[mdlug-discuss] [TECH SUPPORT REQ] OBD-II typical readings

[Aaron Kulkis]

Robert Meier wrote:
> Aaron,
>
> Thanks for your suggestions.
>
>   
>> Perhaps your Mass Air Flow sensor is reading high
>> (engine computer thinks more air is coming in than
>> actually is, and thus directs for more fuel than is needed).
>>     
>
> OBD Gauge does not include that measurement (or my car is unequipped).
>
If you have fuel injection, you probably have one. It will
be an electrical sensor in the duct that connects the air
filter housing to the throttle body.

If it's say, a little gummed up or a bit of rust, such that
it "sticks" a little bit high after an acceleration, it
could cause a problem like this.

I first became aware of the impact this thing has on earlier
car that had a bit of dry rot on the ductwork between the
MAF sensor and the throttle body. When pressing the
throttle down heavily, the the air pressure from the air
intake all the way back to the throttle body drops
significantly. (Under low throttle, the butterfly valve
in the intake side of the throttle body significantly
obstructs the airflow into the throttle body, and so
there's a significant drop in air pressure across the
butterfly...but when the butterfly is open, the manifold
vacuum extends all the way through the air-intake system)

Well, anyways, a section of the air ductwork was made
of rubber, and molded like a soda straw that has a
mechanical "bend" point...and in one of the crevices,
dry rot and vibration caused an air leak. So with a
sudden throttle position change, the vacuum behind the
crack increased suddenly, and a bunch of air would
rush in through the crack, bypassing the MAF, and so
the mixture would suddenly get REAL lean--engine RPM's
would drop significantly, manifold vacuum dropped,
the crack closed, the MAF would be sending accurate
readings, air-fuel mixture went back to spec, the
engine would rev, increasing manifold vacuum, starting
the whole cycle all over again.

I could still accelerate, just VERY slowly.

A couple feet of duct tape solved the problem. :-)

>> By different means, a dirty air filter can also cause
>> an overly rich mixture.
>>     
>
> Air filter looks clean and was just changed less than 3Kmi ago.
>> 75,000 miles is getting up there.
>>     
>
> Two of my prior cars were at 120Kmi and 140Kmi when they
> were totalled by collision.
>
>   
I hear you... my current car has 131kmi on it, and the
oil consumption is what I would expect from an engine
with only 20kmi.

What I meant was 75,000 is getting to the point where
if any "regular services" which are recommended at
intervals of 40,000 or less start to cause problems
if they have been neglected.

Back in college, I had a friend with a Honda Civic...
the body was heavily rusted. The engine had 200,000
on it and was still going strong. At the time, that
sort of lifetime was extremely rare for an engine
made in Michigan, but he told me that it wasn't
uncommon for the Honda engines.
>> When was the last time the following services were performed:
>>     
>
>   
>> Fuel Filter Replacement
>>     
> 1Kmi after start of problem, and 1Kmi before now
>
>   

Wow.

>> Spark Plugs Clean and Re-gap OR Replace and re-gap
>> Spark Plug Wires Replace
>> PCV valve Clean or Replace
>>     
>
> 60Kmi (15Kmi ago)
>
>   
>> Throttle Body Clean
>
> unknown

Probably unrelated. It's a fairly simple procedure
if you want to do it yourself -- a can of throttle
body cleaner is cheap, like $5, making the main cost
to be nothing more than time. No special tools nor
expertise is required. In contrast, having it done
by a mechanic will cost $50 or more. I had it done
once at a quick oil change place, and it was $60.
As the vehicle was my first fuel injected engine, I
paid for it because I didn't know anything about
the procedure.

I had it done by a mechanic at an oil filter place
once, and it provided an immediate and significant
improvement. Engine idle smoothed out as soon as
the procedure was done, as well as power output.

Basically, you take the air duct off of the intake
of the throttle body, and first brush out some of
the major crud which has accumulated inside the
throttle body. Once that is done, the procedure
consists of introducing throttle body cleaner into
the throttle body while the engine is running.

The engine will race when the large quantities of
cleaner are introduced (it's similar to fuel).
The can I got says to open up the butterfly valve
and spray inside the throttle body for 10 seconds,
then let the engine run for 10 seconds, and repeat
until the can is used up. With my engine, that
turned out to be rather impractical -- the engine
really revved when I tried to do that, and without
a tach, I was afraid of redlining it by introducing
so much of this stuff while the butterfly was
open at the same time. And when the fan would
kick in, the hot air would blow back towards the
throttle body, and the engine would stall for
lack of power. So every time it stalled, I would
give a 10-seconds spray while the engine was
stopped. After a few repetitions, I observed a
significant reduction in the amount of crud on
the inside of the throttle body (most likely
condensates from the vaporized oil and whatever
else gets sucked out of the crankcase through the
PCV valve into the front of the throttle body).

In contrast, the guy at the quick oil change place
had a nifty little set-up, essentially some sort
of plastic tube and a flow-control valve (think
of an IV drip in a hospital), with the open end
of the tube attached just inside the throttle
body opening, or perhaps even poked just past
the butterfly valve.

Doing the operation myself took me about 3 hours
(although it would probably be maybe 60-90 min
when I do it the next time). Oil shop guy did
it in 30-45 minutes. The price difference was
significant however. The oil shop charged $60.
Doing it myself cost me $5 and a few paper towels.

If you do it yourself, do NOT use carburetor
cleaner. The price difference between carb
cleaner and the same size can of throttle
body cleaner is negligible -- so if carb cleaner
were appropriate for throttle bodies, I would
think that it would just be called carb and
throttle body cleaner. Since it isn't, I'm
going on the assumption that the two products
are similar, but different in some important way.
>
>> Air Filter Replace
>>     
>
> 72Kmi (3Kmi ago)
>
>   
>> Spark plugs are cheap ($2 each), so I replace those ever
>> 12-24 months. Aging spark plug wires can result in weak
>> ignition, even if you don't have any misfiring.
>>     
>
> Unfortunately the Saturn plugs are difficult to get at to replace.
>
So it's probably been a long time. I'm going to be out
of town for the next two weeks, but if you want, I can
take a look at it when you get back. Sometimes there
are amazingly simple (but non-obvious) ways of getting
to hard-to-reach spark plugs.

The Fiero was notorious in this respect -- undo an
engine mount and rock the engine forward. Then
someone realized that you could take a rear tire
off, and then another couple of bolts would allow
the wheel well skirt to be removed, and you would
be staring right at the spark plugs.

A similar trick might be possible with your vehicle.

>> A gummed up PCV valve allows the inside of the engine block
>> to hold high pressure gases (and also lots of nasty stuff
>> that you don't want in your oil).
>>     
>
>   
>> Fuel filter... it's amazing how a used up fuel filter can
>> rob you of both mileage and power. I'm not sure if it could
>> cause overly rich readings, however.
>>     
>
> As this was actually overdue for replacement,
> it was replaced first,
> but I would expect a clogged fuel filter to restrict fuel flow
> and behave like a "fuel starvation" lean injection.
>
> Still investigating,
>