Rough Running 535 is

[duncan363]

I have a 1987 535 is with a mild cam as purchased from a previous owner. I recently had to replace the coil when it died at a stop light idling. Now as a constant speed its power is uneven (bucking mildly), however it idles like a car with a cam would and acceleration is smooth. Could this be clogged fuel injectors? I store this car in the winter, however it started right up this past weekend after sitting 4 months. I don't know when the injectors were last replaced, however the previous owner kept this car up very well. The car has 195,000 miles.

[Jeron]

I'd start with plug wires then plugs. Watch the engine running in the dark to check for sparks then pull and inspect the wires and test them for resistance.

[duncan363]

I will check the plug wires tonight after dark. I was looking at a BMW catalog today and it mentioned in a side bar that a bad oxygen sensor could cause drivability issues including surging (it does feel more like surging than missing). I performed the oxygen sensor test per Bentley's manual and at startup from cold was reading .42 volts and it climbed for first couple of minutes above .5 volt. The Bentley manual stated range of voltage is 0-1 VDC. Once it got warm all of a sudden the voltage was jumping around, so much that it was over ranging my digital voltmeter. The engine started running rougher (I assumed this was the mild cam in the engine) coincident with the voltmeter fluctuations. The voltage readings were way out of the 0-1 VDC range, and up and down several times a second. I have had my share of mis-diagnosis, were I put in the new electrical part, and no change to the problem. A new oxygen sensor runs ~$140. Does this sound like the problem to you?

[Jeron]

Some digitals cant handle the fast fluctuations of the O2. I would try a cheap analog multi-meter.

[duncan363]

I retested the O2 sensor with an analog voltmeter. Starting cold it was reading 0.2 VDC and as it warmed it increased to around 0.5 VDC. Once it was warm at idle it began cycling every 3 seconds down to 0 VDC (short period) and then recovering to just above 0.5 VDC (remaining at 0.5 VDC for a longer period), and continuing to repeat. I ran the engine speed up to 1500 and 2800 rpm and the same cycling from 0.5 to 0 VDC occurs except the cycling is quicker, instead of 3 seconds it is more like 1 second. Maybe I am missing on a cylinder resulting in unburned O2, so the system goes more fuel rich. I didn't get to check the plug wires for arcing dark last night, but will tonight. Thanks for your help.

[Jeron]

The O2 is supposed to cycle.

You should be able to find some generic testing procedures online. Such as creating rich and lean conditions and watching for proper change in v.

I wish I was better informed about O2 sensors, but I fear I am about to become so while diagnosing my mother-in-laws Camry O2 problem.

[duncan363]

I checked the the spark plugs wires with the engine running in the dark. I can see mulitple arcing locations, so I have ordered a new spark plug wire set and a new set of plugs. Thanks for your help.

[Jeron]

The simple solution is the best.

[duncan363]

I replaced the spark plugs and the spark plug wires. Now the 535 runs like it should. The removed spark plugs still looked really good, but with the new wires there is no more surging during steady driving speeds. Thanks again!!!