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Discussion Starter #1
Hi everone. I have recently got a Saab 900 16v turbo areo 1985. I diddn't get the chance to really give it some beans on my test drive as it was very busy at the time. Now i've had the chance to open it up a bit I now have a problem. When i reach near full boost on the turbo guage somthing seems to be shutting down the fuel untill boost is lost then switches everything back on.
Is this a common problem? I've had a look under the bonnet at all the plumbing to see if it looks ok. I have noticed a small valve that is disconected, It's on the left hand side inner wing (same side as the fuse box) and it looks like it had an electrical fitting on the top and a vacume hose on the side. I've got a manual on the way but is this the 'Pressure Transducer'?? Would this have been unplugged for a botched job to overide another broken thing?
Any ideas on where best to look? Waste gate stuck shut? Shagged APC or fuel pressure valve?

All ideas greatly appreciated - solutions even better!

Cheers, Geoff
 

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the pressure switch is under the dash , you can reach it through the left hand speaker grill , you can turn it up or splice the 2 wires together to stop the cut out and save your teeth from the steering wheel :cheesy:
 

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mothman said:
When i reach near full boost on the turbo guage somthing seems to be shutting down the fuel
If the overboost cutout is activating when the dashboard boost gauge needle reaches the end of the orange sector, then there is a fault. How much boost is the gauge showing when the engine cuts out?

Is the car modified?

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Fuel Cutoff Switch (Last Ditch Safety switch) is set to cut power to the Fuel pump at 1 bar.. obstensibly 15 psi..
That's approx Half way thru the Red zone on the Dash guage.
Fuel cutoff under such conditions feels like yr gonna plant your face on the windshield.. No joke.
Don't fool with it with insufficeint knowledge.
Sure the Engine can sustain more Boost.. But the Transbox has proven problematic with OEM levels of Power :)
 

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Discussion Starter #7
The fuel cut off it happening in the red zone not to sure where abouts as i was looking where i was going - I was gathering serious momentum!! I would have thought it would pull the boost to the end (or very near) of the red part then bleed off the waste gate. Your right about face in the windscreen! Glad i wasn't picking my nose at the time - i'd have given myself a labotimy!! AS far as I know it's all bone stock. I've looked through my full servive history and there is nothing about this problem. If i've got more boost than i should maybe someone has wiggled about with the APC pots. Can i keep this hi boost and get rid of that cut off - I'll try not to be too harsh on the tranny!
 

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When the car is still stock the gauge should only get to the border between orange and red. It sounds like someone has been tweaking it. This is fine, but you need to know what they have done since if they have messed it up you could do serious damage. I don't know a lot about APCs as I don't have one, but I think if you open it up there should be a small seal on each of the pots. If this is broken they have been tampered with. Full instructions on modding the APC system can be found on www.900aero.com . You also need to know what base boost is set at - I reckon the best thing to do is to take it all back to stock levels then turn it up slowly - that way you get some control over what's going on. Either that or stick it on a rolling road now to see that fuelling is fine. You can bypass or recalibrate the fuel cut off the way Alan says in his post above. Do a search for more details on any of this stuff.
 

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hi mothman. When i first got my 900T i was hitting the overboost switch all the time.
I solved it by turning the P pot on my APC down slightly (after marking where it was). However when i did this i found that the APC still had a tamperproof seal over it (it was like a big bit of plastic that covered the pots that was held on my 2 little metal loops at the ends). I broke the seal off.
This adjustment stopped me from hitting the boost switch on all but very cold/fresh mornings, but seemed to affect the rate at which the boost went up.

I have since adjusted the wastegate actuator arm on the car and that knocked my boost down enough that i have been able to turn the APC pot back to where it initially started. The car is much nicer now!
Adjusting the wastegate arm was a much easier job than i initially though. I made mine 2 turns longer to lower it (might go back one turn shorter and see where that puts me now that the weather is warmer)

As for the boost guage, it does seem very offputting how saab only designed the stock car to boost 2/3rds of the way up the guage! What is the rest of it there for then :cheesy:
Mine now goes just into the red. Overboost switch is about a 1/3rd of the way into the red on my car. Eventually i will fit my proper guage but havent managed yet!
 

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Discussion Starter #10
Fuel cut off valve

Hi everyone. I've turned the fuel cut of valve round a bit (it was at 11 o clock and now its at 3 o clock) and this has stopped the fule cut off. I know it's a botch job as it's boosting 3/4 in to the red:evil: but pinking like crazy. Is this sound the same a knock and detonation? I sertainly doesn't sound to good. Am i right in thinking I have either a faulty APC (i had a look at it's untampered with and still has the saab seals on it) or could my waste gate be stuck shut? How can i check the waste gate. Can i do that with only limmed tools and the turbo still in the car? Would spraying some penetrating oil on it free it up? Does the turbo charging APC stuff work in this order
APC, if that fails the waste gaste takes over and failing that the fule pressure switch? If this is so would this now point to the fact i have a waste gate problem?

Sorry about all the questions. Thanks for all your help so far - it's already running alot smooter!
 

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That's going to be an extremely expensive botch job. Something wrong in the turbo/APC system is causing you to overboost thus triggering the safety cutoff so you 'fix' it by removing the safety cutoff?!! Turn that overboost switch back down, disconnect the APC solenoid which should get you down to base boost only. Now see if you're overboosting in which case the wastegate is at fault. If you get base boost only, you need to start troubleshooting the APC bit rather than the wastegate.
 

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That pinking is knock/detonation, and it is not good at all. Disconnect the APC now, get a calibrated boost gauge, measure your base boost. Should be 5 to 6 psi. Adjust it if necessary by lengthening (to reduce) or shortening (to increase boost) the wastegate actuator arm (2 full turns = 0.5 psi roughly). When that is set right, if you then plug the APC back in and it goes mad, you know the APC is the problem.

Edit - what he said :D
 

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Uhhhh NO.. the Fuel cutoff switch is Factory set at 1 bar.. 15 psi.. that IS half thru Red on virtually evrey Saab Boost gauge ..unless it's Broken..
Easily Verified with an Accurate industrial type NOT boy Racer :) gauge.
I believe each turn (IN) onna adjustment screw on the Switch = 1.5 psi. Again use yr Reference gauge for setup if in doubt
PS: McMasterCarr (web site catalogue) sell V good ones starting at $10 !!...do a Google search then work yr way thru their massive catalogue) .. a seriously recomended Vendor.
Not clever to either disable or go past 20 psi for this switch..It IS a final safety.. Engines are expensive to fix.. But the Transboxes are even Pricier and a real bugger to get at.
 

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Discussion Starter #14
BaBOOM!!!!!!!!!

Ah, ****! i've blown the engine! - Too bad, it was a really nice one too, low miles,1 prevoius owner (a vicar), oh well........
Only joking, so i'm running 16psi ish on a stock old car it aint gonna last long. Cdaly, jezzadee, i think you've got the right idea as far as responsible fault diagnisis. Gonna keep a beady eye on the old boost omiter and drive like a granny till i've sorted it. Mind you, there is somthing calming about driving the big old saab that makes me not want to go for the big boost anyway - i've got a Turbo Kawasaki for that mega rush!!!:evil: :evil: :evil:

Thanks for all the help, i'll keep you posted.
 

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Discussion Starter #15
sensorship issues

look at that now it reads as if i've said a rude(er) ((even a botch job on the spelling)) word than a actually did!
 

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Discussion Starter #16
apc/solenoid

I've had a look at the APC box and it still has the tamperproof seals on it. I unpluged it (putting the car in base boost?) and went for a drive. It still hits the fule cutoff - i've turned it back down by the way. Is this actually base boost? - i've just re-read the post and the apc box is not a solenoid, so...... If i am in base boost then i recon i must have a stuckshut waste gate. Looking at the old MOT sheet my car only did about 80 miles last year, could the wastegate have rusted shut and could be freed off?

Nearly there i hope, thanks for the help so far

Cheers
 

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definetly sounds like you have too high a base-boost. Baseboost is set by the wastegate itself. The APC adjust boost above this level, and reduces it to base-boost if it sees knock.

Now, you hit the cut-off when you pull the connector on the APC-solenoid that sits on the top of the radiator?
Then your wastegate is either wounded to hard and giving you way too much boost, or is stuck tight, because it hasn't been on the road... First, reduce base-boost, take it for a drive. If it still boost too high, the wastegate need taken care of...
Daniel.
 

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Assuming your APC system is intact, next time you find a nice uphill that you can push your foot down for a good 30 seconds, just touch the brakes on the way up and floor the accelerator. Notice the max boost on the APC gauge ... if it's in the middle of the orange section, that's good. Higher than that then you've got high basic boost and possible overshoot.

I bypassed my fuel cut off switch by simply disconnecting the vac line that T's off into that switch. No need to splice the wires ;) You can use the T-off to add a calibrated boost gauge and get some definate readings on your basic and top boost. I have a calibrated gauge, which is a great aid for any car that's been fiddle with by any of the former/current owners :cheesy:

Let us know what your "basic boost test" shows on the standard gauge. As inconsistent as that gauge is, it will still give us an indication of what's going on with your boost. If you want to be really proper about it, yes, remove the power connector to the APC solenoid above the radiator ... but, as I've said, for a Q&D test, just touch the brakes and floor it :evil:
 

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Sounds like a stuck waste gate or air leak on the actuator feed. With a cold engine reach under the turbo housing and feel for a rod ( the actuator) you should be able to push it towards the engine slightly and feel it spring back - if not it's seized. Check the 2 hoses that go from the turbo outlet to the solenoid and the solenoid to the actuator - if they are split the actuator won't get any boost pressure and won't work when needed. The Turbo system on a Saab is designed to fail safe - something is seriously wrong when if fails like this.
 

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Discussion Starter #20
over boost

I've checked the wastegate arm and that moves ok, i have also disoncted the pulg to the APC solinoid and taken out the APC box. I still get boost about halfway into the red zone then the fule cut off takes over - so where do i go from here? I've undone everything that makes it computer controlled and it still does all the same stuff as when i first got the car. Any ideas? I think i'm going to have to bypass the fule cut off otherwise (i dont want to - it's there for a reason).
Maybe i've just got a fast one, or it's had a different turbo on it at some point.
 
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