87 Pontiac 2.8L FI

[[email protected]]

Hi;

This one has an IAC error (code 35) and it says it's lean I think (code
44). I don't think the mixture is that far from stoich, as it passes HC
and CO. Problem is it fails NOx. EGR ports are good. It also does not
purge the EEC, there is a whoosh when you open the gas cap.

The canister purge, EGR solenoid and TCC run off the same line, which
is up, has 12 V. The main problem has something to do with EGR so we
went there first. The convertor locks up we believe, but the system
never goes into closed loop operation. After a several mile drive the
light still flashes fast in run test so I'm not so sure about the TCC.
I haven't personally driven it.

Afterward, we grounded the gray wire to the EGR and it did not actuate,
later we found that it is fed with ported vacuum, and there was on
vacuum at idle. So we left the gray wire shorted to ground and revved
it, sure enough it actuated and the engine stumbled.

We moved on to the IAC, removed and inspected it. It seemed to be at or
near maximum extention. Retracted the plunger and reinstalled leaving
the electrical connector unplugged. Of course the engine ran at a high
idle, and slowed down when the IAC was plugged back in, so we know it
goes one way.

It is surprising to me that ported vacuum is fed to the EEC, not
manifold vacuum, I thought it purged right at startup. Either I'm
mistaken or they changed it.

There are four vacuum hoses at the throttle body The placement seems
logical, The other two do have direct manifold vacuum.

Is this correct or is there a clog ? The vacuum I can feel is indeed
ported vacuum, not simply there when the RPMs are up, so I don't think
it's a clog.

The EGR has an air bleed, when it is not actuated the ported vacuum is
there. This may be normal because there is an air filter. Is this
normal ? A leak in that spot could concievably be taking the IAC out of
range, throwing the code 35. Or is the IAC the more likely culprit ?

With a code 44 though, I guess the possibility exists that it needs an
O2 sensor. Should I go there ? There should be no reason an IAC or EGR
malfunction should throw a code 44, or is there ? Also an O2 error
should not cause it to throw a code 35, or should it ?

Usually I can handle these things, but this one is a bit wierd. It
might not be unique so therefore I post. I see where maybe it could be
one simple thing with this combination of symptoms, or of course it
could also be the ECM.

Any ideas ?

Thanks in advance.

JURB

 

[Phil]

Hi;

This one has an IAC error (code 35) and it says it's lean I think (code
44). I don't think the mixture is that far from stoich, as it passes HC
and CO. Problem is it fails NOx. EGR ports are good. It also does not
purge the EEC, there is a whoosh when you open the gas cap.

Any ideas ?

Thanks in advance.

JURB

Swap the lambda (O2) sensor & retry....

 

[D_Zoot]

First, some background information that might help some of this make
sense:

Idle air control is handled by the ecm and position is indicated in
counts. 0 counts would be fully closed, 255 would be fully open.
Generally at a hot idle counts average 15 to 45. You need some manner
of scan tool to see what the counts are. The ecm will set a code 35 if
the ecm reaches 0 counts or 255 counts and the idle speed is still not
within the desired idle speed.

Code 44, lean condition, is determined by monitoring the oxygen sensor.
Bias voltage on the o2 circuit is 450mV. Anything below 450 is
considered lean, anything above is considered rich. Since we can't
hold fuel control exactly at 14.7:1 all the time we vary the fuel
rich-lean-rich-lean and hopefully it will average out at stoich. If
you tap into the o2 line with a scope or meter, you should see this
happening, above 450, below 450 and so forth. The ecm sets a 44 when
it has reached a certian point in it's fuel control compensation table
and the o2 remains below 450mV.

So, based on the code 44, it's likely that the o2 is stuck below 450.
This could be either a bad sensor, which is common, or you could
actually have a lean fuel mixture. One of the things that could cause
lean fuel mixture is a vac leak. Something that bears creedence to a
vac leak theory is the code 35. A vac leak could also cause higher
than normal idle that the ecm cannot compenstate for. So, check
carefully for vac leaks (don't forget to check in the engine base,
since on a 2.8 an intake runner leak will be under a valve cover or in
the lifter valley), if no vac leaks turn up, try an o2 sensor.

Egr - I'm not convinced you actually have an egr problem at this point.
Lean fuel will cause your NOX to be high due to the elevated
combustion chamber tempurature. Get the fuel trim issue sorted out
first. The small amount of vac you noted at the egr "filter" is
actually the solenoid bleeding off vac to the valve when the ecm
commands the egr closed.

Purge - Purge is ported. Generally we don't want purge at idle as it
can cause idle quality issues since we don't really know how much fuel
vapor we are going to pull from the canister at any given time. On
older vehicles (pre-OBDII) off idle purge is commanded on via the
solenoid and the purge amount is controlled by the ported vac. The
higher the throttle rate the more purge we can allow without noticing
any change in the way the engine runs. If it's pulling a good vac in
the fuel tank it sounds like your fuel cap may not be venting to allow
fresh air to enter the system. But there are several venting schemes
used, and since you didn't state what this is for a car I can't be more
specific.

Good luck!
D

 

[Bob Kos]

PCV Valve?

It's a common source of vacuum leak and very often overlooked. Especially
if it's recently been changed with the INCORRECT part ( oil change shops are
good for this ).