Rogviler

This isn't a huge issue, but it would be nice to track it down just because it's kind of annoying.

The car: '84 4000 S quattro

The symptoms:

-After a cold start the engine will run fine for a mile or so, then bog down for 10-20 seconds, especially if I've come to a stop, and then run fine after that. It's worse and goes longer the colder it is outside. In fact it usually doesn't do it much during the summer, but is still there (maybe a second of bogging).

-When it's stumbling, pressing on the gas causes it to get worse or not do anything, won't accelerate past 2500rpm or so, no power. When not pressing the pedal the engine seems to run just fine.

-If I pump the gas pedal rapidly it runs okay and accelerates normally. If I'm in traffic I have to do this to get me past the stumbling period without getting run over.

May or may not be related:

-Idles at 1000rpm fine from a cold start, but if I stop after driving a block or two, up to a mile, the idle will be around 1200rpm, then slowly settles back down if I sit. Once it's warm it doesn't do this.

-The problem seemed to clear up once when I had taken the distributor cap off and put it back on, then it returned. It also seemed to clear up when I replaced the distributor (same cap and rotor), and then it returned again. It's hard to say for sure because it was during the summer, but like I said, normally it would still stumble a little and it didn't in that period.

I do have some cracks in my intake boot, but I sealed them completely and it didn't help. When I get around to it I have a new boot to install. No other vacuum leaks that I can find. It's somewhat similar to what I had with my other 4kq, which had both a bad plug wire and some intake leaks, but that ran really poorly right from the start and it never cleared up, even when it was warm. This is like the baby version of that.

It hasn't ever gotten worse, it just appeared suddenly a couple years ago and has been there ever since. The fact that there's such a small window when it happens has made it difficult to track down, but since it is small I haven't spent a ton of time on it.

Any ideas, let me know.

-Rog

socalclimber

Make sure distributor rotor is good or new Bosch. I've seen the rotor break down and cause ignition stumbling, but it's usually worse, the hotter it gets. If there are any cracks on the strip from rotor center contact to outer rim, replace rotor. They're cheap, get a new one.

Also, check temp sensor and O2 sensor. Is your air-fuel mixture on the money (measure mA current on differential pressure regulator, aka DPR)? Do you have VW 1315 test harness (or made your own) to measure DPR with amp meter?

Old O2's get sluggish. BITD, I even made an extension set of test leads for my volt-ohm-amp meter, and put meter in passenger seat of car, so I could see DPR current while I was driving, to make sure it was working, and what is was doing under varying conditions.

For example, DPR current reverses during deceleration to cutoff fuel to injectors, and this depends on RPM > 1200-1400 and thottle idle microswitch closed (foot off the gas). Wonder if this is malfunctioning? If so, you would see the DPR current go negative for a second or however long condition exists, and engine would stumble as it has no fuel.

Seeing what your DPR current is reading is really key, as many of the tests of KE-jetronic rely on this, like unplugging O2 sensor and grounding signal wire, or plugging a 15K Ohm resister in place of coolant temp sensor and seeing what DPR does, or closing full-throttle switch to see if DPR current goes up 3-4mA to richen mixture, or what happens during warm up. Bentley has all the tests to do for your car.

Vacuum leaks, if any, of course, all have to be fixed, esp. intake boot.

Microswitches all in order, esp. idle microswitch?

Do you have single-wire O2 sensor, or 3-wire? How long since replaced?

Lastly, if all of the above check out, it could be air flow sensor potentiometer on side of air housing (if you have one). It aids in cold start acceleration by altering signal to ECU of the movement of air flow sensor arm. It is black, has a 3-prong connector, and is usually opposite or near DPR.

socalclimber

BTW, this is a great book on Bosch FI and applies to all older VW/Audi FI systems (<1990):

http://www.amazon.com/Bosch-Fuel-Inj.../dp/B000GKONP2

This and Bentley manual are essential IMHO to really understand and tune Bosch FI systems.

Rogviler

I keep thinking about this and it doesn't seem like a no-spark or bad spark issue, so for the moment I'm going to set aside the distributor-related stuff. It could've easily been observer bias anyway.

If I had to say what it's like in one sentence it would be that it's leaning out for a few seconds right when the thermostat opens.

I'm going to have to do some unplugging and testing when I get around to it.

Like I said, it's probably the least of my worries, as long as it doesn't get worse.

-Rog

socalclimber

Have you measured DPR current (mixture) to see where it's at cold, then during warm-up, then warm?

I only mentioned the ignition stuff, as ignition system has to on the money before testing anything in FI system IMO, sounds like you got that covered.

How many miles on O2 sensor?