Hi guys, I've reverted my GTA ('88 350/A4) back to stock. I put the stock cam back in, stock fuel pressure regulator back on, stock TPI tubes the works. The only things left not stock are the ford blue 24# injectors and the accel short spark plugs.

I burned an AUJM chip with injector flow rates set to 24# and put it in.

Car idles great, sounds as perfect as stock. Anything off idle the motor surges really bad.

In park, if you mat the accelerator the motor will stumble a bunch and sometimes just stall, sometimes itll stumble up through the RPMs.

I've attached a TunerPro RT datalog, probably want to start looking around sample 1000.

Why does the O2 constantly read like 50mv? It only goes above 50mv when I hit the throttle. Not getting hot enough? Just junk?

You can see the MAF readings surging with the motor.

Well anyways, I hope someone has some insight as I'm really quite defeated.

Matt

 

I hooked up the wideband today.

Anything off idle goes off the charts lean - causing the stumble.

I reduced the injector flow constant, showed some improvement but even as low as 16lb/hr off idle was lean.

I reverted the flow constant to 22lb/hr and adjusted the MAF tables up. Persistently lean even until the MAF table was maxed.

Increased the MAF Scalar for more headroom and increased the max air flow vs RPM. Could not add enough fuel for it to not go lean.

Inspected FPR vacuum hose to make sure it wasnt plugged. It checked out. Verified 6* base timing.

Need to get a fuel pressure gauge to verify proper FPR function.

Any ideas? Ignition problem?

 

I was/am dealing with a similar problem on a Crossfire Vortec (Corvette Crossfire intake adapted to a Vortec 5.7L), I haven't installed the wideband yet, but the NB shows similar low voltage as you are seeing. The plugs show a rich condition though.

My problem was bad enough that I couldn't get it to rev. A combination of short plugs and old spark plug wires seemed to be the majority of the problem, I can now rev it, and even move the truck under it's own power, but there are other issues, that I need to take care of.

What do your plugs show? Numbers on a laptop screen don't always show the whole story.

 

I pulled the #1 plug today before I started doing anything and it was a nice tan, I was really expecting to see some soot or a white plug but of course not. That would be too easy right?

The wires on the car are almost brand new taylors. I am considering swapping the accel plugs back to stock-style AC-Delcos, though I ran these plugs before when I had a zz4 cam and they worked flawlessly.

Im also considering swapping the SVO 24's for stock style 350TPI injectors but I'm going to wait until I can get a fuel pressure test gauge.

This all started when i swapped the ZZ4 cam for the stock cam and swapped the Holley AFPR for the stock FPR. Since im 99.999999% sure the cam is installed correctly, I'm suspecting the fuel system, a vacuum leak from intake re-assembly or improperly lashed rocker arms.

Would an intake vacuum leak behave like this?

 

Check the fuel pressure, even more so when whacking the throttle open (hood open in the driveway). The Holley FPRs for TPI have an issue where the upper seat will flip sideways and lower the fuel pressure. Loss of regulation results.

RBob.

 

Thanks for the advice RBob. The holley regulator is no longer on the car. I still want to get a pressure gauge and check it though to rule it out.

I'm wondering now if its an ignition problem. A miss would cause lean readings correct? This surging just seems too regular to be random miss-firings though.

My plan is to check the fuel pressure, then check all the spark plugs individually, maybe just do a compression check while I'm at it.

 

New plugs today - stock style R45TS delcos.

Checked the fuel pressure, 44psi with vac undone, around 38psi at idle, jumps to 44-45ish when given throttle. Pressure does not leak down, so no leaky injector.

Advanced the timing a few degrees, no difference. Pulled the drivers valve cover and #1 spark plug, brought to TDC feeling through the plug hole. Damper degree marks seem accurate. Intake and exhaust rockers both closed at pretty close to 0deg on the balancer - so I'm very confident the cam is installed correctly and not a tooth off.

Verified plug wires were connected correctly.

Idle still reads off the gauge lean. Spark plugs look tan and fine. No exhaust leaks upstream or around the o2 that I could find.

The interesting thing is that right after you start the car before it starts to settle its idle down the wideband reads right around 14.7, then once the idle settles a couple seconds later it just shoots off the lean end and the motor begins to run rougher.

 

Swapped ignition modules today, no difference.
Made sure all TPI bolts were tight and no vac leaks.
Played with the timing a bunch with no success.

Changed the chip to ARAP, initially thought the problem was solved as motor revved instead of fumbling, however steady throttle has the same off-the-scale lean surge.

At steady throttle above idle as the motor surges and stumbles you can hear a pretty regular bang/poof through the intake like its backfiring.

I am completely out of ideas. I put the car away in the polebarn as it doesnt look like itll be running any time soon. Probably will just pull the TPI motor out and be done with it.

 

89 iroc 5.7 idles but touch throttle it dies,
new ecm, new iac, new filter(fuel), new tps, Iam stumped, any help?

 

You mentioned that you reverted your engine FROM something else. From what?
I had stall out issues which were all related to bad AE settings. The "pump shot" was not happening soon enough, long enough or with enough volume and it would die.

I'd be suspicious of other factors are still in play covering AE that are still out of whack, given the non-stock injector package.

 

This might sound dumb but...did you try running the car with the factory tune?

 

When you checked the timing, does your car have a disconnect on one of the distributor wires that goes to the ECM? To properly set BASE timing without interference of the spark tables, you need to disconnect that wire first. I believe it is brown with w black stripe. Otherwise you are never truly setting your base timing as the ECM was in still in control.