wlw-19958

Hi There,

I am working on my son's 1994 Camaro and frankly, I am at a loss.
The engine runs terrible! It idles okay (most of the time) but it has NO
power. The engine stumbles and it will not rev. above 3500 rpm. It has
trouble getting above 2500rpm.

The car is mostly stock. The only modification is a K&M air filter system
(and the skip-shift has been disabled).

When my son brought me the car, it was running terrible and setting codes
for the high and low resolution pulses from the opti-spark. So, I replaced
the distributor. At the same time, I put in new platinum plugs, new spark
plug cables, Hoses, Belt, Water pump and a new timing gear/chain set
(even though the old set wasn't that badly worn).

The codes have gone away but it still runs terrible.

I've scanned the system with my Genisys scanner and I don't have any
codes (except for the skip-shift) and all the sensor data looks good (at
least I nothing I see jumps out at me as being out of spec.).

Fuel pump pressure is a little low (40.5 psi) and exhaust back pressure
is slightly high (1.8 psi at idle and 3-4 psi at 2500 rpm).

I've recorded some of the info (freeze frame) in my scanner so I can look
at it again (and again) to see if I can see if something is wrong but so far,
I don't see anything wrong.

What do you experts suggest?

Thanks in advance!
-Blue Chips-
Webb

P.S. sorry for the short post but dinner is ready and I got to go...

Injuneer

Can your Genisys produce a complete data log for a reasonable period of driving under varying throttle position and load? Can often find problem source be watching the interaction of the various sensors and PCM parameters.

If you can't, look at the "sticky" post at the top of this forum's topic listing, regarding Scan9495 free scanning software.

Under what conditions is the fuel pressure 40.5 psi? With or without vacuum compensation? Have you checked it under load at max possible RPM?

What do the long term fuel corrections look like, in various operating Cells?

wlw-19958

Hi There,

The fuel pressure test is first done with the engine NOT running.
I just energized the fuel pump with the ignition a couple of times
to get the base pressure (as per the manual's instructions). The
pressure is supposed to be between 41 to 47 psi and my measurement
is a little low (40.5 psi then again, my gauge might be off by 1/2 a pound).
When the engine is idling, the fuel pump pressure drops to 35 psi.

The Genisys system can record for a period of time and is limited
by the total amount of memory. I have recorded a few runs but
as of yet I don't see anything a miss but maybe you will.

I am curious about the long term fuel trim. From my recordings,
the long term fuel trim varies almost as much as the short term fuel
trim. It varies from 128 down to as low as 112 at times. At full
throttle, both the short and long term fuel trim will stabilize at the
base 128 value. At the same time, the HO2S shows high voltage
(about 800mV). Other times, the HO2S show typical up/down
voltage variation.

Good Luck!
-Blue Chips-
Webb

Injuneer

Priming to 40psi is generally not a problem. You need to start it and let it idle. Remove the vacuum compensation line. That's where the spec is 43.5psi, with the 41-47psi tolerance band. Reconnect the vacuum line and an 8-10psi drop, proportional to intake manifold vacuum is normal.

The key is - what does the pressure look like when the engine starts losing power?

It would also be interesting to see if the exhaust back pressure continues to increase quickly as you move from 2500 RPM to 3500 RPM.

You might want to check the Scan9495 software, or TTS Datamaster. They are both capable of producing extended run time data logs, and produce files which can be opened in Excel, facilitating manipulation to try and establigh cause and effect relationships.

The long term fuel trims (LTFT, also called BLM = block learn multipliers) are developed for a wide range of RPM vs. engine load. Engine load is represented by manifold absolute pressure (MAP). You end up with 16 "cells", each defined by RPM on the horizontal axis and MAP on the vertical axis. There is a left bank LTFT and a right bank LTFT stored in each cell.

As you move the engine through the range of RPM vs. MAP, the LTFT shown on the scanner will change as it moves from cell to cell. The scanner should also tell you which cell the engine is operating in. Cell 16 is used for idle. Cells 1 through 15 range from low RPM/low MAP (Cell 1) to high RPM/high MAP (Cell 15). There are two additional cells, mathematically derived from the other 16 using a weighted average. Cell 17 is generally used for decel, and Cell 18 is generally used for open loop operation under load, and some cases of power enrichment (PE) mode = roughly "wide open throttle".

When the throttle reaches certain limits of RPM and throttle position, the fuel control will transition from "closed loop" to "power enrichment" mode. A/F ratio target changes from the closed loop stoichiometric value of 14.7:1 to a richer A/F ratio, required to produce peak torque and HP. While the actual target A/F ratio is a calculated value, generally you will see it in the area of 11.7:1. Actually a bit on the rich side, but probably programmed that way to minimized the risk of running lean at WOT. Reprogramming the target A/F ratio calculation to achieve a ratio closer to 13.0:1 will usually net about 10-15HP over the factory calibration.

The O2 sensors are still operating, and at WOT readings of 850 - 920 mV are not unusual.

Since the stock, narrow-band O2 sensors are not accurate at anything other than 14.7:1, they are ignored in PE mode. Since the sensor feedback will be ignored, the short term fuel trims are locked at 128, and the PCM uses the LTFT from the last cell it used before entering PE mode. EXCEPT.... if the LTFT in that cell was subtracting fuel (less than 128) the LTFT will default to 128. If the LTFT in that cell was adding fuel (more than 128) it will use that value. There are also instances where the control will default to Cell 18 at WOT.

wlw-19958

Hi There,

Thanks for the responses. The Fuel Pump Test I was following is from the
GM 1994 manual (via the diagnostic flow chart). I will re-test the fuel
pressure as you recommend. Unfortunately, the hose on my fuel pressure
gauge isn't that long and I'm not sure if I can read it while driving. I'll see
what I can do. I'll just mention that I did change the fuel filter (seeing it was
the original and this vehicle has just over 200K miles; plus it couldn't hurt).

I have been looking at the Scan9495 software posts and I have downloaded.
I've update one of my laptops with NET framework 4.0 (my laptops still use
XP). I will have to get the cable components and build one. Of course, this
will take some time. But it looks like a neat program and I look forward to
trying it out (once I get the components and fabricate the proper cable).

Thanks for explaining about the Block Learn Multiplier. My scanner does
have BLM cell and BLM enable outputs (I wasn't sure what they were until
you explained it). I will re-scan the system and include the BLM data and
report back.

Also, your explanation of the WOT fits the data I've been getting from my
scanner.

I also should check the EGR valve. It is unfortunate that its location is back
under the cowl. The manual suggests pulling the EGR valve and checking
the passages for blockages. Any suggestions on how to check them
without having to pull the intake off?

To add to the "poor performance" description, the engine will surge and
buck when it tries to accelerate and when the peddle is first depressed,
there is a noticeable sag before the engine starts to pick up. Performance
is so bad, a fat chick on a tricycle could outrun it! (My apologies for my
politically incorrect comparison).

Thanks again for the suggestions and the education. I hope with your
help, I'll get this beast back on its feet.

Good Luck!
-Blue Chips-
Webb

walt355

Im gonna go out on a limb here, I had a buddy had one do the same thing after an opti swap on a 94. Ran but real sluggish, no RPM over 3000ish and rough start. Turned out he got the spline off on the opti when he was installing it.

Injuneer

That does happen. Even to people who swear they did it the right way.

What brand Opti did you buy? Did you loc-tite the rotor screws before installing it?

wlw-19958

Hi There,

I have replace the opti twice. Once with a new after market one and
once with an ACDelco remanufactured one. Neither time was it forced
onto the splines. I was careful to align the fat spline with its corresponding
slot in the cam. So, I don't think I got it wrong but if nothing else turns
up, I'll check it again.

No, I didn't use any Loctite® because I didn't know one was suppose to.
I can see that screws would work loose over time but would they come loose
when first started? Would this happen to two different opti's from two different
manufacturers exactly the same way? I don't know but I once heard that doing
the same thing over and over and expecting different results was one of the
symptoms of insanity. Anyway, at this point I really don't think it is the
distributor. Initially, I thought like you guys and that is why I went back in
and replaced the distributor. Just to be on the safe side, I pulled the timing
cover and checked the gears to make sure they were properly aligned when
I went back in the second time. All was correct. That is why I turned to
you guys for ideas.

I don't want to sound ungracious, but I initially thought the same as you guys
and that is why I went back in to the opti and the timing gears but I didn't find
any "smoking gun" so to speak.

Thanks for your ideas and I hope we will be able to diagnose the problem.

Good Luck!
-Blue Chips-
Webb

GaryDoug

In case it isn't obvious, you don't have to build a cable. Any OBD1/ALDL cable will work but will cost more. The one from Red Devil River is $55 shipped. There are others as well.

wlw-19958

Hi There,

Quite true but I don't mind waiting and saving the $40+. Also, I do have my
Genisys® scanner and I can graph the data on it. All I have to do is
to learn all the bell and whistles.


Good Luck
-Blue Chips-
Webb

wlw-19958

Hi There,

I finally got some time to run another scan on this beast. This time,
I recorded the BLM information. At idle, the BLM is "enabled" and is
16. When I start to accelerate, the BLM goes to 18 and stays there
(this is with throttle angles of 3% up to 85%). At throttle angle 85%
and above, the BLM "enable" changes from "yes" to "no" and the BLM
cell stays at 18. The only time the BLM changes from 18 to 15 is just for
one frame (when I came off WOT). Just to make clear, I lift my foot
off the accelerator during each up shift and the BLM stayed at 18 the
whole time except the one time I was in 4th gear and I lifted my foot
to coast briefly (as mentioned above).

I don't know what else I can tell you. Please ask questions and I'll tell
you what I can from the recordings I have.

I haven't tried the scan9495 yet. I'm unclear as to the procedure for
installing the USB drivers. The user guide is written for Windows 7 and
my laptop is still running XP.

Thanks for you help so far!

Good Luck!
-Blue Chips-
Webb

Injuneer

PM GaryDoug - he will respond to your question on the drivers, and the requirements for XP.

"16" and "18" are cell numbers. The BLM's are the numbers stored in that cell, for left and right engine banks. BLM enabled changes from yes to no because you have moved past the limits (combinations of RPM than throttle position) that put the PCM into "power enrichment" mode. As noted, it will use Cell 18 in PE mode, if the BLM'
s in the preceding cell were negative (below 128), or it will use Cell 15, but lock the BLM's at 128.

"BLM Enable" indicates whether the LTFT's will be adjusted based on the STFT's. When you are in PE mode, it is normal for this to indicate "no".

wlw-19958

Hi There,

Thanks for the response. I can understand about the WOT situation
(where the BLM dis-able and uses just cell18). What about at other
times? I mean, when the throttle is somewhere around 20% to 25%?
I was getting BLM cell 18 at those time too.

Also, someone suggested I check the fuel pressure while the engine
was running. I get 34psi at idle and when I disconnect the vacuum
from the regulator, it jumps to 42psi. Unfortunately, the hose on my
gauge doesn't permit me to see it while driving. Do they make extension
hoses for these or will I have to cut-n-splice a section of hose onto
my gauge (or buy another gauge)?

Good Luck!
-Blue Chips-
Webb

GaryDoug

The procedure should be the same for installing the drivers on WinXP. Download the FTDI file "CDM20824_Setup.exe" and run it. Or if the pc is internet-connected, just plug in the cable and let the pc find a driver.

Injuneer

"Engine running" fuel pressure, with and without vacuum line connected, are within GM spec.

Does the scanner indicate the PCM is in "closed loop enabled"? If not, it's going to operate in cells 16, 17 and 18. To enable closed loop, coolant temp has to be above ~140° F, both O2 sensors have to show "ready" and engine has to have been running for at least 206 seconds.