Adjusting AFM bypass screw

[micbrody]

Car starts fine; idles fine when initially started. When I take for a drive, it drives fine, but when I come to a stop sign/light, the RPMs drop way lower than idle (below 300-500); it doesn't happen all the time. Seems to not happen when I coast a while to a stop.
My first thought was that I had some sort of minor leak in brake booster in that when I was stopping, I would be actuating brake booster, and engine would be exposed to unmetered air; but after many experiments driving, its unrelated to pressing brake pedal.
My next hypothesis was that oxygen sensor was sending signals that were not "healthy" for engine performance. I had just changed oxygen sensor from the one that came with car when I purchased a few years ago. A PO had definitely "screwed" around with AFM bypass screw (no cover). With original oxygen sensor I did adjust the screw to give me an average of 0.45ish on voltmeter.........however I realized I did this adjustment at idle. With new oxygen sensor, my readings were all over the place......but related to flaky wire from oxygen sensor (voltage changed with slight movement of wire). So I just installed third oxygen sensor. Voltage stable with wire tug.
I then proceeded to try to adjust bypass screw: I wedged some clump of aluminum foil at throttle screw to keep RPMs around 2000. I started screwing and unscrewing bypass. Seems to be within 0-1 volt, but lots of range, but I think it was close to averaging around 0.5 ish. I then removed aluminum foil so car would be at idle. The voltage dropped to almost zero. (0.05 ish). I ran out of time to screw around more; however my brain has been thinking about it (and I read a few more posts).
I have questions for experts on the board:
1) when measuring voltage, I disconnected oxygen sensor from ECU line and directly measured voltage relative to ground. Is this correct? Or should I have tapped into connection, leaving ECU to still see signal?
2) I started thinking more about throttle position switch. Is purpose of switch to tell ECU to "pay attention" to oxygen sensor during throttle up only? But when switch at rest (idle), don't look at oxygen sensor (meaning engine doesn't care if it is running lean at idle); and at full throttle, ignore oxygen sensor and let engine run rich? All this can be summarized: is it normal for oxygen sensor to show lean conditions at idle, but 0.45ish only at throttle up? Or should it always be 0.45ish?
3) assuming I do get bypass screw "dialed in" correctly , will everything change if I reset idle with throttle body idle screw?

[RRoller123]

The TPS mostly keeps the car from backfiring, by shutting down the injectors when you take your foot off the gas at speed, otherwise, they would keep dumping too much fuel in, based upon air flow. My understanding is that when the ECU sees the TPS "idle" position, it immediately sets the injectors to idle flow volume.

[azruss]

I think your issue is the throttle body idle speed adjustment screw. This is different from the throttle stop screw. This screw is supposed to have an o-ring on it. It sounds like your o-ring is either cracked or missing.

[micbrody]

The throttle body idle screw does have a o-ring. Idle is constant and steady at engine start; and after stopping , the idle eventually returns to normal. Not sure as to why you think this o-ring(or lack of) can explain it.

[RRoller123]

Because if the ring were missing it would allow Unmetered air into the plenum. Check all the hoses going to the intake and intake plenum, for cracks or missing/disconnected, and also make sure that your Charcoal canister has a small cap snapped onto the hole on top, i.e that it is not missing. It meters air in too.

Pete

[micbrody]

My charcoal canister is missing the little cap. Should I just cover it with tape? Or maybe tape with pinpoint holes?

[RRoller123]

I think maybe the vendors have one, or certainly someone here on the forum has one available. Don't just cover the hole! Try "spiderparts" here on the forum, John has nearly everything!

[micbrody]

I apologize in advance for my long winded posts. The engineer in me wants to fully understand this car!


I took a tiny, tightly bound wad of steel wool and stuffed into hole of vapor canister (missing cap). I then took two thin pieces of electrical tape across hole to keep wad from migrating out of hole. (But still can see steel wool, so not sealed)
So theoretically: I decreased the functional size of hole, decreasing the amount of unmetered air coming into intake plenum via vapor canister.

I took for a drive. I "think" much better. Early in drive (car not up to operating temp), it did dip about 400ish rpm below idle at a stop; but quickly returned to idle. After that one time, it would, at most dip 200 rpm below idle at stop and not all the time.
I did another experiment:
I would drive car for 30 seconds at 2500-3000 rpm, and then let it slow down with engine engaged (foot off of gas); down shift; near end of stop I would put in neutral. RPMs barely, if at all, dipped below idle at stop.
I then repeated, but after 30 seconds at 2500-3000 rpm, I would put in neutral and coast; RPMs dipped at least 200 below before returning to idle a few seconds later. If I gunned engine while coasting, same thing would happen when releasing throttle.

So two question:
1) is it normal for it to slightly dip below idle for a few seconds, as long as not a dramatic dip , if engine speed/load suddenly becomes absent?
2) if it is abnormal: I have two theories:
a) at high throttle, vacuum in plenum is low;but when engine suddenly slows down, vacuum is suddenly increased, and an excess of unmetered air is entering the system before rest of system can equilibrate
b) oxygen sensor takes a few seconds to adjust firing duration of fuel injectors when it suddenly sees a sudden change in air/fuel mixture when disengaging throttle?

Addendum:
I just took car out again after about a 20 minute break. It started immediately, but at a very low RPM 300-500. After about a minute it, the RPMs slowly rose to around baseline 1000ish. I interpret this as: my wad of steel wool definitely affected the amount of unmetered air. I need to now run car up to operating temp; and then go through procedure of resetting idle and throttle screw. In addition, I had previously adjusted AAV to be more open when colder; might have to revisit that.

[RRoller123]

Have you verified that the Throttle Position Switch is adjusted and is actually working? That can do this too.

[micbrody]

It did check the continuity on the switch per procedure. Seems to be normal. I guess I can check from connector to ECU harness when I have time (maybe bad connector ?).
This all happened (or at least I noticed) when I installed new oxygen sensor and tried to adjust bypass on AFM.
I don't think oxygen sensor is involved with start-up; so the low RPM on hot start might be pointing to some unmetered air leak? I did spray air hose and intake plenum with carb cleaner and did not notice RPM changes. I will systematically go through air intake again.

[azruss]

My issue was when pulling to a stop, the idle would slowly get lower. Sometimes it would stay low. most times it would die and wouldn't want to start again. If the idle got low, i could hit the gas and get it back to normal where it would slowly drop again. I chased the problem to the idle screw on the throttle body. It had no o-ring and was tightened all the way down. Every once in a while, my idle gets weird again and i have to adjust the screw again.

[RRoller123]

It did check the continuity on the switch per procedure. Seems to be normal. I guess I can check from connector to ECU harness when I have time (maybe bad connector ?).
This all happened (or at least I noticed) when I installed new oxygen sensor and tried to adjust bypass on AFM.
I don't think oxygen sensor is involved with start-up; so the low RPM on hot start might be pointing to some unmetered air leak? I did spray air hose and intake plenum with carb cleaner and did not notice RPM changes. I will systematically go through air intake again.

The O2 sensor only works after coming up to full temperature, and even then, has only a small impact on the A/F ratio.

This topic comes up so often, that I compiled a small list of the various FI Manuals that are out there and available. Send me others that you may be aware of and I will add them to the list:

For those interested in further studying the Bosch L-Jetronic system, there is an amazing amount of quality material available on the web. All these references below come up with a Google search on the term: “Bosch L-Jetronic”

Brad Artigue’s FI Guide is excellent, and is contained in his book; “Maintaining The Italian Roadster"

This link from the BMW folks has a very good PowerPoint type presentation on the components, etc.

http://www.firstfives.org/faq/ljet/jetronic.pdf

This one, from the VW folks, has a good component description too, but also a good trouble shooting manual:

https://www.type4.org/manuals/ljet/

Here is Bosch's own manual, a long, detailed slog through the swamp for sure. Probably best printed out for studying. I think it is the best of the bunch for a complete in depth study of the system, as it includes selected engineering design data, explaining some of the design decisions they made.

http://foxed.ca/rx7manual/manuals/BOSCH ... Manual.pdf

[micbrody]

Today I removed the T-coolant thermo sender. Even though it was mounted in top hole, the T was filled to top with coolant.
With the temp probe removed, I tested the resistance at different water temperatures. All resistances seemed good.

I drained some radiator fluid; inspected coolant T. It was installed correctly with restrictor side on thermostat side.
I re-installed the temp probe in bottom hole; filled and burped system.

The car started ; but lots of variability at start eventually it warmed up and remained constant at RPMs about 900. When I gunned the engine, it revved fine; but when throttle released, it again dipped below 900 (to 500), before recovering to 900.

I sprayed all potential leak sites with carb cleaner; pinched AAV when car warm to see if maybe it was still open.... pinched vacuum line to booster.....,etc... etc.. etc.. Nothing changed RPM. My conclusion: AFM might be flaky.

I removed; tested the resistance compared to an extra AFM I had; definitely the resistance was different than extra AFM; more importantly: resistance would jump from 120 ohms to 1500 ohms to 200 ohms with very tiny movement of AFM door; the extra afm had variable resistance; but from 120-300 over same type of movement. I opened up cover. Tested the resistor board. It just is not conducting much in the low end. I had previously (about 1.5 yrs ago) refurbished (moved the swing arm; cleaned (de-oxyed) the resistor board ; It worked fine; but now very little conduction left on the black portion).
I installed the extra AFM. Car started immediately. Idle seemed appropriate over the warm up. I then gunned the throttle and released. RPMs came down to original idle RPM........problem solved.

As for the bad resistor board AFM: for kicks, I ordered some conductive paint ($10 on amazon). I will recoat board. If it works, then this might be a way for others to fix their bad AFMs.