I have a 1991 300TE that seems to pull nice and strongly between 1/2 and 3/4 throttle, at full throttle it seems like it bogs down or something. If I let off of the gas a little bit the power seems to return.

I'm currently leaning towards a possible lack of fuel pressure which is causing it to starve for during quick acceleration. You can feel somewhat of a bump in the power at WOT and I believe this is because there is a switch or something to provide WOT enrichment, but it is still nowhere near the power it should have.

I have recently replaced the plugs, distributor cap, fuel filter, and it has a good OVP relay. I also tried swapping a known good EHA valve and that, if anything, made it worse.

My question is, could a lack of fuel pressure cause somewhat of a 'notchy' feeling acceleration dependent on throttle position?
I have read that a bad voltage regulator could possibly cause some issues like this, but I believe my regulator is providing a steady voltage.

Let me know what you think

Thanks for the help!!

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1995 E300 Diesel - 298k
1983 240D - ~500k
2015 Porsche Cayenne Diesel - 90k
2013 Audi allroad - 153k

Fuel supply is suspect, check your filters.

The design of the CIS system uses fuel to provide counter-pressure on the Air Flow Meter, and low fuel pressure will result in rich mixture, high pressure in lean mixture (as the fuel pressure makes it more difficult for the airflow to lift the AFM plate and thus move the metering pin). This is supposed to be compensated to some point by the CIS-E system, but there is a limit.

My M103 car ran strong (bought new) until around 50k miles, always on Mobil premium fuel and driven ... spirited, then developed a hesitation like you describe when adding a slight bit of throttle. The dealer tried many things including fuel treatment and walnut-shell blasting of the valve stems to remove carbon (carbon? In my car? NOT) with no changes. I also had the entire fuel distributor replaced, no change.

I found only one fix: RedLine Fuel Injector Treatment (or something like that, but it is RedLine and I currently buy it at O'Rielys). None of the solvent-based "cleaners" like Techron (dealer tried that too) had any effect, this stuff worked, the guys at RedLine (I use their oil anyway) told me that it is a detergent based cleaner, and that solvent-based cleaners are so diluted in the fuel that they don't have a chance, ... plus they're corrosive and you need to change your oil after using it. Anyway, this is the nature of the CIS system, and fuel dribblers don't work as well as fuel injectors (fine spray), especially at low flow rates. I started running this stuff in all of my CIS cars (VW, Audi, and Porsche) and it cleaned them all up.

So it seems like a lean mixture, check first for vacuum leaks, very likely, ... then try a $10 bottle of RL cleaner, ... if that doesn't work then

Good luck,

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Gone to the dark side

- Jeff

Check the TPS. Sometimes they stick or are out of alignment and give the same symptoms.

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1990 190E 3.0L