Throbbing cigar hose, pulsing idle

[arkie]

Car in question is a 1983 300D turbo with circa 174K on the clock. I have had it about a month, and it has always had a bit of roughness or lope to the idle. I have tried quite a few small things so far (all filters, new push-type primer, cracked all of the injector lines), and the idle will sometimes be better when it is cold. For the first time today, I noticed the cigar hose (return line) pulsing in rhythm with the idle. This means the fuel pressure returning to the tank is oscillating - I wonder if this could be the rack dampener bolt (definitely still the old style mounted on the car - not the new gold one)? Any thoughts? I've also got some nailing when it is cold, and a little nailing at mid-range (1,500 rpm give or take) occasionally (I'm thinking time for some new injector nozzles).

The return line will always be throbbing, its from the pulses of the lift pump.

Try adjsuting the rack damper before you replace it.

What RPM does it idle at?

[arkie]

Somewhere around 500-600rpm.

[dieselnutnj]

I would also look at your valve adjustment. On these engines it tends to decrease due to the relationship between the cam and the valves. As the valves wear, they sink deeper into the head and your clearances become zero, and sometimes even keep the valves open a tad. This reduces compression in the cylinders. I have had several that would not even start, and all I did was adjust the valves.

I have one with over 300K on it, and it does its own little "dance" idling especially when warm. I know I have a cylinder with low compression and thats whats causing my wiggle haha.

Also.. another sin all Benzes suffer from.. is odometers that fail, work intermittantly etc. Unless you can document your mileage, you may have more on it then u realize.

[cphilip]

I thought this thread was going to be naughty...

[arkie]

Valves were adjusted my the very reputable indie shop it came from about 2 weeks before I bought it. They told me it needed the engine mounts done, and I plan on doing them pretty soon (they are not compressed but definitely getting mushy), and will do the engine shocks and mounts pretty soon down the road. I have access to a VERY nice hydraulic hydrotest stand that uses a Enerpac hand hydraulic pump and has 3/4" plexiglass guarding it - I plan on picking up an extra injector and lines at a salvage yard and make my own pop tester. Its all a labor of love.

[bgkast]

Originally Posted by cphilip

I thought this thread was going to be naughty...

Me too

[Biodiesel300TD]

If you are experiencing rough idle with a warm engine it's most likely the rack damper bolt. You can adjust it. I would suggest trying to adjust it before replacing the engine mounts. There are two versions of the rack damper bolt. A silver one which is the old style, and the gold one which is the new style and is supposedly better.

To adjust the rack damper bolt: With a warm running engine. Loosen the lock nut. Then back the adjusting bolt out. Then adjust it back in, but only just enough to smooth out the idle. If you go in too far it will cause hard starts. Adjust it in and out a couple of time to make sure it is right where it should be. You can also pull the whole rack damper bolt out and try pushing the tip in. It's spring loaded and if it's really stiff then the bolt is still good, but if you can easily push it in then you need a new one.

Originally Posted by arkie

Somewhere around 500-600rpm.

That could be part of your problem, the idle needs to be 700-800rpm.

[arkie]

Originally Posted by ForcedInduction

That could be part of your problem, the idle needs to be 700-800rpm.

I drove it tonight, and the idle was pretty much spot on 750 rpm (I don't know why I thought it was lower).

[arkie]

Originally Posted by Biodiesel300TD

If you are experiencing rough idle with a warm engine it's most likely the rack damper bolt. You can adjust it. I would suggest trying to adjust it before replacing the engine mounts. There are two versions of the rack damper bolt. A silver one which is the old style, and the gold one which is the new style and is supposedly better.

To adjust the rack damper bolt: With a warm running engine. Loosen the lock nut. Then back the adjusting bolt out. Then adjust it back in, but only just enough to smooth out the idle. If you go in too far it will cause hard starts. Adjust it in and out a couple of time to make sure it is right where it should be. You can also pull the whole rack damper bolt out and try pushing the tip in. It's spring loaded and if it's really stiff then the bolt is still good, but if you can easily push it in then you need a new one.

I've got the old silver style one, but I will pull it and see how it is on the spring-loaded end.

[Biodiesel300TD]

Originally Posted by arkie

I've got the old silver style one, but I will pull it and see how it is on the spring-loaded end.

I'd bet that is your problem. Let us know what you find.

[arkie]

Update - I adjusted the "old style" rack damper bolt and improved the idle GREATLY. One time during extended idle yesterday (when it was 101 degrees here) it got a little worse, so I suspect I can improve things by putting a new bolt in. Overall, it is worlds better, and the amount of adjustment was small - maybe a 1/2 turn in.