how to get smooth decceleration?

[gbsaab]

I was wondering if there are any cures for jerky deceleration; my car (86' T16v) runs great, but at low speeds I find that when I let off the gas the thing just won't run smooth. I also get it a bit when acclerationing in 1st and 2nd. Clean the injectors maybe? I have changed the FPR to a 3.0 bar and have redbox apc and run high octane fuel, along with a few other mods. Anyone else have this problem?

[Mr. Forchielli]

i dont know if this is the same thing, but mine jerks too when i let off the gas at high rpms in low gears, because of the front motor mount.

[Elwood]

Don't forget to check the BPV (hooter) valve.

[fura]

Pardon my ignorance: what is BPV valve? ops:
I have the same problem too.

[Slaab4life]

these cars are both equipped with dashpots on the intake housings? I know its for stopping the butterfly valve from choking the engine on decel, just don't know which models have it though.

[Elwood]

Originally Posted by fura

Pardon my ignorance: what is BPV valve? ops:
I have the same problem too.

Its the turbo bypass valve. It equalizes the pressure on both sides of the throttle body when you decelerate (shifting from boost to vacuum). It keeps all sorts of weird things from happening (pressure waves, butterfly slamming shut, etc). Everything you need to know from Townsend:

http://216.78.172.32/turbo_folder/hootervalve.htm

[fura]

Arghh ... I have no turbo

[gbsaab]

I find that running a low octane (87) fuel helps a bit, though then again, could just be in my head; could this be effecting it at all? Of course running such low octane really messes up my peformance :-? anyhow, I'll give a different bypass a try.

[Saabster900S]

perfect soloution! Match revs, and its so much easier to just push the clutch in when you slow down instead of the hoping.

[jasonmah]

Hi. I have a 1990 900ems (non turbo). I also have the problem of the car being quite jerky when lifting my foot off the accelerator pedal. Have tried cleaning the throttle valve and aic. The Saab mechanics don't seem to think that it is to do with the engine mounts. When not in gear and the car is revved, it drops in revs smoothly. ie only a problem when the car is in gear. Have you had any further successes in solving your problem. An annoying problem as the car otherwise runs beautifully. Love any input.
Thanks.

[Matthew]

A friend of mine and I had recent experience of this on his 900T.

Using a set of stock, tatty Bougicord HT leads, his car exhibits jerky deceleration and pickup from part throttle at low speeds. Swapping the old Bougicords to fairly new Magnecor HT leads results in a car that is noticably smoother. No jerky decel or pickup. The difference is particularly noticable when driving around town.

So, try fitting a new set of HT leads. New Bougicords are the safest bet as they're OE fit and robust. I have had excellent results with the Magnecors, but they're a bit more fragile and others 900 owners haven't had the same success.

[djr]

I wonder if my 900 LPT has the same problem. It can have incredible jerky 'kangaroo like' spasms at low revs < 3000 rpm often under load ie accelerating. This problem is intermittent but when with the car lasts for several hundered miles. It can then go completely for another few weeks. It is very very severe and can even jolt the car out of gear. You can 'drive around' the problem by maintaining even revs or driving at high revs >4000 rpm when it seems to be less of a problem.

Several mechanics have now looked at this - no definite diagnosis (even at Saab dealer and local specialists). We have changed air flow meter. At the momemt car is driving well.

I wondered if the high tension leads could in principle cause this sort of problem...

DJR

[900t]

What you mean that at 5000r/min in 1st gear and you suddenly take off your foot the nose bites the pavement?

[jetman]

I'd agree with Mathew. All this sounds like ignition related gremlins.

[Groove]

I spent a few days adjusting the dashpot and test driving, which got rid of a lot of the initial jerk when releasing the accelerator pedal suddenly. It's definitely worth a look as it has improved the jerkiness for me.

[jasonmah]

Matthew, I tried a new set of HT leads but this has not eliminated the jerky deceleration yet . However going along in the same vein, would you recommend changing the distributor cap and rotor button too (distributor cap originally had a layer of something inside the cap but I had cleaned it off), as the next step to try?
p/s is there a way to check if it is a failed motor mount?
Jason

[Matthew]

If the distributor cap and rotot arm are old, then there's no harm in replacing them. They're not expensive parts.

The test for a failed front motor mount is to open the bonnet, put the car in first gear with the handbrake on. Then try to drive away without the engine stalling. Watch the front of the engine. if it rises by more than, say, three inches (slightly arbitrary figure - anyone?) then the engine mount needs replacing.

Does the gearstick move a lot when you lift off the accelerator?

[jasonmah]

I will will replace the distributor cap and rotor button soon at the next service as my odometer stopped working for quite sometime (fixed now - thanks to the posts by others!).

The gear stick doesn't seem to move when the car jerks. I did the test for the front engine mounts and it seems to be OK. The engine does shake a little (horizontally left and right) when the engine is turned off. Maybe the left or right engine mounts causing the jerkiness? Anyway, it's been really good listening to people's ideas and experiences.