Landseer

Adding to the list
Ground strap can do this if loose or corroded
Engine ground can do this if loose.
Partially clogged injectors can do this.

safulop

I will check the electricals as much as possible before moving to the mechanicals.

Temp II sensor was just replaced to try to solve an idle issue; it did help that somewhat but now this cropped up.

Injectors were removed, cleaned, retipped in August. Fuel rail operation was tested then as well.

I will take a look at the relays and the battery connections and hope for the best.

safulop

Well they do have similar hood ornaments.

John Speake

What's the history on the MAF ?

Cosmo Kramer

This is my thoughts if it's leaning out too much under load it will misfire.

safulop

LH and MAF were both bench tested by "the experts" in July, and came back with clean bills of health.

Mongo

THIS. My LH ground was loose at the back of the engine and it caused MONTHS of troubleshooting.

It is worth starting with the grounds first, cleaning EVERY ONE you can get to before focusing on the more tedious.

Mongo

One more thought, how does the harness on your MAF sensor look? Is there any sheething broken away or corrosion present? Exposed wiring???

mark kibort

see if your coils are shorting out against the chassis

check this at night to see if you get any arcing at the sides of the engine bay , where the coils are located.

Speedtoys

They're bolted to it in a metal ring already..how...how would they "short" to the chassis?

mark kibort

go talk to DR TESLA as to "why" or "how" .

Ive come back from the other side to tell you ive seen it! they dont short at the body of the coil, but at the connections. the high voltage will jump to the ground wire if that insulation is weak..... it has the same characteristics as being described. fine to about 4k, then misses to 6k. (bad).... fix the insulation, move the wire a little to not be as close to the center of the main coil output and problem solved.

Mongo

Next thing I can think of are the two ignition exciters at the under very front of the hood when you open it. They are also called secondary ignition modules.

safulop

Could you please describe how your car behaved, when the LH ground was loose?

safulop

Yeah, I've verified that the engine does not miss in neutral or moderate throttle, at any rpm. It seems to be purely a fuel delivery issue.

jcorenman

Not necessarily. What you have here is a a major power loss at higher RPM under load. Power requires air, fuel, spark-- one is missing under high load, high-ish RPM. I think "air" is the ONLY one that can be eliminated, assuming the airfilter is not full of leaves and the throttle cable is not broken. Which leaves fuel or spark.

"Fuel" requires pressure in the rails and a signal to open the injectors. The pressure is fuel pump, injectors involves a bunch of electronics-- MAF, LH, various sensors, ground connections and harnesses. However, none of that electronics stuff much cares about load or RPMs, so I would put that stuff pretty far down the list-- besides which, some it has already been tested or replaced.

So here's the deal with fuel: You've got a pump in the back which continually pumps fuel to the rails, through the fuel-regulator and back to the tank-- a big loop. The flow rate is constant-- whatever the pump can deliver against a 55 psi head pressure-- the fuel regulator. The spec is 2.5 liters a minute, around and around. And whatever fuel the engine needs is taken from that flow-- very little at idle, more at higher RPM and more at WOT, and max RPM at WOT takes almost all of it.

Now suppose your pump has gone poopy, or the filter is clogged, or the inlet screen has gotten plugged with crap. It won't be 2.5L/min, maybe only 1.0L/min.

What happens when you get to 4000 RPM and max throttle? The injectors are now squirting more fuel than the pump can put out, the pressure drops, the mixture goes very lean, and the engine quits making power.

So yes, this scenario definitely fits fuel pump, filter or screen problems. There is a simple test in the WSM for measuring fuel flow at the tank return line, do that before spending more of anyone's time speculating further. Disconnect the return hose form the top of the tank, add an extension and run it into a gallon jug, jumper the fuel-pump for 30 seconds, and measure how much fuel got pumped. For an S4, 1250cc in 30 seconds (2.5L/min). While you are there, pop the sender out and eyeball the screen-- any yuckus in the tank? Change filter, test again, then change pump if needed. They don't last forever and the "modern" fuels are not kind to original pumps.

Spark: You would think that an ignition problem would show up with any load or RPM, right? Not so, as pointed out above this just isn't the case. The reason is simple: To make a spark you need enough voltage to jump the spark-plug gap? And for normal room air that isn't much, maybe 1000 volts (1KV).

Here's the trick question: What is the air pressure at max compression? (15 psi x 10:1 compression). How much voltage is required to jump a gap at 150 psi air pressure? Well over 10KV. Any broken insulation suddenly becomes a very attractive alternative path, and no spark.

So a high-load miss could definitely be an ignition issue. The "firefly" test will find a bad case of broken-down insulation, but you can't do that under load. The best option (IMO) is careful inspection. Or just replace the wires if age and condition are uncertain. They are "consumables", just like fuel pumps...

Coils, likewise, can break down internally-- if the insulating "goo" inside starts leaking out then the spark can jump to ground inside the case, and no spark. This would effect all cylinders on one bank.

The nature of the "miss" can tell you lots: At the transition, when it starts acting up, does it feel or sound like all cylinders are cutting out for a moment and then coming back, a "stuttering" or "lurching"?? (That would be fuel pump or filter). Or only one cylinder cutting out, clearly missing but still pulling? (Plug wire). Or does it feel and sound like if is running on half the cylinders, never cutting out completely but running with half an engine sort of like a 944? (That would be a coil or coil wire).

You need to spend some time testing, there are still too many possibilities.