RPM hunting/oscillating

Jez wrote in news:WfmdnbII9c snipped-for-privacy@pipex.net:

At this point it sounds like misfiring under load, usually ignition related. Most engine problems are due to ignition, not fuel.

Your OBD-II system *should* set an error code if that's the case.

Do you have EGR? Your under-hood sticker will say so if you do.

How old are your plug, plug wires, distributor cap and rotor. Do you have any aftermarket parts?

Hi TeGGeR®, thanks for replying

There is no EGR noted under the hood. I've also checked the Haynes manual which states EGR is only on D15Z3, D15Z6, D15Z8 and D16Y5 (mine's a D14A2).

I'll ask specifically for an ODB-II scan at the local service station.

The plugs are new, I replaced them on schedule last week with Bosch Super-4's (recommended by the store). There were NGK's in there beforehand. Changing the plugs hasn't solved or changed the situation.

As far as the plug wires are concerned, I tested their resistance and all are below 25KOhms (12,10,8,7 longest to shortest). I don't know when they were last replaced. No record so maybe never. Again, not sure when the cap and rotor were replaced. I inspected the cap but I should look at the rotor in this case.

Jez

I should also mention that I don't recall this problem occuring before I did an Oil/Oil filter and air filter change last month. Is there any way that this might have caused it? I did everything carefully and by the book. It's only a thought, the problem could have been there before and not noticed it.

Jez wrote in news: snipped-for-privacy@pipex.net:

Resistance to a tiny test current means nothing if they're leaking HT current to ground due to old insulation.

If they're OEM, there will be a date printed on them. Go check.

Prime culprits for your problem.

The cables are dated 1995 and therefore probably the originals. I'm working on the assumption that the cap and rotor are also original.

I'll look into replacing them when I have some spare time (and money!). If you're interested, I've posted a couple of photos of the distributor rotor and one of the contacts inside the cap:

I gently sanded off some corrosion from the tip of the rotor which is why there is a bit of dust visible.

At this point it sounds like a lean misfire under load. As you depress on the gas you are creating a lean mixture. The O2 sensor should pick this up and compensate quickly. The fastest and easiest way to troubleshoot this is to disconnect the O2 sensor connector that goes to the computer. Start the engine and go up that hill.

Note that you should not drive with the O2 disconnected for a prolonged time. A good ignition coil should be able to compensate for the lean mixture unless it's too lean.