Engine stutter (loss of power) between 4-4.5k rpm only

[rcfanatic]

Hope everyone had a fun thanksgiving!

So about a month ago I began noticing a stutter near and at WOT, between 4000-4500 rpm. It sounds like someone rolling an "r", and there is a loss of power that feels like fuel starvation or an ignition issue. When this occurs my AFR shoots up to around 17. After 4500 rpm it drops back to under 14 and accelerates normally again.

What's odd is that it only happens in this small rpm range, and is dependent upon load/throttle position and engine temp.

I have an m20b27 with 885 head, motronic 1.3 conversion, wide band o2. Cap/rotor, plugs, wires, fpr, coolant sensors, afm, fuel filter are all two years or less. Injectors were reconditioned about 3 years ago when I did the motronic 1.3 conversion. The TPS and CPS are also about 3 years old. I replaced the coil about a week ago but it had no effect on the problem. The car idles smoothly and accelerates smoothly aside from that narrow rev range.


Any ideas?

Thanks,
Adam

[rcfanatic]

No one? Yesterday I checked the flap on the AFM to see if it was getting hung up in one spot but it moved smoothly

[jlevie]

Check the WOT switch in the TPS (electrically). If bad it will cause that.

[rcfanatic]

Check the WOT switch in the TPS (electrically). If bad it will cause that.

Jim, would you mind elaborating on the mechanism by which a bad TPS could cause the symptoms that I'm seeing? The small rev range is what's confusing me

[jlevie]

4500 rpm is the point at which the AFM maxes out on an M20B25 (which is sort of what you have) engine when the throttle is 60% or more open. The WOT switch in the TPS tells the DME to disregard AFM data above that point and use built in maps for fuel. This can also be caused by a flaky CPS, bad engine harness, or damaged harmonic balancer.

[rcfanatic]

The TPS checked out fine. Continuity only with the throttle closed and at 70-90 deg throttle plate position.

The problem goes away after the engine has warmed up. Could this indicate that it is related to a coolant sensor, or a combination of a coolant sensor and something else? It acts like a vacuum leak that requires a cold motor and 4-5k of vacuum to open up. Bad valve cover gasket? I don't get it.

[bimmerboy709]

I had then same issue with my m20. Check if your plug wires are shorting on the exhaust manifold... That was my issue.. Lol

[rcfanatic]

FWIW the TPS was full of oil, but it tested fine

[jlevie]

The TPS checked out fine. Continuity only with the throttle closed and at 70-90 deg throttle plate position.

The problem goes away after the engine has warmed up. Could this indicate that it is related to a coolant sensor, or a combination of a coolant sensor and something else? It acts like a vacuum leak that requires a cold motor and 4-5k of vacuum to open up. Bad valve cover gasket? I don't get it.

At that engine speed I'd tend to think that an intake leak bad enough to cause a problem would cause other, more obvious, problems. When the engine is cold (below 60C) the DME is only using rpm, temperature, and AFM (below 4500rpm if the WOT switch is open) data. If the WOT switch is closed only rpm and temperature data is used above 4500rpm whether the engine is in open loop or not. A bad spot in the AFM trace is a possibility if the problem occurs below 4500rpm.

[rcfanatic]

Bummer, it definitely starts before 4500, so maybe it is the AFM. Just bought it remanned from NAPA two years ago. Guess it's a good time to convert to an MAF

[rcfanatic]

Now it appears to be doing this even when the engine is warm. Is there no way that it could be a fuel starvation issue?

[jlevie]

If the cause were fuel starvation one would expect the symptoms to get worse as the engine speed increased.

[rcfanatic]

OK, thanks. I just wanted to make sure before I pull the trigger on a new maf

[rcfanatic]

Now it appears to be doing this even when the engine is warm.

I should note that although the problem has begun to occur once the engine is warm, it is still for a shorter duration than when cold.

For example, when cold it occurs from 4-~4.5k. When warm it only occurs from 4-4.1k. In both cases the AFR shoots up, there is a fluttering sound, and loss of power.

[jlevie]

Does the econometer react during an event? If so which way does it move?