engine revs and lack of power -w124 2.8

[mattc]

okay symptoms on Natalies estate are as follows. Gearbox is in "D"; car stationary and foot on brake. Engine revs are "hunting" slightly between 500-1000. Feels like car wants to lurch forward constantly (not violently).

Additionally occasionally around town (we have not driven it anywhere above a 40 limit) the engine fails to deliver power - foot to floor but nothing happening.

Of course these issues are intermittent .

Engine has original loom and one aftermarket coil pack. My 3.2 engine runs smoothly - any merit in swapping my packs over? Any potential for my packs to be horlickeds by this action!

Over to the panel for ideas/discussion.

 

[Ian B Walker]

Matt. Do you want to borrow the three coil packs from that "other" engine at my place. Give me a call dude.

 

[Dave Lewis]

I think that concerning these engines with original looms, it's not possible to accurately diagnose any problems until the loom is replaced. Loom failure can produce a whole range of symptoms.

 

[mattc]

Thanks guys - my thoughts were leaning in the direction of the loom. I will call you later Ian

 

[grober]

Ovp Relay?--poor Contacts ---should Provide 12v Power To The Rotary Idle Actuator--- Might Be Worth Giving The Actuator A Clean Also?

 

[imadoofus]

Matt,

Before I change the loom in my car, I had a similar symptom. The car would hardly rev at all. Foot on the gas, and the revs would slowly and stutteringly climb, and the car was almost undriveable. At first, turning off and restarting would fix the problem, but in the end, even that didn't work.

New loom, and no hint of the problem again. I had replaced all my coil packs about a year previously.

 

[mattc]

Ovp Relay?--poor Contacts ---should Provide 12v Power To The Rotary Idle Actuator--- Might Be Worth Giving The Actuator A Clean Also?

Ok, so the OVP is behind the battery right? Simply pull it out and spray with cleaner contact? Where is the actuator and how do I clean it?

p.s. look at my avatar to give you an idea of my technical abilities

Matt,

Before I change the loom in my car, I had a similar symptom. The car would hardly rev at all. Foot on the gas, and the revs would slowly and stutteringly climb, and the car was almost undriveable. At first, turning off and restarting would fix the problem, but in the end, even that didn't work.

New loom, and no hint of the problem again. I had replaced all my coil packs about a year previously.

Piers, typically this morning she has refuelled (told me this morning the tank was sucking fumes) and no problems either before or after fuelling. Clearly changing the loom and coil packs would be a good start but I want to avoid it if possible.

I suspect this will be ongoing as individual things are isolated and eliminated.

 

[Vlad]

The surging you mention is due to a dirty throttle body and is a common issue on the M104 engine. The butterfly flap in the throttle body gets a build up of crud over the years and it stops the flap from closing properly, which in turn confuses the ECU, so it overfuels then underfuels and so on and so on, which causes the surging issue.

Its very easy to clean....just need a rag and some carb cleaner.

In terms of the other issue I would also consider changing your OVP relay as the originals had a design flaw in them and if its still the original its bound to be getting to the end of its life. I would also look at the Mass Air Flow sensor as that could also be explaining the lack of power.

And as with all M104 engines on the W124 its worth checking the engine loom for signs of cracking.....by the way if you change the coil packs, this is the most likely time that the loom will short out as you have to bend it to get the coil caps off.

 

[grober]

The OVP is located behind the plastic cover which covers the engine ECU and abs control units. Often silver coloured its usually identified by having a fuse/fuses on top. Unfortunately the contacts I referred to are inside the relay -they can also suffer from dry joints on the circuit board inside. they can be taken to bits and the board resoldered but I would entrust this to an experienced electronics man who can wield a soldering iron. The other possibility is to substitute a known good one as a test to see if the problem goes away. They are about £70 from the MB dealer IFAIK. This may not be the problem of course its just one of several possibilities.

 

[mattc]

The surging you mention is due to a dirty throttle body and is a common issue on the M104 engine. The butterfly flap in the throttle body gets a build up of crud over the years and it stops the flap from closing properly, which in turn confuses the ECU, so it overfuels then underfuels and so on and so on, which causes the surging issue.

Its very easy to clean....just need a rag and some carb cleaner.

Any pics/diagrams on how to do this?

 

[Vlad]

Where the plastic trunking, over the top of the engine, plunges over the edge into the throttle body...this is where the butterfly flap can be found. The trunking is held onto the throttle body with a large jubilee clip. To access this, its best to have a long screwdriver and to slip that under the injection pipes and get to the clip that way.

Just undo and pull off the trunking. Look inside throttle body and you will see a silver metal disc.......this moves up and down and if you push down gently you will be able to get to the edges of the disc with a rag. Its likely to be caked with black crud, so clean it all off and you should find the the surging gets better. If it doesn't then there could be a couple of other things that need looking at, but as ever its best to start with the simple things first and then go from there.

 

[mattc]

Thanks Vlad, can i use wd40 squirted on the rag or is that not a good idea?

 

[Vlad]

Use Carb Cleaner available from any decent auto factor

 

[kth286]

SLOW down a little: there are another two nuts holding down that trunking across the top of the engine and these need to be removed.

Also there is an air temperature sensor inserted in the corner of the trunking and there is a wire feed to it. The wire needs to be unclipped carefully from the sensor and it is often difficult to see how the clip attachment works, so take your time.

Also there are highly likely to be trouble codes stored and so it is advised to get these read from the diagnostic socket first by a specialist.

 

[imadoofus]

SLOW down a little: there are another two nuts holding down that trunking across the top of the engine and these need to be removed.

Also there is an air temperature sensor inserted in the corner of the trunking and there is a wire feed to it. The wire needs to be unclipped carefully from the sensor and it is often difficult to see how the clip attachment works, so take your time.

Also there are highly likely to be trouble codes stored and so it is advised to get these read from the diagnostic socket first by a specialist.

Removing the 'trunking' isn't hard (or particularly delicate). Undo the two nuts on top of the engine, at the bak of the trunking. Loosen the jubilee clips at each end of the trunking (the one on the right needs a long screwdriver), and the temp sensor wire is just a 'squeeze and pull' thing.

Before I replaced my loom, I had no fault codes at all, so depending upon what the problem is, that isn't necessarily going to tell you much either.