OBD2 datastream - lt fuel trim - +25% - ideas??

I think Jerry may have meant to say 'pulse width counts' (I could be wrong
maybe he missed it altogether). Jerry had emailed me about it earlier in an
effort to help out - he has been very valuable in the past with his input
and experience. For the record, I can't get pulse width information from my
scanner but I can get in from my scope. I haven't tested for pw on this
vehicle before, maybe someone can help with which wires I should tap and
what baseline I am looking for, etc? I'm not sure that checking the pw will
help fix the issue though as I would expect an increase fuel trim to show up
as increased pw (I could be wrong though).

Thanks!
Andrew.





in article ,
aarcuda69062 at wrote on 2/25/07 5:56 PM:

> In article
> . com>,
> "blazerman" > wrote:
>
>> Andrew-does your scantool give you any fuel trim counts, ex.- 128
>
> Read the subject line.

Thanks for the response. I really appreciate it. I will rescan it and let
you know. As for the MAF reading, I didn't think this vehicle had a MAF
sensor. If you can take the measurement some other way, I apologize. When
you say you need a BARO reading are you speaking of the MAP sensor's output
with key in engine off vs idling? As I stated earlier, I don't think there
are any vac leaks but I may have missed something somewhere, the injector
poppets are a definite possibility, PCV valves are fine (test ok, rattle, no
leaks), MAF sensor out of whack is also a definite possibility (I need todo
some impedence and voltage tests I guess), exhaust system is air tight as it
was just redone (problem existed before as well but again, I will double
check).

When you asked what the readings would be at cruise do you mean with the
transmission engaged or in neutral (stupid question - sorry). I was going
to take readings in every gear if that's at all helpful (just take
freezeframes on-the-fly).

Once again, I appreciate all the help and support that you fine folks have
been giving me.
Andrew.



in article ,
aarcuda69062 at wrote on 2/25/07 11:24 AM:

> It might be. Without a BARO reading, it is impossible to tell.
>
> What is the short term fuel trim?
>
> What are the long and short term fuel trims for the other bank?
>
> What is the Mass Air Flow reading?
>
> What happens to the readings during cruise?
>
> Taken at face value, the +25% fuel trim indicates a lean
> condition. This can be caused by vacuum leaks, plugged injector
> poppets, low fuel pressure, wrong PCV valve, skewed MAF sensor,
> skewed O2 sensors, air leaks in the exhaust system ahead of the
> O2 sensors...

In article >,
AWN > wrote:

> Thanks for the response. I really appreciate it. I will rescan it and let
> you know. As for the MAF reading, I didn't think this vehicle had a MAF
> sensor. If you can take the measurement some other way, I apologize.

Shame on me for not using my computer glasses when I first read
your post.

> When you say you need a BARO reading are you speaking of the MAP sensor's output
> with key in engine off vs idling?

Yes, if not shown separately, BARO is the MAP reading key on
engine not running.

> As I stated earlier, I don't think there
> are any vac leaks but I may have missed something somewhere, the injector
> poppets are a definite possibility, PCV valves are fine (test ok, rattle, no
> leaks),

Genuine GM part (PCV), not aftermarket?

> MAF sensor out of whack is also a definite possibility (I need todo
> some impedence and voltage tests I guess), exhaust system is air tight as it
> was just redone (problem existed before as well but again, I will double
> check).

Ignore what I said about MAF, I misread and thought you had a 96.

> When you asked what the readings would be at cruise do you mean with the
> transmission engaged or in neutral (stupid question - sorry).

Not a stupid question. Cruise as in cruising on the road.

> I was going
> to take readings in every gear if that's at all helpful (just take
> freezeframes on-the-fly).

3rd or 4th gear at 1000 RPM, 1500 RPM and 2500 RPM keeps it
simple. A vacuum leak does not show up in fuel trims as apparent
at cruise speeds so the trim numbers are usually closer to
center.

> Once again, I appreciate all the help and support that you fine folks have
> been giving me.
> Andrew.

Thanks again. Here's some atmosph BARO data for you. I live at approx
350ft above sea level which translates to approx 29.16 inHg. I took a
reading of 30inHg which translates to approx 10ft above sea level. Now bear
in mind that I may have my current altitude wrong as I at the base of a
900-1000ft escarpment. Even so, if the MAP was out by 300ft above sea level
equivalent, that wouldn't make a huge difference would it? I would think
we'd be looking for 1000s of feet out to make a wide adjustment?? I'm
actually amazed as to how accurate that sensor really is...

I will go back to the drawing board and do more tests.
Thanks again!
Andrew.



in article ,
aarcuda69062 at wrote on 2/26/07 8:10 PM:

> In article >,
> AWN > wrote:
>
>> Thanks for the response. I really appreciate it. I will rescan it and let
>> you know. As for the MAF reading, I didn't think this vehicle had a MAF
>> sensor. If you can take the measurement some other way, I apologize.
>
> Shame on me for not using my computer glasses when I first read
> your post.
>
>> When you say you need a BARO reading are you speaking of the MAP sensor's
>> output
>> with key in engine off vs idling?
>
> Yes, if not shown separately, BARO is the MAP reading key on
> engine not running.
>

Ok, I went out for some test runs. It was tough to get anything high speed
because it's complete black ice here everywhere tonight. Anyway...

Key in engine off baro reading - 30inHg which is consistent within about
300ft of my local altitude.


Highest rpm run was 1600
Load 13.3
25% LTFT
12 inHg
30mpH
TPS 5%
-10 IGN

Next run...
1250 rpm
16 mpH
2.0 IGN
STFT 11%
LTFT 24.2%
LOAD 5.4%
10 inHg

Idle test at 900 rpm (a little high b/c of my test runs I think)
IGN back and forth from -1 to -14
25% LTFT
TPS 0%
0 STFT
LOAD 3%
10-11 inHg

**TPS ratings are low b/c of icy conditions - I 'crept' up to speed
I didn't list O2 sensor data because there's no issue with high/low voltage
or switching freq

Does this datastream seem pretty much to baseline/normal specs for what you
would expect to see?

Thanks!
Andrew.







in article ,
aarcuda69062 at wrote on 2/26/07 8:10 PM:


>
> 3rd or 4th gear at 1000 RPM, 1500 RPM and 2500 RPM keeps it
> simple. A vacuum leak does not show up in fuel trims as apparent
> at cruise speeds so the trim numbers are usually closer to
> center.

In article >,
AWN > wrote:

> Ok, I went out for some test runs. It was tough to get anything high speed
> because it's complete black ice here everywhere tonight. Anyway...
>
> Key in engine off baro reading - 30inHg which is consistent within about
> 300ft of my local altitude.
>
>
> Highest rpm run was 1600
> Load 13.3
> 25% LTFT
> 12 inHg
> 30mpH
> TPS 5%
> -10 IGN
>
> Next run...
> 1250 rpm
> 16 mpH
> 2.0 IGN
> STFT 11%
> LTFT 24.2%
> LOAD 5.4%
> 10 inHg
>
> Idle test at 900 rpm (a little high b/c of my test runs I think)
> IGN back and forth from -1 to -14
> 25% LTFT
> TPS 0%
> 0 STFT
> LOAD 3%
> 10-11 inHg
>
> **TPS ratings are low b/c of icy conditions - I 'crept' up to speed
> I didn't list O2 sensor data because there's no issue with high/low voltage
> or switching freq
>
> Does this datastream seem pretty much to baseline/normal specs for what you
> would expect to see?

You show a 35% fuel correction on the 1200 RPM run, that is way
lean. The MAP readings, can't tell if thats a simple closed
throttle reading or if they are during decel. If decel, they
should read higher vacuum (lower MAP). Is fuel pressure in spec?
That engine is under fueling by a bunch.
You need to see if the O2 goes high and stays high at WOT high
load...
If I had this in my shop, I'd plumb some propane into a vacuum
port, drive it and see if the propane brings the trims back in
line and if there is a power increase. If so, I'd be on the phone
to Linder for a new spider.

Fuel pressure is fine - 61psi with the key on engine off and dropping to
maybe 55 after 30min. I don't think that a leak would take that long to
drop the psi but I could be wrong. I've never checked the fuel pressure
while driving but I suppose I could rig something up to make that work
without killing anyone around me...

I don't know if that 30% correction was under decel but I would hazard to
guess - no. I tried to lock in all the frame data while under light load.

I lied - I just rechecked and with the TPS flat, I was coasting and decel.
For giggles, that run showed the following O2 sensor data:

Bank1 0.050V
Bank2 0.300V


On another run with the tps at 5% (1449rpm) the O2 readings we

28% adjustment in fuel trim while accelerating

Bank1 0.855
Bank2 0.330

You mentioned the dreaded spider replacement as I knew it might be a
possibility. Would it be worthwhile to get a top end motovac treatment done
after exhasuiting all other options?

Thanks again for your help and patience,
Andrew.









in article ,
aarcuda69062 at wrote on 2/27/07 8:38 AM:

> You show a 35% fuel correction on the 1200 RPM run, that is way
> lean. The MAP readings, can't tell if thats a simple closed
> throttle reading or if they are during decel. If decel, they
> should read higher vacuum (lower MAP). Is fuel pressure in spec?
> That engine is under fueling by a bunch.
> You need to see if the O2 goes high and stays high at WOT high
> load...
> If I had this in my shop, I'd plumb some propane into a vacuum
> port, drive it and see if the propane brings the trims back in
> line and if there is a power increase. If so, I'd be on the phone
> to Linder for a new spider.

In article >,
AWN > wrote:

> You mentioned the dreaded spider replacement as I knew it might be a
> possibility. Would it be worthwhile to get a top end motovac treatment done
> after exhasuiting all other options?
>
> Thanks again for your help and patience,
> Andrew.

Sure, a Motorvac is worth a try.

If anyone is interested or still following, the latest on the beast is that
last night VERY late, I did a Seafoam top end treatment (sucked about half
the can up through the vac line from the booster). I let it sit in the top
end of the motor for about 10 minutes and then I idled the motor for about
20min. The smoke was absolutely unbelievable! I thought someone was going
to call the fire dept! Anyway, I limped it out to a country road and gave
it a little punishment to clean out all the excess carbon. I brought it
back to my 'shop' and scanned it again. The long term fuel trim is steady
at about 7-8% (rpm between 680 and 720) and the short term % is nearly flat.
I anticipate that this is due to a mild vac leak now or maybe a slightly
lazy O2 sensor (it seems to stick low a little so I suspect it may be
causing a slight false lean condition still). I am glad to report that the
problem is no longer 'pegging' the ECM at 25%+.

I anyone is interested in 'filing' away situations for future reference...
This may be a worthy memory. If you suspect a poorly running fuel delivery
system, try this seafoam stuff (perhaps before costly injector/spider assy
replacement) if you've never used it before. The responsiveness and idle of
the motor is much more precise also after treatment and I assume my fuel
economy will benefit also. The manuf suggests following up with 1/3 of the
can in both the gas tank and the crankcase but I only treated the top end of
the engine as it was my problem I suspected. For $11 a can, I have no
complaints! I am going to go with the fact that there must have been a
restriction in one of the poppets or perhaps an EGR issue (either way, the
treatment solved the problem almost entirely).

I am going to replumb all the old dried vac hoses, new PCV gasket/elbow and
valve from the stealership etc. - as well as pull and clean (or replace) my
O2 sensors and see if I can get the fuel trim flat. This has been a very
educational experience of which I have learned a great deal more about the
individual sensor's authority affecting fuel trim and my diagnostic
progressions. Once again, I appreciate all your time with this (everyone
involved - especially Jerry and aarcuda69062). I will get in touch if I'm
successful in completely flattening out my trim numbers but for the time
being, I am confident we nailed the issue.

Take the best of care,
Andrew.








in article ,
aarcuda69062 at wrote on 2/27/07 4:24 PM:

> In article >,
> AWN > wrote:
>
>> You mentioned the dreaded spider replacement as I knew it might be a
>> possibility. Would it be worthwhile to get a top end motovac treatment done
>> after exhasuiting all other options?
>>
>> Thanks again for your help and patience,
>> Andrew.
>
> Sure, a Motorvac is worth a try.