Hello everyone,

I'm looking for some advice:

M104 2.8 loses power (~30%) in certain driving conditions, especially when engine is hot. It seems like the ECU goes in limp mode, as the engine will always run normally after a restart.

This condition occurs when revving the engine over ~4000 rpm (I guess the fault happens only under full load). Anyway, problem continues through the whole rpm range, however the car is drivable and still smooth with less power. After a restart, it runs nicely and with full power, unless I rev it over 4-5 krpm.

There's also a slight difference in exhaust sounds during the power loss (become more howling at full throttle). Sometimes when stopping, idle rpm will be too high (around 1500rpm), which never occurs when engine is running normally.

Replaced: primary ignition (12V) wiring, the ignition coils, spark plugs, high voltage cables to the plugs and the short "boots" for coils. May be a harness issue as it's the original one (except ignition wires).

Neither disconnecting O2 sensor nor unplugging MAF will make a difference.

Any help (especially good HFM knowledge will be greatly appreciated.

/Tabasc0/

Forgot to mention: it does not do that always when revving. It feels like it's distance-related, thus engine temperature might be the key.

Generally, I'd look to a fuel pump with those symptoms. The "howl" is indicative of a lean condition at WOT.

What error codes have been seen when scanning the engine computer/electronics?

I had an issue with rough idle and a slight stumble off the line. Not sure if it would help your situation but its worth a try as its easy as.

I pulled out the MAF and cleaned the element. I did this simply by spraying it with isopropyl Alcohol and let dry. I also did the same for the dirty temp sensor and clean the butterfly valve in the throttle body.

Issues resolved by doing this included:

-Rotten egg smell after WOT.
-Hard cold starts
-Rough idle

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Thanks for replies.

MAF cleaned, intake air temp sensor and throttle body flap cleaned also but didn't make a difference. BTW the engine starts perfectly, no matter hot or cold.

The fuel pump makes the usual "hissing" sound, can be heard around right rear wheel. The car develops some pressure in fuel tank but I guess that's normal.

I haven't been able to read fault codes yet, but I'm looking into that. Is there a way to read codes without special equipment?

Yup, very easy to do. Search for "Arthur Dalton code reader" or similar strings. The code reader requires a pushbutton, a 12v LED, and some banana plugs and wire. About $10 (U.S.) at your local electronics store. Read through all the threads until you understand how to use it.

I'm betting your MAF is dead, given that disconnecting it doesn't affect driveability in any way. The ECU must have already decided to ignore input from the MAF and is essentially guessing at the fuel/air mixture. Had this on my '93 E280 as well.

- JimY

Ok, thanks for the tip.

Now the MAF sensor should be fine: I cleaned it carefully with CRC's MAF cleaning solution and rebuilt the four crispy wires near the ECU to the MAF connector (which is very "nice" to disassemble and put back together btw ...) Then I tested it: I disconnected MAF sensor when idling - the idle became very rough and bouncing. Then I connected it again and the engine idled nice and smoothly.

However, that was not the solution for the problem: the same old power loss raised its head when I drove it hot and accelerated (WOT) to 5000 rpm. But a key cycle will help for a while.

What I found interesting is that acceleration above 5000rpm still feels powerful when problem occurring, but below that rpm it's somehow lazy. For example: when accelerating in 3rd gear from 50mph to 70mph it takes at least as long as accelerating from 75mph to 85mph... Somewhere around 75mph in 3rd at WOT the power kicks in.

I built a DIY code reader and got 4 blinks from pin #8. Where could I find explanations for these blinks?

Argh... stupid me. Four blinks seems to refer to a faulty MAF. And the presence of that code didn't surprise me, as I just described how I had tested the MAF (unplugging while engine running). Well, at least the diagnostics seem to work...

There were no other error codes (just those 4 blinks every time I scanned). I didn't reset it yet but I guess it has no effect on engine functionality...?

Is the ECU able to notice misfiring? If it is, does it store a error code for it?

Cleared the code 4, and now getting only one blink. So I guess the MAF sensor is ok. It seems the OBD will not help me on this one...