Vehicle information:
1995 E320 Sedan
101,000 miles
updated wire harness
new head gasket 8K miles ago
rewired throttle actuator
new motor mounts 5K miles ago
recent plugs (F8DC4) 7K miles ago
new coil to plug connector boots 5K miles ago

Symptom:
Slight intermittent engine burble or hiccup when at idle after engine has reached operating temperature. Hiccups/burbles are random, average time between is 5 to10 seconds. Symptom seems most pronounced when coolant temperature is in excess of 90C. No codes present in DM, HFM/SFI, or EA modules. Engine performs perfectly under all other operating conditions.

What I have tried so far:
Swapped coils, plug boots, and wires with known good from other vehicle, no change.
Removed spark plugs, check for fouling—none found. Replaced with new set of F8DC4 plugs—no change
Removed EGR valve, checked and verified complete valve closure and freedom of movement. Reinstalled—no change. Tried one bottle of Techron in case of dirty injector(s)—no change. Look for obvious vacuum/air intake path leaks—none found.

Anybody have suggestions on what I might try or look for?

J. M. van Swaay

How does it idle when first started cold? How old is the O2 sensor? If it idles fine in open loop but starts acting up after switching to closed loop the O2 sensor may be suspect. I had idle surge on an M104 caused by an ancient O2 sensor. Don't recall if it set a trouble code - don't think so. Swapping in a new part returned the glassy smooth idle the M104 is known for.

- JimY

Plug Purge Valve vac line and retry...

Update:

Jcyuhn: Symptom is not present at cold startup. I'll have to check, but I think O2 sensor is original. Would you recommend replacement as a matter of preventative maintenance?

Mr. Dalton. Thanks for pointing me in the direction of the purge valve.

I plugged the hose on the intake manifold side of the valve, then drove the car. If anything, the symptom seems to be worse. I removed the valve and found about a half tablespoon of what I assume are charcoal granules in the upstream (canister side) valve connection nipple. I suspect that these came from the evaporation canister located upstream of the purge valve. Does the presence of these granules indicate anything? Could they cause the valve to not function correctly? Could a plugged vent line to the canister change the air flow path through the canister thus drawing granules to the valve?

I’m going to look at the canister this afternoon to see if there are any plugged lines. Also, next time the symptom presents itself, I’m going to remove the gas cap and see if that has any effect. Could it be that idle hiccup/burble is the result of excessive vacuum in the gas tank?

Thanks for the help so far.

Any other ideas?


J. M. van Swaay

Update:

Removed gas cap when symptom present--no change.
Checked for unrestricted flow through cannister--OK

Swapped purge valves with known good vehicle. Caused idle surges and unstable idle in good vehicle, but symptom still present in problem vehicle.

Swapped valves back to original cars.

Gave a few squirts of WD 40 to cannister side of valve during run. (recommended in other posts I found on this subject) No change.

Is it possible that the valve is/was indeed malfunctioning but the adaptation has done it's best to correct? Wouldn't this caused an "adaption at limit" code to be stored?

I'll post if I come up with the answer.

J. M. van Swaay

I checked my notes from repairing the car. It was throwing code 13 - rich or lean limit on the O2 sensor. The symptom was more of a fluctuating idle than what I would call a burble or hiccup.

I don't have enough experience with the O2 sensor to recommend replacing it based strictly on age/mileage. Perhaps Arthur or one of the experienced techs can comment.

- JimY

Here is my .02 cents of advice. I would get fuel pressure reading before starting, at idle and after shut off. Hookup the pressure meter to the fuel rail if one available. Sometimes it could be fuel pressure regulator. If the reading are abnormal it could be fuel pressure regulator. it is located on the fuel rail. mine is E420. I am not sure where it would be on E320.

Also, check if any of the vaccum hoses are damaged. white plastic parts are brittle after going through beating from the heat build up in the engine compartment. Do a visual check. I hope this helps.

__________________

Kamal Atwal
2005 Triumph Bonneville T100 (Black/white) [Sold]
2007 GL450 (Dersert Silver)
2009 Toyota Corolla (Metallic Silver)
2002 Tundra V8 - 2WD (Desert Sand)
1995 MB E420 (Silver Smoke) [crashed, now resting in peace at Silverstar Recycling Center]

What was the solution to fixing this problem? I have the same fluctuating idle on my 94 E320. Problem started after replacing a clogged EGR tube and gaskets.

Update:

Checked fuel pressure, values are right on specification per All Data.
3.4 with vac to regulator
4.0 without vac to regulator
no leakdown noted after shutdown

Replaced O2 sensor even though no codes present. (Bosch and MB both recommend replacing at 60K miles, mine had 100K miles)

Symptom still present.

I believe I have reached the limit of my DIY abilities.
I would be happy to pay a shop with testing equipment to diagnose/fix this problem. Before I engage the help of a professional, I would like to have some idea of what the next step might be. Any information from the pros would be appreciated.

J. M. van Swaay

Good you changed the lambda sensor as they go out of spec with age.

The other item that get's tired is the coolant sensor.

This is a vital component in the feedback loop for correct fueling. As you probably know it alters resisitance with temperature, but it gets tired.

It may not be indicating the correct temperature when it should, and therefore incorrect fueling at certain operating conditions.

It is low cost and is easy to change.

Thanks for the suggestion kth286.

I'll change the coolant sensor and post results.

J. M. van Swaay

What is the status? Changing the coolant sensor did the trick?

__________________
New Addition to the family
2013 Passat TDI 5Miles
2004 E500 4MATIC Wagon 95k
1996 E300 Diesel 173k Sold
1993 300E 2.8 230k Sold
1994 E320 White Wagon 373K Sold
Still dreaming about ....500E


Radu Negru

Any results yet...?

__________________
New Addition to the family
2013 Passat TDI 5Miles
2004 E500 4MATIC Wagon 95k
1996 E300 Diesel 173k Sold
1993 300E 2.8 230k Sold
1994 E320 White Wagon 373K Sold
Still dreaming about ....500E


Radu Negru

I did change the coolant temp sensor--no fix. Next step will be to check/clean the throttle actuator right where the plate shaft enters the side of the actuator body. I'm not sure, but perhaps some varnish/build up here could cause sticking. If that doesn't fix it, I might have the injectors cleaned/tested.....

If anyone else has ideas, I'd like to hear them.


J. M. van Swaay

Tap the OVP relay while engine is at idle and look for rpm change or misfires...

__________________
A Dalton