mad-hatter

iTrader: (3)

I have a 2000 model year, built 11/1999, Z28 6MT. MAF has been replaced, K&N filter, baffles removed from lid, MSD wires, brand new TR5IX plugs, and a flowmaster cat back. Initially, I had hesitation around 2500 rpm and ****-poor performance from 2500 up to 5000/5500. Replaced the maf, so the hesitation is now and pull to 5500 is great. However, from the day I got the car to now, at 5500 rpm to red line under load, the car falls on it's face. Pulls great to 5500, but you can physically feel it slowing down it's pull. Is this something in the stock tune that chevy did for "safety", or is deeper than that? LS1 with 853 heads.

I do have codes for cat efficiency, clutch switch, and secondary air injection.

Thoughts?

JD_AMG

Definitely shouldn't be a "Factory safety thing".
While the power starts to drop off around 5500RPMs on a stock LS1 it shouldn't be that noticeable.
When you said you had a K&N filter, that threw a red flag for me. If you over oil them the oil can get sucked into the MAF and coat the MAF wires, causing issues. Try replacing with a paper filter, clean the MAF and see if there is a difference.
If not then the next step I would do is get the codes fixed. The ECU may be doing something like pulling timing due to the cat and air code.

RPM WS6

iTrader: (3)

Have you checked fuel pressure under load? There shouldn't be much drop off in power from 5000rpm-6200rpm (stock rev limit) on a stock engine.

I agree with the above comments about the K&N, I don't use those on MAF-equipped vehicles for this very reason. But it sounds like you fixed the MAF portion of your issue by replacing it, and yet the 5500rpm+ problem still remained, so I doubt these two issues are related.

There is no "safety" setting at any specific rpm, other than the stock rev limiter (6200), that would pull power - even with those DTCs. FWIW, I had the cat inefficiency code for about a year before I disabled it, and there was no difference in power at any rpm with or without the code.

I would start by checking fuel pressure under load, and looking at several PCM values with a comprehensive OBD scanner (such as fuel trims, O2 readings and timing advance for starters.)

mad-hatter

iTrader: (3)

I'm willing to rule out that it's pulling any timing due to the codes as when they are cleared, the issue still remains... If I can get my hands on a decent scanner, I'll take a look at fuel pressure. Part of me believes that it may be my injectors...but at 130k miles?

FormulaZR

iTrader: (7)

Maybe, but a clogged cat will still cause performance issues. Clearing a code won't change that.

RPM WS6

iTrader: (3)

A scanner won't tell you anything about fuel pressure specifically, you need a fuel pressure gauge. Scanner will only show rich/lean based on O2 values and fuel trims, but lean conditions can be due to more than just low fuel pressure.

A cat inefficiency code does not directly indicate any sort of clog at all, I had this code for over a year with the stock cats still in place and there was no clog of any kind. In fact, you would get this same code (P0420/P0430) if you removed the cats entirely and didn't install O2 sims or a custom tune. It just means that the oxygen storage capacity of the cat has been reduced due to coatings on the substrate being fouled or worn/burned off. There may or may not be a clog, but these "inefficiency" codes don't confirm or rule it out either way as they do not indicate any measurement of flow.

myk

iTrader: (4)

Check the fuel pressure...

FormulaZR

iTrader: (7)

Agreed 100%, but based on symptoms it something worth thinking about.

davidcroft

You said you have MSD wires? Stock wires are good up to 1000 rwhp and more. I've seen and heard of random weird misses and loss of power with no explanation and it was the MSD wires. I would personally start there.

Vmack

Has a fuel filter every been changed?

N20zuki

Do you have a check engine light on?

mad-hatter

iTrader: (3)

It was worse on the old plugs and wires. Original plugs and wires with 130k miles on them. MSD wires and NGK plugs...idles a lot better, better throttle response, and it's running better. 0 misfires on the new setup. Had intermittent misfires on the originals.

That I don't know of. Previous owner provided service records, but I can't recall seeing it having been done. I plan on picking one up and replacing it this upcoming week. Previous owner ran this car on a tight budget...way to tight. 100 dollar clutch kit, original plugs and wires, and sold it on 4x bald NT555's. And by bald...I mean 100% slicks.

Yes...see original post.

FormulaZR

iTrader: (7)

Pull the front O2 sensors and see if it revs any better. If it does, it indicates a restrictive cat.

DANOZ28

iTrader: (1)

are you getting any fluttering or sputtering? all i know is my stock 99 springs didnt work well at 5700 rpm. just a thought.

RPM WS6

iTrader: (3)

Has fuel pressure been checked at WOT yet?

mad-hatter

iTrader: (3)

Haven't checked fuel pressure...haven't had an opportunity, but will be checking in the next couple of days. Fuel filter replaced...runs much better. Still has some top end issues, but I'm now moving forward in the fuel system. Injectors are most likely dirty and/or clogged...after seeing the nasty crap coming out of my fuel filter.

Sadly, I have bigger fish to fry than sorting out top end issues... Got hit in a parking lot this evening by some f-in moron not paying attention. Scratched the hell out of the door and put a basketball sized dent in my drivers quarter panel right where the quarter panel meets the door. Accident report has been filed...2nd party admitted fault...contacting insurance in the morning. Unsure where it will go from here. However, until it is time to take it to a body shop, I will continue looking into this issue. Updates to follow.

Thanks for all the help thus far.

I'll attach pictures in a minute when I can get on my computer.

EDIT...pictures attached.

R6cowboy

iTrader: (3)

Wow, that's dirty as hell. Never seen fuel that discolored come out of a filter before. I'd say its more than likely the injectors are clogged, or simply need replaced. Sorry to hear about the parking lot incident by the way. Hope it gets covered and fixed up for ya.

mad-hatter

iTrader: (3)

The previous owner wasn't kind to this car. Treated it like garbage and didn't take care of it. Owned it on a budget. Definitely the wrong car and kind of car to run on a tight budget.

I plan on dropping the tank at some point and flushing my entire system. If I have the time and space to do it, I may attempt it when I pull the injectors.

As a side note...after clearing the codes on the car...the cat code didn't come back after about 20 miles of hard driving. Might not have waiting long enough, but generally the cat code is back on within about 5 minutes of just cruising. Secondary air injection code also went away...also may not have been long enough to report a code. *shrug*

R6cowboy

iTrader: (3)

Dropping the tank for inspection and cleaning is a must idea in my opinion. I'd definitely do that as soon as possible, then run a good fuel system cleaner when back together to help clean the injectors. Fuel that contaminated coming out of the inline filter will definitely cause the car to run like ****, among cause other problems as well.

RPM WS6

iTrader: (3)

If the filter was extremely clogged, then it might have been restricting flow and thus causing a lean condition at WOT/high rpms. The pump may have been damaged somewhat over time, so even with a clean filter it still may not be producing the proper pressure under full load to support fuel needs at high rpm.

Before tearing into the injectors, I'd still recommend checking pressure.