Hi,

I recently inherited my friend's 1995 Z28 that had some motor problems. I ended up putting in a bone stock low mileage used LT1 and bolted on the mods the previous LT1 already had.

Here is the log:

http://dl.dropbox.com/u/8912859/1995...8_89Octane.uni

The motor seems to run pretty well, but since I'm new to LT1's, I'm not sure how much better it can be. I've read a bit about tuning the PCM, and shooting for 128 BLM on both banks. My log shows that in a few cells, but not very many. It seems to be all over the place.

I was running 89 octane instead of 93 like I should be, which might account for some of the extra knock and pulled timing?

The PCM had been in for a few hundred miles since the last reset, so it should have had time to tune itself.

This PCM is stock (I believe). I have the PCM from the old engine that has a tune from madz28.com, but I am interested in understanding what is going on a bit better before putting the tuned up PCM in.

Mods include:
Electric water pump
Hooker Long tube headers
3" exhaust through a flowmaster
Cold air Intake
Throttlebody bypass mod
Various non-engine mods (clutch, etc.)
New O2 Sensors.
New Plugs/Wires (OTVC This time)
Photos: https://picasaweb.google.com/1137637...38899633040082

If anyone can offer any insight, I'd appreciate it.

 

I'll take a look at it. Did you delete EGR (DTC 32), and did you delete AC (DTC 66)?

Let us know how the header wrap works out. Ever since a guy named Tom Byrne's LT1 Z28 burned up many years ago, due to oil soaked header wrap, everyone has shied away from wrap.

 

I appreciate you taking time to look at it for me!

As for the header wrap... A friend who knows much more about race motors than I do warned against vaporlock, etc, and after reading quite a few horror stories I decided it was good to just throw some paint on them to try and inhibit the rust. I'll spring for a set of stainless on my next engine swap.

Like most everyone else, I have aspirations to go for a 383 build on my old block when I can afford it. New baby in the house puts those plans off for a while. Luckily the car came to me with many suspension mods, upgraded t56, clutch, flywheel, drive shaft, etc, so it is ready for some more power when I can manage it. I need to get the stock motor figured out first though.

Worth mentioning is that I have an MSD 6A mounted up, but I have not wired it in to the new motor yet, so I'm on stock ignition and stock coil.

Thanks.

 

Oops, forgot to answer the question...

I have not charged up the AC yet, but I was not getting that code until recently, so something may have gone wrong.

The EGR Code just started showing, and it is intermittent.

I have the full AC system, and full EGR and AIR System in place. I have a spare of everything, so I need to go ahead and swap that out.

Those codes will not put the motor into limp mode will they?

 

DTC 32 means the PCM cycled the EGR valve, and didn't see the expected change in manifold absolute pressure (MAP). Could be a failed EGR valve, plugged EGR passages, leaking EGR vacuum line or a plugged EGR vacuum solenoid.

DTC 66 indicates a problem with the A/C pressure sensor circuit being open or shorted. Could be the harness connector came off. Code sets when the pressure is below -8psi (0.12V) for 10 seconds.

Looking at your data log:
-the knock retard is almost certainly a calculated retard based on the extended use of low octane fuel.

-The things that look OK include the TPS sensor, CLT sensor, IAT sensor, response to the IAC motor, MAP sensor, MAF sensor readings. The engine appears to idle at the correct/programmed speed, which appears to be the stock programming. The PCM control transition from open loop to closed loop exactly where it's supposed to transition, at 206 seconds from startup.

You already picked up on the screwy readings - the BLM's. They are all over the place, with some cells adding fuel, but most of them pulling fuel. When you go into PE (power enrichment = pretty much wide open throttle) mode, the BLM's default to 128, because the PCM is pulling fuel in the cells used before it entered PE. Looking at the O2 sensor readings at WOT, it appears that the right bank reading is typically about 30mV higher (= richer) than the left bank reading. This is consistant with the pattern of BLM's, because in general the right bank BLM's are lower than the left bank, meaning its running richer on the right bank, forcing the lower BLM's to pull it back to 14.7:1.

At this point, you need to track down what is causing the erratic BLM's. You do that before you tune it. Trying to bring the BLM's to 128 without identifying the cause of the moderate split between the left and right side, and the fact its subtracting fuel in several cells (3, 6, 7, 10), while not so bad in others (14, 15, 16), and then the oddball Cell 2 adding fuel (but thats closed throttle decel transition causing that) will not produce the correct results.

Forget about the term "limp mode". Limp mode means the PCM has lost virtually all communication with the outside world, and is using some very basic settings to keep the engine (just barely) running. In general, a couple codes will not produce that result. Each code has a specific effect on the engine. Looking at the 94 factory manual (95 will be similar) for the codes:

DTC 32 - ACTION TAKEN - The malfunction indicator light will illuminate.

DTC 66 - ACTION TAKEN - DTC 66 stored in PCM memory but will not turn ON the malfunction indicator lamp, A/C operation will be disabled....

So, no problems.

 

Thanks for the info, it is much appreciated.

I replaced my O2 sensors recently since they were in unknown condition... used motor and all. It was running really poor. While changing out the plugs, I noticed some of the plug wires were burned on the headers and arcing.

I ran with at least 2 bad plugs on the right side of the motor for almost 100 miles while trying to troubleshoot other things... Maybe I fouled the O2 sensor on that side, which is causing rich readings? Typically a fouled O2 would read extra lean and cause rich condition, right?

I think I'll try to swap them left to right and see if the split changes sides. It'll be Sunday before I can get around to that

I'll report back when I know more. Thanks again.

 

Looking at the o2 data in this form helps (exported from DataMaster EE as CSV, graphed in excel):

http://dl.dropbox.com/u/8912859/1995...8_89Octane.pdf

The Right O2 sensor is definitely reading higher most of the time.

I know some oscillation around stoich is normal, but does this look extreme?

 

So, I charged up the AC this weekend, and in the process managed to realize that what I previously thought was a rattle in the dash was actually a knock on the left bank of the motor.

Knock counts are not increasing under normal operating conditions, but the noise starts around 2000rpm and on up.

My last motor had bent push rods, one that was particularly bad. Could something like that cause my wacky BLMs?

I'll be popping the valve cover off soon to see if I notice anything obvious...

 

"Knock Count" doesn't really have anything to do with the actual noise in the engine. It seems to be a calculated value based on engine operating conditions. You would look at "knock retard" to see if the PCM is pulling timing in response to the knock sensor. The knock sensor is "tuned" to actual spark knock, and then the knock module filters the signal further. A rod knock might not be picked up by the "spark knock" sensor.

BLM's get screwed up when there is oxygen present that isn't supposed to be there. Anything that dumps air into the the exhaust - like an exhaust valve that isn't closing - will screw up the O2 sensor readings, and then the BLM's.

 

Thanks. I'll pull the valve covers and check things out. Something is going on in there.

There is a lot of spark retard, whereas looking at older logs did not really show any except on really fast decel from high rpms.

currently, 5 degrees under various conditions is pretty typical spark retard on my recent log