New to the forum, but far from new to working on cars (and Chevy 350s). I've owned 2 C4 Covettes and my latest project is an '89 Firebird Formula 350. I have two issues and I'm not sure if they're related... hoping someone here can tell me.

I've done a lot of work to the car (more details) below since I got it, fixing obvious issues (it was down on compression in 2 cylinders and was throwing a MAF code and O2 code). That said, I've never been able to get the engine vacuum up to anywhere near factory spec. Also, since doing the work on the car I now have an extremely lean condition (AFR over 15), so I can only run the car for a few minutes at a time to do any troubleshooting. Would the lack of vacuum cause a lean condition in the TPI fuel injection system? I can't find any issues at all with data logging aside from very lean AFR. I've chased down a few vacuum leaks, but I can't find any more.

The Firebird didn't run properly when I got it. It was in limp mode (a.k.a. "get me home" mode), which made sense given the MAF code and O2 code. I assumed one or both needed to be replaced and I'd be on my way. That was 8 months ago and the bird is still just sitting in the driveway. When I got it the car had the Edelbrock Hi-Flow Intake Manifold and Runners, 36 lb/hr Accel Injectors, aftermarket cylinder heads (World 305 heads that I've since had the local machine shop make right for a 350), and was bored .030 over (but not stroked). It has an aftermarket camshaft, but not sure which one (although several measurements at the lifters when I had the heads off, with the correct tools for that job came out to a .490 - .510 lift... no clue about duration). It did have a Hypertech chip in it and I wrote Hypertech about it... all they could confirm is that it was indeed a custom tune and that they haven't done those in about 15 years... which lined up roughly with the timeline I was given on when the engine was rebuilt. So I can't say for certain that the Hypertech chip was actually designed for this mechanical setup, but there's a decent chance of it. Basically I bought someone else's project, and based on my past successes with resurrecting and restoring mechanical blunders that others have made, I naively thought I'd have this puppy roadworthy within the first month.

In limp mode the car ran extremely rich when I got it, so at one point I downgraded to Accel 24 lb/hr injectors (those are still installed) with my theory being if I could get everything returned to somewhat close to factory I could at least get the car driving reliably on the factory tune. That's the only significant mechanical change I've made to the setup (other than replacing parts that tested bad). I also looked up the increased CFM I should expect from the Edelbrock and it's roughly an 8% increase, so I figured the injectors should be roughly an 8% increase as well (24 lb/hr vs. the factory 22 and change). I can only get data logging to work on the factory chip. It will not work on the Hypertech chip, nor will it work on a chip I burn myself with the goodies from Moates, but the car runs much worse with the Hypertech chip installed (stumbles so badly that it won't stay running without extra throttle, so I wouldn't get much from data logging that chip anyway).

I want to be sure I'm not chasing ghosts on the vacuum problem (meaning I just won't get any more with a hopped up cam). If anyone has experience with lack of vacuum causing an extremely lean condition that would also be helpful, because it would indicate that I need to keep chasing engine vacuum as my primary problem.

Thanks a bunch!

 

Sounds like a modded car trying to run on stock tune or mildly modded tune. Not always gonna work well. If no other electrical gremlins or mechanical issues, then just needs tuning

 

Yep, that's the path I started down (tuning), but I posted something in the DIY PROM forum and didn't get any replies. In reading through threads over there the general consensus seems to be that you get everything mechanically sound first, then you start tuning.

That may be why I'm getting no replies over there. My engine vacuum is very low... like around 10 inches at idle. If I increase idle and timing slightly it doesn't change, so I figured it'd be worth asking about experience with bigger camshafts here because maybe I need to get more aggressive with adjusting idle and initial timing.

The car does still have a functional EGR. It had EGR on it when I got it. When I had the heads machined and a 3-way valve job done on 'em, I ordered new intake manifold gaskets for an EGR... only to find that the gaskets the previous owner had installed were the style that blocked off the crossover. Since I had already done other troubleshooting on the EGR I elected to install the gaskets I'd already purchased when I reassembled everything. I've had a few suggestions to remove the EGR because that can cause vacuum problems... not sure if others with larger camshafts have any experience with both setups.

I've got full, consistent fuel pressure at the fuel rail... the injectors are effectively brand new... no other indicators that anything is wrong except for low engine vacuum and running extremely lean.

I'll add a little more background on my post over in the DIY PROM forum in hopes to move that conversation along. In the meantime, if there are any other suggestions please keep 'em comin'.

Thanks!

 

Cam will change vacuum. Tune being off can change vacuum slightly.

its not the vacuum causing poor running, its the chip bin file not having the right values for fuel and timing to make the motor happy. Cam is changing air flow requirements for the engine and thus fuel needs to change. Maf can adjust some for this but is not perfect

i’ve seen some heavily modded cars run ok on stock tune files with basic changes. Ive also seen cam only stock motors run lean with maf and require alot of changes.

also make sure your coolant temp sensor is working, fuel is based on that as well

 

Got it. Very helpful, thank you! I haven't looked in a while, but last I checked the temp gauge was working (so I assume this means the coolant temp sensor is fine). I do have the service manual so I can review that if there's a different diagnostic I should run to be safe.

Thanks!

 

Theres two sensors one inthe head and one in the manifold. Head was for the gauges, manifold for the ecm. Basically can ohm it to find the resistance at two known temps and compare it to sensor calibration specs you can find online. Do it at ambient and say normal operating temp

its probably ok but can happen. I’d do chip tuning of the open loop fueling and maf settings first

 

Very helpful info, thanks again!

Whoever did my rebuild put one temp sensor in each cylinder head, nothing on the intake manifold.

I assume the passenger side sensor is the one for the ECM (since it’s closest to the computer)?

Any good resources you could point me to for getting started with AFR and MAF tuning?

 

I found this which should be very helpful in tuning: https://www.gmcmi.com/wp-content/upl...EFI-System.pdf

I do have an air temperature sensor on the manifold. In reviewing the service manual I see where it references taking in both Manifold Air Temperature and Engine Coolant Temperature as input parameters, but haven't found reference to confirm if it's monitoring air temp or coolant temp for the ECM/ECU.

 

Just gotta dig thru the diy prom board stickies and search threads. Theres lots of good info in there just gotta read and find it.

if you are using tunerpro rt and are generally familiar with the tables and parameters available to you in there, you are good to go.

everyone has their methods but start with your stock bin file and change the constant And basic stuff like injector size, fan temps, idle speed, turn off/disable egr and air if you dont have them, that sorta stuff

wideband o2 Sensor is desirable for air fuel readings. I like to also disable closed loop, theres an enable temp setting just change it to max temp. Attempt to start car and see if it runs/idles. Note afr readings. You’ll want to adjust the open loop fueling vs coolant temp values for this typically since its a cold start. Idea is to get it near operating temperature, once there you can mess with maf table 1 values to get desired idle afr close and adjust your idle speeds, timings etc to get it to idle smooth. Reset iac and throttle opening screw as needed.

from there maf table 1-4 basically do all the part throttle fueling. Wide open throttle is done with table 4-6 if not maxed out, but some mild mod cars can max it out, so i much prefer to use wot power enrichment percent corrections vs rpm. Adjust accordingly til afr is near 12.5-12.8 ish.

timing table can be smoothed out and adjusted as needed as factory tables are kinda rough with adders not included in the main table like power enrichment timing. I like to 0 out the power enrichment and other adders/subtractors mainly, and add all those values into main table so its clear. May leave some timing adders for cold temp run assistance

thats basically it in a nutshell. Not overly difficult just abit time consuming. Once fuel is close on the wideband gauge you can enable closed loop if you want. I usually dont. If you do then you can fine adjust maf table values to get the BLM and INT values back to 128. Thats the magic number

 

Closing the loop on this. My main issue ending up being a bad pickup coil inside the distributor. I had done the ICM, plug wires, cap and rotor... everything but the distro. Once I dropped in a new Delphi distributor the car started running much better. The bad pickup created a misfire condition. I've spent a fair amount of time working out a few other issues (primarily exhaust problems, then realized I had to wire in a heated O2 sensor)... and now, with the help of Ryan at Sinister, it's got a custom tune and it's very happy!