SS#502

Having some weird issues with car after warm up and I was reading that the MAF can be disconnected while it's running for troubleshooting. So can the car be driven with it disconnected? Issue with car is after it warms up, about 10-12 minutes, it stumbles and I lose my tach signal but still runs and feels like it's trying to shutdown. If I hold gas pedal in steady position and throttle threw eventually it comes back to normal after 30-45 seconds... I've changed every sensor but the MAF and the MAP, cleaned them both and reinstalled. New opti, wires, plugs, coil, Icm and O2's (ACDELCO's), original PCM with no tune and 157K miles. Any help would be greatly appreciated!

hrcslam

Yes, if your VE tables are good it'll run. With your mods (according to your sig) it should run and drive on the stock tune.

Catmaigne

iTrader: (6)

It will stay in open loop but it should run fine.

hrcslam

This is not true. It will go into closed loop with the MAF disconnected.

If you have a MAF failure (like disconnecting it) it will revert to SD mode using the VE tables to calculate injector duty cycle then it uses the O2's to fine tune stoich AFR.

Catmaigne

iTrader: (6)

I thought SD was disabled in the stock tunes for MAF cars. I noticed the scalar is unchecked in tunerpro but I guess it reverts back to it with no MAF input? The more ya know...

So if it runs fine without the MAF plugged then that would be your culprit. If not there's the MAP and IAT to look at.

SS#502

Ok, so I cleared the codes, disconnected the MAF and it drives better but now after it warms up it does a "HARD STUMBLE" upon warmup and then reverts back to normal throttle response with no loss of the Tach signal now. I scanned the PCM again and only codes for the MAF P0100 and P0412. No misfire codes for O2's like before. I'm guessing with the Cats aren't getting hot enough @warmup to burn off unspent gasses and the O2's sense that and cause PCM to lean it out. Would that sound right???

hrcslam

EDIT: I mis-read your post. Check the ICM. They tend to have a heat soak related issue. They'll pass the bench tests, but then fail when hot on the head. Loss of Tach with stumbling sound ignition related, only after warmed up leads me to guess the ICM is about done.

You can try spacing the ICM off the head a little bit to avoid heat soak and see if that improves things.

hrcslam

The VE tables are used in MAF mode, but not primarily. The VE tables serve to provide the engine with a back up if the MAF fails. But they are also referenced in MAF mode for sudden WOT and some other scenarios. I'm not sure the exact algorithm though. I wish I wrote it down that one time I saw it.

After I did my engine build I did my VE tables in MAF mode and it made a HUGE difference in drivability under 2K. I don't know why because the car should be using the MAF in that scenario, but the VE tables were referenced enough to effect how it drove.

SD mode ignores the MAF completely.

SS#502

Thanks hrcslam, I thought about the ICM too and did the spacer trick a while ago. I've replaced the ICM like 5-6 times and just put a new one on about 2 weeks ago. This miss and stumble has been an on going problem. I live in Hawaii and no one put here tunes LT1's anymore. What would be a good program to use with Apple products if you know of any???

hrcslam

I'm not sure about the OBD2 tuning software. The most popular approaches to tuning an OBD2 LT1 that I've seen are, Mail order tuning (not sure who), 24x EFI Connections/ LS1 PCM swap, or OBD1 conversion.

What brand ICM did you go with? Seems when it comes to the LT1 ignition system, going OEM (AC delco , Delphi, GM) is your best bet; they are double the cost, but more than double the reliability. My entire ignition system is still stock minus plugs. If you don't plan on going more than 7K rpm keep it stock IMO.