Throttle sensitivity - '89 3.2

[GeorgeM]

Post rebuild, the gas pedal/throttle in my '89 3.2 appears to have a sensitivity that it did not have before the rebuild.

For example, when driving slowly like in a parking lot, it's hard to accel and decel slightly without bucking. There's a sensation like there's play in the throttle before throttle spring applies tension.

The linkage in the car was untouched since before the rebuild, and the linkage under the car appears normal.

I've replaced the throttle springs and even loosened up the cruise control cable to take that out of the equation, but no dice.

Anyone ever had this issue? Any insight would be appreciated. Thanks!

[steely]

I was going to mention the 2 springs on the bellcrank on top of the engine, but you mentioned those.
The same bell crank and the one at the pedal cluster and side of the transmission by the driver side rear wheel all have plastic (nylon?) bushings that can dis-integrate. Maybe one check those out?

Sensing throttle play is one thing, but the bucking has me stumped / am thinking response issue like a vacuum leak in that case, unless I misunderstand.

[r-mm]

Did you adjust the throttle linkage post rebuild?

You gotta make sure the throttle closed switch is closing. Even if it closes when you really snap the throttle closed it might not be closing when you gently get off the gas.

While you're down there, might as well make sure you're getting WOT. Lotsa cars aren't (mine included pre-adjustment).

[Arne2]

Two things to check, that I have experienced in my '84 3.2 that had that bucking problem. One was the AFM with a worn contact path (search for AFM track arm), the other was my aftermarket chip. The chip had a lot to do with it in my car, reverting to the stock chip cured it completely.

I don't know if either if those things are your problem, but they are things to consider.

[jlex]

My chip experience was the opposite. On my '85, bucking was almost totally eliminated by installing a Steve Wong chip.

[GeorgeM]

Thx for the inputs.

I did not adjust. I'll ask my mech that did the rebuild.

I do have a Steve W chip, which I also had before the rebuild, but a different one now with 20/21 cams and 1 5/8 exhaust. I really think it's a mechanical issue... just feels like that.

[COLB]

Worn AFM was the source of my bucking issues.

[Tippy]

You're experiencing intake reversion caused by the larger cams.

Steve W. can retard the timing in those areas to get rid of the bucking sensation, but retarding timing plus intake reversion together might make it drive poorly.

I'd chalk it up to a snappier, rev-happy motor.

[prschmn]

It's likely that the 20/21 cams and the chip aren't quite a match. SW does usualy push timing
under light/part throttle and those cams won't like it much. He should be able to give you a simple
revision. I just built a similar engine and it's fine with closer to stock timing.

[Quicksilver]

I'm suspecting your problem is vacuum related or a by product of a 'bigger' cam. It is hard to guess without seeing how drastic it is and what RPM ranges are effected.

A worn AFM track can cause bucking but it isn't apt to be just off idle.
A vacuum leak can can cause a range of bucking but full throttle won't be affected.
The throttle linkage obviously can effect smooth throttle response.
Mixture adjustment could be the culprit as you come off the idle switch and the ICV stops helping things along.

The order I would check things is:
- I would start by visually checking the throttle actuation. Obviously you'll need a helper for this. You want to make sure it isn't touching anything anywhere and that it is going fully closed. (Check fully open while you are at it. It can't be part of this issue but it is a common issue to not fully open the butterfly and you are right there.)
- Check the actuation of the throttle valve switch.
- Check that all grounds to the engine and transmission are connected correctly. (Weird things can happen with a loose or missing ground!)
- Perform the idle speed adjustment procedure. (The DME can mask an idle maladjustment because it compensates automatically to make it idle at the right speed.)
- Try a stock chip. (The later chip with the 880 RPM idle will help with the lope-ier cam.)
- Do the CO adjustment procedure. (This requires a wide band O2-sensor/AFR meter setup so either you need to borrow one or just switch over to it.)
- Check for vacuum leaks. This one can be ugly and involved which is the only reason I listed it last.

A copy of the "DME Test Plan" manual would be a real help in all this but all of the harder tests can be found by searching the site so it isn't absolutely required.
It has been mentioned above about the effects of a larger cam. Rougher running at low RPMs is a well known effect. If you increased the engine size it would somewhat offset that effect.

[steely]

I'd assumed the rebuild was stock - missed that little detail about the new cams.

[DRACO A5OG]

Post a video, OP.

+4 Cams, I suspect, the reason I stuck to 964 Cams for my 3.4 build but will still dyno with Steve Wong SWChip.

[GeorgeM]

Thx again. I have my work cut out for me, I guess. New gas pedal and bell housing bushings on the way since they've never been swapped out. Will let you know how it goes.

[Quicksilver]

Originally Posted by GeorgeM

Thx again. I have my work cut out for me, I guess. New gas pedal and bell housing bushings on the way since they've never been swapped out. Will let you know how it goes.

Confirm before buying stuff. I'm coming up on 247,000 miles on my car and the gas pedal and the bushings are original. Maybe it would be nice to go through them but not an issue at that much age.

Your issue came about suddenly so it should be something that was messed with.

[DaveMcKenz]

Hi George,
I would also consider speaking with Steve W. Remind him of the exact nature of your rebuild (cams), and the kind of problem you are having. I've heard it called the "Parking lot Buckaroo". He may be able to modify your chip to eliminate it. Remember the chip controls fuel and timing, and that can give you symptoms that seem mechanical.
Obviously, you are trying to eliminate mechanical problems already, such as throttle linkage and vacuum leaks.
I wish you the best of luck,
Dave

[michael lang]

Originally Posted by Tippy

You're experiencing intake reversion caused by the larger cams.
I'd chalk it up to a snappier, rev-happy motor.

George
What Tippy said. I have the same condition and it only happens at very low rpm and in 1st gear. As soon as the rpm level is above 1500, I don't experience that "bucking". This did not start until after I changed the exhaust and installed 964 cams in my engine when I did my top end. At first, I thought it was something that I did wrong when I set the timing. Then one day I was at an AMG training class and the instructor was talking about the differences in the 156 and 158 engines and that's when I figured out there really isn't anything wrong with my engine it's just requiring more air/fuel at the very low load. When I go to a chip, I will explain to the programmer the things that I am experiencing and what my expectations are so the chip can be set up accordingly.
I hope this is helpful to you.

[GeorgeM]

Thanks to all for the great input. I finally got a chance to work through this....

I contacted Steve Wong and got some good tips as well.

As Arne and Wayne mentioned, it ending up being the AFM wiper track. Steve sent me the info on how to adjust the wiper tip position so that it sweeps along a new arc and that did the trick.

I also swapped out the bell crank bushings (what a PITA!), so I got some better feel from that, but the culprit was obviously the AFM.

Thanks again!!

[sf911]

Could you share Steve's info on how to make the adjustment to the wiper tip ?
Mine bucks at 2100 rpm with no load , partial throttle .
Thanks .

[GeorgeM]

Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk

It covers the 944, but same setup. Go to the "Refurbishing your AFM" section for the procedure.

My wiper assembly was mounted all the way down on the shaft, so I had to use the "move south" procedure.

The hardest part was removing the plastic cover, which is held on with silicone sealer.

[sf911]

Thanks George , the adjustment looks easy enough once I get the cover off . I'll let you know if it works .
Scott .