Engine surging on idle

[satbasi]

Hi my w140 s320 has an issue when idling. The engine idle seems to be a bit unstable-surging. Ive put new plugs leads & coils on. It runs way better than before but this surging issue seems to still be there. Cleanes maf no difference. On other forums people have menstioned ovp regulator/crank position sensors.

What are your thoughts? Its a 99 320 m104 engine

 

[carat 3.6]

Dirty throttle body, or the wiring loom to it is degrading.

 

[satbasi]

Hi carat 3.6, my understanding was that the wiring loom issues were with cars up to 96/97, after which merc fitted an improved wiring loom. This is a 99 treg, so last year the w140 was built. Also looking at wiring loom it looks healthy-its not brittle or cracking anywhere??

 

[satbasi]

Just to add, the car is a weekend car only so not driven much. 2 days ago i drove it after about a week and it was alsmost cutting off at one point. Drove it today and was much better. Seems to drive better when warm

Have noticed a bit of oil on plugs when i took them out to inspect

 

[carat 3.6]

Hi carat 3.6, my understanding was that the wiring loom issues were with cars up to 96/97, after which merc fitted an improved wiring loom. This is a 99 treg, so last year the w140 was built. Also looking at wiring loom it looks healthy-its not brittle or cracking anywhere??

I tend not to use the year as a guide when it comes to the wiring loom issue, yours sounds ok but I always prefer a visual check to be sure. There's always an exception to the rule.

Just to add, the car is a weekend car only so not driven much. 2 days ago i drove it after about a week and it was alsmost cutting off at one point. Drove it today and was much better. Seems to drive better when warm

Have noticed a bit of oil on plugs when i took them out to inspect

I'd try cleaning out the throttle body with brake cleaner. Did you replace the 3 coil pack extenders?

 

[satbasi]

Hi yes i did replace them. All genuine bits from merc dealer, coil packs were bosch ones from kms.

Ive changed oil & filter few days ago. Will be replacing fuel filter over weekend too. Whilst doing this will also clean throttle body.

Do i need to take throttle body off to do so or clean it whilst on car?

 

[Terry_]

I'd check the purge valve is working.
It has mot stamped on the top of it.
This will cause hunting at idle

 

[satbasi]

Hi wgmb, where is the purge valve located?

 

[carat 3.6]

Purge valve is located behind the abs unit.

 

[Vlad]

Start with cleaning the throttle body first....its very common the M104. Make sure you get the rim of the butterfly flap in the throttle body nice and clean. I normally push down on it gently to open it and then spray some carb cleaner and clean the rim with a cloth. Its normally black and sticky with oil, dust etc etc.

 

[satbasi]

Right an update..cleaned throttle body, replaced fuel filter & air filter. The car now runs slightly better but not quite 100%

I tested to purge valve which seems fine

Then i ran engine & disconnected maf..the car almost cut off but then ran fine on idle & seems to run better with the maf disconnected. So the question is does this mean the maf needs replacing? It runs slightly better without maf. Is there a more definate way of testing?

 

[satbasi]

Hi all, the problem is still the same & its annoying me. Just been thinking could it be a vaccum leak? If so, where are the common areas to look for one on a m104 pls?

 

[hfmscan]

Hi all, the problem is still the same & its annoying me. Just been thinking could it be a vaccum leak? If so, where are the common areas to look for one on a m104 pls?

Hello,
places where the engine can take a lot of air left. Typical leak through the valve of the EVAP (MOT) to show you.
Any vacuum tube, jumped out of the intake manifold ...
violation of seals, etc.
all air leaks looking carburetor cleaner using a particular mode of operation of the engine ...
also usually leaks on error codes indicates lean. you have an error?

PS: unless there is W140 with engine 104 production 1999?