Chopface wrote:
> Caroline wrote:
>
>> "Chopface" <thename_is_slick@yahoo.com> wrote
>> > I've got a '91 Civic Si with the stock 1.6L MPFI setup and a manual
>>
>>> tranny. I put an AEM cold air intake on in August and I don't suspect
>>> any problems from it directly. It has 136k miles and is well maintained
>>> as of late (IMHO).
>>>
>>> Over the past few days the car has developed an idle problem. I have no
>>> check engine light indication. The ambient air temp. has been around
>>> 30-40 deg. F lately. The fast idle seems normal when I start the car,
>>> but after I have driven the car up to operating temp. (either cruising
>>> on hwy. or around town) the idle bounces repeatedly from 750 - 1250 rpm
>>> in neutral with the clutch out. I also seem to be getting lower gas
>>> mileage (low 30's vs high 30's).
>>>
>>> I changed my coolant this summer, and never bled the system. I just
>>> squished my upper rad. hose a bunch after filling it up and running it.
>>
>>
>>
>> I think I learned the somewhat-hard way that bleeding must be taken very
>> seriously, or air pockets and so inadequate cooling results around vital
>> components like the Idle Air Control valve (a.k.a. EACV).
>>
>> I'd just add that it will likely take at least 40 minutes to get the
>> radiator fan to come on the first time (which the manual says to do when
>> bleeding) and so do a thorough, proper purge.
>>
>> I did a bleed this summer, and the ambient temperature was 79 degrees F.
>> Forty minutes is about how long it took for the fan to kick on. One is
>> supposed to let it come on twice.
>>
>> Remember that both www.autozone.com and the UK site
>> http://www.honda.co.uk/owner/workshopmanuals2.html
>> have detailed guides for your Honda. At the UK site for my 91 Civic (1.5
>> liter), I used the Concerto manual a lot. It's engine is the same or
>> identical to my Civic's.
>>
>> I took off and disassembled my Civic's EACV this summer while
>> troubleshooting idle problems, but could have spared myself this trouble
>> had I first simply purged the cooling system of air.
>>
>>
>
> Thanks for the advice Caroline. It seems kind of strange to me that it
> took your car 40 min. for the fan to come on. I think mine comes on once
> the car is nice and warm and it is sitting idling.
>
> What do you think of the other poster's response about my 02 sensor? Is
> it reasonable for it to fail after 14 years and 136k miles? AFAIK it is
> the original and I have heard different sides about replacing 02
> sensors. I've heard that it is the sort of thing that is either working
> or it isnt, and I've also heard it can slowly degrade in sensing
> quality. I could see a bad 02 sensor causing my problems, but I have
> also have heard the many stories about air in the cooling system causing
> similair problems. I guess I hope there's not something wrong with my
> car (running too rich?) that caused the sensor to go bad if it is indeed
> bad.

it could be the sensor, but it's likely you'd get a code in that case.
insufficient coolant however is a common problem. most people are in
the habit of just looking at the expansion bottle. if there's a small
leak, any contraction with the cooling that takes place on switch off
sucks air back through the leak, not fluid from the expansion bottle.
so the coolant level can easily drop without being obvious. and
unfortunately, the fans come on ok if there's low coolant because
they're switched from a seperate circuit from the engine temp circuit,
so again, it looks like everything's ok. the trouble is, that if the
/engine's/ temp sensor is not fully bathed in coolant, the ecu goes rich
assuming cold start mode, and gas consumption goes way up. it cooks
cats too, so you need to fix it asap.

>
> Could my loss in mileage be caused by having some air in my cooling
> system? I may be really lucky, but I have had to drain and refill my
> radiator a couple times with my current car and a couple with my old '91
> Civic DX sedan, and I haven't really had idling/mileage problems
> previously. I did my timing belt (and water pump) earlier this summer
> and haven't had idle issues until now.
>
> I did forget to mention that my idle is a little off with regards to my
> power brakes. Stepping on the pedal causes my idle to dip maybe 100 RPM
> , and then it adusts up to maybe 50-75 RPM below my normal (no lights no
> brakes, etc.) idle. Maybe this is normal, but I don't remember it on my
> old Civic. I have been figuring it is something with the booster, but
> maybe it is related to my recent idle fluctuation problem. I have done
> the check for vaccum at the check valve on the booster vaccum line and
> it was okay.
>
> It looks like I can get a Bosch OE style sensor for around $30 on the
> net. If I can get one cheap locally I might just buy one. I just fear
> the bolts on the manifold heat shield. I've never tried to unfasten them.
>
> Mark