Can anyone tell me if these numbers are normal or not? My brother and I are trying to figure out why the truck doesn't run right. It's an S-10 with a painless harness and a mild LT1. It simply doesn't want to idle and dies within a few seconds of start up when cold.

Have 2 pcms on hand;
B-body pcm has an unknown tune... may be changed for the cc503 cam, lifters, no emissions etc.
The '95 F-body PCM is bone stock.

B-body...
DTC 29 - Air injection pump
Both o2 sensors are reading about the same .75-.08 Millivolts. ( I DO have leaky collectors that need to be fixed, not sure how that effects it)

Fuel Trim = 160 at idle, and drops to about 130 with foot on the floor revving.


F-body...
Barely idles when warm, won't run if motor is cold and had a fuel trim of ~200. It did throw a knock sensor code and reverse inhibit code.


Anyone have any input?

 

Do you have tunercat or LT1edit?

If so, you need to get a known-good tune and install it.

If not, you need to get a mail-order tune.

 

First of all, you'll need to use the correct size injector and fuel pressure that the PCM is tuned for. For the camaro PCM it's around 24lb/min @ 43.5psi

Secondly if it wont run right when started, it isnt the o2's causing the problem. They are on a warm up timer and don't take over in closed loop for about 3 minutes when cold and 42 seconds when warmed up.

I think 160 is the upper limit the pcm will adjust for with the stock tune, so you might actually be past that.

128 is stoich; when the PCM is neither adding nor pulling fuel to achieve the correct stoichiometric air/fuel mixture of 14.7 when in closed loop, based on the feeback from the o2's.

Anything above 128 is lean, so if you're getting a reading of 130 at WOT you're running lean and you could blow up your motor due to pre-detonation.
Also when you floor it, it should go into WOT or power enrichment mode and stop taking fueling commands from the o2 sensors, and refer to tables to proper fueling, so I'm not sure what the deal is with that.

When your fuel injector flow rate and pressure are dialed in correctly you'll see long term fuel trims of around 128 in closed loop.

You problem sounds fairly simple... get a fuel gauge to monitor fuel pressure, and use stock LT1 injectors (or equivelant), then re-diagnose.

Also your AIR pump code is because I assume you don't have that hooked up? It kicks on during open loop warm up and some other conditions to feed extra oxygen to the catalytic converter to better burn hydrocarbons and such.

 

Working on getting tunercat and Datamaster up and running so I can do some tuning and datalogging to figure out what it's REALLY doing. For now, just working off a scanner.

I guess I need to figure out how to determine what injectors I have. Motor was rebuilt before being put in the S-10, and I really doubt the stock injectors ended up back in it. They're orange The actual internals are kind of a mystery... I know what I was TOLD was in there, but the shop was a crook.

Thanks for the mention of o2s not working at first start up... didn't know that. And thanks for the explanation of 128 being stoich.. I had seen the 128 number, but hadn't stumbled on any other explanation.

No AIR, no cat, no egr... it's a pretty stripped down setup... not sure why it wasn't turned off/tuned out in the original tune.

 

If the injectors are larger or smaller than they should be, to make up for it you can adjust the fuel pressure using an adjustable fuel pressure regulator. But i'd just get the right size injectors. Also the 95 camaro uses a 3bar fuel pressure system. Most things now are 4bar (58psi), thats why you really need to check your fuel pressure.

 

Well, figured a couple things out.

Got Datamaster up and running, and took a datalog with the Bbody PCM.

The short term is staying right around 128-130 at idle.
The long term is 160, but with heavy revving, it drops.

Looked into the injectors, and apparently they are 20lb/hr

The .bin file is stock...@ 24lb

Looks like injectors are on the shopping list. Though, out of curiosity, I want to make a WOT pass to see what the duty cycle is.

 

If it's pegged at 160 it's probably actually past that, as 160 is the highest it will report and be able to compensate for (I believe).

The short terms will always fluctuate slightly above and below 128 once the long terms are 'learned", or have dialed themselves in.

You want your long term fuel trims to be right around 128. This means that your fueling pressure/flow rate are correct so it's not relying heavily on the computer to make necissary fueling changes in order to maintain stoichiometry in closed loop.
So either adjust the injector size, or fuel pressure, either way you should also adjust the 'injector constant' in the tuning to what your actual flow rate is (tunercats, lt1 edit).

 

Stock programming for the stock LT1 injectors is 24.9 #/HR

But what the PCM learns from the O2 sensors in closed loop is used by the PCM to fuel the engine on cold start. So bad O2 sensors can affect startup.

160 is the upper limit, but the fact that his short terms are staying close to 128 indicates that the long terms are making the required corrections. If the long terms had max'd out at 160 and it was still running lean, the short terms would increase significantly above 128 to add the required extra fuel.

130 means it WOULD be running lean if the PCM hadn't raised the long term corrections to 130. Once the long terms are stable its no longer running lean, and he's not going to increase the liklihood of detonation (knock). There's preignition (ping - mixture ignites before the plug fires) and detonation (knock - mixture auto-ignites a second flame front in the combustion chamber after the plug fires), but there is no "predetonation".

When you exceed the limits set for closed loop in terms of throttle position vs. RPM, it goes into power enrichment mode. While it no longer takes feedback from the O2 sensors, and can not "learn" in power enrichment mode, it is going to use what it learned in closed loop to fuel the car, in most cases. If the PCM was adding fuel in Cell 15 in closed loop, its going to use the Cell 15 long terms in PE mode. If the PCM was cutting fuel in Cell 15 in closed loop, its either going to lock the long terms at 128 and continue to operate in Cell 15, or its going to switch to Cell 18 and lock the long terms at 128. In the case of a 160 BLM in Cell 15 for example, the PCM will use 160 as the block learn multiplier in PE mode and over-fuel the engine by 25%. While it calculates the PE mode target A/F ratio "from tables", it still fuels in many cases with the BLM's.


It can't run in closed loop, because the extra air flowing past the O2 sensors would create a false lean condition and cause the PCM to dump in more fuel. The LT1 is not like the 3rd Gens, which had the AIR pipes routed to the manifold, and also between the reduction and oxidizing beds of the cat, and the pump was diverted from the manifold to the cat after the engine warmed up and the ECM switched to closed loop. The monolithic cat in the LT1 does not need supplemental air for the oxidizing bed.

There are at least 18 different sets of BLM's stored, for various engine operating conditions. When the engine is idling, its in Cell 16, and uses those BLM's. Once you move the throttle, it moves to a different Cell, and uses different BLM's. That's why yours are changing when you rev it.

There's a section in my online Scanner writeup explaining how all this works, and what your data should look like:

http://www.injuneer.com/ScanMast.html

The 160 BLM indicates the PCM is adding 25% extra fuel to keep the engine from running lean. If you are running 20 #/HR injectors, and the PCM has the stock injector program value (24.9 #/HR), you have the explanation for the 160 BLM's right there. 20 x 1.25 = 25. It has to use the BLM's to increase the injector pulse width by 25% just to get the 20# injector to supply the amount of fuel the stock injector would supply.

What do you think you will see if you check the duty cycle?

As noted above, if the PCM raises the long term to 160 (max limit), and its still running lean, it will start elevating the short terms until it gets enough fuel. He's at/near 128 on the short terms, so the 160 long term is doing the job.

 

Thank you for clearing up what I said about the BLM's.
A BLM > 128 indicates a lean condition IF corrections weren't being made; or rather it shows the degree of corrections being made.

 

Well, adjusted the tune to match the yellow top 20lb injectors that it has and took it out for a datalog. This screen cap is the worst it got, but it passed 90% duty cycle at about 3500rpm.

Don't pay too much attention, as I already have some bigger injectors, I just wanted to see what it had been running like.

 

The duty cycles calculated by DataMaster seem to be too high. They do the correct calculation, involving pulse width and RPM, but they always use "BPW", which appears to be some sort of base point from which the PCM developes tha actual PW. Don't know if that's correct, but I've seen DataMaster logs that indicate more than 100% DC, without the engine running lean.