Uneven idle, hesitating when accelerating, power is down

[little_e]

The title says it all. I've had this problem for a while - idle is uneven - engine is shaking a bit, it almost feels like it's missing, at certain throttle positions the car hesitates when accelerating, and - at least I feel - the car should be quicker. A friend owning a w124 E36 says the same.

The car in question is a 1993 500E, 171k miles.

What I have done:
Replaced S29/3 Idle switch at accelerator pedal (Mercedes)
Replaced S16/1 Neutral Safety Switch (Mercedes)
Replaced Brake Lamp Switch (Mercedes)
Replaced vacuum seals between lower and upper intake (Mercedes)
Replaced rubber between ETA and MAF (Mercedes)
Replaced a few vacuum hoses here and there, checked the entire vacuum system in the engine compartment
Replaced gasket between intake and ETA (Mercedes)
Replaced intake gaskets (Mercedes)
Replaced distributor rotors (Bosch)
Replaced distributor caps (Bosch)
Replaced distributor dust covers (Bosch)
Replaced spark plugs (Mercedes) - they are the correct for the car (Bosch F8DC4)
Replaced spark plug wires and coil wires (Bosch)
Replaced the oxygen sensor (Mercedes)
Replaced air filters (Mercedes)
Replaced ignition coils (Bosch)
Replaced catalytic converters (brand new from a company in Germany)
Replaced LH-module with an early one with WOT

I checked the MAF with STAR C3: Values are 1,50V and 21,8 kg/h at idle. Voltage rose to 2,0-2,1 at 3000 rpm.
I have checked the fuel pressure; it's 3,65 bar with vacuum @ idle, rising for a second if I press the accelerator and then going back to about 3,65 bar (with vacuum).
After shutting the engine off it stays at 3,65 bar for at least one hour (that's when I disconnected the metering device).

What I have not done/ought to be investigated:
Smoked the vacuum system
Tested the EZL

As far as I know I don't have any fault codes now.

What else haven't I checked? Where do I go from here?

[emerydc8]

I experienced a similar acceleration issue with my E420 last month. This happened right after I changed the spark plugs with new MB F8DC4, gapped to 1mm as per Klink's SOP. I noticed a rough idle which seemed to go away after driving, but there was still a hesitation while accelerating, especially when the engine was hot. It was almost as if the ignition module was retarding the timing as it tried to accelerate. I could tell it wasn't accelerating linearly. At first, I actually wondered if the transmission was slipping.

I installed another new set of factory F8DC4 plugs and the problem completely went away. So I don't know if it was a bad plug, right from the factory, or whether one of the suppressors just wasn't making good enough contact. I checked all the wires for resistance and they sat right at the 2K Ohm range, so I don't think the suppressors are bad.

Has anyone experienced a bad MB plug right out of the box? Are German-made plugs better? Neither of the plug sets I installed was from Germany. Both were from France. In any case, plugs are cheap. Maybe try installing a new set in addition to checking your insulators behind the rotor caps.

If you think the cats may be plugged, I did a post somewhere on this forum on checking the back pressure using a vacuum gauge tapped into where the HVAC pods sit.

[gsxr]

MAF values, as a VERY ROUGH guide, will be: 650rpm idle = 20kg, 1000rpm = 30kg, 2000rpm = 60kg, 3000rpm = 90kg. Within ±10% of those numbers would be fine. If you see anything radically different, the MAF is suspect... the oiling doesn't help either.

The catalysts should be silent when you whack them hard with your fist. If there is any rattle, the cats are also suspect. A proper test requires an exhaust backpressure gauge that you can view while driving at full throttle (this is important - can't check at idle, can't check in your driveway). That may explain the reduced power, but not the uneven idle.

The uneven idle could be a vacuum leak somewhere, a smoke test is the easiest method to find leaks. In the meantime, keep driving the car, give it some full-throttle time through the gears, and check codes frequently on all modules (with SDS / HHT-Win, use the Quick Test).

Oh, it would help if you mentioned the year / model being discussed here...

[little_e]

The car is a 1993 500E

[Eno]

Have you pressure tested all the injectors to check if they are working properly?


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[little_e]

No.. how do I do that

[little_e]

Status today is that the car runs better, power is up, however you can feel that there are some small misfires at idle only - and only then.
The car has got zero fault codes.

What could be causing this? A bad EZL? Could there be something I overlooked?

[Trae]

Did you test resistance in each suppressor @ 2,000ohms, and were the F8DC4s gapped to 1mm as per Klink?

The only time I have experienced misfire @ idle, not counting caps and rotors, was because I had a bad suppressor or too narrow a gap spark plug.

[gsxr]

It's definitely not the EZL. Top items to check are what Trae listed above...

[little_e]

Actually no. EPC lists the spark plug gap to 0,8 mm. I gapped them all to 0,9 mm.
It's not a bad misfire (cylinder cutting out completely), it's just... more like a small miss, and this is happening only when the engine has heated up.
I will check the suppressors when spring comes back to Norway - the car will be put into winter storage tomorrow

Edit: I’m not opposed to regapping the spark plugs to 1,0 mm, however what is the reason for doing that when Mercedes recommends 0,8 mm?

[TerryA]

The factory recommendations can always be improved upon. Trust us and GAP the plugs to 1mm.

[gsxr]

Edit: I’m not opposed to regapping the spark plugs to 1,0 mm, however what is the reason for doing that when Mercedes recommends 0,8 mm?

There's detailed discussion on this in a thread a few years ago, if you search you can find it. Short version is, when the engine is hot and the mixture is lean, the gap is too small and the spark created has trouble fully igniting the mixture. Opening up the gap helps reduce the hot-idle intermittent hiccup. It may not cure it, but it should help. From memory, the Klink/Jono recommendation was to open up to 1.0-1.1mm.

Also... if your engine mounts are old, that can amplify the feeling of the hiccup. Bad news is, replacing mounts is not a fun job, and the only good mounts available now are OE/dealer, at ~$450/pair from discounters. Search for threads on this if you need to replace the mounts.

[maw1124]

Did you test resistance in each suppressor @ 2,000ohms, and were the F8DC4s gapped to 1mm as per Klink?

The only time I have experienced misfire @ idle, not counting caps and rotors, was because I had a bad suppressor or too narrow a gap spark plug.

A year or so ago, my guys noticed a “slight miss” that I hadn’t, and replaced caps and rotors as a result. I’m not mad at that, but I’m keeping this post handy, as next time I replace plugs, they’ll be gapped to create a larger spark. Question: can simply running a hotter plug help?

maw

[gsxr]

Question: can simply running a hotter plug help?

maw, a hotter plug would help keep the plug clean, if fouling were an issue. But that is a different issue, and generally not a concern. I run a colder plug than normal in one of my cars and have never had any issues with fouling. In general the M119 has no issues with fouled plugs unless something is really messed up on the engine. Put another way, heat range and gap are adjustments/variables for two very different concerns/issues.

[little_e]

Did you test resistance in each suppressor @ 2,000ohms, and were the F8DC4s gapped to 1mm as per Klink?

The only time I have experienced misfire @ idle, not counting caps and rotors, was because I had a bad suppressor or too narrow a gap spark plug.

Excuse me for being a retard, but do you mean the resistance in the suppressor should be 2000 ohms, or should I set my ohm-meter to the 2000 ohm-setting and see what I get?
Again, I'm sorry for being a total tard

[Trae]

As long as you test the resistance from the cap to the end of the boot that attaches to the top of the spark plug it should read 2000 ohms of resistance +/- 100 ohms.

[little_e]

I forgot to mention - can this problem be - at least - partially caused by a bad MAF? When I got the car it had some reusable air filters with oil, and the MAF was not looking pretty (dirty, oily etc). It was cleaned with MAF cleaner and the values in Star C3 are correct - but still?

[JCM1]

Hi Little-e:
Rennlisters are strongly against the use of reusable oiled air filter elements for exactly that reason - oil gets on the platinum wire and despite a high temperature residue can remain. I had used an oiled fabric air filter and now run a paper filter in my 928 GTS and have had no MAF issues.

All the best,

Joe

[gsxr]

Over-oiling those type of filters can cause problems, but if PROPERLY oiled, they don't hose up the MAF. I have K&N's in a couple of cars with zero issues. The problem is almost always caused by people not following the clean / re-oil instructions and grossly over-oiling. If you watch the K&N manufacturing video, it's interesting to see how little oil they apply to new filters.

Anyway - if kg/hr values are normal in live data, it's highly unlikely the MAF is causing you any grief.