Ignition breaking down under load

[someguy360]

Hi All,

Not strictly tuning related but I know we have a few wiring gurus on here that know their way around delco wiring etc.

Sorry for the long story.

I'm absolutely running out of ideas on this one, I've tried just about everything.

Warm 304 (Head work, compression work, 286 cam, edelbrock dual plane etc)

Engine was out, did a wiring loom relocation to smooth the bay and hide the wiring, relocated ignition module, relocated map etc.

Ever since putting it back in it's been running super lean (tune hasn't changed) and feels like it's missfiring as soon as I touch the accelerator and bounces from normal to lean to normal etc. It really really struggles to rev past 1500rpm and coughs and splutters (in fact putting my foot down it won't go past about 1800-2000rpm in neutral it just surges and splutters).

Engine won't stay running unless I give it a fair bit of throttle (regardless of IAC position) otherwise I have to wind the throttle stop screw out to about 5% throttle just to hold idle.


Before the basics get suggested the following has been replaced (both as part of upgrades and troubleshooting).

*New fuel pump
*Fuel filter
*Fuel regulator (Adjustable and set to 3 bar)
* Spare balancer installed at TDC to confirm it hasn't slipped and set base timing at 10 degrees in diagnostic mode.
* Dizzy (put a brand new one in)
* Ignition Module (tried 3)
* Cap And Rotor
* Spare set of injectors
* Checked all earths from body harness, engine harness and battery and added additional earths with no change, battery is even earthed both to the block and the body from the boot.
* Moved ignition module to 3 different spots on metal and even ran an additional earth lead to it's chassis.
* Tried 2 coils
* Verified in Tuner Pro (as you can see in the video) that Coolant Temp, Manifold Temp, TPS, IAC etc are all reading correctly.
* Verified the coil when running is getting a constant 13.4 volts at the positive terminal.
* Spare PCM
* Reflashed Tune.
* Put the timing light on each lead to confirm it's firing consistently
* Compression tested when hot all as expected and even.
* Ran a noid light on each injector plug to confirm pulse.
* Tune runs in open loop so onboard o2 sensor for the ECU shouldn't be a factor.


I've attached a video showing it's behavior and the sensor readouts in TunerPro. (As you can hear it's stumbling/missfiring/hunting)
https://youtu.be/h1L4qZbLkbE

Any suggestions would be welcome and appreciated before I tear my hair out. Car has to be at the track in just over a week and I've been battling this for almost a month.

Was up until 1am stripping the wiring loom insulation down to try and find any wiring mistakes I might have made but everything looks as it should, injectors are banked correctly, obviously in tuner pro all my sensors are reading where I'd expect them to etc.

With the fuel system being entirely rebuilt and still not making a difference to behaviour I'm leaning more towards it being electrical/spark related.

[vlad01]

Did you check the MAP sensor? I have had this happen a few times on Falcons, dodgy MAP that was stuck at low Kpa readings and like you describe, as soon as you try to accelerate it just falls flat and misfires like crazy, but if you feather it really carefully you can get the car to drive.

[someguy360]

Did you check the MAP sensor? I have had this happen a few times on Falcons, dodgy MAP that was stuck at low Kpa readings and like you describe, as soon as you try to accelerate it just falls flat and misfires like crazy, but if you feather it really carefully you can get the car to drive.

Map vacuum readings look about right in tuner pro for my cam (aka not much at idle) but that doesn't mean it's working perfectly.

I might try with it unplugged tomorrow and see if it makes a difference (obviously it will run excessively rich, but it may shed some light).

Thanks

[vlad01]

It should go into Alpha N mode if you unplug the electrical connector and if the tune is pretty close it should run almost like normal (does on my cars).

The other thing to look out for is a kinked hose on the MAP. This might bleed enough air pressure though to get going and idle ok but too slow to respond to throttle which does the same as you mentioned.

[yoda69]

With your wiring tidy up in the engine bay did you keep the earths from the ECU separated as per factory and to separate locations on the block?
If you join noisy earths (O2 heaters, ignition module, alternator…) with sensor earths (MAP, TPS….) to the same earth location you can have problems. They all go to the block back at separate locations.
I’ve had a problem in the past when someone tidied the wiring and joined all earths together to one location.

[brindo]

Another thing to consider with the MAP sensor, if it has been relocated to try to keep it higher than the port on the manifold where the vacuum is sourced. And keep the vacuum hose to it so that it can drain freely back into the engine if necessary. If you ever get a slug of water in that line from moisture build up it can cause erratic readings.

[h4x0r]

I improved my wiring loom recently and it started running rich because the voltage increased from an average of 12.4 volts up to 13.8 volts. I just retuned it to settle it down. It wasn't immediately obvious to me what happened but the clue was right in front of me the whole time. Anyway go fix your vacuum leak so the car runs nice again

[someguy360]

Thanks for the suggestions everyone.

Yoda69 was on the money, I moved the two system earths of the PCM to their own location on the block and kept the sensor earths separated and it's all smooth sailing now.

Took almost 2 months to figure out what was going on but we got there, thanks everyone!

[yoda69]

Good to hear it’s sorted.