I have a few questions about a few of the topics you talked about.
1. What kind of exhaust are people getting good results with? Also is it even worth the hassel (im sure its a pain in the ass) to do headers too. Can you even get to all the manifold bolts without lifting it up a little bit?
2. Intake. more specifically, throttle bodies and tb spacers. It seems every time I read about whether or not the spacers actually help, it seems to be mixed reviews. Any input or experience on that?
3. PCM tuning. What do you use to tune with on these vehicles? Im an import and bike guy. So I'm used to a standalone or a power commander.
I have an intermittent sputter right now, but I'm sure it just because the o2 sensors are reading all sorts of whacky. I gutted my cat (
)because it came loose and was rattling around. I would hate to see my AFRs and I dred looking at the plugs when I pull them out. I work 60hrs a week, ride my bike alot, and just didnt want to spend the money until I knew what my plans were.
Thanks alot for the help.
1. You can put a set of headers, but not worth the hassle and money you spend on installing them. And yes you have to disconnect the engine mount to raise the engine to clear parts in the engine bay to swap the exhaust manifold and i your not lucky, most cases the exhaust manifold gasket fails is what i have found.
2. You have a 2000, so forget about the Throttle body spacers. The spacers work great in the TBI system, but a waste of money and time on the SCFI system that you have. The poppet injection nozzle is in your upper intake platinum in-front of your intake ports on the heads, so the spacers really do nothing for performance.
If you would like to get a better throttle response, remove the throttle body blade restrictive plate and the MAF sensor screen. The other improvement also is to modify your air cleaner intake box. When you open up your air cleaner box, you'll see the restricted tube. Just simply cut out the front area of the box where the tube is and make sure you cut below where the air cleaner sit's or your going to trash your air cleaner box. (only held on with one bolt)
http://gmcowners.com/gmc-forum/showthread.php?t=10246
3. I am using HP tuner on my Jimmy since i am a tuner junky and i found that the 96 and 97 is not supported by the HP tuner so i had to get creative by updating my PCM system with a newer PCM. You 2000 Jimmy will be just a plug and play system for the Hp tuner, but it's expensive so you may want to just send your PCM out to have it tuned by
http://www.pcmforless.com/.
You will find difference in the performance and economy to your Jimmy. If you have any question about the tune you can call them up and they will be more then happy to answer any questions you may have, and it's the PCM tuning that will make the biggest improvements after you have modified your exhaust and intake.
As for your exhaust, you really need to replace your Catalytic converter, a bad converter will set your check engine light on and will fail your next E test. I am running a full
http://www.magnaflow.com/ system including the Catalytic converter. I believe i have the 3 inch cat that goes in to a 2.5 inch single exhaust that run the same as the stock pipe did with a more aggressive tone.
As for your engine having a sputter, good chance that it's related to the Catalytic converter being gutted out and your pcm is running on a open loop.
PCM Basics 101:
Executive summary: The PCM controls the fuel injectors by using signals from the engine sensors, like MAP, MAF, RPM, ECT, and others to pick out the right value from the right lookup table to use in the fuel injector pulse-width calculation. The PCM injector pulse-width calculation also uses the INT and BLM correction factors based on O2 sensor feedback when it is in closed loop. O2 sensor correction is not used in open loop.
Note that there are 2 kinds of open loop. Right after a cold or hot start, a scan tool will report that the PCM is using open loop fuel control. This version of open loop is described in the next section. Once the PCM decides that conditions are right, it switches to closed loop fuel control, which is described below the open loop discussion. But even after this switch, if you lean on the throttle hard enough, the PCM will switch to power enrichment (PE) mode, which is another form of open loop. In PE mode the scan tool may report that the PCM is in closed loop, but it should also report that learning is disabled. Learning disabled means that the PCM is ignoring the O2 sensors, which is the same as saying that the PCM is using a version of open loop fuel control.
Open loop
When you fire the beast up, whether the engine is stone cold or fully warmed up, the PCM starts out in open loop. This is because cold (or just warm) O2 sensors don't work very well, and they take a little while to get good and hot. Most LT1 O2 sensors have built-in heaters to speed up the process, which is why they have 4 wires coming out of them. Also, cold engines are happier when they are run richer than the fixed closed loop air/fuel ratio target. Since the O2 sensor signal is not reliable right after the engine is started, the PCM has no way to find out whether the engine is running too rich, too lean, or about right. But it still needs to take a stab at the right injector pulse-width.
When the engine is running in open loop at light to moderate loads (idling, city driving, highway cruising), the PCM uses the AFR table for the pulse-width calculation. The AFR table uses the ECT (colder engines need to be richer than warm engines) and MAP (higher loads require a richer mix than light loads) sensor inputs to point to the desired AFR for those temperature and load conditions. As always, the MAF sensor tells the PCM how much air the engine is inhaling. The PCM knows how big the injectors are (the amount of fuel is delivered per millisecond of pulse-width), so with the MAF sensor output (the amount of air inhaled by the engine) and the AFR from the table (the desired air/fuel ratio), it can estimate its best-guess injector pulse-width. Remember that the PCM has no idea whether or not this guess-timated pulse-width is anywhere near right. It could be causing the engine to be very rich, or very lean, or anywhere in between.
Remember that engines like a richer air/fuel mixture when they are cold (richer than the fixed 14.7:1 ratio used in closed loop), and when they are asked to make more than just a cruising level of power. So the AFR table values for high MAP (heavy load) and low ECT (cold engine) are lower (richer) than the values for low MAP and normal operating ECT. This means that "cold engine enrichment" and a bit of "power enrichment" are built right in to the table. This quasi-power enrichment may be used in open loop when you maintain your freeway speed up a steep hill in high gear.
When is the engine ready to switch from open loop to closed loop? The current theory is that the PCM decides to switch based on some combination of ECT and engine running time (3 minutes?).
Closed loop
When the engine temperature and time-since-the-engine-started timer agree that the O2 sensors are warmed up and ready, the PCM switches to closed loop (at least this how I think it works). Just as it did for open loop, the PCM calculates the closed loop base pulse-width using numbers programmed in lookup tables.
While the target open loop air/fuel ratios are contained in the AFR table, the closed loop air/fuel ratio is hard-coded in the PCM to be 14.7:1. The MAF sensor tell the PCM how much air the engine is inhaling, the injector constant and injector-offset vs. voltage values tell the PCM how much fuel the injector will flow for any particular pulse-width, so the PCM just does the math to calculate the appropriate injector pulse-width.
Closed loop operation may refine the injector pulse-width calculation using the 2-dimensional VE tables, which use MAP and RPM as the 2 inputs. In the 94-95 f-body (OBD-I) PCM there are 3 different VE tables, one for cranking/starting (0-340 rpm), one for low rpm (400-2000 rpm), and the third for high rpm (2000-7000 rpm). If used, these tables would account for the fact that an engine does not necessarily use all the air it inhales for combustion. For example, engines with long duration cams pass some of the inhaled air right out the exhaust valve during overlap.
Even in closed loop, the injector pulse-width calculation is really just a guess based on the MAF sensor, the injector constant and offset, and (possibly) VE table numbers. If the pulse-width is not right, the fuel/air mixture will be too rich or too lean. It's very important to remember that the PCM tries to keep the AFR at the "ideal" ratio of 14.7 while it's in closed loop.
This is where the O2 sensors come in. They generate an electrical signal for the PCM that represents the amount of oxygen in the exhaust stream, which is proportional to the air/fuel ratio. This signal, which is the feedback that "closes the loop", tells the PCM how good a job it's doing on the fuel mixture. The PCM uses this feedback to calculate a correction number, sometimes called the integrator, or INT for short, or (more accurately IMHO) the short term fuel offset. The PCM uses the short term fuel offset to adjust the pulse-width calculation so that the air/fuel ratio homes in on 14.7:1.
I hope this helps you get a better grip on your Jimmy's PCM.
since the 4L60E transmission has a very low first gear.