328 Idle Exhaust popping diagnosis long

Hi again JRV,

Noticed over approximately the past month "popping" at idle and slight surging when car warm. During warmup period as drop from fast idle to normal runs rough @ 900 RPM for about 30 seconds then returns to 1000 RPM. No loss of power while driving. Turning on A/C (once warm) drops RPMs about 100 and seems to lessen popping sound.

Airflow sensor has silver plug (? factory) in mixture adjustment hole.

(1988.5 328 completely stock 9400mi started weekly/driven monthly)

To date my endeavors have included:

* Removal of all NGK plugs (~400mi/12mo old), blasting clean, then confirming proper gap (0.024").
* Removing caps and wires. Amazingly, could not demonstrate continuity on 5 of 8 (!) replaced with new wireset and extenders confirmed continuity before reinstallation. Wire-brushed cap/rotor contacts. Confirmed spark with timing light (rear bank only) tonight.
(thought this was defacto my problem but unfortunately not)
* Removed Microplex, checked reistances across terminals (not sure of individual meaning, but both identical). Cleaned, reinstalled & confirmed grounds.
* Frequency valve buzzing appropriately.
* Cleaned wiring harness plug contacts for cold start, warm-up regulator, frequency valve.
* Removed bellows hose and wiped out throttle body (not really dirty) and airbox & perimeter of air intake plate (same).
* Removed O-2 (lambda) sensor relay from ECU plate (ergo my earlier posting). Pulled fuse (good) and top didn't notice any burned connections, etc. Reinstalled with new fuse.

None of the above made any particular difference.

Starting to get somewhere tonight measured O-2 sensor voltage when warm-start: fluctuating 0.4 to about 0.9mV. However, once blipping the throttle for the first time and returning to idle digital voltmeter would flash 0.24-something- then "OL" for overload.

Disconnecting the O-2 sensor resulted in about a 200 RPM increase in warm idle (to 1200-1300) and complete resolution of popping in exhaust.

Not sure what I've stumbled upon here - dying O-2 sensor? Have refrained from playing with any other settings (throttle body stop, mixture). Only temporal event I can recall is dropping cat to replace gaskets about 2 weeks ago but as I recall symptoms were present prior to this. Went through old FC archives some symptoms similar others not would prefer not to throw parts at problem.

Suggestions where to proceed from here? (unforunately do not have exhaust gas analyzer nor easy access)

Thanks as always,
Carl

Hi Carl,

there is a way to somewhat set the fuel mixture (very poor way, but possible) using the voltage oscilations at the frequency valve and an analog volt/dwell meter..

I'll have to get my Bosch Books back from JL to write the procedure inteligibly.

Yes the silver plug you referenced will need to be removed and the small screw beneath it. You'll also need a long 3mm Allen.

Actually there is a very long indepth conversation in the old FC archives about 308/328 Mixture setting procedures between myself and someone else (can't remember who).


Oh ....the relay is a standard Porsche part...it's a 928 overvoltage protection relay.

Regards, JRV

I think method for resetting mixture involves reading O-2 sensor voltage and adjusting to 0.5mV (?) It is in the Probst Bosch book (which I have been scrutinizing over the past 48hrs).

Do any of my symptoms suggest faulty component (O-2 sensor, relay) that I should replace?

Thanks again JRV,
Carl

Oh PS JRV I printed out your description of setting mixture (9 pages, very thorough) from FC archives yesterday.
Carl

Hi Carl,

way to soon to suggest faulty part imo. The mixture itself will make the O2 Sensor Voltage vary widely. Mixture can & oes fall off on it's own, needing periodic corrections.

>>I printed out your description of setting mixture (9 pages, very thorough)<<

LOL...without flames I probably could have written it in 7 pages...

(without flames...what a great line!)

My thoughts on this two-fold: without change in mixture adjustment, this problem arose spontaneously (which is why I suspect faulty part). Or, old plug wires slowly failed until became symptomatic then replacing with new changed previous state of tune - in the other direction.

Certainly the former agrees with your assessment. JRV, what do you think of the relatively inexpensive exhaust CO testers (Gastester $230) on the market? Sufficient for home mechanic (obviously not as thorough as your 5-gas machine).

Carl

Also must comment on how *fast* you have responded to my problem - wonderful!

Mixture can just fall out of adjustment for no apparent reason, although I would suspect normal wear &amp; tear of use plays a part.

I'm not familiar with the Gastester, although I know we have discussed many types in the past.

As a setting tool many home testers may work. The thing I don't like about the testers that don't read Hydrocarbons (HC) is that they can't be used as a Setting Tool AND a Diagnostic Tool. To diagnois &amp; isolate issues one needs HC readings as well as CO readings. If the Gastester reads HC then it will be a handy tool to have around.

Regards, JRV

Sorry JR,

I'll get he Bosch books back to you tomorrow. Have been busy with the CV boots.


JL

Carl,
On a properly set-up car, with a properly funtioning 02 sensor, the voltage range should be between 0.2 volts (lean) and 0.8 volts (rich)...

A voltage reading, such as the what your measuring likely points to a bad O2 sensor...Steady state idle conditions should produce 500 milivolts. A voltage test is the confirmation step...Using a CO/HC meter is really the best way to dial in the mixture. The Gastester you refer to works reasonable well in terms of giving you a relative reading...though repeatability is suspect. Idealy, you would adjust for the lowest possible HC, which translates to the highest possible combustion efficiency.

Regards,
David

JRV &amp; David you guys are great.

JRV, went though searches on FC archives for "HC", "PPM", "emissions" etc. didn't find anything specific regarding tester equipment. Gastester device only measures CO. But did realize (not sure others do) how much of a loss your departure was.

David, I cannot explain why I get such a fluctuating reading at idle after blipping throttle; but the fact that I do get a reasonable reading *before* blipping throttle makes me slightly hesitant to condemn sensor. Any other multimeter testing (resistances? voltage at higher RPMs?) I can perform?

That said, being in the middle of MS few resources exist locally. JRV if I still lived in Houston I'd be dropping by.

At this point have choice of funding a new sensor ($130) [or relay?] or pursuing proper diagnosis/equipment. Want to be a mechanic here...but also want to effect a fix. Any good sources for used 2 gas analyzers for <$1k?

(and in the back of my head remember reading incessantly on old board about ignorant savages who played with adjustments they didn't understand..!)

Carl