95 E320 M104 poor idle and flat spot - PeachParts Mercedes-Benz Forum

W124coupe · #1 06-04-2006, 06:23 PM

Hi, I have the above problem - flat spot is around 3,500 RPM and then picks up and flies at 4,000 RPM and above. Idle speed is steady but misfiring (only when hot).

Car is new 2 months ago to me but dealer records show harness replaced 2003, O2 sensor and MAF 2004. Also coils replaced plus ECU coil output stage rebuilt by local inde late 2004). The OVP is the old part number but appears OK (fuse, contact and soldering all fine, voltages measure OK)?

I did the B service plus all fluids and replaced plugs and boots. Also read the codes and fixed a broken knock sensor lead which improved things a lot but, having driven another E320, this one ain't right! (swapped MAFs with this one and reset adapt - no change).

No codes from the reader now.

Cleaned and checked throttle flap, intake temp sensor, resonance flap clear and moves with vac. Cleaned up and checked fuel vapour recyc system (valve clear and "beats" OK, cleaned by Arthur D's WD40 method). Can't find any vac leaks.

Please advise what to check/look at next.....thanks!

W124coupe · #2 06-04-2006, 06:30 PM

I forgot to say the car does not have EGR and has the cable throttle (linkage all checked and adjusted OK as per the CD).

Arthur Dalton · #3 06-04-2006, 06:41 PM

What are you using for plugs?

W124coupe · #4 06-04-2006, 06:46 PM

Arthur, a new set of Bosch F8DC4s went in with new boots.

Arthur Dalton · #5 06-04-2006, 06:48 PM

Pull the vac hose off the fuel regulator and see if there is any fuel evident there..

W124coupe · #6 06-04-2006, 06:53 PM

OK done that - took the hose off both ends - its dry.

Arthur Dalton · #7 06-04-2006, 06:56 PM

You seem to have covered the bases..
May want to try tapping the OVP while the car is at idle and look for rpm changes or rough running..
Those old # OVPs are crap and they are the feed for ignition, which automatically drops power to the injectors when there is an gnition power feed glitch...

W124coupe · #8 06-04-2006, 07:10 PM

OK will do. I supose I should swap it out for the new part as a preventive thing anyway judging bt the posts on here?

Could the OVP affect running at the flat spot (which should also be the torque peak from the specs for this engine)?

Could the problems be intake or injector air leaks? What should I spray around to find a leak of that sort?

Arthur Dalton · #9 06-04-2006, 07:14 PM

Vac leaks would be my next suspect...look at all the lines and I would even go so far as disconnecting some [ such as purge,tranny upshift delay, air pump, etc ] and retrying with each disconnected..
What modules did you get codes from?

A scope on primary/secondary would be nice..............

W124coupe · #10 06-04-2006, 07:30 PM

No codes at all - clean. The car doesn't have the DM so read pin 8 - single flash - and also the SRS and convenience pins - all clear.

I worked through the vac component connections I could find, removing one at a time and plugging the hole - didn't find anything that fixes the idle issue but didn't try the 3,500 rev drive (yet) with each disconnected.

Not sure what's best to use to look for manifold leaks (rumours around that carb cleaner affects seals?) - I have a DMM with long leads to be able to monitor the O2 sensor output.

W124coupe · #11 06-04-2006, 07:34 PM

I don't have a scope - will need indy for that. Looking back at the invoice, all three coils were replaced when the ECM was refurbed (or at least it says so!).

They all look similar and measure the same Ohms for primary and secondary but I can't test them firing (or at least I don't know how?)

Arthur Dalton · #12 06-04-2006, 07:51 PM

I have a DMM with long leads to be able to monitor the O2 sensor output.
>>

you can catch the 02 sensor sig wire right under the passengers floor mat..
The test here is to pull the vac hose off the fuel reg and watch for high[rich] o2 voltage, then pull a vac line and watch for lean condition. You are using the engine as a test platform for the sensor by manually changings the a/f mix and seeing if the 02 responds...unplug the sensor at the connector under the mat when doing this so the ecu can not correct/override your manual inputs..
Are you sure they changed the boots/resistors under each coil .. very important !!

..also, did you get a code from pin 14 , if UK even has that.......................

W124coupe · #13 06-04-2006, 08:08 PM

OK I understand - test with the O2 signal wire disconnected so the ECM can't compensate. Will do. I will also use a water spray around the manifold in case there is a gasket leak or something.

My car has only 4 of the reader pins connected on the diag connector - pin 8 for the ECM and then pins for SRS (6), A/C (10) and the IR/convenience module (12). Pin 14 is empty.

The only other problem I have found with the car is low A/C refrigerant (so the aux fans don't come on at slow speed as the high pressure switch doesn't fire). This does mean I get the temp cycling up to 105C in crawling traffic but I'm getting a recharge with dye later this week to sort that one.

I will post back with what I find from the vac tests.

Thanks for your help.

W124coupe · #14 06-04-2006, 08:11 PM

Re the plug boots - all 3 under the coils are new Bosch.

The coil leads & caps to the other 3 plugs have not been changed recently - can't see a date code on them - do they have a short life as well?

Arthur Dalton · #15 06-04-2006, 08:25 PM

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>>

OK

You can put a cabin sw in parallel with the a/c pressure sw so you can manually work that.. or you can use a 1100K resistor across the blue sensor at the thermostat housing that will drop the cut in for high fan to 100..

Anyway , the boot problem has always been the ones under the coils..
I still would be suspect of OVP.. that might show up with the tappy tap test [ or a scope on the coil common 12v feed. for sure.]