Hi all,
I have been doing LT1 swaps in Porsche 944's, 951's and 968's for a few years now. I've been using low mileage junkyard engines, and only OBD1 LT1s. Until now all of them have started up and ran great after fixing some settings in the PCM. My most recent one though has been having some issues, and I'm not very familliar with Chevy stuff since I've been working exclusively on German cars for the past 30 years or so.

At about 5100 rpm the engine seems to missfire very badly, almost as if it's hitting the rev limit, but I have the rev limit set to 6000. It does move arround a little bit, sometimes it will miss slightly at 4800 then get worse as it gets up to 5100, sometimes it will go to 5300 or so, and every once in a while it will rev cleanly right up to the 6000 rev limit. Also (I don't know if it's related or not, but I figure I'll give all the info I have) if I push the clutch while decelorating the engine will almost always die, or if it doesn't it just barely catches itself before it dies.

I have a DataMaster log of some accelation runs from earlier this morning that I'll upload in about fifteen minutes and I'll post a link to them. As for details on the swap, I'm using all stock sensors except the MAF which I'm deleting, and I just did a new optispark a few days ago (trying to fix this miss). I have TunerCat for making all the needed changes in the PCM. I'm deleting the MAF, deleting the anti-theft, and raising the rev-limit a touch, but thats about it.

Please help!

 

Ok, heres the link to the DataMaster log:

6-7-04 968

Also during that run I had a fuel pressure gauge and the pressure did not drop a touch.

 

I have the same prob man

I beleive your problem is a four letter word that is spoken of very much around here...........Opti

 

Heh, too bad for you, but actually I just replaced the opti yesterday and it does the exact thing.

 

Well Let me know what you find...........when you figure it out

 

z28.com forums said it's the coil most likely, so I'll change that tomorrow and let you know.

 

Is it a missfire, or more of a hesitation? Does it feel like you're running outta fuel up top?

Mine was doing that up untill the other day. Turns out the dead O2 sensor that I already knew about was the cause. I had dismissed them since the O2's aren't used at WOT, but it was obviously the problem. I watched the O2 millivolts on my Scanmaster as I went through the gears, and they'd start out at 920 like they were supposed to, but would plummet down to the 400's after about 3k rpm's. It only became a problem in the higher gears, most notibly at the top of third, since the millivolts had a longer time to drop. It felt just like I was hitting a fuel cut-off.

Put in new O2's the other day, and now it runs hard all the way to redline. O2 millivolts never drop below 910 even at redline now either.

This may not be your prob, just throwing out a possibility.

 

i had the same prob, felt as if it was hitting the rev limiter. replaced plugs/wires and did intake manifold leak fix and solved the problem. Im pretty sure mine was the intake manifold leak. Oil was reducing the octane rating in my fuekl causing knock and this 'misfire/hesitation'

 

I love how LT1's have so many ways to have the same exact symptoms.

I think my dead O2 was causing it to run in open loop mode, and off the pre-programmed fuel tables. Since my O2 millivolts almost always dropped around the same RPM, I'm wondering if the fuel maps that were pre-set in it only went to a certain RPM? Does anyone have any insight into this?

 

I am just learning TTS Datamaster but I noticed that you are throwing some DTC codes (15, 16, 42 & 43)

DTC 16 - Low Res fail (error code for Opti)
DTC 42 - Ignition control circuit, grounded circuit
DTC 43 - ESC Fail (knock sensor circuit)

Also noticed that you had a high knock count.

These are the 3 codes you need to focus on. Your problem is probably one or more of those items.

Hope it helps. Good luck.