Mercedes-Benz Forum banner

1 - 20 of 23 Posts

·
Registered
Joined
·
10 Posts
Discussion Starter #1
Hello all,

This is my first Merc, I mainly mess around with 80's bimmers and have a couple e30s and an e28. I decided to try my luck with this 420sel, as it was running and driving and was only 350$. The car has a list of problems that I will address later, but the most prevalent mechanical imperfection is the extremely weak acceleration. The car starts on the first try most of the time, and if not it always fires up on the second crank. It idles at a steady rpm, but the engine is visibly not running as smooth as it should be. When in park, or neutral the car happily revs up when I blip the throttle, but when it is under load, it might as well be a 4 cylinder. Applying 1/3 throttle or wide open throttle makes no difference (car won't accelerate harder under increased throttle input) the car leisurely accelerates but it does not bog down like others have experienced. Climbing a relatively steep hill, I keep my foot to the floor and it barely is able to do 25. Taking it onto the highway, it took over half a mile to reach 75 mph. The engine does not sound like it is misfiring, but I believe it is running slightly rich as the smell of un-burnt fuel is prevalent by the tailpipe. I checked the throttle linkages and they seem fine and the air filter is clean. I don't believe the catalytic converter is clogged because even after a long drive, it wasn't that hot (or glowing red). I went to examine the spark plugs today and the plug wire pulled straight out of the rubber boot and was significantly corroded, after sticking the wire back in, I didn't notice any difference in how the engine ran. I purchased new spark plugs and plug wires and will install them tomorrow, hopefully this will remedy the problem. If anybody has any other suggestions, please feel free to let me know. The car has the M116 v8.
 

·
SuperModerator
1986/1990 W126
Joined
·
13,366 Posts
Welcome. That is a cheap car! Well done, lol. Something is very wrong there but at least it's running. Yeah plugs and wires of reasonable quality, non resistor plugs preferably but at this point you won't want to be spending too much money I wouldn't think.
There's a strainer at the fuel inlet that can be blocked, in the fuel distributor. There's a strainer on the outlet on the fuel tank under the car with an odd large size thin walled socket required. Often blocked off the tank has had ethanol fuel or other crap in it.
The usual filters of course.
Then there's vacuum, which gets a bit more complicated..
But go easy and cheap first I would. Hope you get somewhere with it.
 

·
Registered
Joined
·
10 Posts
Discussion Starter #5
The air intake system is clean. Just replaced all the plugs and wires, didn't change anything. It idles a little smoother though
 

·
Outstanding Contributor
1990 350SDL, "Grandpa's Roadster" Project Car
Joined
·
3,010 Posts
Does it make black smoke when you mat the throttle? What did the plugs look like?
 

·
Registered
Joined
·
10 Posts
Discussion Starter #9
Does it make black smoke when you mat the throttle? What did the plugs look like?
When I mash the throttle a puff of bluish white smoke comes out of the exhaust. Some of the plugs looked fine, others were wet with oil/carbon buildup. When the car was idling, I covered the intake hose with my hand and it had weak suction.
 

·
Outstanding Contributor
1990 350SDL, "Grandpa's Roadster" Project Car
Joined
·
3,010 Posts
Some of the plugs looked fine, others were wet with oil/carbon buildup.
Pull all the plugs, line them up in order and post good photos of the firing ends. You can tell a lot about the health of the engine from the plugs.
When the car was idling, I covered the intake hose with my hand and it had weak suction.
What does the "economy" gauge tell you?
 

·
Registered
Joined
·
10 Posts
Discussion Starter #11
Pull all the plugs, line them up in order and post good photos of the firing ends. You can tell a lot about the health of the engine from the plugs.

What does the "economy" gauge tell you?
I'll try to get pics or the plugs tomorrow. I can hold the revs around 1,500-2,000 and the economy gauge doesn't move. It only seems to respond when I aggressively depress the accelerator.
 

·
Premium Member
560SL 1986 244k miles astral grey / black
Joined
·
389 Posts
I'll try to get pics or the plugs tomorrow. I can hold the revs around 1,500-2,000 and the economy gauge doesn't move. It only seems to respond when I aggressively depress the accelerator.
For me this is a hint towards a quite substantial vacuum leak. Due to this loss in vacuum your AFM doesn’t move enough to push the the FD plunger upwards allowing more gas to the injectors whilst acceleration under load.

Make or buy yourself a smoke tester and repair any leaks, vacuum is a very essential part in the smooth functioning of the engine and a must do before you look at anything else.

Good luck!
 

·
Outstanding Contributor
1990 350SDL, "Grandpa's Roadster" Project Car
Joined
·
3,010 Posts
the economy gauge doesn't move.
Which end does it not move from? At idle it should be all the way to the economy side and when matted it should move toward the opposite end. If it stays on the economy side then either it isn't connected to the right place or your throttle isn't opening (or the air flow plate or something else is throttling the intake.)
 

·
Registered
Joined
·
10 Posts
Discussion Starter #14
Which end does it not move from? At idle it should be all the way to the economy side and when matted it should move toward the opposite end. If it stays on the economy side then either it isn't connected to the right place or your throttle isn't opening (or the air flow plate or something else is throttling the intake.)
The gauge sits at the economy side (black part) most of the time, but moves a little when I blip the throttle to the 1/2 way point. I am weary to put my foot to the floor as I don't want to blow it up.
 

·
Registered
Joined
·
10 Posts
Discussion Starter #15
2608641


Here are the old plugs. I took the top of the air cleaner off and revved the engine from the throttle body, the diaphragm seems to open and close as it should.
 

·
Outstanding Contributor
1990 350SDL, "Grandpa's Roadster" Project Car
Joined
·
3,010 Posts
Those plugs are nasty. The bottom one is what they should all look like. I'd suggest you pull the new plugs and take a photo of them.

Long shot: are the plug wires on the right plugs? Have you done a compression check? When you crank it over with the starter, do all the cylinders sound the same?
 

·
Outstanding Contributor
1990 350SDL, "Grandpa's Roadster" Project Car
Joined
·
3,010 Posts
Just one more thing... I plead ignorance of the configuration of the air intake system. The air flow meter is upside down compared to what I'm familiar with. That said, I don't think you can see the throttle plate the way it is. Shifting at 3000 RPM and sluggish performance is consistent with a throttle plate that is not opening all the way. (Doesn't explain the nasty plugs, though.)
 

·
Registered
Joined
·
10 Posts
Discussion Starter #18
Those plugs are nasty. The bottom one is what they should all look like. I'd suggest you pull the new plugs and take a photo of them.

Long shot: are the plug wires on the right plugs? Have you done a compression check? When you crank it over with the starter, do all the cylinders sound the same?
I replaced the plugs and plug wires a couple days ago and made sure I connected everything correctly, the car idles a little better now. I am actually from Florida, but am attending school out of state (West Virginia). I have a nice garage full of tools back home where I created "the beast" (my swapped e30), but here I have very primitive tools. I do not have compression testing equipment here. The throttle plate is the large gold tinted disk on the intake manifold that moves up and down when the accelerator is pressed, correct?
 

·
Registered
Joined
·
10 Posts
Discussion Starter #19
I finally got the car registered, so I can drive it around and better diagnose the problem. When driving the economy gauge seems to do a lot more than when at a standstill. When I floor it, it pegs in the red, when I reach the point where increasing throttle application doesn't result in more acceleration the gauge goes from about halfway towards the red zone, seemingly proportionate to how much throttle I give it.
 

·
Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
Joined
·
4,005 Posts
I wonder about vacuum leaks. One line that I would definitely check is at the back of the intake manifold. Way back there and down kind of low you'll see a fitting with a line coming off it, sort of following along the brake booster line. The trick here is that there is another line below it on the same fitting, hard to see because it points down. It goes down to the transmission. Get a mirror and a strong light and make sure you have a vac line at this second port and test it for leaks.

And I'd follow every line I could see and either test them or at least visually inspect all along their length, including the rubber connectors. Don't forget to look at the two or three lines coming off the throttle valve deep under the front of the intake manifold.
 
1 - 20 of 23 Posts
Top