Won't go over 4800RPM, stock ECU...FIXED!

[akorcovelos]

So I've been trouble shooting this issue for a while now, and the season is quickly approaching. Engine was rebuilt two years ago and has run flawless for two seasons until this issue started at the end of last season.

Car will not rev over 4800 RPM at WOT in any gear. If I nurse it partial throttle it will go to redline, but as soon as I hit WOT it falls on its face and won't go over 4800 again. It acts like its cutting ignition at 4800, very consistent in where it falls flat.

What I've tried/tested:

swapped the entire ignition system minus coil from a good running e30. Wires, plugs, dist.& rotor

unplugged o2

tested ICV

swapped in a known good TPS

replaced CPS

Fresh fuel

Swapped ECU chips (had an ebay chip, went back to stock)

good battery

New fuel filter


I'm really at a loss here. I seem to have eliminated most of the electrical culprits, haven't tested CTS or replaced the coil yet.

Fuel? I haven't swapped in the good FPR yet. The fuel pump is silent, so I doubt it's a failing pump. Would fuel delivery be that predictable in terms of RPM cut though?

[TheTacoMan]

Ive heard of issues of with non- oem cps sensors being faulty and I myself had an issue where the cps was not close enough to read the pulley and would have a revlimiter at 4k as well.

[akorcovelos]

Well the stock one ohm out OK, but the wire was worn through on one spot so I was sure it was the cause. I can measure the gap, but I'm fairly confident that's not the cause since it the exact same symptom between the two cps's.

[akorcovelos]

Also just did a compression check and it was near perfect 150 across all but one cylinder, and #3 was at 140. I assume it just needs a valve adjustment on that cylinder so I'll do then all just to eliminate that. Going to pull vacuum reading next.

[akorcovelos]

Vacuum is solid at 17-17.5".

[TobyB]

Try the AFM,

and check fuel pressure.

Both of those can do what you describe...

t

[jlevie]

This does sound like it could be a fuel delivery issue. Tee a gauge into the rail supply line, zip tie it to a wiper, and see what the pressure is when the limit is reached. Also check the WOT switch in the TPS at the DME connector.

[akorcovelos]

I already tested the TPS, and swapped in a known good one, all good there. I'll throw a fuel pressure gauge on it tomorrow, with some fresh plugs and a known good coil.

Is there a way to test the AFM?

[gnarsteezman]

check MAF

[hachirokuae]

does it have a water temp sensor on the side of the head like m42? Because it can be the problem, like in this vid http://www.youtube.com/watch?v=VBWWT...re=context-chv

[jlevie]

I already tested the TPS, and swapped in a known good one, all good there. I'll throw a fuel pressure gauge on it tomorrow, with some fresh plugs and a known good coil.

Is there a way to test the AFM?

If the WOT switch in the TPS is closed, AFM data is not used above 4500rpm.

When you tested the TPS did you test at the DME connector? The TPS could be good but a wiring fault is preventing the signal from reaching the DME. That input on section of the DME could also be bad.

[akorcovelos]

I haven't tested anything at the ECU end of the system aside from swapping the stock chip back in. The TPS tested good for all idle circuit and WOT circuit. I'm heading out to test the AFM per the Bentley now.

[akorcovelos]

FIXED!

Tested the AFM and it had a dead spot at WOT. Swapped in a good spare and it happily hit 7k through all the gears. Thanks to everyone for the help!

[TobyB]

If the WOT switch in the TPS is closed, AFM data is not used above 4500rpm.

Yeah, agreed- and yet, 3 or 4 race cars with the OP's diagnosed problem (open circuit at full flow)
did what the OP described- loss of power just ABOVE that threshold. Maybe as high as 5k.
What the ECU does is very much 'black box'- it might
not respond to the data, but it responds to the
loss of coherent data in very odd ways...

empirically,

t