Sorry for the long post, but I hope a little extra time reading will be made up for by allowing shorter answers.

Symptoms:

1. Engine starts, idles, and shuts off fine. A little smoke when cold, but none when starting warm. 142k miles on engine.

2. Glow plugs generally relight shortly after ignition and stay on 20-60 seconds. From other threads, I’m led to believe this is normal on 606 engines, as long as they don’t stay on for an extended time.

3. Acceleration to highway speed (65 mph), whether gentle or aggressive, is generally smooth and smoke free. A slight but perceptible hesitation (but no smoke) begins anywhere from 1400 to 2200 rpm and continues to about 3000. Cruising at 3100 rpm (75 mph), performance is great.

4. The real problem is when rpm drops from 3100 to about 3000-3050 (e.g., when I have to slow slightly because of other vehicles ahead) AND I try to maintain speed by depressing the accelerator. The engine hesitates severely, rpm continues to drop despite the attempt to accelerate, and it smokes (grey to slightly bluish grey) like a beast.

4a. If I try to maintain speed in 4th gear, it will slow to under 2500 rpm (bucking and smoking the whole time) before smoothing out enough to accelerate again.

4b. If I floor it, it downshifts, rpm kicks up to near 4000 and it accelerates smoothly (with little smoke) back to 75 mph.

4c. If I let off the accelerator and let it glide down to 2400 rpm, it will do so without smoking and I can then gently accelerate back to 3100 rpm.

5. Check engine light is on. Actron CP9135 OBD2 tester (borrowed from AutoZone) read two codes: P0120, TPS/Pedal position sensor A circuit malfunction; P0380, Glow plug/heater CKT A malfunction. Cleared codes after reading, but CEL returned on next drive.

6. Have run Diesel Kleen (1 oz/3 gal) in last 3 tanks; symptoms remain.

After reading a number of other posts, I’ve come up with a list of things to check/clean/fix:

A. Vacuum pump, vacuum transducers, vacuum system leaks
B. Vacuum control valve to main intake baffle; VCV at EGR valve
C. EGR valve (clean)
D. Vacuum manifold sensor line
E. Electrical connection to VCV’s
F. Pressure control flap, intake resonance flaps (carbon buildup)
G. Intake manifold (soot/sludge buildup)
H. Fuel lines (air leaks)
I. Fuel and air filters
J. Fuel tank vent
K. Injection pump timing
L. Engine control unit

My questions:

1. Am I on the right track, or are there additional suggestions?

2. In what order should I do A-L above?

3. I still have my Mityvac from my last Benz (call it a premonition), and I also have a good digital multimeter. Other than the usual hand tools, will I need any other special tools?

4. Assuming I remove the intake manifold for the above checks, and the valve cover (to replace the leaking gasket), is there any other maintenance I ought to do “while I’m at it”? (For example, should I change the glow plugs? I think they’re original, but I dread what this could mean on an aluminum head 606 with 142k miles.)

Thanks in advance for your help!

Paul

I'll bet you it's the:

Pressure control flap, intake resonance flaps (carbon buildup)

And give that EGR a breath of fresh air already.....