RPM hunting/oscillating

Hi chaps,

96 Civic, 78,000 miles, automatic transmission.

I posted about this a few weeks ago. The RPM on my civic is
hunting/oscillating by +-200 when driving at roughly 35mph up a light
hill with the correct throttle position to maintain that speed. It's
indicated on the gauge, by changing engine sound, and by very gentle
thrusting forward whenever the RPM increases. Imagine the RPM needle
going up and down about once or twice per second.

Curly advised checking the TPS (no problem found) and using injector
cleaner which I've done. jim bean suggested checking the ECU codes but
nothing was stored.

Does anybody else know what might be causing this problem? Is it likely
the throttle body needs cleaning.

I used STP cleaner. My local garage said they have some stronger stuff
which isn't available over the shelf. Is that safe to use?

Finally my local Honda dealer said they are willing to sit in the car
for a drive to try and diagnose (free of charge). I'll do this as soon
as I have some time.

Thanks again for any advice. I'd love to get to the bottom of this small
problem.

Jez

 

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.

 

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:

http://homepages.nildram.co.uk/~jainsw/rotor1.jpg
http://homepages.nildram.co.uk/~jainsw/rotor2.jpg
http://homepages.nildram.co.uk/~jainsw/cap.jpg

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.