Acceleration... resistance?
 

Car drives fine and everything seems just perfect, EXCEPT, about 15% of the time when I push the gas hard from a standstill, there will be a slight resistance at about 5-10 mph, then the car just keeps going normally. When I say resistance, I mean, it seems as though something didn't catch right away...
I step on it from a stop, car begins to move, then at in the low RPM range of 1st gear, there is a slight... glitch, undetectable to anyone but me.
Also, I was at the grocery store the other day (Gelsons, none other! ;) ), and going slow in the parking lot, then stepped on it briefly, and there was a loud pop and I could hear and feel a clunk, this happened at about 5, 6, or 7 mph. I was going about 3 mph (still in first gr), I stepped on it, and the car made this loud POP and I could feel it and it held the car back for a brief second (less than 1/2 a second or something).
Again, this doesn't happen all of the time, just every one out of 5 or 10 times when I gun it from a stand still, there is jsut something not going/connecting smoothly. I would maybe think this was the transmission, but it doesn't feel as though the trans is slipping or anything. Though I have heard a similar (same?) sound from the trans before when coming to a stop (have also hear other stories from people about popping sounds from MB trans). But I don't think it's related to the trans, but it's possible I suppose. Also, about the same time I began to notice this, I had my differential fluid changed, and I cannot recall if it started JUST after I had it changed, or if it was after. Maybe this means something? I hope someone can shed some light. Thanks.

I know this is hard to explain, but maybe someone can help?

You didn't say what model your car is. If it's the C280 it's probably the electronic accelerator control.

It could also be the ignition coils breaking down. There are 3.

Hmm...
Interesting... never thought of these ideas.

If it's the electronic accelerator control... what does that mean? What is that? I don't have throttle by wire in my C280, that is, it uses a cable, it's mechanical.... Thanks.

for the "pop", suggest checking the health of your suspension components- closely../brake hardware/ exhaust system hangers/ trans mounts/ motor mounts>>something in these systems may be worn or loose

"lag" culprit may be related to the above or be: transmission/ driveline related, or suspension related = weight transfer causing the suspension to abnormally shift (worn bushings, etc.)

good luck & let us know what turns up.
-fad

If your car has the summer/winter mode switch for the transmission, make sure its on the s-position.

No S/W switch here...
but now that the idea of worn suspension components could play a role... I have a little more news.
My car seems less stable at high speeds than it used to. It seems more squirmy and more attention is needed to keep it in the lane. Also, upon flooring and then shift from 2-3, or from 3-4, the back end feels squirrelly (however that is spelled).
What could this be? Maybe they are all related? How and what suspension components should I check? Busings?! Excuse my ignorance, but I really don't know much about suspensions. Thanks!

Your engine in the C280 uses a variant of the 'fly by wire' setup. It does use a cable, but the throttle is all computer controlled.

That aside, I have heard these pop from time to time. Every one was a result of a lean pop. This is a term that is used widely in the industry.

You can have a lean pop from many different causes. One is if the adaptation is going out from an air leak or from the mass air meter being weak. The best way to check is have the fuel system checked for the adaptation. If it is above 1.1, the engine is running on the lean side.

One other thing to check is fuel pressure and delivery. I have replaced a hadfull of fuel pumps on C's.

Could be the fuel pump relay too coudln't it? I still have not changed that. And it acts up from time to time, usually when I'm driving about 15 mph, all fuel supply will cut out for a second, and then be fine, and then cut off for another second, then it just goes away...
And maybe the suspension is a different thing.
Thanks for the insight benzmac, but I have a question. If my car uses a cable, then the point of the fly by wire setup is only to detect throttle position (for whatever reasons, emission, computer, etc.)... correct? In no way can it over-ride the cable driven throttle, right? (My car has no ASR, or even ASD, if that matters).

It can override the cable because the throttle is driven by a motor. The only mechanical link is the LAST 1/4 of travel for limp mode.

It calculates what goes in and then uses a motor inside of the housing to apply throttle.

I know it sounds weird, but take my word for it.

I have only seen the overvoltage relay and the fuel pumps fail, not the fuel pump relays too much on the c280/

Thanks Benzmac! I had NO idea it was like this! I once attempted to change the spark plugs (attempt, was no successful), and i had the housing that goes from the air filter over the motor off and saw down the pipe, had someon push the gas and saw it move open. I guess I would think that my first idea is logical enough! Interesting... does the motor that opens the throttle ever break? That would be interesting...

(maybe this should be a new post...) What causes Limp home mode? And what is limp home mode? Only 2nd gear? What else?

Techs Help
 

Problem is still there, and progressing. But now I hear the pop more and more (along with the slight resistance), I'm beginning to think it is the suspension... and could it be the rear differential? I am definitly sure it's coming from under the car, and it's quite embarrasing because it's fairly loud sometimes!
Can anyone give me an idea?? Thanks a bunch.

I'm not sure if it applies but I"ve been told that when you change the differential oil on a jag (perish the thought) you have to use a special tool that deals with a spring load inside the case. If you don't, you get the same symptoms you are describing...one of the details the jag mechanics like because unless you're a jag tech you don't know this and the rear end very quickly deteriates to rebuild time. The technician who shared this with me said mercedes has a similar design on at least on some models. I don't know how accurate this information is ..just passing it along.

suspension diagnosis isn't easily described in a step by step receipe format.... since there are many parts that interplay (some with more critical roles than others in daily use)

their engineering, & performance interplay must be understood prior, the test drive characteristics/ observations/ conditions noted, specs reviewed, visuals on worn items noted, and.....I've seen on occasion a big lever bar used to test flex points and bushing conditions closer

its best to sit on the shoulder of a compent tech and have him/her explain their inspection process and reasoning for the resulting report card....suspension items do wear over time

-fad

Heavy Foot
 

Brother of The Benz, David
An article in the MBCA Star Magazine made note of combustion chamber pressure when the engine is driven with hard starts, full throttle exceleration.
The author made note of how the valves are slamed closed from the high combustion pressure resulting from full throttle application.
The price we pay for stoplight racing is short valve and valve guide life.
I know this doesn't help you resolve your problem, but the Star information opened my eyes and I'm looking forward to 200,000 miles before the cylinderhead has to be serviced.
Good Luck!
Happy Trails Beep Beep from The Spiderman in Houston!!!

DOnald,
I read that also. Thanks for the advice. What does a job like usually run? A few hundred bucks, correct? But I am *thinking*, although I very well could be wrong, that it's something else. What about bushings, or differential mounts? Transmission mounts? I guess I'll just have to check it out, but I can't duplicate it when I try and show anyone! Either that or no one notices it. Thanks again.

A Few Hundred Bucks?
 

Brother of The Benz, David
There's nothing on a MB engine that costs a few hundred bucks.
An exchange cylinder head is approx $1500.00.
Now you see why I'm no longer Mr.Lead Foot, and haven't been for 100,000 miles.
Happy Trails Beep Beep from The Spiderman in Houston!!!

Well I'm finally getting my lower control arm bushings fixed on Tuesday! THe car is extremely unstable and now feels unsafe for me to drive, especially on the freeway! I just got new tires Saturday, so the new bushings should really add to the ride.

I was on the freeway this evening coming home, and the car just feels out of control! It's all over the road, the steering sucks, it tracks left and right, delays when correcting minor wheel changes, and the back end feels like it's sliding off the chassis sometimes when going fast around slight turns, or a lane change, for example (I am pretty sure that's just a symptom of the severly worn bushings though). Will post results soon, I CAN'T Wait to get this service done!!

Spiderman, that sounds like an interesting article. Do you remember which issue? I would like to read the whole thing about the valves.

Spiderman, I second Souther Son!!

High Combustion Chamber Pressure
 

Brothers of The Benz, all lead feet drivers
Sorry for being so late in replying to your request for the dates in the Star.
Part one July/August 2000
Part two September/October 2000
These two articles are the best published in a long time.
Read and take note.
I've been aware of the condition since being a student at General Motors Institue, Flint, Michigan.Class of '58(five hard years of excellent Automotive Engineering Education.
Happy Trails Beep Beep from The Spiderman in Houston!!!

Spiderman, I read the articles to which you referred by Stu Ritter. Although he does state that the BMEP (brake mean effective pressure) does hammer the valve as well as the top of the piston, (therefore forcing it in the opposite direction) he fails to quantify the extent of resulting wear to valve and seat. And, of course, that is the real question.

If all BMEP resulted in degradation of engine life expectancy, I would think that a diesel should have no chance of making it successfully around the block.

In addition, my LT4 vette has 10.8 to 1 compression, high volume fuel flow injectors, and makes approximately 390 shaft HP. Since this engine also falls under the Federal mandate that all major components must reasonably last 120k miles, I can only assume that the valve components must be of good enough quality to take a fair amount of punishment.

Since you have been educated in this field, can you quantify the effects of BMEP upon modern components? (as you know, sometimes theory and practical application can vary widely).

ttt

PROBLEM (not solved, but...) FOUND!!
 

Ok, well I finally had the lower control arm bushings replaced on Friday. The car drives SO much better now. And the total only came out to $450!

Well after getting my car back (a dream after driving that Pontiac Grand PEICE OF ****!), the difference was astounding. I could actually control the car again. But shortly after getting my back, I noticed the popping sounds were coming quite often. I thought that maybe they just needed some time to set in (?). Well I know they do not. But the pops are there! I can hear them coming from the front end, but I can also hear it from the rear end of the car too! And when I hear these pops, it's pronounced that they are the "acceleration resistance!" So when I hear or feel one of these pops, it happens at the same time the car is having it's "resistance." So I now know what my acceleration resistance problem is!!! And I'm definitely glad it's suspension related.

1. What causes pops in the rear with the suspension? PLEASE don't tell me the rear has the same kind of bushing too...!!

Taking the car back in early next week to have them fix the front bushings again and inspect the rear suspension.

Thanks!!

start off pop and hesitation
 

a few things come to mind
1) the transmission clutch pack is breaking loose and slipping. can be checked by holding the brakes hard and slowly pushing down on throttle. It should hold full throttle torque without any jumping etc. Watch the engine mounts while doing this you may be feeling the engine trying to jump out of the engine bay!

2) A broken/loose flex joint in the drive shaft can cause a jerky feeling as you try to accelerate.

3) Electronic throttles are plagued with defective wiring insulation. The stuff just crumbles off the wires inside the hard covered wiring harness. Early symptoms may include missing under acceleration, missing at higher RPMS, and finally go into limp in mode Benz/Bosch are pretending there is no problem, yet they are replacing lots of wiring harnesses and electronic throttles under warranty. Those of us 'out of warranty' are stiffed.

There should be a NHTSA recall campaign. Ford got hit with several hyndred million judgenment because a $10 ignition part would fail some of the time after several year.

Thanks for the reply! Ok, so we rule out number 3 because my wiring harness was already replaced.

As for number one, when you say to push the brakes and step on the gas a little to check the trans clutch packs, what will I be looking for if that is the problem? Are there any other symptoms that might relate to this problem?

And for number 2, can I inspect this myself? Is this an easy/expensive fix? Again, are there any other symptoms for this? Thanks a bunch!

"As for number one, when you say to push the brakes and step on the gas a little to check the trans clutch packs, what will I be looking for if that is the problem? Are there any other symptoms that might relate to this problem?"

Well I have noticed lately that when I engage reverse and push the gas anything harder than lightly, it will sometimes make a POPPING sound and have an "acceleration resistance."

Does this sound like the transmission clutch pack?

Thanks again everyone, and I'm sorry this has gone on so long.

(for Future Archival Reverence!)
 

Well...
The results are in!!

My ACCELERATION RESISTANCE was a symptom of a worn Front Flex Disc. Part costed me about $60 (from here)... and I don't know what the labor costed, probably about $200. I'll find out later. Thanks for everyone's input and I'm so glad to have this problem solved! This post ALSO helped me to figure out that I had worn lower control arm bushings also. You all made me lose $700!! :rolleyes: ;)

I HAD A C280 THIS PAST SUMMER WITH A POP NOISE ON ACCELERATION AND RAPID DECERATION.
TURNS OUT THE TRANSMISSION CROSSMEMBER JUST WANTED TO BE MUCH TIGHTER THAN THE FACTORY THOUGHT IT SHOULD BE.
THIS HOWEVER WOULD NOT EXPLAIN RESISTANCE TO ACCELERATION OR LOOSE HANDLING.
CHECK FOR WORN OR DAMAGED REAR CONTROL ARMS IN REAR SUSPENSION AND WHILE YOU'RE DOWN THERE CHECK YOUR HALFSHAFTS(THEY MAKE POPPING NOISE AND CAN CAUSE RESISTANCE ON ACCEL.)
MF

Mike,
notice that I solved the problem. But thanks anyway. It was the flex disc (and lower control arm bushings).

BTW, WHY ALL THE YELLING!? I'm sorry, but it's really annoying!!!!! :) ;) :eek: No offense, it's just that the CAPS LOCK IS KIND OF IRRITATING!!!!

Thanks for the info though. ;)