Erratic Idle and Star Diagnosis
 

My car (96c280) has an inconsistent erratic idle or engine speed where it will surge at a stop or as I brake to slow down. I suppose it may do it at higher speeds but it would be less noticeable. I say it is inconsistent because it happens seemingly when the engine is hot or has been restarted after the engine has warmed up already.

I noted the possible causes, MAS, spark plugs, EGR valve, EGR tube, O2 sensors, wiring harness. My question really is that I plan to take the car to the dealer to have it fixed but I'm wondering how the dealer will go about diagnosing the problem. I've read where the posts where the dealers and independents have changed "everything" before fixing the problem (or not). How does the Star Diagnosis system work and does it fine tune the diagnosis in this and other situations? Perhaps the professional mechanics (Stevebfl, Benzmac) have some insight on this?

Thanks.

Hi 96C280, good question. The Star Diagnosis System ( SDS ) is simply a quick and accurate way of checking for trouble codes, and looking at all the input signals, sensor values, etc. of a particular system. In your case the first thing I would look at would be the "Hot Film Voltage" value in HFM-SFI. Mass Air Flow sensors can fail in more ways than one. If the voltage is a steady 5V it would almost certainly cause the surge/hunting you describe. I've only seen a handful of these failures though, mostly on vehicles with bad engine wiring harnesses. Keep in mind that a MAF failure such as this is completely different from the "lacks power/Check Engine light/PO170/173 code complaint. This type of fault develops over a period of time due to degradation of the hot film material. As I said, this would be the first thing I would check, and it would only take a few minutes. Next, I would check ASR, E-GAS, CC/ISC ( depending on what your car is equipped with ) for codes. I would look at the potentiometer voltages in the throttle actuator as well. Again, this would just take a few minutes. I could use SDS to run the motor in the throttle actuator as well, while I watch/listen for smooth operation. I could also road test the vehicle while observing the actual values. The biggest benefit of having SDS is that it saves time. It also allows me to observe several signals/inputs simultaneously.Before SDS we used the Hand Held Tester ( HHT ) which was nearly as good as SDS. Before the HHT came along in 1992/93 I would need to gain access to the control unit to be tested, then connect bulky/unwieldy test equipment and use my DMM to measure voltages, resistances, etc. If I wanted to check more than one signal at a time I'd need additional DMM's. It would also make road testing/observing much more difficult. I hope this explanation helps. Good luck with your car.

Thanks for the info and explanation. I think I was able to isolate the problem. I have access to another c280 and switched out the AMM (air mass meter). After the first trip, the other c280 came up with the CE light and the idle began to hunt. I don't know what the code is but the results are pretty obvious.
In addition, I noticed a very distinct behavior where the idle would not hunt with the AIR CONDITIONING on (steady as rock) but will hunt immediately and pretty dramatically when it is turned off. Does this indicate anything specific? I'd like to know how this system works. Does it indicate any other problem part? Thanks.