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1993 Saab 900S: Unusual Vibration When Stopped in Gear

I have problem that I’ve noticed since I bought my Saab 900 equipped with the automatic transmission and am finally deciding to try to address it. Just to clarify, I’ve cleaned the filter, changed the fluid multiple times to freshen the ATF in the torque converter and it shifts really nicely, actually. The motor is smooth as silk, too. Here’s my problem: with the shifter in “D” at a stop sign or cross walk, and my foot on the brake pedal, the car vibrates strongly. In fact, it’s even popped my hood open on a couple occasions. However, if it’s idling in “P” there’s no vibrations that are out of the ordinary and it holds nicely to 900 RPM either way (I have recently cleaned the IAC valve).

Suspecting my brakes weren’t grabbing well, I recently replaced the accumulator on my ABS system and had the fluid flushed and refilled at a trusted independent import shop. It brakes great, so I don’t suspect it’s the brakes. Since the transmission, motor and brakes are great, I can only suspect the motor or transmission mounts, but that doesn’t explain to me why the problem would occur at specific times only. Has anyone had a similar experience or have suggestions on what to check? Thanks in advance, everyone!

Certainly check out the mounts, both motor and transmission. You are experiencing a rough idle under “load” because there is a drag on the motor when it is in drive. Have you replaced plugs, plug wires, distributor cap, and rotor? If not, do all this basic maintenance and see if it smooths out the idle. You could have some carbon on the runners to the intake manifold.

After the simple stuff your Saab has a tricky Bosch “Jet Tronic” fuel injection system which you need to find a Saab specialty shop to take it from there.

Reading your post, I immediately thought motor mounts. So as uncle said start with the mounts it’s most likley your issue.

Uncle, I have replaced the plugs, wires, cap and rotor recently, cleaned the IAC valve as I mentioned and cleaned the throttle body to boot. I used to have another 900 that had fuel delivery problems and this one seems pretty smooth in comparison, so I’m fairly sure it’s not the FI system giving me trouble. I was researching the symptoms online and at other Saab forums and it sounds like they think it’s at least my solid front transmission mount and possibly the two side hydraulic ones. My goal was to confirm if my suspicions were accurate. I’m guessing, then, that I shouldn’t put too many city (Stop sign) miles on it until I get this addressed?

Just for my own curiosity, does the car not vibrate in park or neutral because the engine and transmission aren’t engaged at that time? As opposed to when the brake is fully applied while its in drive, there is, as you said, a load on the engine, because it’s engaged along with the transmission?

When you go from neutral and engage drive your rpm will drop, something like 100 to 150 rpm. If the AC is on too then the 4 cylinder motor is experiencing a “load” and with lower rpm’s that can make for more vibration. When a motor is running properly and the rpm is still about 800-1000 even under the load the vibration can be noticeable but not so extreme as to be a problem.

If you have bad mounts the rpm drop under load can show up and you’ll see the motor kind of bouncing around as it idles. Since you are experienced with the 900 you should know what normal looks like. You should consider a compression check. A low compression cylinder might cause the rough idle even with the FI and ignition systems working properly.

Have all the mounts checked and get the compression checked and see what you find out.

I’m not sure if it’s relevant, but my A/C doesn’t work right now, I’ve pulled the relay for it anyway. In fact, since it’s a 'vert, I don’t even have the fan on. Just the engine and the radio : ). It’s hard for me to say if say if this is consistent or not (haven’t driven it enough to be absolutely certain), but when I was driving home from work last night, it was noticeably vibrating, but not quite as bad as earlier on my ride into work in the morning. At least the hood didn’t pop, anyway! While I’m throwing darts at a potential problem, maybe it’s the O2 sensor, at least contributing to the problem? Again, I can’t say for sure, since I only claim to be semi-knowledgeable about automotive systems.

Just to clarify, my previous 900 was a manual, so I wouldn’t say my knowledge is 100% applicable, but there is one other piece of info that might clinch the transmission mount as the culprit. Even though it has diminished greatly with a few ATF changes and a filter screen cleaning, I noticed with the hood open, I could see that the power train would move noticeable with as I moved through the gears to cycle the ATF. I don’t want to say jumped or seized, but it was a noticeable movement. That might confirm things, perhaps.


How high does it idle under load? I had two Saabs and don’t remember my idle being 900 rpm when in neutral. It seems a bit high. Maybe it idles so nicely because it is high.
There is a electric load sensor in that car that’s supposed to adjust the idle when under load. Not sure why the electric load would be significantly more when in “D” but maybe there’s something wrong with it. Maybe the idle was adjusted high to ‘smooth it out’ by the previous owner’s mechanic right before it sold.

gsragtop: I will certainly be having a look at 'er tonight to verify or rule out mounts as a problem!

RemcoW: I’m not sure if you can alter or adjust the idle on a 1993 900S - my impression is that all the idle/fuel/air adjustments will come from the ECU, based on readings from the AMM, etc. In any case, what were the year for your Saabs? I’m not familiar with an electric load sensor, but that doesn’t mean it doesn’t exist. Maybe it’s only equipped on manual transmission, turbo models, whereas mine is an automatic N/A. If I’m wrong, feel free to point me in the right direction, though.

This is a while ago so maybe I’m barking up the wrong tree. Mine were turbo sticks. I just seem to remember the idle being lower than 900 rpm.
Just noticed GSR’s idea of looking at the mounts. That sounds like a good idea. I remember when the mounts on one of mine were bad and the car was nearly not drivable but - again - it was a stick.

I’m not familiar with Saab specs, but 900 rpm sounds high to me too.
I know you said you just cleaned the IAC valve, but is the IAC motor working properly?

I’m also wondering why the AC is disabled. Is the compressor pulley spinning freely?

By all means check the mounts. But I’m getting the impression that the root cause may be the high idle. Do you have a repair manual with the proper specs?

I don’t think the OP has mentioned the idle speed. I had a '85 non turbo 900, an '87 Turbo 900 (both with manual trans), and an '00 9-3 with an auto trans. I think the idle speeds on all of them was about 600 to 700 rpm.

Classic symptom of a bad vacuum line or failing brake booster!.

mountainbike, I thought the correct idle should be around 850-900 RPM +/- 50RPM? Not sure how accurate it is, but the tachometer shows the indicator pointing to the bottom of the “1” on 1,000. I’ll check to see if the AC is spinning freely. I disabled it because it doesn’t work at all. I do have a Bentley manual, though. Where should I look in it to troubleshoot my idle?

I’ve experienced a similar problem with a Ford truck with an automatic transmission. It turned out to be a cracked vacuum hose. At idle speed there is very little air supposed to be getting through, so even a small leak in a vacuum hose connected to the intake manifold will make the misture way too lean. It can become so lean the car will eventually stall in fact. And it shows up most noticeably when stopped at idle in the D position. Check all the vacuum hoses and have your mechanic test all the devices controlled by the vacuum hoses to make sure they aren’t leaking. This doesn’t happen in Park because the engine isn’t under load of pushing against the brakes when it is in Park. It could well be something else, but checking the above is probably the first place to look.

Well, I checked for vacuum leaks. I reinforced a few connections, but I didn’t see any obvious culprits. However, I did notice my throttle position potentiometer was a little low on ohms for the closed position (partial throttle and WOT appeared correct), 2300, instead of 2500 in the Bentley manual. I started the car, though, and the idle seemed pretty close to before, possibly 25 fewer RPM or so, closer to 875-900 RPM, still very stead. The AIC valve corrected for jumps from throttle stabbling nicely.

While I was under the hood, I checked the front transmission mount and honestly, the rubber didn’t look perished or cracked. It looked dusty, but in an average state to my untrained eyes. I can certainly take pictures to corroborate. I ensured the bolts for the bracket were snug, which they were. I started the car and, again, it shook pretty well with the brake applied in drive. I’d prefer not to, but until I can diagnose it further, I may just put the transmission in neutral while at stop lights for an extended period of time.

There is more than one motor mount.

jdmere: I know - there’s a left and right hand hydraulic mount too. They didn’t appear to be moving or allowing the engine to vibrate. They were somewhat worn, but not perished by any means.

I wanted to do a further test, so I had the wife alternate between neutral and drive, with the brake on. I was watching the front and side mounts and they seemed pretty solid. The front one may have moved ever so slightly when shifting into drive, but not very much. I don’t think it’s the mount. Granted, it’s not brand new, but it doesn’t look like it’s failed either.

Thinking it could be related to the inner drivers, I had my wife alternate between neutral and drive a couple more times and watched the drive axles (the one nearest me on the passenger side of the engine bay, anyway). I noticed that the end of the shaft, where it joins onto the transmission, appeared to move forward somewhat when my wife shifted from neutral into drive. While in drive with the brakes on, it didn’t continue to move, though the engine was vibrating as before. The mounts remained firm. It was consistent behavior over several attempts. Can anyone tell me if this is typical operation or does it indicate failure or near-failure of the inner driver?

@mphilleo … on the vacuum leak checking, you have to do more than a visual. If all you did was visually verify all the hoses were in place and connected firmly and no obvious holes in them, that’s a good first start, but it’s not enough. The most common type of vacuum leak encountered is not a vacuum hose failing, but the thing the hose goes to, the vacuuum controlled device, that fails. For example the brake vacuum booster has a vacuum hose going to it. It’s a vacuum controlled device. That is something that fails fairly commonly on older cars. When it fails, it’s an internal vacuum leak of the rubber diaphram inside the unit, not something you can see. The only way to tell it is leaking is with a special vacuum tester device. Mechanics will sometimes put a vacuum guage on the entire intake manifold to see if it is w/in spec, but most of us don’t know what it is supposed to be in the first place. So we use one of those vacuum testers on each of the controlled gadgets to see if any are leaking.

These vacuum tester gadgets are inexpensive, less than $15. Harbor Freight has them, as do most all auto-parts stores. These gadgets have a guage and a manual vacuum pump, so you can pump up the vacuuum and watch the guage to make sure it holds steady. If there’s a leak, the guage slowly goes down. It’s pretty obvious when something is wrong.

Your problem may have nothing to do w/vacuum problem, but from my experience, it could, and checking each vacuum controlled device on your car is fast and easy once you acquire the proper tool to do it, especially compared to other things you’ll have to check.

Best of luck.

I have the exact same problem with my 1994 saab 900 convertible(classic body style), although my saab is a turbo. I just stumbled onto this discussion while researching the problem online and it fits my problem exactly. I also have checked mounts, vacuum, brakes etc. and have not come up with a solution…I’ll keep checking the discussion to see if theres a solution or add my 2-cents if i find what i think is the problem