Challenge Engine wont run. Looking for ideas?

The engine in the challenge car was running perfectly before we pulled it to do some rattle can beautification. Now we cant seem to get the challenge car to run right. It fires right up, the rpms race up to about 2500, and then the engine dies as it tried to return to idle and shuts off.

This is the setup we started with ( not the actual challenge car)

The setup was changed to eliminate the EGR system and the Mailbox manifold was installed. The mailbox is just a big box with a big hole in the side. The EGR spacer was eliminated entirely.

So far the MAF sensor and IAC have been replaced with known good parts and no change. Firing order is correct and have pulled the intake apart and re-sealed looking for a vacuum leak.

The engine starts on the button. If I work the gas pedal I can keep the engine running but it wants to die as the revs come down. Probably around 4k it will run.

Out of ideas on what to look for next. There is a E36 M3load of wires in the Mustang harness and no idea where to even start looking there. If the computer is bad, I have do idea how to diagnose.

I need to have this running by the end of this week or make the call to bail on the FL thing.

Pete

What you’re describing sounds like a massive air leak. PCV system still in place? 

What about the sensor signal return ground wire... it’s been a while... but they’re usually orange, and I think on this era 5.0 were attached to one of the lower intake bolts.

Is the EEC-IV diag connector still in place? Scan it, or make a jumper to flash codes? 

Final thought... MAP sensor. 

Is it a ton of work to try fitting the old intake manifold back on?

Two ideas: 

1- Vehicle runs fine in a open loop start cycle then falls on it's face once it begins sampling.  Maybe a MAF, MAP, or O2 sensor?  If you disconnect the O2 sensor and the car continues to run (forced open loop) that will at least isolate a potential issue.  

2- Is the fuel pump getting power after start?  The idea is maybe Ignition is priming the fuel pump, you get up to operate fuel pressure, engine starts, pump loses power, fuel pressure falls, engine stumbles and dies.  

Just the first 2 theories that come to mind.  

Grounds clean? On the equipment i work on we have had people paint them which is fine, but then didnt scuff the paint for grounds and i ended up being called out to fix. 

Change to plan B, The Miata of Shame?

https://grassrootsmotorsports.com/forum/grm/you-too-can-take-advantage-of-the-stampie-rule/153602/page1/

I was expecting someone to come up with a good answer by now and don't like to see my previous post sit at the end of this thread. I don't know the 5.0s at all, but Saron81's comments are what I'd think best. Vacuum leak: besides checking the seals on the intake is there anything else EGR related that was removed -hoses, pipes, covers that could be leaking. Is the system vacuum actuated? Brake booster hose good at both ends? Smoke test the whole intake system for leaks.

I have an old EEC IV tester that I used on some '80s Ford trucks. A 'count the blinking light' sort of thing. It was about $20 at Autozone/O'Rileys/Walmart. I'd mail Friday it if you can't find one.

Lastly, what I've done in cases like this is mentally go over everything I touched since it last ran right. Look at each part and touch it again to verify it is right. Usually ends up one place I got distracted and left a wire disconnected.

Good luck with it.

Shot in the dark, but Ford idle air controllers are more like true love than they are like herpes.

In reply to Saron81 :

Buy this man a chicken dinner! Fortunately one of the team members is a rock star with his own personal smoke making machine and we rigged it up to the intake. Found a massive vacuum leak between the upper and lower intakes. ( some surplus holes we forgot to eliminate¯\_(ツ)_/¯ ) Fixed and now it runs. Still need to road test the gearbox since it is an unknown.

Will report back with road test results in the build thread. Thanks all.

Hooray, i'll be pissed if you bail

In reply to Patrick :

Nowhere near ass pissed as me. 

Speaking from personal experience, unless that motor is modified to run at 7000rpm all day long,  that intake is going to kill all usable power.

Nice!