Hello Everybody,

I have 1997 4DR 4WD Blazer with 232,000 miles.
I love this truck. I have done quite a few things to keep
her tip top and running as smooth as my wallet takes me.

Recently she was hesitating and stalling out
when I would come to a stop only in drive.
P0152 O2 Sensor (High Voltage)
Bank2 Sensor 1

In Park or neutral I could feel the rough idle
but she would not stall.

I found a large vacuum leak at the PCV Valve Cover
and at the PCV orifice on the valve cover. I recently changed
the valve covers on both sides and the driver sides no longer has
a replaceable PCV Valve only an Orifice. I changed out the 93441238
and PCV Valve Cover (Dorman 47026). The plastic and rubber was falling apart.
This stopped error code disappeared immediately as soon as I started the car.
It still has a rough idle when I come to a stop and a small hesitation.

I have done all the general maintenance.
Cleaning both the EGR and Idle Control valves. They looked fine.
Replaced both them new just 10 months a go.
Cleaned the MAF too. New Fuel Injectors.
New Upper and lower manifold gaskets too.
New Map, TPS, IAC sensors. I did all of this only
because everything was original and as preventive maintenance.
I would rather spend a dollar on my truck than a bottle of beer.(sometimes).

Anyway she has been running well.
15-16 city 22-24 highway.

There is still a rough idle at a stop but it will not stall.
The STFT(Short Term Fuel Trims) and LTFT(Short Term Fuel Trims)
are with in reason of each other.

At Idle on Park
Closed Loop
Coolant 193
580 RPMs
Bank 1 STFT 0
Bank 1 LTFT -2.2
Bank 2 STFT .8
Bank 2 LTFT -4.2

1500 RPMs in Park
Bank 1 STFT 0
Bank 1 LTFT -1.2
Bank 2 STFT .8
Bank 2 LTFT -3.4

2200-2500 RPMs in Park
Bank 1 STFT 0
Bank 1 LTFT 0
Bank 2 STFT 0
Bank 2 LTFT 0

Would these readings only indicate that
I have another vacuum leak somewhere
since both Fuel trims basically go to zero
at 2200-2500 RPMs. I have an Actron 9830
that I use to get the live readings.

Thanks for any help or insight.

Mark K

 

Well something went haywire.
For the past day after my fix I drove
the blazer and it responded well.

I monitored the Short term and Long Term
fuel readings and they were well with in spec.
Pretty much same as above.

This morning all of a sudden on the 2nd restart of the day
my readings went off the charts -30 and -35 on both
short term fuel readings. The Blazer wanted to stall just like before.
I had to put it into neutral at stop lights and baby it so it would not
stall out. I checked and I have a pending code P0507 Idle High.

I checked my O2 readings and found out that
Bank2 Sensor1 is constantly at .7 to .9 running rich.
This is a new O2 sensor just put in a week or 2 ago.
The other O2 sensors are functioning properly.

I don't think its a bad O2 sensor. This was the way it was acting
before I changed it out. The previous sensor had almost 100,000
miles on it. It still may be good too.

Getting too much fuel just on that side (Passenger).
The driver's side is acting normal Bank1 Sensor1

The spider was just changed out 11 months ago.
Everything was fine up to about 3 weeks ago.

Any Ideas?

Is it possible I have a Bad IAC Valve causing this.
It does have a hard time getting back to normal idle
when decelerating and trying to come to a coasting stop.
It sometimes will idle high in drive with no effort or my
foot on the pedal. The IAC valve was put in new 11 months ago too.

Thanks - Mark K

 

Well when I get a chance I am going to check
the wiring on the Oxygen Sensor on the Passenger side.
Thats the one that is not oscillating very well.
On average as I watch the Live data it goes from .65 to .85.

This I know is causing a rich condition.
In particular Connections A and B.
I am hoping the connections just need cleaning.

Attached are schematics for the wiring and connections.

 

Well I checked the connections on the Oxygen sensors and they are
accepting a signal and responding back to the VCM. They are doing
what they are suppose to do.

I got to looking through more posts on the forum and came across
something that was similar to my problem.

I went back to my MAF that I had cleaned. I checked the numbers
on my scanner and found out that with the Key On and Engine Off
it was already reading 1.96. Unplugged it was at Zero. I started the
Blazer and it jumped to 8.95. This could be my problem. Even though I had
performed the usual maintenance on it I did not check the numbers.
This seems to be an intermittent problem.
One of the resistors must be giving out. I did this before work so I was not
able to see what it would be reading once the vehicle warmed up. I know that it was running on
open loop numbers so it was idling OK at startup. My problem comes at Closed Loop.
When I get home tonight I'll keep a more careful eye on the actual numbers in Closed Loop.
I can only say that the Forum has all the answers but you just have to keep digging.
This forum is my biggest help for this truck. Don't ever go away. I plan to keep
my Blazer until I croak.

Thanks - Mark K

 

Well upon further review I believe my MAF sensor is OK.
From other posts and responses on the Forum the 1.96
reading for the MAF with Key On Engine Off does happen
on other MAF sensors. My only guess would be the VCM has
some kind of info stored in open loop.

Once my Blazer went into closed loop the MAF sensor looked
to be OK. It was reading normal at Idle in Park. 624 rpms( 4.7 g/s)

I also noticed that in open loop at startup when I revved the engine
the BS2 Oxygen sensor 1 was responding ( Oscillating properly)
It was jumping between .1 and .8 like it should.

Once it goes into closed loop the BS2 Oxygen Sensor 1 starts to flatline.
It stays between .6 to .8
I am thinking that my problem is a sensor that is
sending info to the VCM in closed loop that makes the Oxygen sensor
start reading rich. It does not do it in open loop.
I watched the Engine advance in Open loop and it was at 19-20.
It acted properly and quickly jumping positive. Once it goes to closed loop the the same
type of revving of the engine causes hesitation and I can see the engine Advance
drop to negative numbers then back up to proper positive numbers.
Not sure what that info exactly means?
I might have to take it to a local mechanic who has
more capabilities with digging into my VCM. I only have a basic Actron scanner.
Well I will keep digging.

Thanks - Mark K

 

I went out early this morning to double check
readings on sensors with the Key Off Engine Off.

It was 58 degrees outside this morning.
My IAT was reading 61 degrees but to my surprise
the Coolant Temperature Sensor going to the VCM
was reading 68 degrees. I made sure by unplugging
the sensor and it went to -40 like it should. This could be
my potential problem. I might have more but I will
definitely be changing this sensor out this weekend.
I am not sure how sophisticated/sensitive the VCM
is to the temperature info once it goes into Closed Loop.
If the VCM is getting it's info from the Oxygen sensors
for Injection width I am not sure how much the coolant
temperature plays its part once it is in Closed Loop.
Does the computer take into account the coolant temperature?
Does it change the amount of fuel coming out of the injectors based on closed loop Coolant readings?
Is there a different amount of fuel coming out for a coolant reading at 195 versus 185?
At the moment I am running rich once I go into closed loop. It does start to settle down somewhat once
the Temperature gets up to 188-195. My Blazer usually goes into closed loop at around 145 degrees.

Thanks - Mark K

 

You are really all over the place on this one. Probably why no one is responding.

Let's try it this way. Forget scanners and voltages and fuel trim readings. WITHOUT drawing any conclusions from anything you have peviously done and WITHOUT hooking up your scanner, please describe your CURRENT symptoms while driving starting with a cold engine. Idles rough, black smoke, when, SES light on, etc.?? Be as complete and descriptive as possible. Then we will guide you through diagnosis.

 

Hello Lesmyer,

Your right I think I was rambling on too much.
I will forget about the scanner.

At startup with a cold engine.
Turns on fine no problem. Once it starts idling
I can feel a small miss. Nothing serious.
No codes or pending codes.
Idling around 800-850 in Park

Once the engine is warmed up it drops
the idle down to 600-650 in Park.
I start to feel the miss a little more.

Put it into drive and I can feel a lack of power
at low RPMs driving out of a parking lot.

When I start to come to a complete stop in Drive at a red light
or stop sign I can feel the Blazer really start to stumble,
hesitate and want to stall out, which it has on occasion.
No error codes. I have to play with the throttle in neutral
to keep it going.

Thanks - Mark K

 

Is it setting at least a pending misfire code when completely warmed at the time when you have to play with the throttle to keep it going? If not I think we need to figure out why it is not recognizing the misfire. That may help us find the cause.

Actually there may be a min RPM where misfire code will be set - let me check.

 

I didn't see it in your thread, but if you haven't already done it...you might want to start here https://blazerforum.com/forum/2nd-ge...eakdown-88305/ It's kind of sounding like symptons of the fuel injectors needing to be switched over to MFI (very, very common on these trucks)...but I would suggest starting with the fuel leakdown test first since the fuel pump is another very common problem and at least you can rule that out.