diesel feels like it has a gas misfire - PeachParts Mercedes-Benz Forum

joel price · #1 09-23-2004, 12:47 PM

My 87 300D recently had an injector pump replaced. I had just driven it around town til a recent trip and at 65-70 mph it began to feel as though it was a gas engine misfiring(skipping)--when I turned the AC off it returned to smooth operation---that I think says to me the problem is in the AC--so I have observed the compressor clutch for a lengthy time and it is quiet and I see nothing in it's cycling process that would put such a drag on the engine as to make it feel like a misfire. The AC blows cold-----I am at a loss as to where to go from here --any ideas?
thanks
joel

Mr. FancyPants · #2 09-23-2004, 08:12 PM

I drive the same ride and think also the AC compressor be suspect since they do seem to drag the engine down quite noticibly, especially around town than hiway!

That said, and we know there are no electrical needs from the engine, except maybe switchover valves for ALDA, and transmission, and shut-off solinoid, check that too!

When you say it smooths out after turning the AC off, does the engine miss also between when the compressor switches off for about 20 to 30 seconds before switching back on again, when you do have the AC turned on? (that is how long I've counted between, on an 80+f day)

If so, it would rule out compressor drag, or erratic compressor drag behavior.

Or does it only do it when the compressor is engaged? If so, I would tend to think in this area until proven otherwise. You AC comp could be failing, but you also mentioned a new IP replaced recently and only after your first hiway trip.

You might try running the engine in neutral to higher rpm's with the AC on and see if it starts acting up!

If you haven't found the AC compressor the issue, maybe GSXR or Sixto can clue you in better?

Steve........

psfred · #3 09-23-2004, 08:19 PM

You can feel the AC kick on in that car. Sometimes causes some belt vibration, too, along with a slight drag as though the engine suddenly lost a bit of power. Comes right back when the AC kicks off.

If you get excess vibration when the AC comes on, check the engine mounts. When they go bad, the engine is litterally hanging on a pair of undamped springs, so will shake like crazy with the AC on. Much less noticable with new mounts.

Peter

joel price · #4 09-23-2004, 08:24 PM

Steve
Thanks for your input---When I turn the AC off the misfire sensation goes away immediately. I have tried neutral and high rpm but can't tell if the sensation is there or not---I suspect compressor failure also but in the past when a compressor failed there was always a noise ,roughness or lockup or some telltale sign. This misfire sensation is the only sign this gives as the compressor is smooth as silk and the cluth is very quiet---is there any other troubleshooting I can do before investing in a new compressor?
joel

Mr. FancyPants · #5 09-23-2004, 08:44 PM

Joel: If the AC is working fine then the issue lay elsewhere. PsFred has a point and sounds logical. I have not experienced worn motor mounts but if it where it would mean that this stalling effect you experience would have to coinside when the AC compressor kick's in, and kick's out, giving the engine a jolt each time.

But you describe what resembles to you as a "misfire" and what a misfire means to me, I would doubt would resemble what we have covered so far!

Cant think of to much else! I wonder if the shut off valve wiring is shorting with the AC? Hmmm...That's too long a shot, Hmmm.... It could be a defective IP as this problem started soon after your replacement IP. But you say it only happens when the AC is on, so that doesn't rhyme either! Try to see if the on/off cycle of the AC compressor is linked! If so PsFred's idea may be what you are hunting for!

Good Luck Joel, if I can think of anything else, I'll post..........Steve........

psfred · #6 09-23-2004, 09:25 PM

TAke a look at the left side engine mount (it's impossible to see the right one without taking the sound encapsulation panel off on with the car up on stands!) and see how much space there is between the widest part of the mount and the crossmember. If less than an inch, or if you cannot see the steel insert that the bottom bolt goes into, the mount is shot. So is the right side one, but you have to jack it up to change that one.

These are a PITA on this car, you MUST work from below on the right side (there is JUST enough room to get a short socket and ratchet, flex head only! on the top bolt -- 19 mm).

to replace, remove bottom panel, unclip fan shrouc and lay back on engine, put a floor jack under the oil pan with a board on it so you don't crack the pan, remove the 8 mm hex socket screw from underneath and the 19mm screw from the top. Probably best just to loosen, then lift the engine a bit, remove the screws, lift engine until you can get the mount out, lift engine again until you can get tne new one in (about an inch higher), put bump in slot on the engine carrier, start screws (watch on the bottom one, it can fall into the crossmember is as a bear to get back out), drop engine, tighten bolts. Right side has to be done from below, you will be black grease up to the elbows!.

Also, check to make sure that the belt tensioner spring isn't broken -- the top part and plastic shield will look fine, but the bottom end can be broken clean off and be lying in the sound panel. AC even works, although it will squeal.

I'd bet you have bad engine mounts, it's about time. You will be amazed at how smooth and quiet the engine is with good mounts in there!

Peter

Mr. FancyPants · #7 09-23-2004, 09:34 PM

PsFred: Thank you too for that info. I'm beginning to think maybe my mounts are not 100% but I'm not experiencing Joel's problem either, but I will check clearances tomorrow and see.

Steve............