Diagnosticians Wanted

[JayZ3]

1998 BMW Z3 2.8, single Vanos, 80,000 miles. Relevant maintenance: New plugs, new fuel filter. Fact: No engine codes.

Reproducible symptom:

In second or third gear with the engine at operating temperature, at low RPMs (1,000 to 2,000), with wide open throttle (WOT), the engine will miss once, sometimes twice, before picking up and running normally.

Non-reproducible symptoms and 'feelings':

At 130 in fifth, the engine missed. Once. Haven't repeated the experiment.
There may sometimes be a slight hesitation crossing 3,500-3,800 RPM in fifth gear - This is very subjective, but it may be that as the Vanos kicks, the engine hesitates. Only in fifth. Maybe. It's slight.
Also, it seems that some mornings the engine may take an extra half- or whole-revolution to start.
Acceleration in first gear may be a little jerky; or, the roads may just be a little rough and my foot not so even on the pedal.
The problem is getting worse and has gone from a "Did it just do that?" in the spring, to the reproducible symptom above by the end of summer.

My thoughts:

WOT at low RPM is when a carbureted engine would love the accelerator pump; therefore, I likely have a lean condition that the ECU can't compensate for. The same for the miss at 130. Possibly a small intake or vacuum leak: a hose, CCV, etc. But, a warm engine likes a leaner mixture; it doesn't do it when cold or warming up.
Or, an injector failing; or the fuel pressure regulator; or the fuel pump.
Or, a bad O2 sensor, but that should throw a code.
A bad coil should show up at any RPM, not just in one particular circumstance.
A bad Vanos on an M52 shouldn't be symptomatic at low RPM.

Your thoughts?

[khammack]

First thing I would do is pull the codes.

[JayZ3]

No codes. It was the first thing I did.

[328 Power 04]

What did you use to pull codes? Any shadow codes?
I assume check engine light is not on.

[Tallyho322]

What did you use to pull codes? Any shadow codes?
I assume check engine light is not on.

What is a shadow code? I saw this reference in the INPA diagnostics software but didn't understand what it meant?

Greg

[Mr Bingley]

Sounds to me like you're at least 1000 rpm low for the gears and throttle positions you are in--1000 rpm in 3rd at WOT is not ok and will harm your engine--stop doing that.

[gmushial]

Sounds to me like you're at least 1000 rpm low for the gears and throttle positions you are in--1000 rpm in 3rd at WOT is not ok and will harm your engine--stop doing that.

Painfully true: WOT at 3000 or above, maybe 2500 if you must.

[rf900rkw]

My thought is all same for all of your statements. Why?


Accel pumps where not to fatten the mixture. They were to keep the mixture the same while the air accelerated, building up a vacuum in the ventruri that will eventually suck fuel through the tiny jets. The pump covers for that time lag. Why would fuel injection want or need such a feature?

Why would a failng O2 automaticly set a code? And why would an O2 failed or otherwise have any effect on WOT operation?

Why would a bad coil show at any/every RPM? It's going to show at BMEP peaks, which are predictable and repeatable.

Why could it not be the VANOS? It's always being controlled; every RPM.

And finally, as others have said, WHY in the hell are you banging the throttle open at 1000 RPM?

[JayZ3]

No check engine light. No stored codes. No INPA diagnostic software - a handheld code reader, no laptop.

In reference to the "It hurts when I do this, Doctor." "Well, don't do that." This is a new symptom. I would not have grown accustomed to asking this of the engine had it not always delivered prior to this season.

Story time: I recall, years ago, when I worked in a motorcycle shop, the owner asked the mechanics for a volunteer to give a clinic to female riders (then a new riding demographic) about maintenance. One mechanic volunteered. I saw his presentation and was impressed. Rather than attempt to educate his audience about internal combustion and suspension - to make mechanics out of them - which I would have tried to do; he asked the audience to notice change. If a vehicle is running well, and something changes; then note the change and bring it in to the shop along with the symptoms noted.

In this case, the engine has always delivered pleasing low-RPM torque without missing (and without lugging or knocking). Now the engine delivers pleasing low-RPM torque with a miss (without lugging or knocking). Something has changed.

[deni2s]

And finally, as others have said, WHY in the hell are you banging the throttle open at 1000 RPM?

What's wrong with doing that? Shouldn't ECU manage any unsafe scenarios in this case?

- - - Updated - - -

Rather than attempt to educate his audience about internal combustion and suspension - to make mechanics out of them - which I would have tried to do; he asked the audience to notice change. If a vehicle is running well, and something changes; then note the change and bring it in to the shop along with the symptoms noted.

This is just awesome! I need to stop asking my mom to do such things, as to check for vacuum leaks under the hood, and better explain this