Hello Everybody,

I have a 1997 LS 4WD Blazer with 234,000 miles.
I try to take good care of it and have changed out
quite a few parts over the past 15 years I have owned it.

My question involves the PCM(original computer), the MAF (original) Cleaned every 6 months and the Idle Control Valve(new).
Previously I had a big issue with the Idle control valve. It was erratic,
closing at idle when coming to a stop. It would start to stumble and want to die.
I bought a new one(Delphi) and have changed it 2 times thinking it was bad.

I acquired a Snap-On MT2500 off Ebay which I like very much.
I was able to reset the Idle Control Valve with the MT2500
and do various control functions on the IAC to make sure
that it was working with command from the PCM. All seemed
well but not all of the time. It still gets erratic at times but 90 per cent
of the time it has been doing what it is suppose to do.

The other day I started to get an up and down idle in Park.
It would jump from 550 RPMs to 900 RPMs then try
to settle down to normal. I had my scanner on at the same time.

I did not get any codes, but I watched my MAF readings jump
from a normal 5.5gm/sec to 11.5gm/sec with the RPMs and my IAC counts
jump from a 22 to 60 with the same RPM movement.

My question is what is effecting what first.
Are the MAF readings causing the IAC
to bounce or is the erratic IAC causing the MAF
readings to bounce. Not sure which one is sending
the information to the PCM first since both were not normal.

Maybe the computer just got erratic?

Just add a little more info:
Closed Loop
191 Degrees Coolant
IAT 80 Degrees
Short term and Long Term fuel both normal.
O2 sensors acting normal.
TPS at zero at Idle.
Injector Pulse duration both steady at 3.8ms

Thanks for any help from anyone who knows the sequence.

Mark K

 

The IAC controls air flow entering the engine while at idle, (TPS is unaffected). The MAF measures how much air is going in, regardless of engine speed.


When the IAC opens, MAF will increase.
When the IAC closes, MAF will decrease.


engine at idle, closed loop, engine at normal operating temp,
trans in park or neutral, all accessories off:


These are normal values:
Idle: 650 rpm
MAF: 4.6g/s
TPS: .47
IAC: 18 counts

 

Thanks for the reply Captain,

I guess that would confirm to me that the IAC Valve
was acting up again. The MAF was seeing things clearly.

I have switched out this valve with new ones and old ones.
I t seems to act up no matter what. I am beginning to think
something is up with the computer or the wiring. It is intermittent.
I can't find a rhyme or reason why it does it. I have had my scanner
on many times when this pops up out of the blue and I can't see
any reason why it would do what it does.

Mark K

 

These PCM's rarely fail. When one is replaced, 99+% of the time it's due to a misdiagnosis. If there's a problem with the wiring, it will set a DTC in memory.


Watch B1S1 and B2S1 sensors when the idle starts oscillating. My guess is that at least one of them goes lean, indicating a vacuum leak.