Danwelch

I have a 98 Civic EX that has started to code lately. The Code says P-0505 (Idle Control System.) In addition to the code, it has the following symptoms. When cold it seems to run fine. I can drive it for miles on the highway with no problem. But if I drive it in town (idling, stop and go, etc) after it is fully warmed up, it will generate the code. When it does this, the car gets very "irritable". When you push on the accelerator, nothing seems to happen until you get to a certain point, then it bucks forward. It idles fine - although a tad high - and at high revs it seems to run fine. But acclerating/decelerating it wants to buck.

If I rev the engine gently with the car stopped, the idle will surge up and down about 2x per second. Then as I rev faster, that goes away and all seems fine. It never stalls.

One last point - If I turn the car off and immedately back on, the problem invariably clears up. I may be able to drive another few minutes with no symptoms at all and then the problem will recur.

I swapped out the IACV with a known good one. Didn't help. I replaced the ECT. No luck. I recently got hold of a scanner and checked what I thought were likely culprits (TPS, MAP Sensor etc.) but those readings don't seem to change much when the car is normal vs abnormal. One thing that DOES seem to change - the range of Short Term Fuel Trim % drops markedly. Actually it goes from a negative number to closer to 0 if you consider that a drop. Could this be VTEC related? Is that behavior likely to be a cause or a symptom?

The O2 sensors and the Fuel Trim %'s seem to cycle up & down somwhat. Is that normal? Could that be a vacuum leak? If so, why does it get "fixed" when the car is turned off and right back on? (I searched for a vac leak with propane but didn't find anything.) They still cycle whether the cars is meesing up or not - they just cycle in a different range.

I am now suspecting the ECU, but hate to just start throwing parts at it. Thoughts?

Thanks for your help!
Dan

ezone

The PCM set the code because it couldn't control the low idle (base) speed. Could be vacuum leak, could be sticky IAC, could be some other things, but that is what made the code set..

The surge is from the decel fuel shutoff and is expected when this happens. Figure out how to get the base idle speed down to the proper RPM and it will stop.
Criteria: Warmed up engine + TPS indicates closed throttle + RPM over 2000. Fuel is shut off until RPM is at 1200, then fuel is turned back on. Cycle repeats endlessly because the base idle RPM can't be controlled.
O2 sensor, MAP and STFT should all reflect the surge and vary a lot.
(First O2 and STFT vary rapidly under almost all conditions anyway.)


Pull the snorkel off of the throttle body. Get the engine surging. Find the IAC passage and cork it with your fingers. See if that brings the idle speed down to a more reasonable level.






Is there more than one opening inside there? Cork it too.
Which one made all the difference? (if any) Follow that passage to see where it leads. See why that component is leaking.

Has someone messed with the idle stop on the throttle body? Cranked the throttle open too far?
How about jacked with the bypass screw on the throttle body?


Pinch off vacuum lines? Brake booster vacuum line, pinch that too. Any change?
PCV valve fell apart causing an internal vacuum leak?


Does the engine use a FICV? (I can't see it from here) If it does, that needs to get hot with coolant temperature. I used to see many older cars that had clogged coolant passages causing idle problems because the valve was slow to get warmed up.


Thoughts on my lunch break, HTH

Danwelch

The high throttle makes sense - I have had a problem getting that lowered. I didn't connect that issue to this set of symptoms though. I tried plugging the IACV port with my finger. It pulled a strong vacuum, but I didn't notice much change in the idle speed. I suspect that means that the IACV valve is OK (?) and air is leaking downstream of this valve. Sound right?

You asked if this car has a FITV - it does not. The idle stop has not been messed with lately... I adjusted the idle screw to try and reduce throttle speed, but I was not able to get it below around 900 RPM.

In looking for the problem causing the high idle speed, I am thinking the behavior of turning the engine off might give a clue where to look further. This action invariably fixes the problem at least for a short while. This behavior doesn't seem to be temperature related since I can turn the engine off for just 5 seconds and the problem goes away for several minutes. A vacuum leak doesn't seem like it should change when the engine is turned off and on. So is there something else (some system) that may get reset when the engine is turned off then back on?

Thanks again for your help on this! I think this troubleshooting track makes sense...
Dan

ezone

Yeah. Basic problem is idle is too fast (for whatever reason). The rest are just symptoms of that.



"Lately"?
Do you mean the big slotted screw, or the stop for the throttle cable arm? I hope you meant the big screw.

Kind of wondering if something happened (work done) that might have caused your complaint.



Absolutely that should be some sort of clue, but I'm not sure what to tell you from here. It isn't ringing any bells unless it has to do with the IAC valve. Kinda tough to tell what is going on without seeing it in action.

A thought (not that it would do me any good):
Get it acting up again.....What would it do if you merely flicked the key OFF/ON real quick instead of actually letting the engine shut off? Would that action make it stop surging?



What does it do if the IAC is unplugged?

Danwelch

You wrote: "Do you mean the big slotted screw, or the stop for the throttle cable arm? I hope you meant the big screw." Yes, it was the big slotted screw directly above the throttle. I didn't mess with anything over near where the throttle cable attaches.

You Wrote: "Kind of wondering if something happened (work done) that might have caused your complaint." The only recent work before it started coding was valve adjustment job. I didn't mess with the fuel injection or intake system at all. Can't think of how a valve adjustment may be related...

You wrote: "What does it do if the IAC is unplugged?" I did that some time back and as I recall there was little or no change. I can't recall if it was in its surge mode at the time. I'll have to try that again. I also plugged the IACV port in the throttle body and saw little or no change. Lastly, I tried a new IACV becasue someone assured me that was the problem. It acted exactly the same. Luckily the auto store took it back!

I'll try plugging the various vac ports this weekend and see what happens. (I have to work outdoors so it's tough to do in the evenings this time of year.)

Thanks for all your help. I'll keep you posted...
Dan

ezone

Ok, I can't think of anything there either.

Danwelch

Problem Fixed! A loose vacuum hose - it was the cruise control hose disconnected back at the cruise control adjusting element. Simply re-connecting the hose lowered the idle speed back within range... and Voila! No more surge, just as Ezone had suggested.

Thanks tremendously for the help,Ezone!

Dan

ezone

YW. Thanks for the follow up.