones are cheap, about $50.

Sweet, thanks for the suggestions! Like a dope, I had the radio on this morning when I was messing around. I'll shut it off and see if I hear any noises.

When you say swap hoses on the BPC, do you mean actually change them out?

Klayfish wrote: Sweet, thanks for the suggestions! Like a dope, I had the radio on this morning when I was messing around. I'll shut it off and see if I hear any noises. When you say swap hoses on the BPC, do you mean actually change them out?

No, if you swap two of the hoses' positions (the top one and the middle one, IIRC - there is tons of info on SaabCentral about the trick) you bypass the BPC valve, and if the car runs properly with it bypassed the valve is bad. They are cheap enough though to where I would just replace it if I were you.

When I had my 9-5 eeuroparts.com always seemed to be the cheapest, FYI. genuinesaab.com has some downright awesome performance and better-than-OE trim stuff too.

I have no experience with saabs really but if it was a nissan I would check spark plugs/wires, other ignition stuff, and pressure check for vacuum leaks, PCV valve, and other one way valves.

It doesn't fit your symptoms, so I doubt it's the cause, but I had a weird case of losing boost at high revs in my Miata once. It turned out that the alternator was failing, and that failure was manifesting as a drop in current output at high revs (nominally a 65 amp alternator, but would only do 30 amps at 5000 RPM+). This caused a voltage sag, which browned-out the boost controller, so above 5000 RPM I was only getting the mechanical wastegate pressure.

I agree that yours sounds more like a clogged cat.