My friend has a 5.4L Wipple powered engine/C4 in a 85 Mustang. It was a magazine car called Frightening. It has a Big Stuff 3 system and he ran it in the summer 9.40's @ 140's 1/4 mile, no problem. The problem now is a super rich rough idle, has to two foot it when in gear. No fuel leaking into the pressure regulator hose, 40 psi on the gauge (as it always was) O2 showing 9-11 AFR at idle. Hit the throttle, revs well but big cloud of black smoke. Changed oil, no change. Here's the kicker, while idling, turn off the pump, pressure gauge goes to zero, engine smooths out and keeps running (has run for a minute or more), the O2 still showing rich, have to turn off the ign. switch to cut off engine. Before all this, he could turn off the pump and the engine cuts off in a couple seconds as you would expect. With the Big Stuff gauge display showing while engine is running, all looks fine except for the rich O2. I've used the MegaSquirt and Quarter Horse but not Big Stuff. Even so, I've never come across something like this. Any of you guys seen this?

 

My buddy told me he had to load it on the trailer today, go to his fellow racers shop to get a pan shield put on. He thought he wasn't going to get it on the trailer it ran so bad. However, when he got to the guys shop, it started up and ran perfect driving into his shop. The problem is now stranger than it was before. He hasn't brought it back home yet so we'll have to see what happens on next startup.

 

Have you pulled the plugs and inspected them? Sticking injectors would show up on the plugs. Data logs pulled?

 

Plugs look fine and no misfire and starts easily. Looking at the big picture, I would think there's an intermittent leaking injector and because the blower intake is open and has a flat floor, a leaking injector could effect all of the cylinders. The injector likely sealed while he was backing off the trailer but I have a feeling it will leak again. If it would continue to mess up, removing the blower and intercooler then look into the manifold to see which injector leak when pressurized would be a good next step.

 

We are still working on the car but I did finally see something that doesn't look quite right. With the engine running which still has a poor idle but will go in gear with a stable but rich idle. I see the MAP display shows 70-80 kps which seems high even with the poor idle. My buddies vac/boost gauge is showing about 15 hg. in gear which should be about 50-60 kps. Hummm. With engine off the MAP shows about 118 to 129 kps depending on how warm the engine is. Not right, should be seeing 100 kps with engine off. We did a test. Unplugged the vac. hose on the MAP, used a hand vac. pump, started the engine, pumped the tool until I see 60 kps on the display and the engine smooths out, injector PW drops from 3 ms to 2 or 2.2 ms and o2 leans out. Looks like we have an out of range MAP sensor. He's getting one on Monday and we'll do a re-test.

 

1 atmosphere or 101.325KPA is 29inhg. That means that if the gauge says 15inhg you should be near 50ish KPA. Obviously a bit different depending on where you're at altitude wise but still, it seems to me the gauge is off, the MAP is toast or the wiring is having issues. Perhaps a weird vacuum leak somewhere. You seem on the right track tbh.

 

Definitely sounds like a bad map.

 

My buddy called me back, new MAP, shows 101 kpa on the computer with engine off. Starts, good idle, about 40 kpa on the computer display with 20 hg on the dash gauge, normal AFR on the wide band and cuts off as soon as the fuel pump is turned off. It's ready to go back to the drag strip. I should caught this problem before now but that's the way it goes sometimes. Thanks guys.....Ken.

 

Glad to hear its squared away. Some times it just takes some patience.