Replaced L03 with an L31 in my Firebird and it has been a tuning nightmare ever since

Okay guys, first off pardon my long-winded nature but I'm very detail-oriented. I'll start off with a little background on myself as I am new to this forum. I'm a master technician at a local Chevy dealer and have about 11 years with GM now with experience at all 6 of the original major brands. My mainline experience would be with Chevy and Cadillac focused on light-body and major engine repair. I have been tuning for about 5 years now, but primarily on near-stock vehicles and "converting" them to run on E85. I found out on my current project just how little I know about ecm tuning (I am quite adept with a carb and HEI for a person my age). The swap was rushed and I was not properly prepared, but my L03 was quickly fading and I had 3 easily repairable L31's sitting in crates.

The vehicle and hardware;
It's a 1989 Firebird Formula originally equipped with an L03 and manual trans. The car has 217k miles on it and it is my daily driver that sees 100 miles a day for my commute to and from work. Sometime before I bought it, the manual trans had been ditched for a 700r4 and performed the swap fairly decently except for the tcc wiring and changing to the proper PROM. I chipped the ecu and wired in the trans harness shortly after buying the car. In anticipation of a future port injection upgrade and also as an uprgade for E85 I installed a walbro 255lph high volume AND high pressure fuel pump not knowing that the passages in the TBI unit itself couldn't handle that kind of fuel flow. After discovering my mistake and that the fuel pressure was sitting at over 20psi with no mods to the stock FPR, I replaced it with a billet adjustable unit plumbed to manifold vacuum making it a VAFPR. I set the pressure to 30 psi base with about 22psi at idle. And that brings us to the present setup. I replaced the tired L03 with a used 117k mile L31 bottom end paired with some 96k mile heads. The bottom end was fine and one of the original heads had a crack. The heads came off of a bottom end that had wiped out the bearings due to an intake gasket failure. I used an Edelbrock Performer RPM 4bbl intake with a Holley TBI to carb adapter plate. I reused all of the EFI hardware from the car along with the flexplate and exhaust manifolds and just replaced the distributor gear and used new coolant temp and knock sensors. I installed a GM parformance parts heavy duty single roller timing chain, melling m55hv oil pump and 7 quart oil pan with matching bolt-on pickup tube, and an O.E. style timing cover without the crank sensor hole drilled. I reused the stock L31 balancer and crank sensor reluctor behind it for proper fit. Due to clearance issues with the oil pan I fabricated the front section of the exhaust out of 2 1/4" pipe and bends from the parts store and ditched the y-pipe. The passenger side follows the route of the y-pipe and necks up into the 3" intermediate pipe at the stock location. The drivers side runs straight back until just past the tranny crossmember and makes a shallow bend over to the intermediate pipe attaching right where it bends to follow the driveshaft tunnel. From there it is 3" back to a flowmaster muffler and dual 2 1/2 outlets. The cap and rotor are tan Accel units, the plug wires are Accel super stock spiral-core, the coil is a MSD streetfire, and the plugs are Delco O.E. iridium replacements. The EGR and AIR injection hardware are all gone and disabled in the ECM along with the pins backed out of the connectors at the ECM. The base timing is set at 0 degrees and the thermostat is a high-flow 160 degree unit.

The problem;
The car fired right up and idled fine with a blind tune and ran fine driving in and out of the shop until I fabbed up the exhaust. The initial driveability seemed decent, but when cruising at very light throttle and no load the engine had a random intermittent misfire. The stumble felt like it was either cutting fuel or ignition so the first thing I did was disable the DFCOIt also seemed soggy at WOT in the higher RPM range. After 54+ tunes moving the fuel and timing all over, the engine is running ok and has decent power but it is gradually getting rougher and the roughness is moving into other areas of operation. I pulled the plugs yesterday and they showed signs of either detonation or overly lean mixtures as the insulators showed signs of blistering. I pulled more timing and added more fuel, but to no avail.

Update;
I finally figured out that that a 305 and 350 require matched and specific knock sensors and ESC modules. I will upload a bin image and datalog as soon as I get a chance. Thanks in advance,

Phil

Try an LT1 timing table from a b body car, as the head design for the vortecs came from the LT1 heads. Its a good place to start.

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After 54+ tunes moving the fuel and timing all over, the engine is running ok and has decent power but it is gradually getting rougher and the roughness is moving into other areas of operation. I pulled the plugs yesterday and they showed signs of either detonation or overly lean mixtures as the insulators showed signs of blistering. I pulled more timing and added more fuel, but to no avail.

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Back to basics! Check fuel pressure and replace fuel filter. Tune can not change, mechanical things can...

With all the work done to engine could it have developed a vacuum leak?

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the plugs are Delco O.E. iridium replacements

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Delco OE sounds good! But your in the tech business you have to know to look for snake oil fancy plugs with strange drivibility issues? Do these have and fancy small tips, U grooves or any of that non sense that causes issues constantly? If not at first install then usually within 1000 miles...

You did not mention the EST/ICM? Stock Delco? With all the changes to ignition it might be worth an osiliscope to check.

You'll have to pardon my mistakes in not clarifying a few things in my original post Mark. I have a 5 month old daughter that interrupts a lot. I have put roughly 4k miles on this setup now and the plugs didn't develop the blistered look until recently and I believe the roughness is because the plugs may be trashed now and/or I have caused some internal engine damage like the beginnings of burnt valves or something the like. The fuel pressure is still 30 psi and I removed the intake vacuum from it because the fuel curve is better on this engine with the steady pressure. I have checked multiple times and there are no vacuum leaks. ALL of the ignition save for the plugs were on the L03 engine for several thousand miles with no issues. The coil and wires are a couple of years old actually, but were in good working condition and ran great on the L03 at the time of the swap. The ICM and pick up coil are aftermarket parts store specials that were installed on the old engine a couple of years ago as well. And the plugs are direct replacements for the original platinum units. AFAIK GM has stopped production on platinum plugs in O.E. applications so if you go to the dealer and buy replacement plugs for anything originally equipped with platinum plugs you will receive the iridium units that I am running. The tips on the center electrodes are relatively small like all iridium plugs I have seen to date, but like I said, these are the designated replacements that I install every day with no problems. I didn't disable the DFCO, rather I meant to say that I disabled highway cruise. I also have a narrow band air fuel gauge in the car to monitor the air fuel ratio in real time as I drive. The only lean areas were at WOT (fixed) and at while cruising at very light loads. It actually still acts as if highway cruise, DE, or DFCO are "activating" even though the parameters are not being met. The other issue is making sense of the timing as the knock sensor and EST module are for a 305 and I have noticed that they do not detect detonation on this engine properly (which I didn't realize until recently). I have a hard time hearing audible detonation in this car because it is so loud inside and it also has a minor exhaust leak at the right manifold because it is made for a cylinder head with a 7 bolt pattern and the vortecs just have 6. I have to go for now, but that should clear up a few things.
Thanks again,

Phil

OK so your mentally handicaped from a 5 month old daughter... :laugh: I can relate, done that been there...

You've got so much going on I'm trying to figure out mechanical issues before even getting to tune issues, since your a tech I'm going to through some thoughts your way to find the issue on both.

Exhaust leak at manifold? Is it on O2 sensor side? Just a little air is a bad thing before an O2 sensor.

Fuel pressure is 30 PSI which is about 13 PSI to low for MPFI to start and no vacuum compensation for WOT. Was BPW/BPC set for new fuel pressure in bin? Are the injectors correct size for engine displacement? Injector offsets correct for injectors used? Have you reached the need for 90+ in VE fueling tables? Are BLM readings in the ballpark?

LO3 and L31 Vortec spark plugs are differant, do you have Vortec plugs in your heads?

EST/ICM from GM have 2 differnt modules that can change timing as much as 6 degrees. Aftermarket parts store replacements have been known to change timing as much as 11 degrees. This is also a known part to work when faulty and can pass test, but if test is done repeatedly it will fail one. So where is your timing really on timing marks compared to what bin or data shows? And does it glitch when checking timing? This is a part I would replace with delco 369 module, not 048. Latency tables for the 2 modules are differant, large cap latency is again differant, even stock GM values are not accurate from timing tables to timing marks but at least with a known module you can use known latency settings and get timing close to accurate. Also where is base timing set? Is it set correctly in bin file?

Also what ECM is this now? It was TBI engine to start and now MPFI? What mask ID? What did you do when converting this from TBI to MPFI?

If you think it may be to late and already burnt a valve a compression check would be in order, how is vacuum reading?

Hey Mark...Could that L31 be pulling more vacuum than the swirly port engine he had at a given throttle setting?
he should have the 1228746, and he is still TBI with the Eddy carb manifold, and TBI adaptor, which means hes probably not running EGR.
PMKLS1, Hows the idle quality? you may be throwing the whole setup for a loop with the VAFPR, as the TBI regulator was not a vac operated unit, and did not change with engine vac. I am thinking your injectors are running about 65pph @22 psi and 75pph @ 30psi....thats a pretty big pressure sweep.

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Originally Posted by jameslleary

Try an LT1 timing table from a b body car, as the head design for the vortecs came from the LT1 heads. Its a good place to start.

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I'll second that ^. ve was all over the place for me and my truck ran like doo doo until I loaded a LT1 spark table. Once I did that i was able to quickly tune fueling then tweek spark after that.

Hi. And welcome.
Former GM tech here. 54 chips sounds like a good start on that combo. Some info for consideration:

Stock calibration expects fairly constant pressure differential across injectors. In the case of TBI both ends of the injector are exposed to atmospheric pressure so pressure diff remains fairly constant... OEM is 9-13 psi. PFI injectors use vacuum regulator to maintain constant differential. Delivery end of injector is exposed to manifold vacuum so when vacuum is high, fuel pressure is lowered proportionately. Adding vacuum regulator to TBI means volumetric efficiency tables have to be skewed. Since pressure is lowered when vacuum is high, and stock VE table reduces delivery when vacuum is high, you get 2X reduction in fuel at cruise and idle. Now add in maximum advance found when manifold vacuum is high and you've got the potential for lots of detonation.

Vortec 5.7 engines come with steel plugs... generally commercial graded versions back in the day. Platinum replacements are not stock. Check heat range and plug depth! These heads can really be adversely affected by too long plugs. Lotsa racers running L31 heads have been bitten by this. I've had best luck with Autolites.

There's a post on this forum with a spark map I put into a 300 hp GM crate engine with TBI and Vortec heads. It also might have a stock spark table from a 96 Export G van which was factory equipped with L31 long block and TBI intake. Rare bird. But the complete calibration is available here and that thread may have a copy. It's for a later pcm so it's not compatible with your ecm. Long term I'd consider the pcm from an S10 w/ 4.3 vin W or the 92-95 fullsize truck with auto trans. Better, faster, stronger, and a bit smarter, too.

OEM programming on that old TBI ecm has values representing the time delay between combustion event and exhaust gas passing O2 sensor. Sometimes moving the sensor can cause issues. Simple test is to disable closed loop and see what / if anything changes.

BTW, have you calculated fuel delivery rate for injectors at new pressure? I have info from Delphi listing max pressure allowed for TBI injectors (keep meaning to send to Mark for posting). I'd have to check at home but IIRC it's not as high as people want it to be. Possible issue with injectors misbehaving? Also, if fuel delivery rate is too great then ecm might not be able to properly command minimum injector openings. There's a physical limit to how small the "on" time can be. Scope can show this but so can simple time measurement.

Gotta run... work awaits. More to follow, hopefully.

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Originally Posted by 1project2many

BTW, have you calculated fuel delivery rate for injectors at new pressure? I have info from Delphi listing max pressure allowed for TBI injectors (keep meaning to send to Mark for posting). I'd have to check at home but IIRC it's not as high as people want it to be. Possible issue with injectors misbehaving? Also, if fuel delivery rate is too great then ecm might not be able to properly command minimum injector openings. There's a physical limit to how small the "on" time can be. Scope can show this but so can simple time measurement.

Gotta run... work awaits. More to follow, hopefully.

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I am experiencing this right now, in my case, 13psi causes idle surge, and inability to set minimum idle to 500rpm. my regulator (jet) will only go to 12.5 psi, and I can set everything correctly, and it will not surge, but, anything over 12.5' and the problems start.

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Originally Posted by jameslleary

I am experiencing this right now, in my case, 13psi causes idle surge, and inability to set minimum idle to 500rpm. my regulator (jet) will only go to 12.5 psi, and I can set everything correctly, and it will not surge, but, anything over 12.5' and the problems start.

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it might not like minimum idle set at 500rpm. maybe too much air going through the IAC and not the throttle blades.

Maybe I followed this wrong,

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In anticipation of a future port injection upgrade

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did it ever make it to MPFI? ... is it still TBI?

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Originally Posted by EagleMark

Maybe I followed this wrong,

did it ever make it to MPFI? ... is it still TBI?

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The original post says he is still TBI.
Phil,
I would say, before you do anything else, you neednto install the lower pressure TBI pump, at least for getting the initial TBI Vortec tune in order, but keep the Higher pressure pump in the toolbox, for when you finally do go to MPFI and end up using a different ECM all together, like a 730.

I've run the TPI pump at 18 no issues? But 30 PSI on a TBI is an incredible pressure, I have never gone above 23 so don't know what results would be? If injectors are for engine then I can't see any reason for this kind of pressure? Didn't see any major motor mods... even if they are the 305/5.0L injectors in a 350/5.7L then closer to 21 PSI will do a stock motor... BPW set correctly...

Wow, lots of replies, way more than I expected. I have a limited amount of time so I will just hurry and try to answer all of the questions randomly. The fuel pump has been in the car going on 3 years now and for about 55k miles, along with the FPR and ignition components save for the cap and rotor which are about a year old. I know that I stated a couple years for the plug wires, coil, ICM and pick up earlier, but so much time has passed so quickly that I don't realize how long I have owned the car now. I am using the stock TBI unit and injectors from the L03 along with the '746 ECM and $61 mask. I started with a stock ANTT bin image from another nearly identical Firebird that I used to own. I have compared and referenced many different PROM images throughout the entire process including an ANLU image and the aforementioned G van image. I ran the FPR with a vacuum reference on the L03 for 2 reasons. First, I did encounter idle issues on that engine when running it on gas at a constant 30 psi because the ECM was unable to shorten the injector pulse enough. Second, the sudden increase in pressure (and fuel flow consequently) that correlated with an increase in engine load helped with the AE when running it on E85 since it takes so much more fuel especially on a wet manifold setup. Again, I had no such issues with the L03 using the same parts and setup as on the L31. I have tried both with and without a vacuum reference on the L31 and with a steady fuel pressure of 30 psi it idles fine along with the BLM not jumping around as much under varying loads. The base timing is set at 0 degrees and the value in the bin matches. I checked the actual timing against what the tune is programmed at with the engine at idle and it was right on. I did calculate the injector flow at 30 psi and recalculated the BPW to match. IIRC my calculations led to a BPW of 121 and I have it set at 118 currently after some trial and error. The GM parts catalog has always listed ac delco platinum plugs as the only direct replacements on the L30, L31, & L35 engines in the C/K line. The ac delco part number for all 3 applications was 41-993 and stayed the same even after the changeover to iridium. Regardless, I am using the 41-993 plugs based on the parts catalog for the VIN of the truck the engine came out of. The vortec heads require a long reach plug versus a standard iron headed gen 0 small block chevy plug. The exhaust manifolds are stock L03 units so the O2 sensor hasn't been moved and the leak is on the opposite side. I gotta get going now, I'm off to bed. I think that I covered everything

If I had set this up I would have done the following:

Use Vortec based spark tables as starting point.

Use correct size injectors. Pressure corrected TBI never behaves as well IME. Always suspected wire gauge in injector circuit should be increased.

Dump 160 stat. No need for it. Vortec heads burn well and intake manifold has no coolant based heating. You lose less power and improve combustion efficiency with a higher temp block.

One thought: O2 readings are affected by timing. More advance tends to produce leaner O2 readings. If you've got light throttle issue try backing off timing in that area 2-5 degrees and try again.

Prolly be soggy on top regardless thanks to exhaust configuration. TPI with same manifolds was great due to low end torque without the exhaust restriction causing a problem.

No logs, no real diagnostic trail to follow, lots of assumptions. Hard to offer solid advice.