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
Add pictures here
âś–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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?
--
TeGGeR®

The Unofficial Honda/Acura FAQ
  Click to see the full signature.
Add pictures here
âś–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
TeGGeR® wrote:

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
Add pictures here
âś–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
TeGGeR®

The Unofficial Honda/Acura FAQ
  Click to see the full signature.
Add pictures here
âś–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
TeGGeR® wrote:

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.
Add pictures here
âś–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
TeGGeR® wrote:

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.
Add pictures here
âś–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
âś–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.