Firstfives.org

My son’s (Spark-III) ’80 528i developed two issues which I don’t believe were related. About a month ago after driving for a while and when he would come to a stop the idle would surge up and down quite dramatically. After searching the archives I tried the suggestion of unplugging one of the terminals from the Idle Position Switch and the hunting idle went away just as described. Fast forward to last weekend… he is driving along and all of the sudden the car shuts down, no acceleration; engine just shuts off. Crank, crank, crank, no start. Wait a few minutes, fires right up and away we go… a few minutes later, same thing. Finally get the car home and begin looking around and find that the connector from the wiring harness (30+ yrs. old) to the Coolant Temp Sensor has become brittle and disintegrated. Hmmm, got to have one of those somewhere in my pile of parts… nope, not so lucky. I need to come up with something so I grab a couple female spade connectors and split down the middle leaving just enough to make the connection and put heat shrink tubing on the to fully cover the connectors and insulate from each other. Cut the old end off, crimp the connectors on, and plug into the coolant temp sensor. Still concerned they would stay put without a clip or anything to hold them in place I grabbed my trusty liquid electrical tape and dabbed it all around the connection and filled the well where the connection is made in the CTS. Started the car and she was running perfectly. Took it for about a ten minute drive… no issues. I took the car to work and had a couple appointments, drove off and on all day still no issues. I did get another pigtail connector from NAPA Auto Parts ($9) and last night I was going to upgrade from my jerry-rigging but when I started, I realized that the temporary fix was good and secure so I tossed the spare part in the toolkit in the trunk… just in case.

So, the questions I have are;

a) Were they related? Does the Coolant Temp Sensor interact with the Idle Position Switch in any way or via the ECU?

b) Is it okay to run without the Idle Position Switch fully functional? (car seems to run much better)

One thing I have noticed is with the IPS connected, when letting off the gas, I notice more down-shifting and compression effects (automatic transmission) than with it disconnected. When disconnected it coasts along, idles, and down-shifts less… and no I’m not imagining this, even my son mentioned it without me asking.

Thanks in advance,

You are experiencing symptoms of a 30+yo car in likely (and hopefully) original condition. At this point, anything could be related to anything else. In your case, likely not, but your 'fixes' are likely masking other real engine management failures that are depending on 30+yo equipment to function correctly. and likely not.

The coolant temp. sensor's function is to tell the ECU via varying MICRO voltage what the operating temp. of the engine is over a range from dead cold to normal operating temp and adjust the mixture to achieve stoichiometric air-fuel of 14.7 to 1 (the most ideal combustion ratio for gasoline powered engines). Period. A more problematic condition of this sensor's ability to relay the correct microvoltage is its ground. Read my posts from the archives regarding this. It is imperative for a reliably functioning 30+yo engine.

Your crusty wiring harness is telling you there are other conditions just like that waiting for the most inconvenient time to fail. Take the time to go through everything, inspect/clean/replace every electrical connection related to engine management.

Your comment about surging is likely related to an air leak in the metered air path. There are many places for it to enter. You have to methodically find them, yes, there are likely more than one. Your defeating the idle switch only enriched the idle mixture falsely covering up the likely problem of an air leak.

This engine loves to run rich, but it is not normal. You will be wasting fuel, and emitting emissions the car was not designed to produce.

There are two switches controlled by the throttle shaft. One (WOT) tells the ECU to enrich the mixture when you bury the pedal, the engineering here was to counter a sudden burst of metered air with more fuel. The other is the idle switch (which folks seem to complain about). Its function is to tell the ECU to stop fuel supply when coasting in gear to reduce emissions and conserve fuel. When correctly adjusted it defeats itself and returns fuel supply governed by correct CO adjustments at idle. That was advanced technology 30+ years ago.

When your O2 sensor is functioning properly, the idle CO is adjusted correctly and the position of idle switch is correctly adjusted, the system still works pretty damned good.

This engine will run practically forever, however, the management system will not without updating/repair/replacing relatively inexpensive parts. You have to love the vigilance required but the payback is a reliable 30+yo relic.

I suspect you have some work to do.

Best of luck; it's not that hard, but requires time and dedication.

I think you may have found it. I had a vaguely similar problem on an E24 one time, an '80 with the same L-jet as E12s. I had a broken wire at the FI temp sensor and couldn't give it over half throttle without it dying. What was happening is as the engine torqued and moved, it pulled the wire loose. When at rest or light throttle it made contact.

On the TPS, by unplugging the one lead you are disconnecting it. It's primary function, although there may be others, is to cut fuel with the throttle closed above ~1300RPM since it thinks you're coasting in gear with your foot off the pedal. I'll have to take your word on the auto reactions to it, the only ones I've had with autos have been converted shortly after purchase.

Mike W. wrote:I'll have to take your word on the auto reactions to it, the only ones I've had with autos have been converted shortly after purchase.

And... we are about to do the same to this car, soon... Yes?

The knowledge imparted here (Lenny, Mike) is purely stunning. Stoichiometric, TPS, ECU, WOT, as if I'm attending a lecture on the theory of relativity as applied to tuning the AFM. Meanwhile, I just installed a new plug wire harness, without noting the position of #1 in the distributor. It took an hour to realize the Haynes diagram for US models has nothing to do with my US model. The Euro diagram worked.

If I look at the wiring harness, most of it is encased in vinyl tubing. No idea what 35 years of heat and flexing is doing to the bundle of multicolors inside. I assume your directives concern looking at the wires that leave the bundles, with their connector ends exposed to corrosion, vibration, and the connectors turning to dust. Visually, check all for integrity.

Everywhere a brown (ground) wire connects, check clean and shiny. All of the brass nut sensors with plastic connectors, what ? Don't fix what ain't broke ?

My cars run, just fine, if the same as fifteen years ago and being used to minor glitches, defying remedy, is the definition. Big stuff: No oil or water plumes, quiet valves, timed correctly, 24 m.p.g...35 years.

I'm happy to learn, maintain, prevent, replace, while at the same time thinking there's a, " Deterioration," day sooner than later, deep in the vinyl tubing, a tiny break, realoem, " Ended," along with my trouble shooting abilities.

Spark wrote:

Mike W. wrote:I'll have to take your word on the auto reactions to it, the only ones I've had with autos have been converted shortly after purchase.

And... we are about to do the same to this car, soon... Yes?

I dunno, you haven't replied to my last email.

Mike W. wrote:
I dunno, you haven't replied to my last email.

Sorry... been busy with harvest. Winding down now and should be able to start thinking about the tranny swap again.

SP