944 bucking and chugging when off the gas pedal

VinnyC · 03-02-2009, 06:17 AM

Hello again! This is a great Forum, thank you all for your help!
While I'm still trying to sort out the brake booster issue on my newly aquired '83 944- I've noticed some other issues that need attention.

Seems like when I let off on the gas, the car bucks and chugs slightly in the lower gears. Any idea what could be causing this??

Thanks again in advance!

2Tight · 03-02-2009, 07:53 AM

Quote:

Originally Posted by VinnyC

Hello again! This is a great Forum, thank you all for your help!
While I'm still trying to sort out the brake booster issue on my newly aquired '83 944- I've noticed some other issues that need attention.

Seems like when I let off on the gas, the car bucks and chugs slightly in the lower gears. Any idea what could be causing this??

Thanks again in advance!

The bucking is typical for the US version on the 83 DME, the computer shuts off fuel rather than feather it down on deceleration. It can be eliminated with the Wilkes chip or buying a Euro DME and bypassing the O2 sensor. A failing rubber center clutch will buck also but it happens in both accel and decel.

Zero10 · 03-02-2009, 08:31 AM

Do you mean random surging / bucking WHILE you are off the gas? If so, your TPS is likely no good, the idle contact is not holding properly. You can try adjusting the idle stop screw or adjusting the TPS positioning, but likely it has worn out inside and needs to be replaced.
If you are just talking about the buck when you lift on the pedal, that is completely normal and there is little you can do about it.

VinnyC · 03-02-2009, 09:37 AM

It usually occurs when I lift off the pedal... on decel

VinnyC · 03-02-2009, 10:47 AM

Now, that you have helped identify what could be the cause - I spent an hour searching for a fix....

Found this on http://www.the944.com/shudderm.htm

THE PROCEDURE: Revised 5/2001
Do not continue if your engine is not in good working order. Fix it first.
With the engine off, take an 8-mm box wrench and loosen the nut on the
throttle stop screw. Operate the throttle so you hear the click. Turn out
the stop screw until the click stops. Turn the screw out about a quarter
tun past that point. Tighten the nut and lock it down. Start the engine and
turn the air bypass crew in to adjust the idle down. Set it a little high
at about 1000 RPM. What you have done is completely eliminate the
throttle-closed switch. Simple, maybe 15 minutes.

Has anyone tried this procedure???

Aufenbahn · 03-02-2009, 01:14 PM

You can also eliminate this by disconnecting the electrical connector. I don't suggest this as a good fix. What should be done is to fix the idle speed problem. If your idle is correct you won't get any sudden accel or decel because your air supply (from the throttle and from the IAC) will be down to almost nothing by the time the fuel shut off occurs. The hard part is getting your idle to be correct. I fixed mine last summer by closing off the hose to the IAC. But in cold weather the car stallls too much to drive it that way. I think on my car it's a DME issue or maybe my DME temp sensor. I haven't totally checked everything out yet.

Mask of Destiny · 03-02-2009, 02:12 PM

Quote:

Originally Posted by Aufenbahn

What should be done is to fix the idle speed problem. If your idle is correct you won't get any sudden accel or decel because your air supply (from the throttle and from the IAC) will be down to almost nothing by the time the fuel shut off occurs.

There's a bug in the early DME firmware. Even if idle is set properly, it still has this problem. The only way to fix it properly on these cars is to replace the ROM with an aftermarket chip that has the bug fix.

Basically, when the idle switch is hit the DME cuts off all fuel if you're over a certain RPM because it assumes you want to use engine braking and it can safely use the momentum of the car to keep the engine turning. At low RPMs, it still needs to provide fuel both to maintain idle speed and to preventy any herky jerky action when letting off the gas at low speeds. The early DME doesn't start providing fuel again at high enough an RPM. If the clutch is disengaged, all that happens is the idle momentarily drops too low and the engine shudders a little. If you're creeping along in traffic and left off on the gas it results in some herky jerky action (car doesn't have a lot of momentum because you're going slow and you're in first gear so you get maximum engine braking).

Personally, I've just gotten more careful with the throttle to avoid getting jerked around. Much cheaper than buying a chip (or a late DME and AFM) and less of a hack than adjusting the throttle so it can't hit the idle switch. That procedure eliminates the fuel cutoff completely making engine braking less effective.

VinnyC · 03-02-2009, 02:50 PM

With that being said, which aftermarket ROM chip is the best for this fix? Or if a total DME replacement is available on Ebay, etc, which is the best replacement for an '83 5-Speed car??
and what if any other mods need to be done?

Mask of Destiny · 03-02-2009, 06:26 PM

Quote:

Originally Posted by VinnyC

With that being said, which aftermarket ROM chip is the best for this fix?

I know FR Wilk's chips have the fix as has already been mentioned. Not sure about the others. Note that a lot of the early DMEs didn't have socketed ROMs so installing a chip might be less than trivial. You'll have to crack open your DME to find out.

Quote:

Or if a total DME replacement is available on Ebay, etc, which is the best replacement for an '83 5-Speed car??

Not sure. The 88 model year had higher compression so that's probably out (unless you plan on chipping it anyway). Apparently you can use a BMW DME as well, but I imagine you would need a chip for that as well.

Quote:

and what if any other mods need to be done?

I'm pretty sure you'll need to switch to a late AFM if you install a late DME. Not sure if you need to make any other changes.