LS3 Swap in 1999 C5 - REP Mode Problem

[steve.king]

I've replaced the LS1 in a 1999 C5 with a 418 LS3 with L92 heads, L76 intake, 231/245 dur, 2008 Z06 intake duct and MAF, 90mm ETC, 50lb injectors, etc. It's running a 6 speed manual. The car runs great up to about 5500 rpm then suddenly the throttle slams shut and it goes into some kind of limp home reduced power mode. Sometimes it pops in the exhaust. You can keep the engine running, but it runs really bad. Under hard acceleration it will do it every time. If I accelerate the engine gently, it will rev to 6500+ and not crap out.

The ECM is still alive and communicating during all this. If I key off and restart the engine it runs great again. NO fault codes are set. UEGO trace looks good. Spark timing trace looks good. Throttle goes to 100% during KOEO mode so the TAC module and all seem to be working OK. The throttle scalar was changed from 0.0255 to 0.0192 to accomodate the larger throttle.

I've talked to several experienced tuners in the area, even HPTuners, and no one has experienced this problem before. I was so frustrated with the car I gave it to a tuner for a week and he couldn't find the problem. I think it's the calibration, others say it's hardware.

I've swapped out the PCM and throttle with no luck. I've flashed the factory VIN calibration back in the PCM and started over with no luck. I've swapped to a different MAF and intake duct with no luck. Although this did seem to change where the problem occurred.

I'm leaning towards a MAF/throttle/MAP scaling or compatability problem. I know the PCM measures and calculates air flow using all 3 methods and compares them. The compare table is maxed out at 34. The engine doesn't have to be WOT for the problem to occur, but the MAF reads about 8500 Hz almost every time. The MAF frequency traces on a scope look significantly different between the stock MAF and 2008 MAF, so I figured this might be the problem. I installed the older style 2007 MAF and the engine still shuts down, but the problem occurs at 10,500 Hz instead of 8500 Hz. I disconnected the MAF completely and the engine still shuts down at about 5500 rpm and it doesn't set a fault of any type, not even a MAF fault. This didn't seem right to me.

Has anyone seen or heard of this problem before? What is the maximum MAF frequency for the 1999 PCM? I've heard the 2002 PCM is much improved and pin-for-pin compatible, but is there a reason to upgrade? I've been working on this for several months now and getting desperate. This seemed like a pretty basic and common engine swap when I started, but it's not turning out that way. Any ideas would be greatly appreciated.

[LSX378]

I've replaced the LS1 in a 1999 C5 with a 418 LS3 with L92 heads, L76 intake, 231/245 dur, 2008 Z06 intake duct and MAF, 90mm ETC, 50lb injectors, etc. It's running a 6 speed manual. The car runs great up to about 5500 rpm then suddenly the throttle slams shut and it goes into some kind of limp home reduced power mode. Sometimes it pops in the exhaust. You can keep the engine running, but it runs really bad. Under hard acceleration it will do it every time. If I accelerate the engine gently, it will rev to 6500+ and not crap out.

The ECM is still alive and communicating during all this. If I key off and restart the engine it runs great again. NO fault codes are set. UEGO trace looks good. Spark timing trace looks good. Throttle goes to 100% during KOEO mode so the TAC module and all seem to be working OK. The throttle scalar was changed from 0.0255 to 0.0192 to accomodate the larger throttle.

I've talked to several experienced tuners in the area, even HPTuners, and no one has experienced this problem before. I was so frustrated with the car I gave it to a tuner for a week and he couldn't find the problem. I think it's the calibration, others say it's hardware.

I've swapped out the PCM and throttle with no luck. I've flashed the factory VIN calibration back in the PCM and started over with no luck. I've swapped to a different MAF and intake duct with no luck. Although this did seem to change where the problem occurred.

I'm leaning towards a MAF/throttle/MAP scaling or compatability problem. I know the PCM measures and calculates air flow using all 3 methods and compares them. The compare table is maxed out at 34. The engine doesn't have to be WOT for the problem to occur, but the MAF reads about 8500 Hz almost every time. The MAF frequency traces on a scope look significantly different between the stock MAF and 2008 MAF, so I figured this might be the problem. I installed the older style 2007 MAF and the engine still shuts down, but the problem occurs at 10,500 Hz instead of 8500 Hz. I disconnected the MAF completely and the engine still shuts down at about 5500 rpm and it doesn't set a fault of any type, not even a MAF fault. This didn't seem right to me.

Has anyone seen or heard of this problem before? What is the maximum MAF frequency for the 1999 PCM? I've heard the 2002 PCM is much improved and pin-for-pin compatible, but is there a reason to upgrade? I've been working on this for several months now and getting desperate. This seemed like a pretty basic and common engine swap when I started, but it's not turning out that way. Any ideas would be greatly appreciated.

So you are running a 99 PCM with an LS3 stock ETC?

[steve.king]

It's a stock '99 PCM with a PN 12570790 throttle, which is for a 2008 LS3. This is the one with the silver throttle blade, not brass. The throttle seems to work fine, although it surges a little at idle, but that's tunable. Also running the stock pedal.

[Chevy406]

Engine Diagnostics > TPS Sensor > Calc Airflow - P1514/P0068 Error

The ECM is comparing actual airflow to calculated airflow at various throttle positions. If the actual airflow differs by the amount in this table, then you the ECM goes to Reduced Power mode. With your larger engine you need to increase the values in this table. Start by multiplying the entire table by 1.1 and keep adding if required.

The maximum MAF airflow in a '99 ECM is ~ 54lb/min and maximum frequency is 14,000 Hz.

By the way, post your tune so we can make better informed suggestions.

[steve.king]

This Calc Airflow table is max'd out at 4.1. Plus no faults are being set. I'm at 50 lb/min with my current flow sensor at 5500 rpm so it doesn't look like I can get by the the '99 sensor.

The tune is attached. Log files are attached. The UEGO file shows the conditions when the when the engine problem occured at the top of the page along with a typical PHI trace. The engine was 15% rich when the event occured. I have other traces, but this one is typical.

Thanks for your input.

[steve.king]

bump for help