Partial Throttle "bucking"
09-02-2003, 10:30 PM
#1
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Partial Throttle "bucking"
Hi,
I just replaced my DME chip in my 90 C2 to a stock chip, and the idle is much better. Now, the car has learned a new trick. At partial throttle (driving in traffic, coasting a bit), I can feel the engine buck a little..... it never did this before. It almost feels like I need new motor mounts.... which I probably do, but I never felt this in the 5 months I've had the car.
A little history on the car. It has 91k miles. The Andial mid weight flywheel, new plugs, caps, and rotors. I sent out the injectors to be cleaned, and I replaced the O2 sensor, and ISV.
Thanks for any thoughts.
-Eric
(I read about a low speed bucking issue on 993's where the fuel is pulled by the DME while coasting, and it feels just like that, but i'm wondering if this is an issue on 964's also)
I just replaced my DME chip in my 90 C2 to a stock chip, and the idle is much better. Now, the car has learned a new trick. At partial throttle (driving in traffic, coasting a bit), I can feel the engine buck a little..... it never did this before. It almost feels like I need new motor mounts.... which I probably do, but I never felt this in the 5 months I've had the car.
A little history on the car. It has 91k miles. The Andial mid weight flywheel, new plugs, caps, and rotors. I sent out the injectors to be cleaned, and I replaced the O2 sensor, and ISV.
Thanks for any thoughts.
-Eric
(I read about a low speed bucking issue on 993's where the fuel is pulled by the DME while coasting, and it feels just like that, but i'm wondering if this is an issue on 964's also)
09-03-2003, 05:42 AM
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Hi Eric
It's probably the position of the Idle Switch.
As you gently push the accelerator in, the micro switch disengages. This causes the DME to change out of idle control state (which effects how the ISV operates). If the position is incorrect - perhaps a little late, then the ISV seems to fight a little against the small throttle opening (it tried to maintain idle) and then the drop off is quite sudden (causing the jolt).
Your original chip may have been compensating for this a little by having a bit richer idle mixture values.
You may have to play around with the position of the switch - it should disengage as early as possible on throttle opening, but have a clear ON value when the throttle is fully closed while idling.
Hope this helps.
Ade
Ade.
It's probably the position of the Idle Switch.
As you gently push the accelerator in, the micro switch disengages. This causes the DME to change out of idle control state (which effects how the ISV operates). If the position is incorrect - perhaps a little late, then the ISV seems to fight a little against the small throttle opening (it tried to maintain idle) and then the drop off is quite sudden (causing the jolt).
Your original chip may have been compensating for this a little by having a bit richer idle mixture values.
You may have to play around with the position of the switch - it should disengage as early as possible on throttle opening, but have a clear ON value when the throttle is fully closed while idling.
Hope this helps.
Ade
Ade.
09-03-2003, 12:27 PM
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Adrian,
One of the other rennlisters had a chip from a 278 series DME (i'm not quite sure if its a 911 or 964 DME), which is supposedly fully compatible with my 450 series DME (which is a 911 series DME). Another rennlister that rebuilds the DME's looked through his parts supply, and said the chip would work, and it does seem to be working....... although, the car idled smoother at 1200 rpm............ which i'm sure is way to high..... now it idles a little below the 750 tick mark. It seems a little rougher, but then again, this isn't my VW.... so I expect it's normal......
-Eric
(Ade, i'll look at the setup on my idle switch. I know the switch works, but I have no idea how well adjusted it is. Thank you)
One of the other rennlisters had a chip from a 278 series DME (i'm not quite sure if its a 911 or 964 DME), which is supposedly fully compatible with my 450 series DME (which is a 911 series DME). Another rennlister that rebuilds the DME's looked through his parts supply, and said the chip would work, and it does seem to be working....... although, the car idled smoother at 1200 rpm............ which i'm sure is way to high..... now it idles a little below the 750 tick mark. It seems a little rougher, but then again, this isn't my VW.... so I expect it's normal......
-Eric
(Ade, i'll look at the setup on my idle switch. I know the switch works, but I have no idea how well adjusted it is. Thank you)
09-03-2003, 02:00 PM
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Dear Eric,
1200 is waaaay to high and 750 is too low by around 100 rpm. A DME reset as suggested is a good idea but you have to get that idle up. You know it is not adjustable directly but your CO2 adjust may have been played with due to all the mods. You need to get a gas analyser on the exhaust pipes and check the C02 level. This adjust will change the idle. Probably why you had 1200 rpm. I am still concerend about 100% chip compatibility.
Ciao,
Adrian
964C4
1200 is waaaay to high and 750 is too low by around 100 rpm. A DME reset as suggested is a good idea but you have to get that idle up. You know it is not adjustable directly but your CO2 adjust may have been played with due to all the mods. You need to get a gas analyser on the exhaust pipes and check the C02 level. This adjust will change the idle. Probably why you had 1200 rpm. I am still concerend about 100% chip compatibility.
Ciao,
Adrian
964C4
09-03-2003, 03:02 PM
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When I removed the DME to change the chip, would that have caused a "dme reset"? The DME was out of the car, maybe 15 minutes.... I'm getting good at this.......
Adrian, I don't trust the reading on the tach. The idle around 750 is steady. At one time, in the distant past, a previous owner had the tach taken apart. I know this, because there are three little red leds in a row along the bottom, and I'm assuming this was for a built in radar detector, that is no longer in the car. I wouldn't be surprised if the tach was never recalibrated. So, my 750, might actually be 800, 900 ish........ but going by the tick marks, it was around the 1250 mark with the old chip, and it now around the 750 mark.
I think I need to spend more seat time, to see if this bucking is a problem, or if i'm being hyper critical of changes..... the car doesn't seem slower, but the exhaust tone is definitely different...... and occasionally it does seem to be bucking.......
-Eric
Adrian, I don't trust the reading on the tach. The idle around 750 is steady. At one time, in the distant past, a previous owner had the tach taken apart. I know this, because there are three little red leds in a row along the bottom, and I'm assuming this was for a built in radar detector, that is no longer in the car. I wouldn't be surprised if the tach was never recalibrated. So, my 750, might actually be 800, 900 ish........ but going by the tick marks, it was around the 1250 mark with the old chip, and it now around the 750 mark.
I think I need to spend more seat time, to see if this bucking is a problem, or if i'm being hyper critical of changes..... the car doesn't seem slower, but the exhaust tone is definitely different...... and occasionally it does seem to be bucking.......
-Eric
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09-04-2003, 10:37 AM
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I'm getting a better feel for the problem. It's particularly bad when I first start up the car. I assume this means it isn't looking at the O2 sensor....... It's hesitating, and bucking away until the car warms up a bit.......
-Eric
(I guess my next check is to see what the exhaust analyzer says........ will the O2 sensor cure a problem like this once the car is warmed up?)
-Eric
(I guess my next check is to see what the exhaust analyzer says........ will the O2 sensor cure a problem like this once the car is warmed up?)
09-15-2003, 10:22 PM
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Just a follow up. I put a bulb in the "Check Engine Light" socket, that didn't work before (at the recommendation of another Rennlister), and using managed to get a "1132" out of the DME. I disconnected, and sprayed the connectors to the Knock Sensors, and reset the DME. The check engine light goes out after the engine is started, so hopefully this cures my problems.....
-Eric
-Eric
