u/metsiker

Hey everyone,

I have a 1996 Mercedes C280 W202 with the M104 inline-6 and automatic transmission. I’m trying to track down an intermittent drivability issue and wanted to see if anyone here has experienced something similar.

Current symptoms

Car sometimes stalls when coming to a stop (traffic lights, speed bumps, very low speed deceleration)

Idle is usually stable, but occasionally oscillates slightly

Engine feels noticeably weaker when the A/C is on

Under heavier acceleration the car sometimes feels “loaded” or restricted

Occasionally there is a “pop”/backfire sound under acceleration

Transmission shifts smoothly under light throttle, but under hard acceleration it holds gears to very high RPM

Sometimes I smell something slightly burnt after spirited driving

Important discovery

At first I thought the popping sound was coming from the exhaust.

Today I opened the hood with the engine running and revved it by hand. The popping/backfire sound actually seems to come from the intake side near the MAF/intake boot area. I also noticed the intake hose/boot rapidly expanding (“inflating”) during the pop.

This made me start suspecting:

vacuum leak

cracked intake boot

IAC/idle air control related issue

intake side issue rather than exhaust

I’m going to remove and inspect the intake boot/sanfona for cracks, especially underneath.

What has already been replaced/repaired

Spark plugs are new

Ignition coils are new

Spark plug wires and boots are now new

Valve cover gasket and spark plug tube seals were replaced previously because oil was getting into the plug wells

Engine wiring harness was reportedly inspected/repaired

MAF sensor is new

Transmission fluid/filter service done

Catalytic converter/exhaust work recently done

Injectors cleaned previously

Additional info

I found some oil residue inside the spark plug boots before replacing them

Car runs relatively OK under gentle driving/light throttle

Problems become more noticeable under load/high RPM

White vapor from exhaust seems light/normal condensation, especially in cold weather (not blue smoke)

Questions

1. Has anyone experienced intake backfire/popping on an M104 caused by vacuum leaks or intake boot cracks?

2. Could the IAC valve cause these exact symptoms?

3. Any common hidden vacuum leak locations on the W202 M104?

4. Could this still realistically be crankshaft position sensor related, or does it sound more intake/vacuum related?

5. Is there anything else specific to the M104 I should inspect?

Thanks a lot — trying to keep this car original and eventually get it ready for classic/collector registration in Brazil.