Fresh M20 high RPM loss of power

[RacingTowardsRed]

Hi all, I'm having a bit of a strange issue and I was hoping someone on here might have some suggestions. After sitting for a year and a half, my E30 is running again with a freshly rebuilt M20 (nothing fancy on the rebuild). It pulls hard, sounds great, everything seems normal... Until you get to 5500 rpm's. When the car gets to 5500 rpm's it looses a lot of power, and it happens really suddenly. It's not like it stops revving, it's not that extreme, it just feels like it loses a bunch of power for some reason. What makes this problem even weirder is that it only happens at WOT. I've already tried swapping the ECU, and that didn't fix it. Cap and rotor are pretty new, spark plugs are brand new, fuel filter is pretty new. Could this be caused by the year and a half old fuel I'm burning through? I'm really not sure where to go from here, so any help would be appreciated. Thanks guys!

[hooken m]

That plus maybe your fuel pump is on its way out?

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[canakami]

Definitely sounds like fuel pump. Mostly the wot @ high rpm part.

[u3b3rg33k]

Personally, I'd wait and see what happens after you put new gas in it before spending money on anything else.

It certainly wouldn't hurt to tape a fuel pressure gauge to your windshield though...

[seatown88]

When was your fuel filter last replaced?

I chased a problem for half of the season last year similar (more severe) to yours and it was the filter blocked with rust...

[89 e30]

Bleed your coolant, worked for me

[Gregs///M]

Fuel pump failure is an option yes.

But a poor/old AFM will also cause this problem. If you have not tried a working AFM or rebuilt AFM, you will experience this issue at WOT.

What happens is the internal flap moves across a resistance tract. This sends approx 5 volts to the ECM to indicate full throttle, max air input, etc. Bad AFM's often have voltage drop out at WOT, meaning the signal drives up to 5 volts, then suddenly drop to 0 volts and the ECM thinks the engine is suddenly at idle speed (or sitting still.) Thus the huge drop in power.

Try another AFM or a rebuilt one. Easier than troubleshooting fuel related problems.

[jlevie]

The first thing to check is to see if the WOT switch in the TPS is good (check with a meter). If not that, check rail fuel pressure while driving.

The AFM can't be a cause at 5500 & above during WOT if the TPS is working properly.

[Gregs///M]

The first thing to check is to see if the WOT switch in the TPS is good (check with a meter). If not that, check rail fuel pressure while driving.

The AFM can't be a cause at 5500 & above during WOT if the TPS is working properly.

I've personally experienced the AFM bogging my car under WOT. This is the whole reason that I got into AFM repair. And I know my TPS was working fine.

From my AFM repair endevours, voltage drop out of the AFM near WOT is very common.

[Earendil]

I've personally experienced the AFM bogging my car under WOT. This is the whole reason that I got into AFM repair. And I know my TPS was working fine.

From my AFM repair endevours, voltage drop out of the AFM near WOT is very common.

I thought the logic for WOT was closed looped, and the AFM was ignored in that situation? Same goes for Idle, the AFM is ignored.

Of course, when I saw "idle" and "WOT" I don't mean the physical state, I mean the sensor pick up. If the pedal is mashed but the car doesn't detect WOT, then the AFM matters, and it matters a lot.

[Gregs///M]

I'm not certain the ECM goes into closed loop mode during idle or WOT, I could be wrong. However, I've experienced sudden power loss at WOT that was remedied by swapping to a known good AFM. I've had a few customers with this same problem and solution too.

[Earendil]

I'm not certain the ECM goes into closed loop mode during idle or WOT, I could be wrong. However, I've experienced sudden power loss at WOT that was remedied by swapping to a known good AFM. I've had a few customers with this same problem and solution too.

Before when I said closed loop I meant open. "Open loop" is when the ECU starts ignoring sensors.

Interesting. Perhaps a maxed value (0v or 5v) from that sensor will override open loop mode, perhaps as an indication to the ECU that a sensor, either AFM or TPS, isn't functioning correctly.

[Gregs///M]

Before when I said closed loop I meant open. "Open loop" is when the ECU starts ignoring sensors.

Interesting. Perhaps a maxed value (0v or 5v) from that sensor will override open loop mode, perhaps as an indication to the ECU that a sensor, either AFM or TPS, isn't functioning correctly.

Yes open loop. Not sure the details when the ECM is actually open vs closed loop operation.

[Earendil]

Yes open loop. Not sure the details when the ECM is actually open vs closed loop operation.

I'm positive that in a properly functioning system, that open vs closed loop is dictated by the TPS registering WOT or idle. I do not know what "abnormal" conditions might break it out of open loop though, or override the definition of open loop.

That said, I'm not positive that open loop ignores the AFM. I thought it did, and thought that the ECU just used default "maximum" parameters for fuel and air mixture. But this I am far less sure of. I'm only sure that it ignores the O2 Sensor.

[RacingTowardsRed]

My AFM was taken from our PRO3 racecar and it was known to be working at the end of the season 2 months ago, so that seems unlikely to be the case, but I will swap it out with another just to cross it off the list. I will order a new fuel pump and filter because they could be changed anyways. I'm thinking a trip to the dyno would be pretty revealing, would it not? If it was caused by a lack of air or fuel wouldn't I see a big swing one way or another in the A/F ratio graph? I was also wondering if I may have just been an idiot when installing my fuel injectors after having them cleaned and somehow done it wrong??? Thanks for all the suggestions guys! Interesting discussion about AFM's and whatnot, although a bit beyond my scope of knowledge :)

edit: Also, is there a way of testing my fuel pump? I'd rather not spend $200 on a part I don't actually need.