A little trickery to get a smooth idle (m30)

[gale]

After numerous iterations of valve adjustments, spark plug gaps, fuel injector cleaning, etc., I finally decided to try a different approach to obtain a smoother idle. The boys in Bavaria leaned out the fuel mixture by design, for US emissions, to the point of almost stalling. Mine blows nearly straight zeros for all tailpipe measurements come bi-annual emissions test time.

Remembering a write-up many years ago on the "Big Coupe Group" website, about an e24 warm stalling condition, I inserted a 100 ohm resistor (brown-black-brown) in series with the connector, and a straight length of wire for the 2nd lead, on the DME temp sensor, the blue one nearest the head on the thermostat housing. Instant improvement & no adverse drivability issues that I can tell after a short road test. Mine always idled smooth as silk during cold-start but when it got to operating temp, the idle went to crap. The DME temp sensor measures within spec per Bentley's & once upon a time I measured resistance vs. temp every 5 degrees F as it cooled down and it produced smooth data as it transition from warm to cold, so I figure the temp sensor is good.

The resistor was actually a bona fide factory remedy for the e24 with a BMW part number & everything, only I think they used a 220 or 270 ohm resistor, I forget what wattage, 1/4 or 1/2 watt is sufficient. I used a 2 watt carbon resistor since that's what I had laying around. Somewhere I have the part number for BMW's fix, lost in layers of subfolders.

[Adnan]

Hi,

Adding the 100 ohms would tell the DME that the engine is still cold, assuming a negative temp coefficient sensor. What will that do to fuel consumption and carbon buildup?

The question may be whether Motronic merely adds a percent enrichment when cold or actually uses a different map. Either way, it'll be interesting to hear about your consumption and driveability under hot, loaded conditions.

Please keep us posted.

Thanks,
Adnan

[gale]

Actually, the additional 100 ohms puts it right at the low end of what Bentley's says it should be (ref. e32 pg 130-15). Bentley says 270-400 ohms at 176F +/- 2F. I just took a hard run on the interstate tonite & measured it as soon as I got back. The temp gauge needle was dead-on 12 o'clock (which I previously verified is 182F) & the DME temp sensor measured 178 ohms by itself and 278 ohms with the 100 ohm resistor in series so I'm good for an additional 120 ohms beyond the 100 I added.

As I mentioned on the e32 post, I'm going to hook up a rotary switch under the dash & try to range it in ~50 ohm increments & play with it for a few weeks on the fly. So far I can say the performance feels slightly improved but without a dyno who can say? The engine seems to get up to 6k rpms alot smoother & easier, seems to give it a more balanced power curve feel. Not worried about carbon build-up, I blow the cobwebs out of it daily, I frequent 6k way too often & have seen 7k a few times too many (running Amsoil & safety-wired spraybar banjo bolts)

92 m30, EAT chip, 19lb/hr Mustang injectors, 3.5 bar FPR, stock e28 flywheel/pressure plate balanced as a unit to within +/- 1/4 gram (makes a big difference!)

[RickyJ]

I didn't know about the resistor until the wire on the resistor broke. The resistor was hidden in the sensor boot. It took me awhile to find the problem. I still have a few bald spots where I pulled out my hair.