Start-Up Shudder

[livewirevoodoo]

Here are my symptoms:

Turn the key, on ignition the tach jumps to 1200RPM, then dips to just below 750, i get a wicked 2 second shudder which shakes the whole car, then the tach evens out at 900-950. Engine mounts are bieng replaced in a few weeks, but what are other possible causes?

[arbita1]

I get a slight dip and shudder whenever the rpm's comes back down to idle. I thought it was an inherent problem in these cars.

I think FR Wilk has a chip to fix that.

[livewirevoodoo]

I think the shudder + Wilks fix that you are referring to is for the early model 944. I don't think that would be the cause for my problems though (1987).

As an aside, plugs, wires, cap and rotor and all new within 1 year.

[mscromer]

This is cut and pasted from www.clarks-garage.com. This says it was on 85.5 and earlier, but my 86 did the same thing. It worked for me.

There was an inherent problem with the early 944 DME programming (pre-1985.5 US/Canadian models only). These cars had a momementary fuel cutoff when the throttle was released from any throttle position off idle. However, the tendency was for the idle to drop too low before the fuel cut back in resulting in an idle shudder. The problem seems to be more noticeable on some cars and on others does not appear at all. There are several ways to fix this problem. One is to replace the DME with the DME from a 1985.5 944. The other is to replace the DME computer with an ROW (Rest of the World) DME. And finally, you can adjust the throttle stop (loosen the throttle stop lock nut) until you hear the Throttle Position Switch click. Then tighten the lock nut. This eliminates the closed-throttle switch. Start the engine and adjust the idle adjustment screw to bring the idle back down but, leave it a little high (around 1000 RPM).
There is also a software fix which was developed by FR Wilk (http://frwilk.com/early944/misc/shudder.htm. However, it's typically beyond the capabilities of most shade tree mechanics.

[Eldorado]

Originally posted by mscromer
This is cut and pasted from www.clarks-garage.com. This says it was on 85.5 and earlier, but my 86 did the same thing. It worked for me.

There was an inherent problem with the early 944 DME programming (pre-1985.5 US/Canadian models only). These cars had a momementary fuel cutoff when the throttle was released from any throttle position off idle. However, the tendency was for the idle to drop too low before the fuel cut back in resulting in an idle shudder. The problem seems to be more noticeable on some cars and on others does not appear at all. There are several ways to fix this problem. One is to replace the DME with the DME from a 1985.5 944. The other is to replace the DME computer with an ROW (Rest of the World) DME. And finally, you can adjust the throttle stop (loosen the throttle stop lock nut) until you hear the Throttle Position Switch click. Then tighten the lock nut. This eliminates the closed-throttle switch. Start the engine and adjust the idle adjustment screw to bring the idle back down but, leave it a little high (around 1000 RPM).
There is also a software fix which was developed by FR Wilk (http://frwilk.com/early944/misc/shudder.htm. However, it's typically beyond the capabilities of most shade tree mechanics.

that's if you have your foot on the gas and then release.
not on start up.

his problem is different.

[mscromer]

Originally posted by livewirevoodoo
Here are my symptoms:

Turn the key, on ignition the tach jumps to 1200RPM, then dips to just below 750, i get a wicked 2 second shudder which shakes the whole car, then the tach evens out at 900-950. Engine mounts are bieng replaced in a few weeks, but what are other possible causes?

Hey livewirevoodoo, does your shudder happen only on start up or everytime the engine returns to idle?

[livewirevoodoo]

Originally posted by mscromer
Hey livewirevoodoo, does your shudder happen only on start up or everytime the engine returns to idle?

Never idle, only start up.

Thanks for all the help so far.

[VINMAN]

Have the same problem with mine. Only happens when its cold.
After its running and I restart it seems to go away.

[Slam]

I made the throttle switch mod and it cleanud up similar symptoms. Try it - if it doesn't work, just reset the idle stop. Nothing to lose here.

[Razorback1980]

Sounds like the air/fuel ratio isn't correct during cold operation or the idle control vavle isn't working properly. The fact that it starts out at the proper rpms seems to indicate to me the idle control vavle isn't doing it's job and keeping them up where they should be. That would be where I would start. You may also want to check the voltage of the O2 sensor and ECU temperature sensor to make sure those are within spec.

[livewirevoodoo]

Where can I find the diagnostic plug mentioned here:

http://www.clarks-garage.com/shop-manual/fuel-08.htm

[Razorback1980]

That plug is on the firewall in the wire bundle. I can't remember off the top of my head exactly where but I think it's on left side of the car. If you look at all the wires attached to the firewall, you will see it. It has a cover on it, but you will recognize it. I'll double check the position when I get home and take a picture if you need it.

[Razorback1980]

LiveWireVooDoo...did you find that plug?

[Jbrews]

I’ve been told it’s a combination of idle control valve and vacuum leaks. I’m going to replace lines and valve. Crossing!!

[Monkey Wrench]

You can probably bench test the vale with 12 V applied. Id wash it out but keep the "motor" end up and test. I took a volvo 2 wire one apart to see how it worked, becaue I got curious , and found it never really sealed perfectly which I think is just normal. inside I found a motor it is only capable pof making a part turn, not like a normal motor that spins. the brushes were tabs of copper. not carbon brushes. To get the thing apart I destroyed it. some volvos use a 3 wire one. how it works electrically Im not completely sure. Its a boshce part, probably similar to a 944. perhaps youd find comonality or find some interchangeable.

as you do get blow by the hoses conduct oily air and so they do rot and a leak usually means bad idling so I'd have a good look and some inject smoke ot find any leaks. probably a good idea.

I dont know if that data applies or not but the computer might not be your cheapest option, id start with the hoses. I have a volvo 740 wiht similar issues below 1000 it want to shake and Im wondering if its actually related to the way the exhaust is suspended as it seems to sort of shake like its a resonance of some sort. I thought maybe somehting like the muffler has a natural frequency and it might be matching the natural frequency of the engine near idle. when the two combine maybe it's a combined resonance.

n a volvo or most other front engine cars, the trans and engine both move together so I might feel the motor torquing through the shifter if it was a broiken motor mount , in a porsche the transaxle is completely separated so that would be a different feel .


In my case , I sped up the idle a tad and its somewhat better, worse when cold. above 1000 its fine.