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

[rcfanatic]

Another Update:

Today at WOT it stuttered 3500-4500 rpm, recovered, and then again at 5-5.2k rpm. Engine almost warm.

Also it will now do it at half throttle or maybe even less. The problem is definitely getting worse.

Another thing, twice over the last 3 weeks the engine simply died while idling. Very smoothly, as if someone turned the ignition off. There has also been a few times when it completely fell on its face while accelerating, as if there was a momentary fuel cutout.

I don't understand, everything in the fuel and ignition system is new or refurbished within 3 years, except the fuel pumps.

Could an AFM cause all of these problems and its health degrade so quickly?

[rcfanatic]

i have had this problem in the past with other cars and have this small issue with my m10...

have you checked the timing? for some odd reason, i changed the timing on my m10 and it kinda worked "like a bandaid patch"

i changed most of my fuel parts out, regulator, pump, sending unit and the problem is still there (just slightly)

in the past i dealt with the TPS and it sorta of resolved the issue as well... letting me go to a higher RPM but NEVER WOT.

have you tested your AFM yet? i havent had the chance to do that on my car yet.

but if you do, let us know how it goes

The ignition timing is controlled by the DME and valve timing can only be changed by installing an adjustable cam sprocket. Either way I don't see how either could affect this small rpm range.

[rcfanatic]

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

Cruising at 3k rpm in 2nd gear the gauge reads ~12 mpg. When I do a WOT pull it steady falls and at 4k pauses at 5 mpg until I regain power and it continues on to 0.

[hahaitzskippy]

i have had this problem in the past with other cars and have this small issue with my m10...

have you checked the timing? for some odd reason, i changed the timing on my m10 and it kinda worked "like a bandaid patch"

i changed most of my fuel parts out, regulator, pump, sending unit and the problem is still there (just slightly)

in the past i dealt with the TPS and it sorta of resolved the issue as well... letting me go to a higher RPM but NEVER WOT.

have you tested your AFM yet? i havent had the chance to do that on my car yet.

but if you do, let us know how it goes

[jlevie]

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

[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.

[rcfanatic]

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

[jlevie]

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

[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?

[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

[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]

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

[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]

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.

[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.