300E Engine Dragging??
 

Hey Guys,

My 1991 300E (174k mi) started behaving strangely yesterday. I came off the highway after a 420mi trip and when I hit the first stoplight the engine started struggling. It felt like something was dragging on it, pulling the idle down. Put it in neutral, gave it a touch of gas, and it was fine over 1500rpm. You could also hear an oscillating squeak in the right front of the engine bay. At idle it was shaking badly.

Of course, in Park in my driveway the squeak went away so that I couldn't track it down any further with the hood up. :rolleyes: One thing I did notice also: on the highway it felt like I had to give it more throttle to maintain speed. Don't know if that's related or not...

My first thought was that I had a pulley dragging. But I just pulled the serpentine belt & all pulleys are free & spin easily. I also pulled the valve cover and the timing chain looked ok & seemed tight.

Any ideas here?

Thanks!

Misfire?? like a bad spark plug or wire?

I pulled plug #1 and it was a perfect tan. Had to run & attend to something at that point, but will be going out later to check the other 5. I did notice that the rotor was pretty corroded/burnt for the 8k mi on it. (Actually cap/rotor/plugs/wires all are nearly new @ 8k mi...)

With the belt on & the car in gear, I am getting a bit of noise from what sound like the vicinity of the right side of the dist. I might need to pull the front cover & look at the timing chain a bit closer just to be sure... :confused:

Any/all other ideas welcome...

Check the condition of the distributor cap terminals also. Clean up the end of the distributor rotor. All ignition wires are not created equal or high quality. It's possible that inferior quality ignition wires are failing already. If all the spark plugs have tan deposits then that would rule out ignition problems.

The noise could be due to a failing timing chain tensioner.

Anything in particular to look for with a failing tensioner? Just a loose chain?

When the timing chain in our 1992 300CE was weak there was a disconcerting rumbling noise at low RPM with the transmission in Neutral. I pressed our local MB service supervisor to investigate the noise. One of their experienced techs attributed the noise to the timing chain tensioner. A replacement tensioner resolved the problem.

As you probably know the timing chain tensioner has an internal spring to maintain tension on the timing chain when the engine is not running. Once the engine is started the tensioner works off oil hydraulic pressure. Apparently the faulty tensioner was not maintaining adequate oil pressure allowing the timing chain to slap against the timing chain guides.

Finally got the new chain tensioner in. Engine still runs like crap. It acts like it's missing, but I checked all the plugs and they all look good (like I said, less than 8k mi on the cap/rotor/plugs/wires/etc). Basically the idle is garbage (very descriptive I know :rolleyes:) and the miss continues through the revs (in park). Right when I got off the highway & this started, higher revs would correct the crappy idle, but not anymore. Maybe because the engine was warm? No idea...

So my next guess would be fuel delivery? Unless someone chimes in & has other suggestions.

Did the '91 300E have twin fuel pumps? If so, could one of them have gone to hell? That would explain the hard highway acceleration on that last fateful trip... Or plugged fuel filter?

Any suggestions out there?

Did you ever check the rest of the spark plug wires? I had a spark plug wire come loose on #6 after a long highway trip. Idle instantly went to ***** and it missed really bad at low RPM. Give each of the spark plug connectors a firm push to make sure they are seated all the way.

And yes, 91 did have the dual fuel pump setup. If you haven't changed the fuel filter in the last 30-40K I would do it anyways. It's easy and cheap.

Yeah, I checked all of the plugs and they all looked good. I'm planning on going out tonight & pulling each plug wire with it running to see if there's one not firing.

Would LOVE it if it was that easy. :rolleyes: But here's hoping...

Fuel filter is due at next oil change. I bloody hate changing that thing...

Is there any way to test the fuel pumps w/o doing a flow test??

Just off the top of my head you could have someone turn the key over to prime the pumps while you are under the car. I would think you would be able to tell if one is not running based on noise or vibration?

I can usually hear it from in the car. Just not sure if I could tell that BOTH of them were working... And now that you mention it, I was getting a bit of a thump from the fuel pump(s) awhile back that I could hear when I primed it. went away at some point...

OK. I pulled all the plug wires one at a time to see what cylinders were causing problems. Turns out #4 & #6 aren't firing. However, both plug wires at #4 & #6 were sparking off the head beautifully, so I have spark. I pulled the plugs on those cylinders and they were both bone dry. So no fuel.

Now things get more vague. I pulled the #6 injector and when I crank the car it wasn't spraying any fuel. (It didn't want to start w/ the huge vacuum leak that a missing injector creates). However, if I stuff the accelerator to make it start, the injector will spray a bit.

I have no idea what's normal or what could be causing the no fuel to these 2 injectors. Any ideas/advice?

Thanks.

Its possible your fuel filter failed and contaminants have gotten to the FD, plugging up the FD orifices. Do you get fuel when you crack the lines on the fuel dist?

I crack the #6 injector line at the FD and got the same result; no fuel at cranking, but fuel would come out if I stuffed the accelerator to get it started while cranking.

I highly doubt that the Fuel filter failed. When I say it's due for a change, I'm going on my own preventative maintenance scale: It's been a bit over 30k mi on this filter (changed by me; OEM filter from Phil) and I believe that the MB recommended interval is 60k mi.

I guess verifying fuel pressure is the next step.

Yep. Really hoping it's not the FD....

I finally got my fuel pressure stuff together and did some testing tonight. Passed both the pressure test (5.4 bar) and the flow test (easily over a liter in 40sec). So I guess the fuel pumps are ok.

Does anyone know the testing procedures for the Fuel Distributor? Is there any way to test the injectors w/o special equiptment?

If you have fuel pressure, and you don't get fuel when cracking the lines at the FD...the FD is blocked or has failed internally I would imagine. Which means that def no fuel is getting to that injector. The FI system is completely mechanical....so it's pretty simple in operation.

What I need to do now is crack the lines for #4 & #6 at the FD and trip the fuel pumps by bridging the Fuel Pump Relay. The last test there I did only by cranking with the key, which I learn today is very different: Cranking with the key gives me 2.5 bar of FP and running the pumps straight from the Fuel Pump Relay gives me 5.4 bar.

And of course right when I should be dealing with and/or replacing this car (my daily driver) I stumble across a mint, early 72 SL. Bugger...

I cracked a couple of the fuel lines coming out of the FD and didn't get any fuel coming out when running the FP from the FPR. I cracked one line for a cylinder that had been running & one for a cylinder that hadn't been running, but neither had any fuel coming out.

Am I missing something? :confused:

No fuel at the lines on top with verified pressure...all points to fuel distributor I gotta think. I will be hitting up the junkyards tomorrow and can send one for cost + postage. I think they charge $38.99 + tax..

If it's not too late, I'd take you up on that offer John.

Thanks & let me know the total.

Well, I slapped everything back together & started it and it fired right up. She idled OK, but was breaking up a bit when rev-ed. I may just try to drive it & see how it responds. Needs an oil change & I have a new fuel filter to put in too, if I ever get the time...

Does it running a bit better now after the battery of tests I put it through suggest that it's more likely to be injectors or the fuel distributor?

Took the car out for a couple of runs this week and it runs like crap up to about 2500rpm then it opens up and runs ok. First day I took it out I'd say it was running on 5cyl and the second I'd say 4cyl.

Grabbed a used FD on eBay, so we'll see if that does it... :rolleyes:

Update/New Problem:

So after some fooling around I ended up needing to get a rebuilt FD (don't ask). Phil got me a great deal and I put that in tonight & it ran really good with just a bit of a miss which I immediately tracked down to cyl #1. On top of the new FD several fuel injectors have been replaced inc #1. On inspection of #1cyl I find that the spark plug has been working (not wet, dark brown in color) but that the ceramic is cracked. I run out and get a new plug, put it in, crank the car over and... bastard won't run. :confused: 5min ago it was fine. Swapped one plug out and now it won't run. If I keep my foot @ half throttle or more it'll run, but as soon as I let off it dies immediately. :confused:

I had the exact same symptoms on an M103 a few years back that ended up being a massive vacuum leak (that time it was the big one @ the bottom of the intake manifold under the mix unit). But I can't find any leaks on this one & didn't get near any lines changing the spark plug out.

I've worked on Benz's most of my life & on this particular car for almost 7yrs. I'm no genius, but I know my way around a Mercedes engine. And I'm completely buggered right now.

Any insight, tips, ideas, offer to buy the car would be greatly appreciated. :D