Fluctuating Idle RPM - '87 260E
 

I appreciate any feedback on this.

My car is occasionally idling between 600 and 1500 RPM. It will go up and then drop off suddenly back to normal. Sometimes it will stay at 1500 for more than a minute. It will sometimes idle at 1000 and slowly work up to 1500 RPM - but never higher than that. Then it will sometimes idle where it's supposed to for many minutes at a time. I should say that at every RPM the engine runs smooth and there is no hint of stumbling. The air fuel mixture seems perfect.

The problem is unnoticeable anytime the accelerator is revving the engine past 1500 rpm.

The reason I am running this past the forum here is that I'm very sceptical of the diagnosis of my repair shop. They say the problem is the fuel management computer. I asked them if they meant what is often called the ECU. They said Yes, that's the part. They are going to get me a used one $500.

I've had the ECU go out on other cars. The symptoms seems to reflect the wrong amount of fuel for the available air flow resulting in a very poor running engine, stumbling, and gas smell in the exhaust.

Could it be the ECU? I'm thinking idle control unit or control valve.

Steve Hawkins

I agree with you.

I've read dozens and dozens of cases like yours on this site, and not once has the cure been to replace the ECU.

Based upon the common fixes that you usually read similar to your problem, I'd say it could be a vacuum leak (from a hose or the intake manifold seal), the idle control unit, but my best guess would be the mass air flow meter.

If you have a gauge you can check to see if you have low vacuum. For the intake manifold seal, you can spray water around the gasket and see if you have a leak.

It's interesting that you mention vacuum. I've been hearing a lot of climate control actuator movement going on behind my dash. This is recent also beginning about the same time as the idle problem. It might be related. I mentioned this to the shop.

I'm going to have to talk to the tech who worked on the car and figure out how he came to the conclusion he did. If he didn't actually swap out the ECU which cleared the problem I'm skeptical. Or, if they swapped out the Idle Control Unit and this made no difference.

I don't understand how the Air Mass unit could be the problem though. Can you explain that? For example: the MAS unit says "There's more air coming through, the ECU responds by throwing more fuel through the injectors." This would result in an ultra rich fuel mixture because there is not more air coming through." Remember I've got a smooth running engine here under all loads and RPM's.

Steve

I've noticed that a faulty Mass Air Meter usually acts up at idle.

If this unit is bad, or if there's a vacuum leak, it can't figure out the correct ratio of fuel to air, so you see the oscillating idle going up and down.

The idle speed is controled by the CIS-E control unit on this car. Before replacing the control unit you need to be sure that the throttle switch on the throttle housing is working properly. You could also have a sticking idle air valve. Most times idle problems are not the control unit on this car. Also this car doesnt have a air mass sensor.

Thanks for the information.

I talked with my service people. They actually swapped out the ECU (more specifically CIS-E) with one they had laying around and the problem disappeared. And as you and they say - the idle control is integrated with this unit. Armed with this knowledge I'm willing to plunk down the $500 on a used one. I don't have to give up my old one in this deal.

The throttle switch I can't be sure of but I think it works and here's why I say that. At highway speeds you let off the gas completely and the car begins to slow but about 1-2 seconds later it really begins to slow down. This delayed action does not happen if you rest your foot on the pedal just a little bit. Is this consistent with a working throttle switch?

As for the control valve - I just don't know.

Thanks again for the clarification and I will post again so there is a complete case history through resolution on this one.

Steve

The engine DOES have an air flow position SENSOR & it will cause erratic idle problems as "suginami" has suggested!!

There are 4 sensors that inform the computer how to adust the idle speed. Throttle valve switch, decell switch, hall effect speed sensor, & the air flow position SENSOR. OF course there are other imputs to the ECU but they don't have much effect on idle speed.