After the engine warms to normal temperature, while in D and idling, the engine frequently bogs down to around 400 RPM and recovers for a few seceonds then repeats. Put in Neutral then the problem is almost completely gone. Right after such bogging acceleration is very sluggish as if it's missing a cylinder or two. Normal driving at low or high speed seems normal. Highway gas mileages is around 23 MPG.

I used Sea Foam Deep Creep without any improvement. Then I used FI TB cleaner. I removed TB and IAC and thoroughly cleaned them. TB has light buildup but the IAC was fairly clean. The IAC rotating valve turns smoothly. After cleaning with FI TB cleaner there is some slight improvement. I did notice the idle set screw is completely turned in. Diagnostic code is 1-1-1. Idle speed in Neutral is around 700 RPM.

Q1. Any suggestions as to what to check / suspect next?
Q2. Also, is it possible to test IAC operation? If so how do I do that?

Much TIA,
Lih-Yen

OK, I screwed up!

--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

Thanks for all the inputs. I will try to summarize a little based on what I got so far.

Is it doing this with the AC on?
No, the AC is not working. Whether I depressed the AC butto makes no difference.

I had the same symptoms awhile ago, eventually the car wouldn't go at all, it bogged so badly. Turned out to be the throttle position sensor. It's fairly easy to check with a multimeter. See the FAQ.
This gets on my list of items to check. But the OBD diagnostic did not show anything TPS.

You might want to check your engine temperature control switch. This is not the one that runs the gauge in the dash but tells the computer the engines temperature.
I will check this as well. Based on my understanding, faulty ECT is usually associated with surging idle (toward the hight side), black smoke, poor gas mileage. I don't have these. A recent highway trip of 200+ miles yielded 23 mpg which I think is decent.

What FI system does your car have? Is it LH 2.4? Or Rex/Regina? I don't think LH 2.4 has an idle set screw, does it?
I think it's LH2.4 but not sure. (How do I find out anyway?) It does have an idle set screw, turned all the way in by someone.

As for testing IAC, if it is LH 2.4, isn't there a diagnosic mode that cycles the IAC open and closed?
I just checked with the 700/900 FAQ and there is a mode three test that will exercise IAC. I will do that and see what I will find (looking for code 2-2-3?)

But I believe [in my case] the cause of a drop in the tach reading was linked to an improper connection at the fuse box area. I found that by pulling out the fuse tray and wiggling the wires I found a spot that caused the tach to falter; once I located the connection I cleaned it up and all was fine.
Do you remember which connector it was? I will probably do the wiggle test after I checked other more obvious or easier diagnostic, because mine is not intermittent, it is rather repeatable: after warming up.

I am not leaning more toward the IAC being faulty. A couple of clues:

1. After I cleaned the TB the idle RPM actually dropped a little, perhaps because not the throlle valve closes tighter.
2. The idle set screw was turned all the way in by someone, perhaps to compenstate the faulty IAC (not openning?)
3. I don't think I saw any suggestion that relates to the warm engine only symptoms. I guess this temperature related pattern maybe have something to do with electrical/electronic issues.

Further questions would be, if the IAC is the culprit,
- Is it an ON/OFF device? (if so will make testing easier, won't it?) or pulse width modulated by the ECU?
- If it's ON/OFF, what voltage does it get?
- What are its pinouts?

Much thanks again. You guys are a great bunch.

Lih-Yen

Sorry! I botched the formatting.

What FI system does your car have? Is it LH 2.4? Or Rex/Regina? I don't think LH 2.4 has an idle set screw, does it?

As for testing IAC, if it is LH 2.4, isn't there a diagnosic mode that cycles the IAC open and closed? I think the 240 Bentley describes the steps.
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

What FI system does your car have? Is it LH 2.4? Or Rex/Regina? I don't think LH 2.4 has an idle set screw, does it?

As for testing IAC, if it is LH 2.4, isn't there a diagnosic mode that cycles the IAC open and closed? I think the 240 Bentley describes the steps.
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

I had the same symptoms awhile ago, eventually the car wouldn't go at all, it bogged so badly. Turned out to be the throttle position sensor. It's fairly easy to check with a multimeter. See the FAQ.

Good luck figuring it out.

Is it doing this with the AC on?
There was a service bullitin on this and while I don't think it was a full recall, they were replacing the eprom in the ECU to correct this ( this was done at the dealer for the previous owner of my '93 940t after he complained enough about it )
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale

Robert,

Thanks, but, no, AC is not functioning at this moment. But I will remember turning OFF (and ON) AC to see if there is any correlation.

Lih-Yen

You might want to check your engine temperature control switch. This is not the one that runs the gauge in the dash but tells the computer the engines temperature.
--
90 744ti, Airbox, K/N, Cold air, G-Valve, Dual boost control, VDO, Modified exhaust. 88 745 gle, 99 Camry

I had similar problems last year. The tach would drop and to prevent stalling I shifted to N. I tried all sorts of things to fix it and I did succeed. Because this was intermittent it was tricky. But I believe the cause of a drop in the tach reading was linked to an improper connection at the fuse box area. I found that by pulling out the fuse tray and wiggling the wires I found a spot that caused the tach to falter; once I located the connection I cleaned it up and all was fine. (At about the same time I replaced the rpm sensor)

Thanks, Bryan.

Wouldn't OBD give fault code 1-2-3?
(1-2-3 : Signal missing or faulty to/from coolant temperature sensor, possible grounding short)


Lih-Yen