m103 engine misfires at 2000 rpm...

[Stringer]

....but runs very smoothly up to that point. I've just redone the valve seals and have noticed now that all the plugs are in great condition (no more oiling up). Just dont know where to start as it feels like it is being starved of fuel - but experience has taught me that usually when you suspect fuel, it is invariably ignition.

I've replaced so many ignition parts though EXCEPT the rotor arm, which looked in ok condition.

Is there a simple, shade-tree way of checking fuel pressure? And also the ignition coil?

Here's what i have checked/replaced etc

New HT leads
New plugs
New fuel filter
3 new fuel injectors (after tested at garage)
New Air filter
New temp sensor
'New' OVP (but checked it also with a multi meter)
ALSO checked duty cycle which is about 50% at warm idol
Repaired (resoldered) fuel relay

Any advice appreciated

M, Beirut

 

[Niks]

Does it misfire only during acceleration when you reach 2000rpm or it misfires in any situation at 2000rpm?

 

[Stringer]

if i accelerate from 1500, it starts to misfire when it gets to 2000 - but i have to say that the acceleration - although smooth - is not impressive.
Whats your gut instinct? Fuel or ignition?

 

[RBYCC]

What spark plugs did you install?
Have you checked the fuel pump relay next to the OVP.
FPR also acts as rev limiter and cold start valve...possibly the internal circuitry has weakened?

 

[John Jones Jr]

It could very possibly be the coil. Coils are cheap (or even borrow one from another car), easy job and one basic item eliminated. And as RBYCC mentioned a new FPR is a far better option that a repaired one.

 

[Vlad]

the 300 is prone to cap and rotor issues, so I would change both even if they look ok.

 

[Stringer]

yes, i have checked both the OVP and the fuel pump relay. To answer your question about plugs i am using the standard, recommended bosch h7dc plugs which every shop owner here recommends for these old mercs

 

[Niks]

If it misfires under acceleration and you lack power, it could be your fuel pressure. Have that checked out of you havent already. The fuel pressure should be operating at 5.3 to 5.5 bar (77 to 80psi).

If that all checks out, i would say replace the coil, distributor cap and rotor arm since they are the only bits which you havent already.

 

[Stringer]

yep, all great advice. In fact, i have suspected more recently that it is the fuel pressure. but the real money shot is how do i check the fuel pressure, roadside? Is there a neat little trick? or a cheap gadget? Or, can you check in more basic terms by taking the pipe off from the fuel distributor and checking the distance of its, er, spurt? shut up stringer.

 

[Niks]

Ah that I wouldnt know how to check it roadside. Maybe someone from here knows how to and will tell you.

Dont you want to take it to a garage?

 

[grober]

While it sounds like a high tension breakdown under load I wonder if its an ignition timing issue. Can't remember if you can check the timing with a strobe light on that model or not. Other possibility might be its sign the EZL ignition unit is overheating

 

[Niks]

You could use a vacuum hand pump with a gauge on it? You can use a rubber hose (very tight and no leaks) to connect it up to the fuel pressure regulator and see what the pressure is. Low pressure = found your problem.

This might be helpful...

http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/07.3-1603.pdf

 

[Stringer]

Hi Grober
did you really mean to write EZL - or did you mean coil?

 

[RBYCC]

M103-12V has fixed maps in the EZL.
Euro EZL's have settings to change advance for varied octane grades.
You can check timing with a strobe, but can't adjust same.

Not sure if the OP has checked the vacuum line to the EZL which can affect the fixed maps performance.

Vacuum leaks are typical on any 80's era Merc as the tubing becomes very brittle.
Always best to do a smoke test to find and repair any and all vacuum leaks.

OP appears to have proven fuel supply and misfire is typically ignition.
It will show in reading the spark plugs.
H7DC are correct for early euro engines and gapped at .032.

Rotor and cap are important and coil also.
Coil usually shows problems at higher RPM.

Ed A.

 

[Stringer]

An update for those who showed some interest. I did a 50 mile round trip and half way through had a fouled up plug on cylinder number six. (this pot always burned a little more oil than the others although still has good compression).

I dont think the problem is with the oily pot though. I think the plug fouling is just a hint to what are more cryptic symptoms to a greater problem.

I think my problem is fuel delivery, which is poor on two levels. One is i believe that the pressure is not good in general (which leads to a 'starved' distributor, which in turn leads to my number 6 plug oiling up); but secondly i also think that there is a dry joint somewhere in the circuitry between the relay and the pump as the engine can quite abruptly stop when idling - which i think IS a clue.

Here's my question. What is a smart, step-by-step approach if i am to go with the theory of, at least, POOR FUEL DELIVERY as a primary symptom.

I wanted to know if i put power directly to the fuel pump, perhaps via port 87 on the fuel pump relay - and let it run indefinately - would this BOOST the amount of gasoline to the fuel distributor and at least give me a clue as to whether i am getting closer to defining where exactly is the fault?

And if it did run better, with a direct line to the fuel pump, would this automatically tell me that my FPR has internal faults? or would it indicate something else?

The other thing i am curious to lknow is if i pinch the return pipe from the fuel distribitor, will this increase the pressure int the system - and thus show me that the problem is one of pressure?

thanks for all your help, Grober, Dieselman, John Jones, Niks et al

 

[Niks]

It may just be better for you to get a new fuel pump and fuel pressure regulator. Save yourself the hassle? Least then you know everything as far as the fuel side is all taken care off. It could even sort out your other issue once the pressure is back to normal.