On throttle, car hesitates

Mercedes E500 fourmatic. Hesitates on throttle. Does not accelerate evenly , hesitates especially on inclines. Its been dealer services, had spark plus and cables changed last service. Use only premium gasoline. Does not feel like knocking. I dont see the ful power of the engine right now. It feels like engine fuel/oil mixture. What do you guys think? misfire, fuel injector, fuel/oil mixture. What do you all think makes the engine hesitate when I press on throttle. AFter it picks up speed its okay.

Could need the throttle body cleaned. Hopefully @db4690 will add his comment. He’s our resident Benz expert.

Is there a check engine light?
Any other warning lights?
How many miles on this Benz?

Could be caused by too-lean of a fuel/air mixture. When you step on the pedal the throttle valve opens & allows an extra dose of air into the intake, and that extra air could be enough to cause the mixture to be lean enough that the engine temporarily stumbles. Eventually the computer figures out via the O2 sensor it is too lean and adjust the fuel injection, so it seems to run better.

Could be something else too of course. I think if I had that problem – assuming this is an OBD I design similar to my 30 year old Corolla , computer controlled fuel injected — I’d do a thorough check of the vacuum system to make sure none of the hoses are leaking, and none of the vacuum operated devices are leaking, especially the brake booster. Next, I’d check the pcv and egr systems. Next would be an ignition system check-up. Still not found? Engine compression check. Suggest to anticipate this may take a good deal of shop time to figure out.

Yeah I would vote for throttle body cleaning too and checking fuel pressure. On a carburated car, you’d replace the accelerator pump leather when this happened.

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Excellent suggestion, if OP’s car is carbureted. I had a weird problem with a similar symptom on my carbureted truck, after much investigation and grief, turned out the fuel bowl wasn’t being properly vented.

OP, suggest to tell us more about your car’s engine configuration. Model year? Engine ID? Carb or fuel injected?

I’m guessing this is around a 2004 with 5 liter V8 and AWD. So completely modern, no carb.

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No light yet. it has 148K miles

Its a 2005 model 5 liter v8, AWD. No carb

I need to bring to the dealer shop. Last service new plus. But not new cables. I dont think it can be cables because its not like a misfire. It hesitates on throttle especially on gradient slopes in SF. I have never seen the car so power deprived when in the past on throttle it pushes driver backwards because of the engine power. Air/fuel mix is what I suspect. What is the first item for change. Most probably they will touch o2 sensor at shop. Whats OBD? Sorry I am not folllowing

  1. 5 liter. V8. Fuel injected. 4matic. 148K miles .

I’d find a good independent MB shop, sure there are several in SF. Very complex car, need a good mechanic to diagnose it.

The issue in SF is that indepdent MB shops are no less expensive than a dealer. I have stuck with MB dealer shop given discounts he throws my way.

Then I’d stick with the dealer, assuming they can solve this problem.

OBD == on board diagnostics. Applies to computerized powertrain car designs. There are two versions, OBD I and OBD II. OBD I to about 1995 model years, OBD II afterward. Your 2005 car is OBD II. This means your car’s computer provides a way for a shop w/proper diagnostic equipment to easily measure the engine’s fuel-trim parameter, an excellent way to determine if air/fuel mixture is incorrect. I’ve experienced power loss & pinging when driving uphill on my Corolla at one time, turned out to be caused by an overly-wide spark plug gap. So your symptoms could be an ignition system issue as well. MB’s require specialty MB diagnosis & repair procedures and equipment, so your best bet is to hire a shop with a lot of MB experience and let them do their thing. If you don’t know which shop to use, ask other MB owner-acquaintances which shop they use.

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I don’t know anything about Mercedes Benz cars… but could the fuel pump be dying, or on its way out?

MB, like every other car company that sells their product the USA, must comply with OBD II standards and a bevy of environmental standards. So it stands to reason a 2005 MB drivetrain design can’t be that much different than any other 2005. Your hunch of a failing fuel pump has merit imo, definitely worth mentioning as a possibility.

I wasn’t suggesting that it had a carb, just remembering back to the 70s.

Do you know any good independent MB repair shop in SF? Last guy I knew took early retirement and closed shop.

I am going to guess that someone who has a screen name of Texases is not in San Francisco .

You said the independents are just as costly as the dealer so just use the dealer.