My mother bequeathed her well-maintained 84 300D (253K) to me. It's been park and unused for over two years. I had a Mercedes specialist in CA get it in running condition for the 1600 mile trip to Houston. The car ran very well with lots of power running 75 mph most of the way (sometimes 90). After two tankfuls, I noticed the engine would shake wildly at idle. I also noticed that I only got 20 mpg. Put in 6 oz of Lucas for every tankful from then on.

What was done to it in CA: New tires, battery, air filter, alignment, engine and tranny oil change.

What I have done to it in Houston:
1) Valve adjustment (Dieselgiant procedure). All valves were tight with 2 exhaust and 2 intake almost w/o clearance. Noticeable improvement but still rough idle. Wish I had this done in CA but car was running well then.
2) Two cans of diesel purge (Dieselgiant procedure). Significant improvement in power. Idled well for a while but after driving for 20+ miles, started shaking again at idle. MPG increased to 23.
3) Adjusted idle but no help up or down so I put it back where it was originally

In order to determine whether the injectors are faulty, I cracked open 1/4 turn the injector supply at each injector. 1,2,4 did not further deteriorate the rough idle. What is interesting and not expected was that cracking open 3 and 5 actually improved the idle??? (or did it reduce the shaking because it also stopped contributing power and therefore "balanced" the engine??) I'm suspecting underperforming fuel injectors or bad fuel delivery (IP perhaps?).

Another symptom, I notice is a constant knocking sound towards the front. I stethoscoped the vacuum pump and don't hear it there. It seems to be coming from 1 or 2 cylinder (upper). Knocking not noticeable when 3 and 5 injectors cracked. As an aside, I have a 87 SDL that idles smoothly with little vibration. I definitely have a sick 300D. Any ideas guys?

Do you know when/if the injectors were last replaced?

@Craig, as far as a I know they have never been replaced since she bought the car 12 years ago. I am hoping it's just a fuel delivery problem. I am concerned with the constant knocking. Hopefully, its not anything in the crankcase.

I doubt the knock is from the bottom end if it goes away when you crack an injector. The drop in mileage also makes it sound like a fuel issue. I would probably try replacing those injectors first. I guess it could also be a problem with the injector pump, but that seems less likely.

Googling injectors now. They are so expensive. I wish I had another 300D that I can swap injectors with just to ascertain before I spend the money. Thanks for the input!

If this were my car I would do a compression check. Running high speed long distance with very inadaquate valve clearances on some cylinders may not have been good. You stated the engine was running well before you left. Plus when checking the valve clearances at the destination some valves had virtually none.

Do you remember the cylinders that had virtually no valve clearance and if it was the exhaust valves? Although you also said the engine smoothed out after a couple of cans of diesel purge temporarily. . Also have a look for any bubbling in the radiator with the engine ideling.

Backing off the injector lines and getting the effects you were indicates something is up. Rather than buying or spending money on injectors. Just moving them around should clear them or otherwise..Simply the problem cylinder moves with the injector it is the injector.

since you followed the DG purge, you likely already changed them, but what is the condition of your fuel filters?
also, have you tried driving with the fuel cap off?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

@Barry123400. Unfortunately, I do not remember the cylinders. There were 2 exhaust and 2 intakes that had no clearance and all others were tight.. Yeah, I regret not getting the valves adjusted in CA ().

Will do a compression check like you suggested. I'm curious as to what the readings will be since there's a good amount of blow by. I wish I had checked that in CA as well just so I have a baseline as to whether the high speed long drive with mal-adjusted valves cause problems. Being on a tight budget, I plan to buy the Harbor Freight compression tester. Will it suffice, or do I get what I pay for?

The Diesel Purge worked for a while and that got me elated but bummed out later. A roller coaster day for me today.

I will check the radiator for bubbling like you suggested. Are you ruling out the water jacket being breached (cracked engine)? The oil still looks like pure oil. No sign of water.

Yup, I plan to move the injectors around tomorrow. 3 and 5 actually reduced the shaking. I thought that was interesting. Like I said earlier, they may actually have "balanced" the engine by not providing power. 1,2,4 had no effect. If the injectors are the problem, I am thinking that I have potentially three bad ones. I'll report my findings. Thanks for the input

@Vstech. I used new filters (main and pre-filter) when I did the DG diesel purge. It didn't take long for the shaking to disappear once the purging started.
I also meant to check the tank strainer for algae being that the car was not used for over two years but I did not have the large socket required. Looking at the old pre-filter, it was somewhat greenish. It may be that the injectors have gotten clogged again by algae but looking at the new pre-filter, it looks clean and free of debris, so I think that rules out the algae.

I will try to run without the gas cap on. Just curious though as to what it eliminates?

Thanks for the input.

The fuel tank is vented, if the vent is clogged, it's possible to have issues.

Running without the cap allows the venting to occur.

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With the cast iron head just checking on a head gasket leak. Since you did not seriously overheat the engine the head itself is probably fine.

I was trying to get my mind around the fact of the diesel purge being able to cover something like a bad cylinder. Initially I thought no but then again diesel purge exhibits very smooth running tendencies when present.

Even this is offset if as soon as diesel fuel was reintroduced the roughness of idle should have returned if the compression was seriously flawed. It seems far more typical instead for diesel purged faulty injectors to return very soon to their previous condition.

The harbour freight compression checker is so so but can do the job on a occasional basis. I would want better if I used one every day. It kind of reminds me of the real lack of quality we used to complain about with chinese products. Many members like myself own one.

Another good place to grab a better brand one in good used condition for about the same money is on ebay if not in a rush.

If a guy is a little short of disposable funds is completly understandable. If it where me and even if I have the funds. I would focus intently on changing the injector on the cylinder that stopped knocking when you backed off the injector line fitting. If the knocking ceases on that cylinder and moves to the new one you will not only be lucky. It will be worth going on. I suspect that is what you were going to do anyways.

Of course some bad injectors are a possibility. A little strange that more than one would let go over a 1600 mile trip though. Then again that car may have seen duty cycles on that trip that your mother would never have subjected the car to... 254 k on original injectors if never serviced is no shame either.

Hi,
I'd do a compression test. Harbor Freight has a cheap diesel tester that worked fine for me. A trigger type remote starter switch makes the job easier. I tested mine through the injector holes. Gave me a chance to look inside the prechamber and check the injector ends. I took the hard lines off, covered the IP with a plastic baggie, and pulled all the injectors before testing. The engine should at operating temp before starting according to manual - but cold testing is also useful.

As for the injectors, you can have them tested (spray pattern & pop pressure) at M&D (other injector shops also) on for about $10 each. But unless instructed otherwise, they like to rebuild all injectors - for the extra income and to be on the safe side.

LQK pull-a-part often has a couple of diesels in stock. A set of used injectors with run about $50 with the core, add-on charges, sales tax etc.

I don't know that your 300D has one - but on my 1985 300TD, adjusting the rack damper bolt fixed my idle problem.

Good Luck

Joseph

How do the motor mounts look?

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also something to check is the motor mounts. my 83 td knocked like a vacuum cleaner salesman until I replaced the mounts. take a look at the driver's side motor mount.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Lots of feedback since I last checked. Finally got to work on the car today. Went around town buying up 1-1/16 deep sockets (for fuel injector removal) from Advanced Auto, O'Reilly's and Lowes. Only the Lowe's Kobalt brand fit perfectly (around $10).

So here is what I did today.

1) Ran with gas cap removed. No help.
2) Checked radiator for bubbles with engine running and revving. No sign of bubbles
3) I had removed the fuel lines two days ago making sure to cover injectors and IP with plastic bags. I thought my 27mm socket would work but it didn't so the car sat for two days. Today, I reconnected fuel lines since the HF diesel compression tester instructions said to bring engine up to operating temperature before doing test. The engine started right up and was actually idling smoother (???) but still hear what sounds like a misfire. Cracked the injectors one at a time and each one degraded the idling as would be expected of a "normal" engine. Took the car for a 2 mile spin and lo and behold, the same symptoms reappeared. Cracking the injectors revealed no change in 1,2,4. 3 and 5 actually smoothed out the idling just like I reported in initial post.
4) With engine hot, I did the compression test.
Cyl 1 380
Cyl 2 245
Cyl 3 350
Cyl 4 380
Cyl 5 350

Cyl 2 fuel injector was the only one that looked different from all others. The chrome around the injector nozzle had brown varnish while the others had clean chrome.
5) Marked each fuel injector as to their original position. Switched 2 and 3, and 4 and 5. I did this because the original 3 and 5 smoothed out the idling. in 3) above. The engine started right up and ran smoothly as in 3) above. Cracking each injector degraded the engine normally as in 3) above. Took the car for a 2 mile spin. When I pulled into the garage the same symptoms reappeared. Engine shaking and rough idle as in 3) above. Cracking the injectors showed the same as in 3) above. 1, 2, 4 had no change. 3 and 5 smoothed out the idle.

Could cyl 2 being significantly lower than the others be causing the rough idle? Could the reappearance of the symptom after a 2 mile drive indicate a bad IP, or fuel injectors?

Thank you all for your input.