Part-throttle stumble

[Doug914]

Hello everybody.

My freshly rebuilt 2.1 (a 2.0 with 96-mm p&s's and stock FI) has about 800 break-in miles so far and runs pretty well...with the exception of a stumble that's noticeable as you pick up the throttle from cruise. I had the mixture set by a mechanic, and the plugs look good, with light tan deposits on the electrodes and just a little soot on the "ring" part of the plug.

It didn't have the stumble about 400 miles ago. I'm wondering, since it probably needs a valve adjust about now, if excessive lash here could cause the stumble. All the vacuum connections seem to be airtight.

Also, if I don't give it, say, 2000 revs off the line, the engine loads up and nearly dies. I've richened the idle via the pot on the ECU just enough to eliminate the surge. It's a minor inconvenience, and driven appropriately it's not a problem, but it does tend to catch me out at the most embarrassing times!

Many thanks in advance for any insight.

[Dave Bell]

"All the vacuum connections seem to be airtight"....

I have thought that a number of times and was later proved wrong.

Just yesterday I tighened up the already "tight" throttle body screws on the 1.7 Djet I am fiddling with and eliminated some mid-range stumbling that had me baffled... if you are running lean on the FI system, most times for me it has turned out to be an undetected vacuum leak.

just my experience... your problem may have a different source

- Dave

- DAve

[pbanders]

Part-load stumble and bucking are usually due to a worn throttle position sensor. You can make it better for a while by opening it and cleaning the accelerator contacts, but ulitimately the best solution is a new switch.

Brad Anders

[This message has been edited by pbanders (edited 05-10-2001).]

[vw@micron.net]

I had the same problem when I bought my 76 2.0 two months ago. I installed a new Bosch throttle switch and it fixed the low rpm steady throttle stumble. Well worth it.
Steve

[Doug914]

Thanks for the input, everyone. I'm pretty sure it's not the TPS, since I replaced it about a year (4000 miles) ago. I'll recheck all vacuum line thoroughly, reset the position of the TPS then maybe look at the advance mechanisms in the distributor. Would a vacuum leak cause the off-idle phenomenon I described?

[pbanders]

OK, so let's say it's not the TPS, and it's not a bucking at constant throttle, but a stumble on transition as you say (read: we should have listened to you ). Have you checked to make sure that you have a compatable setup of MPS, head temperature sensor and ECU for your car? I ask because it sounds like you're car is lean and you are getting poor transitions. Do you have any light backfiring during overrun, or while coasting on downhills?

To check on your components, take a look at my page at:

http://members.rennlist.com/pbanders/djetparts.htm

If you have access to an A/F meter that you can use while driving the car, check your part-load mixture, it should be fairly rich (in comparison with newer cars) at about 13.6 to 13.8:1 (that's where my car is). If you are seeing readings like 14.5:1 and higher, that might be the cause of your problem.

Another thought is whether the accelerator function of your TPS is working. Try disconnecting the TPS and driving the car (it will still start and drive). If you don't see any difference, maybe you have a wiring harness or connector problem.

Brad Anders

[Doug914]

Thanks for the input, Brad. Really, the problems started after the 2.1 upgrade/rebuild. Driveability was quite good in every respect beforehand, so I'm assuming that all the FI parts are correctly matched (I know the line about "never assume...." so I should probably check everything against your chart...nice work, BTW!)
I should really go out and get the air/fuel-ratio gauge you suggest to help diagnose these things. I think my biggest problem is that I know just enough to THINK I can solve the problem. What's so puzzling is that it was running quite well after my mechanic "set the mixture" but has become progressively worse as the miles pile on.

I'll let you know how it goes. These glitches sure do take the fun out of driving my toy, but I'm determined to put it right. Thanks again.

P.S. No hint whatsoever of backfiring on the overrun. Idle quality is quite good. It's a high idle (about 2000 rpm) when cold that settles down to about 1000 when warm...I think this is normal.

[This message has been edited by Doug914 (edited 05-11-2001).]

[pbanders]

Uh, oh - how did your mechanic "set the mixture"? Don't tell me he adjusted your MPS - if so, I'll explain. Hopefully, he simply jacked up your fuel pressure to 34 psi to accomodate the 2.1 rebuilt.

Brad Anders