924Board.org

[howlingdogsteve]

Hi All,

I just looked at a 924S that I'm thinking about buying.
Overall car is in nice shape, however the idle seems pretty rough.
Car runs strong but when you come to a stop light it is shakey.
Also, I rev it and let off the gas fast, for a second or so feels like it could stall (down to 750RPMs and then back up to 1000).
Kinda reminds me of decel valve problems on old VW's.

Owner tells me he had new engine mounts put in and new balance shafts.
Any ideas what's causing this?
Should I stay away from this car?
It's really clean and drives very nice.

Thanks

[moorepower]

If it's running rough at idle, my first guess would be vacuum leak. If it has the original hard plastic lines with rubber connectors, then that's most likely (one of) the problem(s). Could also be plugs/wires/cap/rotor.

If it's just vibrating at idle, then I'd suspect motor mounts. Even if he replaced them recently, if he used the cheap aftermarket type, they could be bad already.
_________________
Dan.

'88 924S-LE (Luddite Edition)
Manual steering, manual sunroof, manual windows, AC delete, cruise delete, M030 Konis, 25.5mm torsions, 26.8/20 sways

[gohim]

Maybe you heard him wrong about the balance shafts being replaced, and what he really said was that he had the balance shaft belt replaced.

In that case, I would say that whoever put the belt on installed it out of time, which could explain the rough idle/shaking at idle.

Get written service documentation. If the Seller cannot show you written documentation showing what has been done and when, you have to do a FOES (front of engine service) immediately if you buy the car. That is going to cost you $2000-$2500, and should be a bargaining point, since paying for the car, and paying for the FOES will exceed the actual cash value of the car should it be wrecked.

[howlingdogsteve]

Odd thing is once your above say 1200 RPM it's smooth.
Also, if at idle I blip the accelerator really fast, there's a hesitation.
No hesitation when driving though.
Is that normal for these cars?

[gohim]

No, that does not sound normal.

Obviously this car has some maintenance issues. Sounds like they may be Owner/Service Caused. May not be difficult to repair, but all maintenance on a 924S is expensive.

[9xx]

[mhart]

does this only happen when you rev it and let off the gas? If so, my guess would be that the AFM is worn out. If I'm correct, what's happening is the "barn door" in the sensor is getting stuck open, so the ECU thinks its getting more air than it actually is, and as a result, it's running rich (which causes it to lose power and bog down before the door closes
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'87 924 S - No sunroof and a few modifications

[Ian924s88]

Mine does that exact same thing sometimes, when its good and warm it doesn't dip as much.. I wouldn't worry about it really... If I tap my gas pedal fast, it dips down and does the shaking. and sometimes sitting at a light it will decide to take a dive, but never stalls. Runs great otherwise, I'm sure its something like others have mentioned. Either a vacume or idle sensor needs some cleaning. Mine runs to nice to tinker with right now, I'd wind up making it worse haha.
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1988 Blue 924s

[Rasta Monsta]

Bad IAC, vacuum leak, worn contacts inside AFM.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[howlingdogsteve]

What is IAC?

[Rasta Monsta]

Idle air controller.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[bcblase]

This could be another manifestation of the "944 Shudder" that affects many of the 2.5L engines.

I had the same problem with my 924S when I first bought it...

I replaced the motor mounts
I replaced all the vacuum lines
I corrected the balance shaft belt alignment
I replaced the cap, rotor, plugs and wires
I reset the wiper arm on the mass air flow (MAF) sensor

I still couldn't get rid of the "idle shudder", so I bought the FR Wilk replacement DME chip. It helped!

Here's what he has to say about it.

"On deceleration, the overrun [coasting] fuel cut-off frequency is too close to the idle frequency. In other word, fuel is cut-off when the throttle is closed and does not start up again until the engine drops below 1,200RPM. Fuel does not start up soon enough to keep the RPM from dropping below normal idle. The overrun fuel cut-off frequency should be set higher at 1,600RPM. It is designed to save fuel. Mine dropped to about 600RPM with a nice big shudder."

HERE"S MORE INFO:

http://www.the944.com/shudderm.htm
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1987 Porsche 924S - track toy
1986 Mercedes 190E 2.3-16v - autocross
2007 F-150 5.4L Lariat Supercrew - tow beast
1994 Volvo 850 Race Wagon - 24 Hours of Lemons Car
2001 BMW 325xi - daily driver

[masterdave]

i have the same issue, the owner is most likely lying about the engine mounts or they have worn out since he put them in, the explanation for the shaking at that speed is actually a musical one. any thing that shakes has a frequency, the natural frequency of the engine below 1300ish rmp is low enough to be mechanically amplified by the car its self even with a new mount it will shake a bit. once you pass that 1300ish mark the frequency get high enough to not be noticed how ever the car is shaking a bit. the slowing problem is due to the mechanical fuel injection system which takes a second to slow down, computer technology was not great in the early 8-s when they designed the car. as well if the oil is old it will provide a bit of resistance even pure inertia keeps the engine from slowing down. good luck and i would highly advise the car. sounds like its in good shape and on any note they are simple to work and and tons of fun.

[Paul]