My engine has gone asthmatic

[mattc]

Noticed yesterday my engine was a bit sluggish. driving home it did not seem to want to pull more than 2500 revs. at home on the drive in N it would rev, unwillingly to 4000.

Its not misfiring as such (atleast not like it does when a coild pack fails), rather its more like a bit of juddering and is generally struggling for any get up and go.

Have changed MAF sensor which did nothing.

What would the panel suggest I look at next please - and given I am mostly clueless please spell it out in detail with a How to!

thanks in advance

matt

 

[wemorgan]

Have you checked/replaced the air and fuel filters?

 

[mattc]

Air filter is pretty clean - when I changed the MAF I ran the engine without the filter connected and the result was still the same.

I have not checked the fuel filter and am not sure when it was last changed.

 

[Dieselman]

The fuel filter would be unlikely to restrict fuel in neutral as the engine just doesn't require much then.

I'd probably go for an air leak. Pull a plug out and have a look at the colour.

 

[mattc]

Any particular plug or are you suggesting they will all be discolouring?

 

[Dave Lewis]

Perhaps a sensor has gone out of spec through age - water temp sensor in the water pump housing (about £25) and the air temp sensor on the plastic intake duct downstream of the MAF (£12). Is it on the original engine loom?

 

[mattc]

I have a spare water temp sensor - I will fit that. Did not know about the air temp sensor - can i get that from ECP or do i have to go to Merc for it?

 

[Dave Lewis]

I got one from ECP for £12 a couple of days ago, and it has improved idling and throttle response. It's a snap fit in the intake duct, 30 secs to change, so worth a punt. Do you have the original engine loom?

 

[mattc]

Think car is still on original loom.

Car has no problems starting and ticks over happily - its under load that the problems seem to manifest. I will swap the sensors from the other car and see whether that affects either.

DM-you mentioned air - where should I be looking for this?

 

[Dave Lewis]

Think car is still on original loom.

This is the problem with these engines - since the crumbling loom is inevitable at some stage, it is difficult to diagnose any running problems unless it is known to be good, or replaced, as the symptoms of a bad loom can be varied and intermittent. One could spend time and money replacing various sensors, investigating air/vacuum leaks, etc to no avail. Since the ECU controls ignition and fuelling, it relies on accurate information from the various sensors, and that information reaching the ECU. The loom is a ridiculously expensive item for what it is, but once fixed, will cause no more problems. Since having mine replaced last year, I have replaced most of the sensors too, as these go out of spec with time.

 

[w124coupe]

1994 320 - assume this is the HFM engine?

You should read the stored fault codes to at least get a pointer to the problem - there is a thread on here for a home made blink tester easily made up for under £10 which also allows codes to be reset (after running with the MAF disconnected for instance!) so you can just work on the "real" ones.

post #4 in here: http://www.mbclub.co.uk/forums/showthread.php?t=29373

Re the loom, any or all of the wires to the various sensors and to the coil packs can harden and the internal insulation turns to dust when the wires are moved around. Worth cutting into the insulation on the wires to the coil packs to see the state.

If the loom was replaced, its worth checking to see if the two knock sensors were correctly re-attached as they are a bit of a pig to get to (one knock sensor wire on my car was neatly folded up with the coil leads!).

 

[Dave Lewis]

its worth checking to see if the two knock sensors were correctly re-attached as they are a bit of a pig to get to (one knock sensor wire on my car was neatly folded up with the coil leads!).

Where are these located?

 

[grober]

Other possibilities are
failure/malfunction of the variable inlet manifold actuators/flaps
OR ALTERNATIVELY
failure malfunction the inlet cam timing adjuster.

 

[lynall]

Here http://www.detali.ru/cat/oem_mb2.as...GM=717.450&CT=M&cat=520&SID=15&SGR=075&SGN=04



Lynall

 

[w124coupe]

Where are these located?

Front and back below inlet manifold (easy to see but hard to reach).

There are lots of possible reasons for your symptoms - check the loom and then reset all the codes. Drive the car and then read the newly stored codes to let us help you narrow it down...

The alternative is to guess and fiddle for a while and hope to get lucky!

Could be (just a sample):

Loom (short(s) to one or more sensors or to coil(s)), plugs, plug leads, plug "boots" under coil, coil failing under load, bad new MAF (cheap copy?), knock sensor(s), water temp sensor, air temp sensor, cam sensor, fuel problem (pump, blocked filter), O2 sensor. Or, of course, a combo of 2 or more.

Make a blink tester (very simple and cheap) or go to a dealer/indie/Bosch centre and get the codes read, reset and then after driving and experiencing the fault, read again.

 

[Roger Jones]

How many miles on the car?

 

[mattc]

136K - lots of history and bills. I think I will build a reader and then start swapping individual sensors from the estate one at a time to see if that makes any difference.

Ultimately the estate has an identical loom which was new last November so I could swap that over to see whether it makes any difference but I am loathed to disturb the existing one if it NOT the source of the problem (though I understand the need to replace it for the long term.)

 

[Roger Jones]

Estate? Identical engine?

I ask my mechanic to check the loom on my 1996 3.2 Coupé at every service. It's done half your mileage and has yet to exhibit any visible degradation or manifest any problems, but given all that I see discussed and recommended on various forums I am ever more anxious about the loom and inclined to get it changed. It seems to be one of those items to which "if it ain't broke, don't fix it" may not apply.

 

[kth286]

Car could have gone into "limp home" mode which does limit the revs etc.

Agree you need to read the codes as a first step.

 

[recycled]

Have you tried an inhaler in the engine?
Sorry could not resist